Writing assignment
Jonathan van Leeuwen
Seeing Color? Where? Current Standing in Projector and
Associator Grapheme-Color Synesthesia Research.
RM Neuroscience and Cognition
Writing assignment
Jonathan van Leeuwen
Supervisor:
Dr. Paffen, C. L. E.
Second reviewer:
Dr. van der Smagt, M. J.
1 / 35
Writing assignment
Jonathan van Leeuwen
Abstract
Synesthesia is a neurological condition where a perceptual experience of a stimulus triggers a new perceptual
experience, which is not caused in non-synesthetes. For instance, in grapheme-color synesthesia, a black
grapheme “a” can induce the perception that the grapheme is colored “red”. It has been a matter of controversy
whether the synesthetic percept arises due to bottom up processing of the initial stimulus or if the stimulus needs
to be fully processed and subsequently induces the synesthetic percept by top down processing, due to evidence
apparently supporting both views. To explain these diverging findings, a distinction was made between
“projector” and “associator” synesthetes by Dixon, Smilek, & Merikle (2004). The former perceive the induced
color in the outside world, on top of the grapheme, while the latter perceive the color in their minds eye. This
articles reviews research which has addressed whether projectors and associators differ in neurobiology and/or
cognition as well as how differing methods of differentiation between projectors and associators influence
results. Results indicate that there are indeed cognitive and neurobiological differences between projector and
associator grapheme-color synesthetes and that the method used to distinguish between the two may influence
the results of the experiments. Suggestions for increasing reliability and accuracy of projector/associator
differentiation for future research are also discussed.
Keywords: Synesthesia; Projectors; Associators; Grapheme-color synesthesia; Differentiation; Bottom-up
processing; Top-down processing.
Introduction
“I see vague pictures of Bessel functions from Jahnke and Emde’s book, with light tanned j’s,
slightly violet-bluish n’s, and dark brown x’s flying around. And I wonder what the hell it
must look like to the students”. This quote from the Nobel prize winning physicist Richard
Feynman aptly captures the perceptual phenomenon known as synesthesia (Feynman, 1988).
Synesthesia is a neurological condition in which a perceptual or cognitive stimulus triggers a
sensory percept, then the sensory or cognitive processing of this stimulus triggers a new
sensory percept which is not directly caused by the original stimuli. This synesthetic percept
can arise in the same or in a different modality as the inducing stimulus. The stimulus that
causes the synesthetic percept is generally considered the “inducer” and the synesthetic
percept the “concurrent” (Grossenbacher & Lovelace, 2001). Thus, for a grapheme-color
synesthete seeing the letter “a” as “red”, the letter “a” would be considered the inducer and
the perceived color “red” would be the concurrent. Synesthesia was first described in 1812 by
an Austrian physician (Jewanski, Day, & Ward, 2009), however, the scientific community lost
interest in the phenomenon during the mid-twentieth century (Howells, 1944) as it was
explained as normal conditioning between two stimuli by behaviorist. It resurfaced in the late
1990’s (Baron-Cohen, Burt, Smith-Laittan, Harrison, & Bolton, 1996; Baron-Cohen &
Harrison, 1997) and was widely popularized by Ramachandran in 2001 (Ramachandran &
Hubbard, 2001a, 2001b, 2001c). Since the resurgence in the late 1990, it has become an
increasingly hot research topic in cognitive neuroscience and psychology, demonstrated with
a search in Scopus with “synesthesia” showing 64 articles published between 1960 and 1999
2 / 35
Writing assignment
Jonathan van Leeuwen
while the number of published synesthesia articles between 1999 and 2013 is more than 690.
Since synesthesia is thought to be a variation of normal sensory processing, it should therefore
be possible to explain this phenomenon with current cognitive models. Its ability to explain
normal cognition is thus the main driving force behind the increased popularity in synesthesia
research. The increase in new research technologies like electro encephalography (EEG) and
functional magnetic resonance imaging (fMRI) for cognition research has also made
synesthesia a hot topic in cognition and neuroscience research. A Number of different types of
synesthesia have been described, with the more common types being time and space
synesthesia, where synesthetes experience the months as arranged in a particular spatial order
(Brang, Teuscher, Ramachandran, & Coulson, 2010), week days inducing colors (Julia
Simner et al., 2006), sounds inducing colors (Banissy et al., 2012) and graphemes inducing
colors (Brang, Rouw, Ramachandran, & Coulson, 2011; Gheri, Chopping, & Morgan, 2008;
Jürgens & Nikolić, 2012). Grapheme-color synesthetes have been classified as projector and
associator synesthetes (Dixon et al., 2004) as a method of explaining divergent findings in
grapheme-color synesthesia. This review will start by giving a general overview of graphemecolor synesthesia, followed by a review of current neurological and cognitive literature on
projector and associator synesthesia research. How differing methods of differentiating
between projectors and associators causing non-repeatable results or lack of results, as well as
recommendations for future approaches to differentiating projectors and associators will be
given. Finally, this review will look at how the projector and associator differentiation can be
used in non-grapheme-color synesthesia research.
Synesthetic Classification
Certain criteria are used to differentiate between synesthetic percepts and normal associations.
One of the main criteria for a synesthetic percept is that it should be stable over time (BaronCohen et al., 1996; Eagleman, Kagan, Nelson, Sagaram, & Sarma, 2007). For instance, a
hypothetical grapheme-color synesthete with a particular “inducer”-“concurrent” coupling,
will always have the same concurrent for that particular inducer. In other words, if for a
synesthete the letter “a” induced the concurrent color “red”, the letter “a” would always
induce “red” and never “blue” for this hypothetical synesthete (Jamie Ward, 2013).
Synesthetic percepts should also be automatic, but depending on attention and awareness the
strength of the synesthetic experience varies (Mattingley, Payne, & Rich, 2006). The
“inducer” should also always precede the “concurrent”. In grapheme-color synesthesia this
means that the grapheme should always precede the perceived synesthetic color.
3 / 35
Writing assignment
Jonathan van Leeuwen
Traditionally, it was thought that synesthesia was unidirectional, thus that for grapheme-color
synesthetes only the grapheme could induce a color; and a color should not induce a
grapheme percept (Martino & Marks, 2001). However, recent evidence suggest that at least
under some circumstances, synesthesia appears to be bidirectional (Kadosh, Kadosh, &
Henik, 2007; Gebuis, Nijboer, & Van der Smagt, 2009).
Figure 1: Illustration of synesthetic experiences. A: Grapheme-color
synesthesia (letters and numerals with color) (“Synesthesia,” 2013). B:
Time-space synesthesia (Smilek, Callejas, Dixon, & Merikle, 2007)
Determining if someone has synesthesia was previously done by asking the subject
about synesthetic percepts and then label the person as either a synesthete or as a nonsynesthete (Ramachandran & Hubbard, 2001b; Ward, Li, Salih, & Sagiv, 2007).
Subsequently, subjective reports were usually followed up with consistency checks to be sure
the synesthetic percept was stable over time (Dixon et al., 2004; Rouw & Scholte, 2007). In
order to define synesthesia more consistently, Eagleman et al. (2007) developed an online
synesthesia questionnaire battery. This synesthesia battery includes a synesthesia
questionnaire, a color consistency check, a speed congruency check and a number of other
sub-tests (Eagleman et al., 2007). Since the introduction of the questionnaire by Eagleman et
al. (2007), this online tool has become quite popular and is currently the standard method for
determining synesthesia (Gebuis et al., 2009; Hupe, Bordier, & Dojat, 2011; Melero et al.,
2013; van Leeuwen, Petersson, & Hagoort, 2010). Besides the obvious advantage of
increasing the reliability of classifying synesthesia with a standardized test, this online test
also keeps track of all test results derived from synesthetes filling in the questionnaires. It is
therefore possible to do statistics on large groups of synesthetes as well as differentiating
4 / 35
Writing assignment
Jonathan van Leeuwen
between subgroups, making this a powerful tool for synesthesia research now and in the
future (Novich, Cheng, & Eagleman, 2011).
Bottom Up and Top Down Processing
How synesthetic percepts emerge from stimulus processing has been the subject of interest
since the 1990’s. One of the main questions was whether synesthesia was caused by bottom
up or top down processes (Ramachandran & Hubbard, 2001b; Smilek, Dixon, Cudahy, &
Merikle, 2001). If a stimulus is processed from simple to more complex features and/or from
lower to higher cognitive areas, this would be considered bottom up processing. Conversely,
top down processing is when stimulus processing is influenced from higher cognitive areas,
e.g. a Kanizsa triangle where one sees a whole triangle when part of the sides are actually
blocked (Frisby & Clatworthy, 1975). Ramachandran et al. (2001b) used an embedded figures
task, a screen filled with inducing and non-inducing black graphemes in which the inducing
graphemes were then organized such that they formed a figure when combined, and the
participants were asked to determine of the display contained said figure. In this task the
synesthete outperformed controls and Ramachandran et al. (2001b) took this as evidence for
bottom up of processing of synesthetic percepts. Different research groups have also found
experimental evidence supporting the assertion that synesthetic percepts are caused by bottom
up processing (Palmeri, Blake, Marois, Flanery, & Whetsell, 2002; Smilek et al., 2001).
Contrary to these findings is research indicating that synesthetic percepts are influenced and
might even be caused by top down modulation. For instance, Palmeri et al. (2002) had their
synesthete attend to a Navon stimulus (e.g. a “5” made from small “2”’s), while reporting
their synesthetic percept. If attending the global “5”, the synesthete reported to experience the
color percept related to “5” and if the synesthete attended to the small “2”’s, the color percept
relating to 2 was reported. This was taken as evidence for top down modulation affecting
synesthetic percepts. Similar findings showing evidence for top down processing have also
been reported by others (Ramachandran & Hubbard, 2001c; Rich & Mattingley, 2003; Dixon,
Smilek, Duffy, Zanna, & Merikle, 2006).
Cognitive Models of Synesthesia
A number of models have been proposed to explain how synesthetic percepts are formed. The
two most influential ones are the cross activation model (Ramachandran & Hubbard, 2001c)
and the disinhibited feedback model (Grossenbacher & Lovelace, 2001). Other less known,
but promising models, have also been proposed, like the cascade cross tuning model (Brang,
5 / 35
Writing assignment
Jonathan van Leeuwen
Hubbard, Coulson, Huang, & Ramachandran, 2010), the re-entrant feedback model (Smilek et
al., 2001) and the two stage hyper-binding model (Esterman, Verstynen, Ivry, & Robertson,
2006; Hubbard, 2007). Interestingly, some of these models suggest that abnormal brain
connectivity is the underlying cause for synesthesia, while other models maintain that
synesthetes have similar brain connectivity but that the synesthetic percept is caused by
functional differences.
Based on their findings of a pop-out effect in synesthesia, combined with the fact that
the grapheme area is located adjacent to the V4 color area, Ramachandran & Hubbard,
(2001c) argued that the synesthetic experience arises from early cross activation of areas
located spatially near each other in the brain (Ramachandran & Hubbard, 2001a, 2001b,
2001c). Ramachandran & Hubbard (2001c) proposed that synesthesia is caused by a failure of
pruning in the grapheme and V4 color area. Synesthesia has been shown to run in families,
which suggest that there is an underlying genetic cause (Baron-Cohen et al., 1996), which fits
in line with the pruning hypothesis of the cross activation model of synesthesia. Later
neurobiological studies also found increases in white matter in grapheme/color areas for
synesthetes compared to non-synesthetes (Rouw & Scholte, 2007). Their model posits that
when a synesthete sees a synesthesia eliciting grapheme, the grapheme area becomes active,
this area then automatically cross activates the V4 color area via horizontal connections and
the synesthetic experience is created (Ramachandran & Hubbard, 2001c; Hubbard, Brang, &
Ramachandran, 2011). The cross activation model suggests that synesthetes have abnormal
brain connectivity compared to non-synesthetes. While structural differences might explain
some effects of synesthesia, some argue that non-synesthetes can also experience synesthetic
percepts, evidence for this comes primarily from drug studies (Hartman & Hollister, 1963;
Marek & Aghajanian, 1998). If drug induced synesthetic percepts are indeed causally the
same as synesthetic experiences this would indicate that abnormal brain connectivity is not
necessary for explaining synesthesia.
The disinhibited feedback model suggests that synesthetes have less inhibition of
feedback signals, which in turn activate the synesthetic percept (Grossenbacher & Lovelace,
2001). It proposes that feedforward pathways from inducers converge with feedforward
pathways from concurrents in a multimodal cortical area. This multimodal area is then
thought to send feedback down the inducer pathway. In non-synesthetes this feedback only
travels down the inducer pathway and seeing a grapheme does therefore not induce a
synesthetic percept. For synesthetes on the other hand, it is thought that lack of inhibition
causes both the inducer and concurrent pathways to be activated by the feedback mechanism
6 / 35
Writing assignment
Jonathan van Leeuwen
(Figure 2). The activity in the top down activated concurrent pathway is thought to underlie
the synesthetic percept (Grossenbacher & Lovelace, 2001). This model suggests that there is a
functional difference between regular people and synesthetes and not a structural difference as
proposed by the cross activation model (Ramachandran & Hubbard, 2001c). The strength of
this model lies in its ability to explain synesthesia with current known brain mechanism, e.g.
feedforward and feedback mechanisms (Fahrenfort, Scholte, & Lamme, 2007; Lamme &
Roelfsema, 2000; Schmolesky et al., 1998). It does not rely on structural brain abnormality in
synesthetes as a way to explain the synesthetic percepts, but on functional differences
between synesthetes and non-synesthetes. The apparent synesthetic percepts caused by drugs
can be explained with the disinhibited feedback model, as drugs could cause reduced
inhibition and therefore activate a concurrent pathway in non-synesthetes (Marek &
Aghajanian, 1998). The disinhibited feedback model also gets support from research showing
higher order top down influence on synesthetic effects, for instance evidence showing that
synesthetic percepts can change due to attention (Palmeri et al., 2002). Interestingly, evidence
showing that synesthesia is not purely unidirectional but bidirectional also lends support for
the disinhibited feedback model of synesthesia (Gebuis et al., 2009; Kadosh et al., 2007).
Gebuis et al. (2009) used a number and color priming task to investigate bidirectional
synesthesia. They found that both numbers (inducers) as well as color patches (concurrents)
caused interference for detecting colors or numbers, respectively. Using EEG measurements
Gebuis et al. (2009) found that the effects found for bidirectional synesthesia originated in the
parietal lobe, indicating that the synesthetic percept does not arise in early sensory areas but in
higher processing areas. The disinhibited feedback model can explain this with neural activity
originating from the concurrent pathways activating spatial areas in the parietal lobe causing
activity to feedback into the inducer pathway causing bidirectional synesthesia effects.
The cascade cross tuning model is an updated model of the cross activation model
(Brang, Hubbard, et al., 2010; Ramachandran & Hubbard, 2001c). In this model Brang,
Hubbard, et al. (2010) propose that synesthesia is caused during hierarchical grapheme
analysis (Dehaene, Cohen, Sigman, & Vinckier, 2005; Grainger, Rey, & Dufau, 2008).
Hierarchical grapheme analysis posits that graphemes are processed from low complex forms
to higher integrated forms sequentially: simple components (e.g. curves and lines) of a
grapheme are analyzed, then they are combined in a bottom up fashion into the final complex
grapheme representation (Rey, Dufau, Massol, & Grainger, 2009). However, if there is
ambiguity as to what letter is seen, top down activity can influence how these features are put
together and which letter is finally perceived. The cascade cross tuning model therefore
7 / 35
Writing assignment
Jonathan van Leeuwen
argues that low level stimulus features cross activate the V4 color area. It suggests that each
feature then elicits its own associated color. When a grapheme is completely processed the
final synesthetic percepts stabilizes and the synesthete experiences the grapheme as having a
particular color. One prediction this model makes is that partially seen graphemes should
elicit a synesthetic color experience, but that this color does not necessarily have to be the
color associated with the complete grapheme. According to the cascade cross tuning model,
simultaneous incremental grapheme and V4 activity modulated by top down processes cause
the synesthetic percept, while the cross activation model posits that the activity from the
grapheme area to the V4 color area occurs after grapheme processing in a pure bottom up
fashion. Evidence from studies showing that synesthetes with a strong synesthetic color
percept show similar colors for similar graphemes support the cascade cross tuning model
(Brang et al., 2011).
Figure 2: Three of the cognitive models for explaining how synesthetic percepts might arise. (A) The cross
activation model posits structural differences between grapheme areas and color areas that result in grapheme
areas activating color areas. (B) The re-entrant feedback model suggests that higher order conceptual areas
feedback to earlier grapheme areas as well as color areas, thereby influencing the perception of the letter and
causing color percepts. This model does not require structural differences between synesthetes and nonsynesthetes and explains how cognition can influence synesthetic percepts (C) The disinhibited feedback
posits that feedforward information from grapheme areas converge in higher cortical areas, then feedbacks
into color areas due to the lack of feedback inhibition. This model does not require structural differences
between synesthetes and non-synesthetes. Figure originating from (Mulvenna & Walsh, 2006; Hubbard et al.,
2011)
Building on findings that conceptual information about graphemes influences perceived
color, Smilek et al. (2001) proposed the re-entrant feedback model. This model argues that
higher order conceptual areas (e.g. anterior fusiform areas) influence earlier processing areas
like the grapheme areas and color areas in a top down fashion. Evidence for their model
comes from studies showing that ambiguous and/or conceptual information influence digit
detection tasks (Smilek et al., 2001). One example is the conceptual CAT task, were the
perception of the “A” in “CAT” as “A” or “H” influences the perceived color (Ramachandran
& Hubbard, 2001c). Several other studies have also found evidence for top down modulation
8 / 35
Writing assignment
Jonathan van Leeuwen
of the synesthetic percept, supporting the re-entrant feedback model (Rich & Mattingley,
2003; Dixon et al., 2006).
Based on structural results showing that synesthetes also show neurobiological
differences in the parietal lobe (Rouw & Scholte, 2007) as well as trans-cranial magnetic
stimulation studies indicating that the parietal lobe is necessary for grapheme-color binding in
synesthesia (Esterman et al., 2006), the cross activation model was modified to the two stage
hyper-binding model (Esterman et al., 2006; Hubbard, 2007; Robertson, 2003; Rouw &
Scholte, 2007). The two stage hyper-binding model posits that graphemes are first cross
activated in grapheme-color areas but subsequently need to be bound together in parietal areas
for the synesthetic percept to arise. For a schematic overview of the cross activation, reentrant feedback and disinhibited feedback model see Figure 2.
To summarize these models, the cross activation model, the cascade cross tuning model
and the two stage hyper-binding model suggest that there should be increased structural
connectivity between grapheme areas and the color areas. The other models posit that there
should be functional differences in brain areas related to synesthesia, but not necessarily
dependent on structural differences. Some of the models explain apparent bottom up effects,
while others explain top down effects of synesthesia. Most of the models can also be adjusted
in such manner that they could account for both types of processing (see Hubbard et al., 2011
for an example). However, as mentioned, similar experiments have sometimes produces
differing results concerning bottom up and top down processing. The next section will turn to
one proposed explanation for these diverging findings as which is also the main topic of this
review.
Projector and Associator Origin
Dixon et al. (2004) proposed a dichotomy in personal synesthetic experience as a way of
explaining why both bottom up and top down effects are found using similar methods. Color
grapheme synesthetes often describe their synesthetic experience either as seeing the color on
the grapheme itself (Ramachandran & Hubbard, 2001b) or in their minds eye (Dixon et al.,
2004). Dixon et al. (2004) distinguished between these two types of synesthetes and named
the synesthetes that see the color on the paper “projectors” and the synesthetes that see the
color in their minds eye as “associators”. They used subjective reports as a method of
distinguishing between projectors and associators. By using their criteria for projectors and
associators, the quote by Feynman (1988) would classify him as a projector synesthete. Dixon
et al. (2004) gave their associator and projector synesthetes a synesthetic stroop interference
9 / 35
Writing assignment
Jonathan van Leeuwen
task and found that projectors showed larger interference caused by synesthetic color
compared to the veridical color. The associators showed the reverse effect, i.e. larger
interference caused by veridical color compared to the synesthetic color (Dixon et al., 2004).
Based on these results, they suggested that the underlying cause for synesthesia differs
between projectors and associators. It was proposed that cross activation between areas
involved in conceptual representation of numerals and colors might account for associator
type experiences (Ramachandran & Hubbard, 2001c; Smilek & Dixon, 2002; Dixon et al.,
2004). Projector type experiences could be caused by reentrant feedback, caused by an
interaction of feedforward activity from V1 to grapheme areas, then activity might feedback
from grapheme areas into early color specific areas when an inducing grapheme form is
recognized (Dixon et al., 2004; Grossenbacher & Lovelace, 2001). Considering the
differences between projectors and associators, Dixon et al. (2004) stressed the importance of
differentiating between them when doing synesthesia research to increase reliability and
validity of findings.
When Dixon et al. (2004) proposed their view on synesthesia, very little fMRI
research had been done on synesthesia. Currently, fMRI research is flourishing and a number
of fMRI studies have been done to identify the neural underpinnings of synesthesia, including
fMRI research for the projector/associator dichotomy (Hupe et al., 2011; Sperling, Prvulovic,
Linden, Singer, & Stirn, 2006; van Leeuwen et al., 2010; Melero et al., 2013). This review
will consider the current cognitive and neurobiological findings regarding associator and
projector synesthetes and how the current research fits with the explanatory model proposed
by Dixon et al. (2004). It will also take a critical look at how the literature on associators and
projectors has differentiated between these types and how research on this topic may be
improved in the future, for instance by expanding the projector associator distinction to
include other types of synesthesia (besides grapheme-color synesthesia).
Differences Between Projectors and Associators
The synesthete in the experiment by Ramachandran & Hubbard (2001b) was described as
seeing color out in space overlying the real grapheme. Thus for this synesthete the synesthetic
color perception was similar to seeing a colored grapheme. Other synesthetes describe their
perception as a feeling that the grapheme elicits a percept of a color in their mind’s eye
(Dixon, Smilek, Cudahy, & Merikle, 2000). It has since been proposed that synesthetes who
perceive color in the outside world and synesthetes who perceive color in their mind’s eye
have differing neurobiological causes for their synesthetic percept (Dixon et al., 2004). A
10 / 35
Writing assignment
Jonathan van Leeuwen
projector percept might be explained by one neurobiological model while an associator
percept might be explained by another model (Dixon et al., 2004). Since the article by Dixon
et al. (2004) on the difference between projector and associator synesthesia, a number of other
articles have been published that have specifically investigated the differences in cognition
and neurobiology for these two types of grapheme-color synesthesia (see table 1).
Cognitive Differences Between Projectors and Associators
The distinction between projector and associator synesthesia was first made by Dixon et al.
(2004). They divided their group of synesthetes into a group of projector synesthetes which
saw the color as projected onto the viewed grapheme and a group of associator synesthetes
which saw the color in their mind’s eye. Using a stroop grapheme-color task they showed
inducing graphemes in congruent and incongruent synesthetic colors. The results indicated
that projector synesthetes showed increased stroop interference from the synesthetic colors
compared to the veridical colors. Contrary, the associator synesthetes showed increased
interference from veridical colors compared to induced colors. Indicating that synesthetic
experiences can interfere with normal visual processing of graphemes, although this
interference effect appears to be mainly for projector synesthetes and not for associator
synesthetes.
Ward et al. (2007) replicated these behavioral results with a similar stroop task and
again showed that projectors had increased interference from induced colors and that
associators had increased interference from veridical colors. In their experiment, Ward et al.
(2007) showed synesthetes four inducing or non inducing graphemes on a screen, with an
inducing target grapheme which they had to detect in the middle of the other graphemes.
Projectors did not differ significantly from associators at this task. If projectors had a real pop
out effect like Ramachandran & Hubbard (2001a) proposed, it would be expected that
projectors outperformed the associators, at least in the non inducing crowding experiment as
projectors could have used the color to identify the grapheme while associators would be less
able to do so. Having thought of this Ward et al. (2007) had the participants indicate any color
they perceived during this task. Neither the projectors nor the associators saw the colors they
should for the particular graphemes, that is, if color was perceived it did not match the normal
grapheme-color association. Indicating that there was no true pop out effect of synesthetic
color perception for projectors or for associators. Interestingly however, projectors indicated
that they did experience some form of color perception, while associators did not report any
color perception. This would indicate that there is early low level grapheme processing
11 / 35
Writing assignment
Jonathan van Leeuwen
influencing synesthetic percepts for projectors but not for associators. These results fall in line
with the CCT model of synesthesia, at least for projectors (Brang, Hubbard, et al., 2010), as it
would appear that projectors show some form of incremental grapheme-color processing.
Gebuis et al. (2009) tested for bidirectional effects in grapheme-color synesthesia and tested
for differences between projectors and associators. Although they did not find evidence for
bidirectional differences between projector and associator synesthesia, they did find
differences in EEG results when they split their group based on behavioral data, which they
took as evidence for the low/high distinction proposed by Ramachandran & Hubbard (2001c).
However, the participants were not tested with a stroop task to test if the projector/associator
groups showed the characteristic priming effects expected based on the results by Dixon et al.
(2004) and Ward et al. (2007), Gebuis et al. (2009) also used subjective reports to distinguish
between projectors and associators. The lack of significant differences between projectors and
associators found by Gebuis et al. (2009) might be caused by the method of differentiation,
but this will be discussed more in a later section. Brang et al. (2011) did an interesting study
which tested the assumption, based on the CCT model, that similar graphemes elicit similar
colors. They got 52 synesthetes to accurately define the colors they perceived for a number of
graphemes. Subsequently, they calculated a measure of similarity between colors and
graphemes. Similar graphemes did indeed have similar colors, as expected, which indicates a
bottom up effect of synesthetic percepts, which fits nicely with the CCT model of synesthesia.
Brang et al. (2011) also defined their synesthetes on a range from 5 to -5 on a
projector/associator scale, with projectors on the positive end of the scale and associators on
the negative end of the scale. They then correlated the letter/color similarity score for each
synesthete with the projector/associator score of each synesthete and found a significant
positive correlation. The synesthetes scoring high on the projector/associator (projectors)
scale thus show the highest grapheme color similarity, while the lower scoring synesthetes
(associators) show less grapheme color similarity. Saiki, Yoshioka, & Yamamoto (2011) used
a type/token paradigm to test whether associator synesthesia was caused by extreme forms of
normal association between stimuli. A type is a category specific stimulus, e.g. defining a
stimulus as the letter “A” or a color as “red”. Token representations are formed by binding a
type to an episodic memory objects, e.g. the letter “a” is “red”. Normal associations are token
representations, thus if a synesthetic experience is found to be caused by token association it
would indicate that it is an extreme form of normal association. Saiki et al. (2011) presented
inducers above or below a fixation cross followed by color patches that were either at the
correct location (same location as inducing grapheme) or incorrect location (different location
12 / 35
Writing assignment
Jonathan van Leeuwen
than inducing grapheme). They also trained control subjects on color grapheme associations.
Their results indicated that while controls did show token-based responses, associator
synesthetes showed type-based associations between graphemes and colors. This implies that
associator synesthesia is not merely an extreme adaptation of normal associations but appears
to be caused by a different mechanism. Unfortunately they did not include projector
synesthetes. Rich & Karstoft (2013) did an experiment on projector/associator synesthetes.
First they did the projector/associator stroop task to determine if their projector synesthetes
did indeed show increased interference from induced colors compared to the veridical colors.
Secondly they used a variant of the embedded figures search task (Ramachandran & Hubbard,
2001b) to test for pop out effects of synesthesia induced colors. Falling in line with previous
results (Dixon et al., 2004; Ward et al., 2007), they also found that projectors showed
increased interference from induced colors compared to veridical colors, while associators did
not. In their second experiment, they did not find any evidence for a pop out effect compared
to matched controls. However, one of the two projectors did show a non-significant effect for
pop out. Rich & Karstoft (2013) did show that synesthetes were better overall on detecting the
embedded figure, especially when more distracters were used. They concluded that
synesthetes show increased ability for grouping objects, but that they did not show a true pop
out effect.
These results suggest that projectors rely more on bottom up processes than
associators do as projectors show increased interference from induced color, higher similarity
between grapheme shape and induced color and experience color perception under conditions
were associators do not. The results also show that the synesthetic experience is not just
extreme versions of normal object associations. These differences in cognition between
projectors and associators may be caused by associators simply having weaker connections
between grapheme areas and color areas. If these connections were stronger, associators
might become projectors. On the other hand, it might also indicate that the underlying neural
cause for the synesthetic percepts differs more fundamentally between projectors and
associators. With differing underlying neurological causes for projector and associator
synesthetes. These hypotheses will be investigated in the next section which focuses on
neurobiological differences between projectors and associators.
Neurobiological Differences Between Projectors and Associators
The first imaging study to specifically test for neurobiological differences between projector
and associator synesthetes was done by Rouw & Scholte (2007). They used diffusion tensor
13 / 35
Writing assignment
Jonathan van Leeuwen
imaging (DTI) and fMRI to distinguish differences between synesthetes and controls as well
as between projectors and associators. Using a projector/associator questionnaire they scored
synesthetes between 4 and -4, with a high positive number indicating strong projector
synesthetes while a low negative number indicated strong associator synesthetes. During the
fMRI scan, participants watched inducing and non inducing graphemes. Both the projectors
and associators showed increased white matter coherence in the left parietal lobe. A
significant positive correlation was found between white matter coherence in the inferior
temporal lobe and score on the projector/associator questionnaire. This area is next to the
fusiform gyrus, previously implicated in the perception of visual stimuli (Cohen et al., 2000;
Kanwisher, McDermott, & Chun, 1997) and is adjacent to color perception areas (McKeefry
& Zeki, 1997). Rouw & Scholte (2007) also found increased BOLD responses in the right
inferior temporal lobe. However, no differences were found in BOLD response between
projectors and associators. Using a free viewing task and a repetition suppression task, van
Leeuwen et al. (2010) found increased BOLD effects for synesthetes in the left parietal cortex
as well as the right fusiform gyrus. No significant differences were found between projectors
and associators during the free viewing task. The repetition suppression task showed that
synesthetic colors could suppress BOLD responses in the left superior parietal lobule,
however not in areas previously found related to veridical color processing. In the repetition
suppression condition differences were found between associators and projectors, with
associators showing increased activity in the left superior parietal lobule. They argued that
their results support the two stage hyper-binding model by Hubbard (2007) which suggests
that synesthesia activates color areas and that the grapheme percept and color percept are
subsequently bound in the parietal cortex. Based on this, the results by van Leeuwen et al.
(2010) indicate that synesthesia is caused by both bottom up process as well as higher order
binding mechanisms, but associators appear to really more on higher order parietal binding
areas for their synesthetic percepts (van Leeuwen et al., 2010). Continuing their research
between projectors and associators, Rouw & Scholte (2010) used MRI and fMRI to measure
structural and functional grey matter (GM) differences. They used the same method for
defining projectors and associators on a continuous scale as previously described (Rouw &
Scholte, 2007). Overall, an increase of grey matter was found in the parietal cortex for
synesthetes. Synesthetes also showed increased BOLD responses in parietal as well as frontal
areas. Projectors showed increased GM compared to associators in early sensory areas, e.g.
V1 and pre-central gyrus, as well as in the frontal cortex, while associators showed increased
GM in higher order areas involved in memory, e.g. the hippocampus and the parahippocampal
14 / 35
Writing assignment
Jonathan van Leeuwen
gyrus. The functional results showed that projectors did not have significantly increased
BOLD responses compared to associators during free viewing. Associators did show
increased BOLD responses compared to projectors between fusiform areas and hippocampal
areas. Associators also showed increased BOLD in the right parietal cortex compared to
projectors. Taken together, the results by Rouw & Scholte (2010) indicate that projectors rely
more on early sensory regions, while associators show effects in later cognitive areas. This
indicates that projectors and associators use different mechanisms for producing their
synesthetic percepts. It has been demonstrated that grapheme areas, color areas and parietal
areas are activated in synesthetes during the synesthetic percept (Rouw & Scholte, 2010;
Specht, 2012; van Leeuwen et al., 2010). How these areas interact was tested by van Leeuwen
et al. (2011) who used dynamic causal modeling (DCM) to determine temporal onsets of these
areas for projectors and associators. The synesthetes were given a projector/associator score
from 8 to -8, similar to Rouw & Scholte (2007, 2010), with projectors on the positive side and
associators on the negative. By defining regions of interest based on previous research as well
as indicating different activation patters, DCM analysis creates a probability for each of these
predefined models. van Leeuwen et al., (2011) used a letter shape area in the fusiform gyrus,
the V4 color area and the superior parietal lobule as predefined ROI’s on which to base the
DCM analysis. Based on previous research van Leeuwen et al., (2011) hypothesized that the
functional connectivity pattern should either be bottom up activation from grapheme areas to
color areas followed by binding in the parietal lobule, or top down activation due to grapheme
areas activating the parietal lobule which in turn would activate the color area. The
participants had to watch inducing and non-inducing stimuli during the experiment.
Interestingly, they found evidence for both the bottom up and the top down processing model.
Not surprisingly, there was a high correlation between projector/associator score and the
calculated bottom up/top down score. Showing that projectors are more likely to use bottom
up processing with the grapheme area activating the color area then activating the parietal
area. While associators are more likely to show top down processing with the grapheme area
activating the parietal area which in turn activate the color area. These results again imply that
projectors and associators differ substantially in the processing of the synesthetic experience.
Melero et al. (2013) used structural MRI on associator synesthetes and controls to
determine GM and white matter (WM) volume differences between these groups. They found
increased GM for synesthetes in hippocampal areas, the right temporal cortex, the left inferior
parietal sulcus and frontal areas. WM volume increases were found in frontal areas for
synesthetes. These results show that associator synesthetes show differences in areas related
15 / 35
Writing assignment
Jonathan van Leeuwen
to higher cognitive functions and areas related to binding mechanisms. They did not find
differences in the fusiform gyrus, which is the location of the color area. Based on the results
showing that associators use top down process and not bottom up processes these results are
not unexpected.
Evidence from neurobiological studies on projector and associator differences show
that parietal binding and temporal grapheme areas are important for synesthetic percepts.
During free viewing tasks projectors and associators do not appear to show differing BOLD
responses, indicating that the areas they use overlap during sustained synesthetic percepts.
However, structural and temporal measurements show that projectors rely more on early
sensory areas while associators rely more on parietal binding and temporal memory areas.
Indicating that projector synesthesia is more related to bottom up sensory processing while
associator synesthesia is more related to top down cognitive processing. Not all the
neurobiological studies showed differences between projectors and associators, but one which
did not, showed differences between higher and lower synesthetes. The higher/lower
distinction has been used for grapheme-color synesthesia research and it has been suggested
that the projector/associator distinction maps onto this distinction (Dixon et al., 2004). The
next section will therefore look at evidence for the higher/lower distinction and its
relationship with the projector/associator distinction.
Higher and Lower Synesthetes
Another distinction which has been made for grapheme-color synesthesia is the higher/lower
distinction proposed by Ramachandran & Hubbard (2001c). They proposed that higher
synesthetes show more conceptual synesthetic experiences, i.e. a number should elicit the
same color whether it is seen as a numeral or written out as a word. Higher synesthetes are
also thought to have more conceptual synesthesia types, for instance spatial form synesthesia
were days or weeks are arranged spatially. Lower synesthetes on the other hand should not
experience the same induced colors for conceptually similar but physically different stimuli
according to the higher/lower distinction. Ramachandran & Hubbard (2001c) suggested that
lower synesthetes had increased cross activation between grapheme areas and color areas. For
higher synesthetes they suggested that the synesthetic percept was caused by increased cross
activation between the angular gyrus and the superior temporal sulcus. Thus synesthesia
elicited in higher synesthetes is thought to be more conceptually driven while synesthesia for
lower synesthetes is stimulus driven (Ramachandran & Hubbard, 2001c). Hubbard, Arman,
Ramachandran, & Boynton (2005) used a crowding task combined with fMRI to differentiate
16 / 35
Writing assignment
Jonathan van Leeuwen
between higher/lower synesthetes. They found a correlation between V4 activity while
watching graphemes and the ability to detect the grapheme in the crowding experiment. They
thus defined lower synesthetes as those who were the best at detecting the grapheme in the
crowding experiment.
To test whether projector/associator synesthesia was the same as lower/higher
synesthesia, Ward et al. (2007) used the same stroop task as Dixon et al. (2004) used, to
differentiate projectors from associators. Ward et al. (2007) showed that projectors had higher
stroop interference from induced colors while associators showed higher interference from
veridical colors, replicating previous findings (Dixon et al., 2004). They then used a crowding
task based on the one used by Hubbard et al. (2005). Projectors did not outperform associators
on their crowding task, which indicates that the projector/associator distinction is not the same
as the lower/higher distinction. However, it should be noted that projectors did report
experiencing synesthetic color percepts while associators did not. Gebuis et al. (2009) tested
for bidirectional effects of synesthesia in projector/associator synesthetes with a number-color
and color-number task. Their results did not show any significant differences between
projectors and associators. However, based on their behavioral data they could split the
synesthetes into a group with high priming effects and a group with low priming effects. The
group with high priming effects showed EEG effects at parietal areas as well as frontal areas,
the group with low priming effects only showed EEG effects at frontal areas. They interpreted
these findings as lower synesthetes showing lower perceptual effects and higher conceptual
effects, while higher synesthetes only have higher conceptual effects. Their findings thus
supports the idea that the projector/associator distinction is not the same as the lower/higher
distinction. However, it should be noted that Gebuis et al. (2009) differentiated projectors and
associators as a dichotomous group based on questionnaires asking about their perception.
Taken at
face value, experiments testing whether the projector/associator
differentiation maps onto the higher/lower distinction appear to indicate that these two
different classifications are not the same. However, associators and projectors do show
perceptual and neurobiological similarities to the hypothesized higher/lower distinction.
Projectors experience color during fast grapheme search tasks as well as showing
neurobiological effects in early sensory areas while associators do not experience color during
fast grapheme tasks and show neurobiological effects in higher conceptual areas. Considering
that Skelton, Ludwig, & Mohr (2009) showed that merely asking synesthetes about their
17 / 35
Writing assignment
Jonathan van Leeuwen
Table 1: Articles published that specifically test for cognitive and/or neurobiological differences between projector and associator synesthetes.
Participants
Synesthesia
Determination
Subj. rep
P or A
Determination
Subj. rep
Results
Projectors
Higher interference from induced
colors compared to real colors
Results
Associators
Lower interference from
induced colors compared to
real colors
Supported model
Projectors
Re-entrant
feedback
Supported model
Associators
Cross activation
Synesthetic stroop task
18 (18F; 0M)
7 Projectors
11 Associators
Subj. rep &
Consistency check
12 item P & A
questionnaire
DTI and fMRI. Inducing and
non-inducing stimuli
Increased structural connectivity in
the right inferior temporal lobe
compared to associators
R - inferior temporal cortex
L - parietal cortex
Bilateral - frontal cortex
Sub. rep
Synesthetic stroop task &
conscious/unconscious
visual crowding task
Replicated Dixon et al 2004. Color
experience when stimuli was
unconsciously perceived
Questionnaire &
Consistency check
Sub. rep
EEG. Number  Color &
Color  number priming
task
***** “Lower”: Showed parietal
and frontal priming effects
Reduced structural
connectivity in the right
inferior temporal lobe
compared to projectors
Replicated Dixon et al 2004.
No color experience when
stimuli was unconsciously
perceived
***** “Higher”: Showed only
frontal priming effects
Cross activation
(more than
associators)
Cross activation (Less
than projectors)
Rouw & Scholte 2007
14 (10F; 4M)
7 Projectors
7 Associators
Subj. rep &
Consistency check
Cross activation
Disinhibited feedback
Ward, Li, Salih &
Sagiv 2007
14 (13F; 1M)
6 Projectors
8 Associators
Frontal cortex
Parietal cortex
Late and early
processing for
“lower” synesthets
Late processing for
“higher” synesthets
Gebuis, Nijboer, van
der Smagt 2009
21 (19F; 2M)
7 Projectors
6 Associators
8 “MS” Projectors*
Questionnaire &
Consistency check
Subj. rep and
P&A
questionnaire
fMRI. Inducing & noninducing stimuli.
Repetition suppression task
No completely overlapping areas for
veridical and induced color.
Increased activity in the left superior
parietal lobule
No completely overlapping
areas for veridical and induced
color. Increased activity in the
left superior parietal lobule
Ventral occipital areas
L – superior parietal lobule
Integrated model
Integrated model
van Leeuwen,
Petersson & Hagoort
2010
42 (42F; 0M)
16 Projectors
26 Associators
Subj. rep &
Consistency check
(R & S 2007)
***
sMRI & fMRI. Inducing and
non-inducing stimuli
Increased GM in parietal cortex.
Increased activity in areas related
stimuli processing and frontal areas
Increased GM in parietal
cortex. Increased activity in
areas related to memory
L – superior parietal cortex
Frontal cortex
Temporal cortex ****
Early processing
models
Late, integrative
processing models
Rouw & Scholte 2010
52 (Gender)**
16 Projectors
36 Associators
(Eagleman et al.,
2007)
(R & S 2007)
***
Color identification of a
number of similar and
dissimilar graphemes
Positive correlation between
strength of projection and grapheme
color similarity.
Same pattern as projectors, but
lower than projectors
Cascade cross
tuning (Stronger
than associators)
Cascade cross tuning
(weaker than
projectors)
Brang, Rouw,
Ramachandran &
Coulson 2011
19 (17F; 2M)
8 Projectors
5 Associators
6 “MS” Projectors*
Questionnaire &
Consistency check
(R & S 2007)
***
fMRI. Dynamic Causal
Modeling. Viewing
graphemes
Activation pattern:
Grapheme area  V4  Parietal
area
Activation pattern:
Grapheme area  Parietal area
 V4
Early processing
models
Disinhibited feedback
van Leeuwen, den
Ouden & Hagoort 2011
8 (4F; 4M)
0 Projectors
8 Associators
(Eagleman et al.,
2007)
Subj. rep
Graphemes presented above
or below fixation point
followed by a color patch
NA
8 (7F; 1M)
2 Projectors
6 Associators
Questionnaire &
Consistency check
(Edquist 2006)
Synesthetic stroop task &
Modified embedded figure
task
8 (7F; 1M)
0 Projectors
8 Associators
(Eagleman et al.,
2007)
Not reported
sMRI & DTI
12 (11F; 1M)
5 Projectors
7 Associators
Method
Brain areas
NA
NA
NA
Fusiform grapheme area
Superior parietal lobule
V4 color area
Authors
Dixon, Smilek &
Merikle 2004
Color association is type based.
Qualitatively different from
normal associations
NA
NA
Integrated model
(qualitatively different
color associations)
Saiki, Yoshioka &
Yamamoto 2011
Replicated Dixon et al 2004. Overall
no evidence for pop-out.
Better at grouping items.
Replicated Dixon et al 2004.
Better at grouping items
NA
Early/late
processing models
Late processing models
Rich & Karstoft 2013
NA
Increased GM in areas related
to emotional/attention
processing, projecting to
parietal areas
NA
Areas suggest late
integrating processing
models
Melero, Melian, Lago,
Pajares, Tamames, &
Linera 2013
Temporal cortex
Frontal cortex
Parietal cortex
Occipital cortex****
* Mental screen projectors. ** Gender not reported. *** Rouw & Scholte., 2007, P & A questionnaire. **** See relevant article for a complete list of areas. ***** No difference between P & A. Split group into
“lower” and “higher” synesthetes. Sub. Rep (Subjective reports). P & A (Projector & Associator). All studies used grapheme color synesthetes.
18 / 35
Writing assignment
Jonathan van Leeuwen
perception is not very reliable. While van Leeuwen et al. (2011) showed a correlation between
projector/associator strength and functional connectivity results indicating that the
projector/associator distinction should not necessarily be dichotomous. It would therefore
appear that the method of differentiating between projectors and associators might influence
subsequent results, which could explain why Gebuis et al. (2009) failed to find similarities for
projector/associator synesthesia and lower/higher synesthesia. How differentiating between
projectors and associators may influence experimental results is the topic of the next section.
Projector and Associator Differentiation
The early studies on synesthesia used subjective reports combined with consistency checks,
which is considered an indicator of genuine synesthesia (Baron-Cohen, Wyke, & Binnie,
1987), to distinguish between synesthetes and non-synesthetes (Baron-Cohen et al., 1996;
Dixon et al., 2000; Ramachandran & Hubbard, 2001a). One problem with subjective reports is
that they are not standardized and are open for interpretation. This can result in low test-retest
reliability (Edquist, Rich, Brinkman, & Mattingley, 2006). This problem was tackled by
Eagleman et al. (2007), by creating a standardized online synesthesia test battery, which is
currently used to determine synesthesia in a number of studies (Brang, Hubbard, et al., 2010;
Novich et al., 2011). However, many studies still use subjective reports and consistency
checks despite of these advances (Gebuis et al., 2009; Rich & Mattingley, 2010; Rouw &
Scholte, 2007; van Leeuwen et al., 2010). It must be noted that Rouw & Scholte (2007) also
created a questionnaire which often is used, but this questionnaire is not an online aggregating
questionnaire and only tests for grapheme-color synesthesia (Brang et al., 2011; Paffen, van
der Smagt, & Nijboer, 2011). The test-retest reliability of their questionnaire is also not as
high as it could be if it had included illustrations depicting synesthetic experiences (Skelton et
al., 2009).
When Dixon et al. (2004) introduced the projector and associator synesthesia
dichotomy, the normal procedure was to base synesthesia categorization on subjective reports.
They therefore split their groups based on subjective reports of the spatial location of the
synesthetic experience. If the synesthetes reported seeing the induced color on top of the
grapheme they were classified as projectors. If they saw the induced color in their “mind’s
eye” they were classified as associators. Since the introduction of the projector/associator
distinction for grapheme-color synesthetes, researchers have distinguished them using three
different methods: questionnaire for dichotomous grouping, subjective reports and a
questionnaire for grouping on a continuum (Edquist et al., 2006; Gebuis et al., 2009; Rich &
19 / 35
Writing assignment
Jonathan van Leeuwen
Karstoft, 2013; Rouw & Scholte, 2007; Skelton et al., 2009; van Leeuwen, den Ouden, &
Hagoort, 2011).
To examine test-retest reliability for the projector/associator distinction Edquist et al.
(2006) split the synesthetes into associators or projectors based on three questions. The
synesthetes had to describe their perception as: “out there in space”, “in my mind’s eye” or
“neither”. Edquist et al. (2006) found that synesthetes showed low test-retests reliability in
how they described their synesthetic percept. Some synesthetes also reported that their
synesthetic experience could not be classified as projector or associator and answered
“neither”. However, they could be mental screen projectors, which are synesthetes who
perceive the color as if it was projected on a screen in front of them but not on the grapheme
or only in their mind’s eye (Rouw & Scholte, 2007; van Leeuwen et al., 2011). These mental
screen projectors thus differ from both projectors and associators when it comes to the special
location of their synesthetic percept. Based on these results, Edquist et al. (2006) argued that
subjective reports are not a reliable method for distinguishing between projectors and
associators. However, their results might be caused by difficulty in relating these statements
to their perception (Skelton et al., 2009). Skelton et al. (2009) used the descriptive synesthetic
experience questionnaire (DSEQ) to distinguish between projectors and associators. Just like
Edquist et al. (2006), they found low test-retest reliability using purely descriptive statements.
They then made the illustrated synesthetic experience questionnaire (ISEQ) by adding
illustrative pictures to the questions relating to special position of their synesthetic experience.
The ISEQ showed significantly increased test-retest reliability compared to the DSEQ in its
ability to reliably distinguish between projectors and associators (Skelton et al., 2009).
Unfortunately, researchers have yet to adopt the ISEQ for distinguishing between projectors
and associators as no study has yet used the ISEQ. These findings highlight the importance of
standardization of questions and the need for clear unambiguous descriptions of projector and
associator type experiences. It could also explain why one of the synesthetes tested by Edquist
et al. (2006) reported experiencing vivid colors, but could not relate to any of the questions.
Using the same questionnaire as Edquist et al. (2006), Rich & Karstof, (2013) split their
synesthetes into projectors and associators and tested for pop out effects of synesthetic colors.
van Leeuwen et al., (2010) also used an adapted version of the projector associator
questionnaire for splitting their participants into a projector/associator dichotomy (Rouw &
Scholte, 2007).
As mentioned earlier, the use of subjective reports for distinguishing between
projectors and associators was introduced by Dixon et al. (2004). Following in their footsteps,
20 / 35
Writing assignment
Jonathan van Leeuwen
a number of other articles have used subjective reports to distinguish between these two
groups (Gebuis et al., 2009; Saiki et al., 2011; Ward et al., 2007). Thus, based on the
synesthetes’ reports of seeing the color out in space or merely seeing it in their “minds’s eye”,
they were split into projectors and associators. Ward et al. (2007) replicated the findings of
Dixon et al. (2004): they again showed that projectors had increased interference from
induced colors compared to veridical colors. Saiki et al. (2011) also used subjective reports as
the method for differentiating the synesthetes, however they only had self-reported associators
in their group of synesthetes. Using this method of differentiation, Gebuis et al. (2009) were
unable to find differences between projectors and associators. They also split their group
based on low or high stroop interference effects. When split on using behavioral measures,
their two groups did show EEG differences. Interestingly, the groups that were split up based
on the behavioral data contained an equal amount of self reported projectors (Gebuis et al.,
2009) and if the projector/associator distinction is the same as the low/high distinction, which
it appears to be, their findings underscore that subjective reports are not always reliable.
The last method used for differentiating between projectors and associators uses a
slightly
different
approach,
using
questionnaires
to
assign
the
synesthetes
a
projector/associator score on a continuum (Rouw & Scholte, 2007). By using a number of
questions which relate to projector and associator type experiences and having the participant
rate each statement on a likert scale, the summed associator score is subtracted from the
summed projector score and gives each synesthete a projector/associator score. If the score is
positive synesthetes relate their synesthetic experience more with the projector descriptions, if
the score is negative synesthetes relate their synesthetic experience more with the projector
description. Thus instead of splitting the synesthetes into dichotomous groups, this method
treats the projector/associator distinction as a continuum. The strength of this approach lies in
its versatility, it can be used to group synesthetes dichotomously (van Leeuwen et al., 2010)
or group them on a continuum allowing for a correlation approach (Brang et al., 2011; Rouw
& Scholte, 2007, 2010; van Leeuwen et al., 2011). Interestingly, all studies that used a
continuum score and a correlation approach found significant differences between projectors
and associators. van Leeuwen et al. (2011) had a number of self reported mental screen
projectors in their participant group. Using the continuous projector/associator score, self
reported associators showed negative score, self reported projectors showed high positive
scores, while mental screen projectors scored between the other two groups. This again
highlight the importance of correctly distinguishing between synesthetes, as a mental screen
21 / 35
Writing assignment
Jonathan van Leeuwen
projector might be grouped with associators or with projectors if dichotomous grouping based
on subjective reports is used.
As of yet, no study has used a behavioral measure to distinguish between projectors
and associators. This might be due to the fact that the distinction is based on how an
individual synesthete experiences the induced percept. However, as has been demonstrated,
subjective reports of synesthetic experiences are relatively unreliable (Edquist et al., 2006),
although projectors and associators appear to show some reliable behavioral differences
(Dixon et al., 2004; Rich & Karstoft, 2013; Ward et al., 2007). This may result in the
assumption that a participant group consists of only associators or projectors (Saiki et al.,
2011) while they could contain both, and it might also cause the groups to be divided
inaccurately. Another drawback of using subjective reports is that it only allows predefined
distinctions. Contrary, all the studies that have used the method proposed by Rouw & Scholte
(2007) or an adaptation thereof (van Leeuwen et al., 2011), show significant results. By
scoring projector/associator synesthetes on a continuum, synesthetes that experience both
forms or that cannot readily identify with either description can also be included (Edquist et
al., 2006; Ward et al., 2007), thereby increasing the validity of the results. Considering the
behavioral and neurobiological evidence (Brang et al., 2011; Rich & Karstoft, 2013; Rouw &
Scholte, 2007, 2010; van Leeuwen et al., 2011; Ward et al., 2007) it appears that projectors
and associators do indeed differ in how the synesthetic percept arises. Projectors show
evidence that their synesthetic percept is primarily caused by bottom up processing due to
neurobiological differences in early sensory areas (Brang et al., 2011; Rouw & Scholte, 2010;
van Leeuwen et al., 2011). Associators show evidence that their synesthetic percept is
primarily caused by top down processing due to neurobiological differences in later
association and memory areas (Rouw & Scholte, 2010; van Leeuwen et al., 2011). However,
as will be discussed, it appears that projectors and associators do not rely solely on bottom up
or top down processing, respectively, but depends on the projector/associator strength. Using
a continuous score for projectors and associators appears to be able to reflect the relative top
down and bottom up differences between the two groups better than a dichotomous distinction
(van Leeuwen et al., 2011).
Another possibility is that both projectors and associators rely on bottom up and top
down processes. This possibility suggests that two, or more models can explain synesthetic
percepts, i.e. cascade cross tuning for early bottom up processes and disinhibited feedback for
top down processes. Even though both projectors and associators might rely on both models,
the stronger perceptual projector synesthetes might rely mostly on cascade cross tuning while
22 / 35
Writing assignment
Jonathan van Leeuwen
associators rely more on top down processing models like the reentrant feedback model. The
next two studies support this idea. van Leeuwen et al. (2011) showed that the probability of a
synesthete using bottom up processing vs. top down processing was not dichotomous. Not all
projectors were equally likely to fit the bottom up model, but the likelihood of them fitting
one of the models was related to how high they scored on either the projector scale or the
associator scale. This suggest that the groups should not be readily split into dichotomous
groups. Similarly, if projector solely use bottom up processing and associators solely used top
down processing, it would be expected that projectors would show color/letter similarities
while associators would not. Brang et al. (2011) showed that this is not the case: color/letter
similarity did not show dichotomous grouping between projectors and associators, but rather
correlated with individual projector/associator strength. Suggesting that the projector
associator distinction might not be sensitive enough to detect the subtleties in the individual
experiences of synesthesia (but see Jürgens & Nikolić, 2012).
With previous results in mind, it would be preferable to stop using subjective
descriptive reports to distinguish between projectors and associators as it has been shown to
have low test-retest reliability as synesthetes might describe their synesthetic experiences
differently. Using a premade questionnaire to ask about experiences and subsequently
splitting the synesthetes into a dichotomous projector and associator group would be better
than subjective descriptive reports. However a dichotomous distinction removes a lot of
individual variety and does a poor job of accurately representing synesthetic perception as
they are not always classifiable as either projectors or associators. This is rectified by
classifying projectors and associators on a continues scale, as it would reflect individual
synesthetic perception more accurately by taking into account more aspects of how they
perceive their synesthetic percept. A version of the questions used by Rouw & Scholte (2007)
with illustrating pictures, would probably be the most reliable and would provide a valid
method of distinguishing projectors and associators (Brang et al., 2011; Skelton et al., 2009;
van Leeuwen et al., 2011). That these differences map onto the known processes of visual
perception is an important aspect of any model of synesthesia and the next section will
therefore look at visual and attentive processing differences between projectors and
associators.
23 / 35
Writing assignment
Jonathan van Leeuwen
Processing Differences Between Projectors and Associators
Visual Processing
An important aspect of any model that wants to explain visual or perceptual phenomena, is
that it maps onto the structural and functional understanding of the brain. That is, it needs to
have ecological validity. Therefore, in the context of models explaining synesthesia it is
important that it takes into account how the visual system processes information, from the
basic visual constituencies of a stimulus to the eventual conscious percept of a stimulus.
The visual system processes stimuli in two main pathways, the ventral path and the
dorsal path (Lamme & Roelfsema, 2000). One of the interesting aspects of the visual
processing for synesthesia is that different types of synesthetes might not only process
synesthetic inducing stimuli in differing ways (e.g. top down or bottom up processing), but
evidence indicate that they use areas related to different visual pathways (Melero et al., 2013;
Rouw & Scholte, 2007, 2010).
The parietal lobe is important for the synesthetic percept for both projectors and
associators (Hubbard & Ramachandran, 2005; Rich & Mattingley, 2002; Rouw & Scholte,
2007; Specht, 2012; van Leeuwen et al., 2011; van Leeuwen et al., 2010; Ward, 2013). This
area is part of the dorsal pathway for visual processing (Lamme & Roelfsema, 2000; Walsh &
Butler, 1996), which is primarily related to attentive vision (Mesulam, 1999). Interestingly,
van Leeuwen et al., (2011) showed that projectors show activity in color areas before parietal
areas, while associators show the opposite pattern. This might indicate that the synesthetic
experience for projectors starts pre-attentively and then binds to the inducer when processed
in the parietal lobe, as proposed by the two stage hyper-binding model. Contrary, associators
appear to need attention areas before the synesthetic experience starts to arise (van Leeuwen
et al., 2011). Current neurobiological findings indicate that temporal areas are important to the
synesthetic experience for associator synesthetes, but less so for projector synesthetes as
previously described (Melero et al., 2013; Rouw & Scholte, 2010). As the temporal cortex is
part of the ventral visual processing stream, this suggests that associator type experiences
might arise from the ventral pathway. As most of the research looked at relative differences
between projectors and associators, it is difficult to determine if projectors also use the ventral
pathway as the results indicate that associators use these areas more than projectors. However,
this does not preclude the possibility that projectors rely more on these areas then nonsynesthetes when viewing inducing graphemes. Suggesting that the temporal areas might be
important for both projectors and associators.
24 / 35
Writing assignment
Jonathan van Leeuwen
The dorsal stream mainly processes spatial, functional and movement aspects of the
visual stimuli (e.g. location and direction) (Mishkin & Ungerleider, 1982; Lamme &
Roelfsema, 2000) and involves the parietal cortices. It gets most of its information from the
magnocellular visual neurons, which contain information about movement and edges (Walsh
& Butler, 1996). The ventral pathway is primarily involved with object identification and
perception (e.g. what a stimuli is) (Mishkin & Ungerleider, 1982; Lamme & Roelfsema,
2000) and goes to the temporal cortices. Contrary to the dorsal pathway, the ventral pathway
gets most of its information from the parvocellular neurons, which code for fine details (high
spatial resolution) and color (Walsh & Butler, 1996). Interestingly, it appears that the
information transfer in the dorsal pathway is faster than the ventral pathway (Schmolesky et
al., 1998). Thus, this would suggest that associators use both the ventral and dorsal pathway
with predominantly top down processing while projectors primarily use the dorsal pathway
with bottom up processing (Brang et al., 2011; Dixon et al., 2004). This suggests that even
though the phenomenological aspects of associator and projector synesthetes are quite similar,
the underlying causes appear to be spatially, temporally and functionally different. If this is
indeed the case, it has implications for future synesthesia research, as this suggests that
similar experiences might have different neural causes.
Attention and Priming
A point of debate for synesthesia is whether attention is necessary for inducing the synesthetic
percept. Different models make different predictions as to whether attention is needed. In
bottom up models attention might or might not be necessary, while in top down models
attention is needed to induce synesthesia. As with a number of the experimental results
regarding synesthesia discussed earlier, experiments done to test for the contribution of
attention in synesthesia have also found diverging results, with some results indicating that
attention plays a large role (Mattingley et al., 2006; Rich & Mattingley, 2010), while other
results indicate that attention is hardly needed for creation of the synesthetic percept (Brang,
Hubbard, et al., 2010; Ramachandran & Hubbard, 2001c). However, this pattern might again
be explained by splitting the synesthetes into projectors and associators, thereby showing that
attention might indeed be necessary for both projectors and associator synesthetes, but it
appears that projectors need less attention than associators.
As mentioned earlier, a number of experiments have found top down effects for the
induction of synesthesia (Dixon et al., 2006; Palmeri et al., 2002; Smilek et al., 2001). This
suggests that the synesthetic percept can at least be influenced by attention mechanisms.
25 / 35
Writing assignment
Jonathan van Leeuwen
Mattingley, Rich, Yelland, & Bradshaw (2001) used masked inducers to see if unconsciously
perceived inducers caused a synesthetic percept. Their results indicated that unmasked
inducers caused a synesthetic percept while the masked inducers did not. However, masking
disrupts conscious perception and does not influence attention to stimuli, but their results do
imply that conscious perception of the inducer is necessary for synesthetic percepts. In a latter
experiment Mattingley et al. (2006) manipulated attentional load during a letter priming task
by making the participants do a second task which required either high or low attentional
load. Falling in line with their previous findings, higher attentional loads reduced synesthetic
priming effects. Attention as a necessary factor in creating synesthetic percepts was also
found by Rich & Mattingley, (2010). They used a modified attentional blink task and found
that attended inducers created color congruency effects while non-attended inducers did not.
If the synesthetic percept can arise pre-attentively, inducers that are not attended will
still elicit a synesthetic percept and it would suggest that the synesthetic experience originates
from early bottom up sensory processes. Pre-attentive synesthesia would result in a pop out
effect of inducing graphemes when viewing a screen filled with inducing and non-inducing
graphemes as they would be caused automatically by the physical shape of the inducer.
Synesthetic pop out, bottom up effects have been reported (Ramachandran & Hubbard,
2001c) and synesthetic colors can be induced with brief grapheme presentation times (Spruyt,
Koch, Vandromme, Hermans, & Eelen, 2009). However, when their experiment was
subsequently replicated with a more robust methodological approach no real pop out effect
was found (Hubbard et al., 2005; Rich & Karstoft, 2013; Ward, Jonas, Dienes, & Seth, 2009).
These findings are also supported by a study showing that synesthetic inducers did not help in
a chromatic or achromatic grapheme search task, as well as participants only reporting a
synesthetic percept when inducers were attended (Edquist et al., 2006). Alvarez & Robertson
(2013) presented colored inducing graphemes and made participants indicate the color of a
subsequently presented color patch. Using this approach they found that the largest priming
effects occurred for inducers which were colored in the same color as they induced, e.g. a red
inducing grapheme printed in red. These results were taken as evidence that induced colors
use the same underlying substrates as veridical colors. However, most of these experiments
did not take into account the projector/associator distinction and the possibility that these
differing sub types of synesthesia might rely differently on attention mechanisms. For
instance, Ward et al., (2007) found evidence indicating that inducers that were not perceived
did not cause interference, for either projectors or associators. However, projectors did report
experiencing some form of color percept, even when they could not identify the displayed
26 / 35
Writing assignment
Jonathan van Leeuwen
grapheme. Similarly, projectors have higher color/letter similarity than associators, indicating
that they rely more on early sensory areas when creating the concurrent (Brang et al., 2011).
Evidence from fMRI research has also shown that projectors rely more on sensory areas and
that functional time course of inducing the concurrent is different between projectors and
associators (Rouw & Scholte, 2010; van Leeuwen et al., 2011).
The experiments which have used stroop tasks for investigating differences between
projectors and associators all show similar and reliable findings. Projectors show more
interference from induced colors than veridical colors while associators show the reversed
(Dixon et al., 2004; Rich & Karstoft, 2013; Ward et al., 2007). These stroop interference
results again suggest that projectors show more bottom up processing and should therefore be
less reliant on attention for their synesthetic percept. As most of the synesthetes in
experiments that specifically look for projector/associator differences have a larger number of
associator synesthetes (140) compared to projector synesthetes (74), it is likely that most of
the studies which do not distinguish between these groups will have a higher number of
associators. If projectors showed pre-attentive effects in their experiments, these effects would
therefore most likely be averaged out due to the low number of projectors in the study
compared to associators (table 1). Not only are projectors and associators probably mixed in
these experiments, they are underrepresented in general (Brang et al., 2011; Rich & Karstoft,
2013; Rouw & Scholte, 2007; van Leeuwen et al., 2011; van Leeuwen et al., 2010), even to
the point of being absent (Rich & Mattingley, 2010).
Taken together, it appears that attention and priming works differently for projectors
and associators. If the synesthetic percept for projectors is indeed caused by the cascade cross
tuning, it could explain why most experiments show a small or no effects of inducing
synesthetic color and priming when attention is manipulated. Partially seen or unattended
graphemes could therefore elicit different colors than a fully detected or attended grapheme
(Ward et al., 2007). This would explain why experiments fail to find priming effects when
attention is manipulated and/or stimuli is masked.
However, due to lower number of projectors, differing methods of distinguishing
between projectors and associators and the possible effect of cascade cross tuning for
projectors, the exact role of attention for projectors and associators needs further research.
One approach might be to use EEG to test if reentrant feedback is necessary for eliciting
unconscious but detectable neuronal responses to eliciting graphemes (Fahrenfort et al.,
2007). If projectors really need less attention than associators for the synesthetic percept to
arise, it would be expected that synesthesia inducing graphemes show larger occipital/parietal
27 / 35
Writing assignment
Jonathan van Leeuwen
EEG differences with non-inducing graphemes for projectors compared to associators when
reentrant feedback is disrupted.
Projector and Associator Synesthesia in Non-Grapheme-Color Synesthesia.
As has been argued, the projector/associator distinction appears to be able to explain a range
of diverging findings for synesthesia, including cognitive and neurobiological differences.
However, Dixon et al. (2004) proposed this distinction for grapheme color synesthetes, but
did not try to extrapolate this distinction to other forms of synesthesia. Due to this, all current
studies looking at projector/associator differences have only used grapheme-color synesthetes
(table 1). As synesthesia has traditionally been seen as a unitary, though heterogeneous,
condition, the differences between projectors and associators might indicate that multiple
causes may underlie synesthesia. If projector and associator color grapheme synesthetes do
indeed differ in underlying neurological and cognitive causes for their synesthesia, other types
of synesthesia might also have differing underlying causes. Since there has been no research
done with the projector/associator distinction for different types of synesthesia, it is hard to
speculate on how different synesthesia forms might reflect the projectors and associators
distinction, but attempts at speculation, if cautiously, are important for expanding on possible
research areas. For instance, non-sensory type synesthetes, for example space-form
synesthetes, might primarily consist of associator synesthetes as the inducer is caused by a
cognitive concept and not a sensory stimulus. Sensory type synesthetes, for example soundcolor synesthetes or vision touch synesthetes, might comprise of combination as the inducer
might be caused by a cognitive concept or due to a sensory stimulus, as is seen in graphemecolor synesthesia. This would be supported by evidence showing that projectors rely more on
sensory areas while associators rely more on memory areas (Rouw & Scholte, 2010).
Interestingly, sound-color synesthesia does indeed appear to be quite heterogeneous
with regard to how it arises. Some sound-color synesthetes only experience color related to
spoken words (Nunn et al., 2002), while others experience color percepts with all sounds
(Head, 2006; Ward, Huckstep, & Tsakanikos, 2006). It has therefore been suggested that the
speech-color synesthetes rely on linguistic representations of the word, while the pure soundcolor synesthetes rely more on the perceptual properties of the stimuli (Frith & Paulesu, 1997;
Simner, Glover, & Mowat, 2006). Using the projector/associator differentiation, the former
could be labeled as an auditory-color associator, while the later could be labeled an auditorycolor projector. Interestingly, it has been shown that pure auditory-color synesthetes show
qualitatively different early EEG responses to inducing auditory stimuli compared to healthy
28 / 35
Writing assignment
Jonathan van Leeuwen
controls (Goller, Otten, & Ward, 2009). These results of early auditory effects fit nicely with
the cascade cross tuning model and suggest that these synesthetes process their synesthetic
experience in a similar manner as grapheme-color projectors, albeit in a different sensory
area. Interestingly, projector type experiences have also been reported for space-form
synesthesia (Smilek et al., 2007). This might indicate that there are similarities between types
of synesthetes (e.g. grapheme-color, sound-color) who have the same projector or associator
classification, e.g. a grapheme-color projector synesthete might show similarities in cognition
and neurobiology with a sound-color projector. If correct, research on other types of
synesthesia probably also suffers from low reliability as they draw their subjects from
different subject pools, e.g. projectors and associators. The application of the
projector/associator distinction for other types of synesthesia could therefore increase
reliability and validity of future synesthesia research, by making sure that the subjects with
different underlying causes for synesthesia are not grouped together. However, using
subjective reports to distinguish between projector and associator synesthetes is not ideal to
say the least. An adapted version of an illustrated projector/associator questionnaire as well as
an adapted stroop task could be used to differentiate projectors and associators with other
types of synesthesia.
Just as the mechanism for grapheme-color projectors appears to be cascade-cross
tuning in the visual cortex, other sensory projector synesthesia might be caused by cascadecross tuning in the relevant sensory cortex. The relation between different associator
synesthesia forms might be slightly different. As projectors might rely on the relevant sensory
cortex for cascade-cross tuning, associator synesthesia mechanisms appears to be related to
the temporal cortex. This might be the same for all associator synesthesia forms considering it
would be the same high order conceptual mechanism that leads to the association between
two stimuli, for instance hyper-binding. Thus different types of projector synesthetes might
show larger differences in neurobiology, due to the spatial location of the sensory areas, while
different types of associator synesthetes might be more similar, due to the use of memory
areas. This interpretation is purely speculative as no research has yet been done to investigate
projector/associator differences in non-grapheme-color synesthetes. Further research is
needed to determine if this is the case.
Limitations in Projector/Associator Research
Certain limitations and confounding factors have become obvious when reviewing the
projector/associator literature. The first one being the method of differentiating between
29 / 35
Writing assignment
Jonathan van Leeuwen
projectors and associators. Using subjective reports has been shown to be unreliable, while
illustrated questionnaires and continuous scores appear to more adequately describe the
synesthetic percepts. A number of studies used a continuum score for projectors and
associators based on questionnaires and found that most grapheme-color synesthetes fit nicely
on a continuum, even though they might not subjectively subscribe to being either projector or
associator. Combining this continuous approach of defining projectors and associators, but
adding illustrations to the questionnaires to improve test-retest reliability would be the next
step in reliably distinguishing projectors and associators. Another limiting factor is caused by
the assumption that synesthesia was more prevalent in females than males (Barnett et al.,
2008; Rich, Bradshaw, & Mattingley, 2005). This is reflected in the participant inclusion for
synesthesia research. Current research on grapheme-color synesthesia has included 216
synesthetic participants (although some of the same participants were used for multiple
experiments) with 148 being female, 16 being male and 52 being of unreported gender (table
1). However, findings based on more robust methods have found a prevalence ratio of 1:1 for
female and male synesthesia (Banissy, Kadosh, Maus, Walsh, & Ward, 2009; Brang,
Teuscher, et al., 2010; Jamie Ward, 2013). This shows that almost all of the research done
with projector and associator synesthetes has been done with females while males have been
massively underrepresented. Not only is the female/male ratio skewed, but also the
projector/associator ratio is severely skewed. With 140 participants being scored as associator
synesthetes and 74 being scored as projector synesthetes, the discrepancy in total number of
participants is caused by some not being scored as either projector or associator (table 1).
However, no study has yet tried to determine the ratio of projector and associator synesthesia
or if there is a relationship between gender and projector/associator synesthesia. Thus, future
studies on projector/associator synesthesia should try to reflect the natural occurrence rate of
synesthesia and strive for equal female/male ratio inclusion as well as strive for a higher
inclusion rate of projector synesthetes. As well as having skewed participant inclusions, all
current research done on projectors and associators has been done on grapheme-color
synesthetes. Interestingly, it appears that other types of synesthesia can also be differentiated
based on the projector associator distinction. However, as this has not been directly tested as
of yet and future studies would have to determine how and if the projector/associator
distinction can be applied to other types of synesthesia.
30 / 35
Writing assignment
Jonathan van Leeuwen
Concluding Remarks
It is apparent that grapheme-color synesthesia is indeed a very heterogeneous condition, even
more so than supposed by Dixon et al. (2004). With evidence showing that there are indeed
differences in grapheme-color synesthesia between projectors and associators. Not only do
their synesthetic percepts differ, they also show cognitive and neurobiological differences.
Research done on grapheme-color synesthesia which have not differentiated between
subgroups, draw their subjects from two different subject pools, thereby causing diverging
results. The use of the projector/associator differentiation has the possibility of explaining
these diverging findings in grapheme-color synesthesia research, possibly even for other types
of synesthesia. Interestingly, it appears that projectors and associators do not appear to be as
dichotomous as originally thought. The results from experiments using a continuous score for
projectors and associators indicate that projectors and associators are not two completely
separated groups, but fall on a continuum. The continuous scores also manages to include the
third group of grapheme-color synesthesia, mental screen projectors. It therefore appears that
grapheme-color synesthesia is not caused by only bottom up processing, neither by only top
down processing, but rather a combination of the two. Neither do these results indicate that
projectors are solely the cause for bottom up effects and associators solely responsible for top
down effects. Rather, it suggests that both projectors and associators rely on both bottom up
and top down processes. With the importance of bottom up processing appears to correlate
with projector strength while the importance of top down influences appears to correlate with
associator strength. Currently, the cascade cross tuning model appears to be best able to
explain the bottom up effects (Brang et al., 2011; van Leeuwen et al., 2011) while some form
of reentrant feedback could explain the top down effects.
As differing methods of differentiating between projectors and associators reduce
reliability, which therefore causes discrepancies in research results, correctly and reliably
classifying projectors and associators is important. The dichotomous distinction between
projectors does not appear to be adequate, thus classifying projectors and associators on a
continuous scale using an illustrated questionnaire currently has the most promise in capturing
the heterogeneous nature of grapheme-color synesthesia by reliably scoring projectors and
associators (Edquist et al., 2006; Rouw & Scholte, 2007; van Leeuwen et al., 2011).
By switching from a purely observational and behavioral approach, synesthesia
research is now seeing an increase in research determined to detect the underlying
neurological cause for synesthesia. Even though there is still a long way to go for truly
understanding the underlying neurological and cognitive aspects of synesthesia, research on
31 / 35
Writing assignment
Jonathan van Leeuwen
grapheme-color synesthesia has come a long way since the late 90’s early 00’s and
performing synesthesia research as a method of understanding normal cognition has all the
appearance of only becoming more popular in the future.
References:
Alvarez, B. D., & Robertson, L. C. (2013). The interaction of synesthetic and print color and its relation to visual
imagery. Attention, Perception, & Psychophysics, 75(8), 1737–1747. doi:10.3758/s13414-013-0520-3
Banissy, M. J., Kadosh, R. C., Maus, G. W., Walsh, V., & Ward, J. (2009). Prevalence, characteristics and a
neurocognitive model of mirror-touch synaesthesia. Experimental Brain Research, 198(2-3), 261–272.
doi:10.1007/s00221-009-1810-9
Banissy, M. J., Stewart, L., Muggleton, N. G., Griffiths, T. D., Walsh, V. Y., Ward, J., & Kanai, R. (2012).
Grapheme-color and tone-color synesthesia is associated with structural brain changes in visual regions
implicated
in
color,
form,
and
motion.
Cognitive
Neuroscience,
3(1),
29–35.
doi:10.1080/17588928.2011.594499
Barnett, K. J., Finucane, C., Asher, J. E., Bargary, G., Corvin, A. P., Newell, F. N., & Mitchell, K. J. (2008).
Familial patterns and the origins of individual differences in synaesthesia. Cognition, 106(2), 871–893.
doi:10.1016/j.cognition.2007.05.003
Baron-Cohen, S., Burt, L., Smith-Laittan, F., Harrison, J., & Bolton, P. (1996). Synaesthesia: prevalence and
familiality. Perception, 25, 1073–1079.
Baron-Cohen, S., & Harrison, J. E. (1997). Synaesthesia: Classic and contemporary readings (Vol. xiii).
Malden: Blackwell Publishing.
Baron-Cohen, S., Wyke, M. A., & Binnie, C. (1987). Hearing words and seeing colours: an experimental
investigation of a case of synaesthesia. Perception, 16(6), 761–767.
Brang, D., Hubbard, E. M., Coulson, S., Huang, M., & Ramachandran, V. S. (2010). Magnetoencephalography
reveals early activation of V4 in grapheme-color synesthesia. NeuroImage, 53(1), 268–274.
doi:10.1016/j.neuroimage.2010.06.008
Brang, D., Rouw, R., Ramachandran, V. S., & Coulson, S. (2011). Similarly shaped letters evoke similar colors
in
grapheme–color
synesthesia.
Neuropsychologia,
49(5),
1355–1358.
doi:10.1016/j.neuropsychologia.2011.01.002
Brang, D., Teuscher, U., Ramachandran, V. S., & Coulson, S. (2010). Temporal sequences, synesthetic
mappings, and cultural biases: The geography of time. Consciousness and Cognition, 19(1), 311–320.
doi:10.1016/j.concog.2010.01.003
Cohen, L., Dehaene, S., Naccache, L., Lehéricy, S., Dehaene-Lambertz, G., Hénaff, M.-A., & Michel, F. (2000).
The visual word form area Spatial and temporal characterization of an initial stage of reading in normal
subjects and posterior split-brain patients. Brain, 123(2), 291–307. doi:10.1093/brain/123.2.291
Dehaene, S., Cohen, L., Sigman, M., & Vinckier, F. (2005). The neural code for written words: a proposal.
Trends in Cognitive Sciences, 9(7), 335–341. doi:10.1016/j.tics.2005.05.004
Dixon, M. J., Smilek, D., Cudahy, C., & Merikle, P. M. (2000). Five plus two equals yellow. Nature, 406(6794),
365–365.
Dixon, M. J., Smilek, D., Duffy, P. L., Zanna, M. P., & Merikle, P. M. (2006). The role of meaning in graphemecolour synaesthesia. Cortex, 42(2), 243–252.
Dixon, M. J., Smilek, D., & Merikle, P. M. (2004). Not all synaesthetes are created equal: Projector versus
associator synaesthetes. Cognitive, Affective, & Behavioral Neuroscience, 4(3), 335–343.
Eagleman, D. M., Kagan, A. D., Nelson, S. S., Sagaram, D., & Sarma, A. K. (2007). A standardized test battery
for the study of synesthesia. Journal of Neuroscience Methods, 159(1), 139–145.
doi:10.1016/j.jneumeth.2006.07.012
Edquist, J., Rich, A. N., Brinkman, C., & Mattingley, J. B. (2006). Do synaesthetic colours act as unique features
in visual search? Cortex, 42(2), 222–231.
32 / 35
Writing assignment
Jonathan van Leeuwen
Esterman, M., Verstynen, T., Ivry, R. B., & Robertson, L. C. (2006). Coming unbound: disrupting automatic
integration of synesthetic color and graphemes by transcranial magnetic stimulation of the right parietal
lobe. Journal of Cognitive Neuroscience, 18(9), 1570–1576.
Fahrenfort, J. J., Scholte, H. S., & Lamme, V. A. F. (2007). Masking Disrupts Reentrant Processing in Human
Visual Cortex. Journal of Cognitive Neuroscience, 19(9), 1488–1497.
Feynman, R. P. (1988). What Do You Care What Other People Think’ Further Adventures of a Curious
Character. London: Unwin.
Frisby, J. P., & Clatworthy, J. L. (1975). Illusory contours: Curious cases of simultaneous brightness contrast.
Perception, 4, 349–357.
Frith, C. D., & Paulesu, E. (1997). The physiological basis of synaesthesia. Baron-Cohen, Simon [Ed], 123–147.
Gebuis, T., Nijboer, T. C. W., & Van der Smagt, M. J. (2009). Multiple dimensions in bi-directional synesthesia.
European Journal of Neuroscience, 29(8), 1703–1710. doi:10.1111/j.1460-9568.2009.06699.x
Gheri, C., Chopping, S., & Morgan, M. . (2008). Synaesthetic colours do not camouflage form in visual search.
Proceedings of the Royal Society B: Biological Sciences, 275(1636), 841–846.
doi:10.1098/rspb.2007.1457
Goller, A. I., Otten, L. J., & Ward, J. (2009). Seeing sounds and hearing colors: an event-related potential study
of auditory–visual synesthesia. Journal of Cognitive Neuroscience, 21(10), 1869–1881.
Grainger, J., Rey, A., & Dufau, S. (2008). Letter perception: from pixels to pandemonium. Trends in Cognitive
Sciences, 12(10), 381–387. doi:10.1016/j.tics.2008.06.006
Grossenbacher, P. G., & Lovelace, C. T. (2001). Mechanisms of synesthesia: cognitive and physiological
constraints. Trends in Cognitive Sciences, 5(1), 36–41.
Hartman, A. M., & Hollister, L. E. (1963). Effect of mescaline, lysergic acid diethylamide and psilocybin on
color perception. Psychopharmacologia, 4(6), 441–451.
Howells, T. H. (1944). The experimental development of color-tone synesthesia. Journal of Experimental
Psychology, 34(2), 87.
Hubbard, E. M. (2007). Neurophysiology of synesthesia. Current Psychiatry Reports, 9, 193–199.
Hubbard, E. M., Arman, A. C., Ramachandran, V. S., & Boynton, G. M. (2005). Individual Differences among
Grapheme-Color
Synesthetes:
Brain-Behavior
Correlations.
Neuron,
45(6),
975–985.
doi:10.1016/j.neuron.2005.02.008
Hubbard, E. M., Brang, D., & Ramachandran, V. S. (2011). The cross-activation theory at 10. Journal of
Neuropsychology, 5(2), 152–177. doi:10.1111/j.1748-6653.2011.02014.x
Hubbard, E. M., & Ramachandran, V. S. (2005). Neurocognitive Mechanisms of Synesthesia. Neuron, 48(3),
509–520. doi:10.1016/j.neuron.2005.10.012
Hupe, J.-M., Bordier, C., & Dojat, M. (2011). The Neural Bases of Grapheme-Color Synesthesia Are Not
Localized
in
Real
Color-Sensitive
Areas.
Cerebral
Cortex,
22(7),
1622–1633.
doi:10.1093/cercor/bhr236
Jewanski, J., Day, S. A., & Ward, J. (2009). A Colorful Albino: The First Documented Case of Synaesthesia, by
Georg Tobias Ludwig Sachs in 1812. Journal of the History of the Neurosciences, 18(3), 293–303.
doi:10.1080/09647040802431946
Jürgens, U. M., & Nikolić, D. (2012). Ideaesthesia: Conceptual processes assign similar colours to similar
shapes. Translational Neuroscience, 3(1), 22–27. doi:10.2478/s13380-012-0010-4
Kadosh, R. C., Kadosh, K. C., & Henik, A. (2007). The neuronal correlate of bidirectional synesthesia: A
combined event-related potential and functional magnetic resonance imaging study. Journal of
Cognitive Neuroscience, 19(12), 2050–2059.
Kanwisher, N., McDermott, J., & Chun, M. M. (1997). The fusiform face area: a module in human extrastriate
cortex specialized for face perception. The Journal of Neuroscience, 17(11), 4302–4311.
Lamme, V. A., & Roelfsema, P. R. (2000). The distinct modes of vision offered by feedforward and recurrent
processing. Trends in Neurosciences, 23(11), 571–579.
Marek, G. J., & Aghajanian, G. K. (1998). Indoleamine and the phenethylamine hallucinogens: mechanisms of
psychotomimetic action. Drug and Alcohol Dependence, 51, 189–198.
Martino, G., & Marks, L. E. (2001). Synesthesia: Strong and Weak. Current Directions in Psychological
Science, 10(2), 61–65. doi:10.1111/1467-8721.00116
33 / 35
Writing assignment
Jonathan van Leeuwen
Mattingley, J. B., Payne, J. M., & Rich, A. N. (2006). Attentional load attenuates synaesthetic priming effects in
grapheme-colour synaesthesia. Cortex, 42(2), 213–221.
Mattingley, J. B., Rich, A. N., Yelland, G., & Bradshaw, J. L. (2001). Unconscious priming eliminates automatic
binding of colour and alphanumeric form in synaesthesia. Nature, 410(6828), 580–582.
doi:10.1038/35069062
McKeefry, D. J., & Zeki, S. (1997). The position and topography of the human colour centre as revealed by
functional magnetic resonance imaging. Brain, 120(12), 2229–2242.
Melero, H., Peña-Melián, Á., Ríos-Lago, M., Pajares, G., Hernández-Tamames, J. A., & Álvarez-Linera, J.
(2013). Grapheme-color synesthetes show peculiarities in their emotional brain: cortical and subcortical
evidence from VBM analysis of 3D-T1 and DTI data. Experimental Brain Research, 227(3), 343–353.
doi:10.1007/s00221-013-3514-4
Mesulam, M.-M. (1999). Spatial attention and neglect: parietal, frontal and cingulate contributions to the mental
representation and attentional targeting of salient extrapersonal events. Philosophical Transactions of
the Royal Society of London. Series B: Biological Sciences, 354(1387), 1325–1346.
Mishkin, M., & Ungerleider, L. G. (1982). Contribution of striate inputs to the visuospatial functions of parietopreoccipital cortex in monkeys. Behavioural Brain Research, 6(1), 57–77.
Mulvenna, C. M., & Walsh, V. (2006). Synaesthesia: supernormal integration? Trends in Cognitive Sciences,
10(8), 350–352.
Novich, S., Cheng, S., & Eagleman, D. M. (2011). Is synaesthesia one condition or many? A large-scale analysis
reveals subgroups: Subtypes of synaesthesia. Journal of Neuropsychology, 5(2), 353–371.
doi:10.1111/j.1748-6653.2011.02015.x
Nunn, J. A., Gregory, L. J., Brammer, M., Williams, S. C. R., Parslow, D. M., Morgan, M. J., Gray, J. A.
(2002). Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words.
Nature Neuroscience, 5(4), 371–375. doi:10.1038/nn818
Paffen, C. L. E., van der Smagt, M. J., & Nijboer, T. C. W. (2011). Colour?Grapheme Synesthesia Affects
Binocular Vision. Frontiers in Psychology, 2. doi:10.3389/fpsyg.2011.00314
Palmeri, T. J., Blake, R., Marois, R., Flanery, M. A., & Whetsell, W. (2002). The perceptual reality of
synesthetic colors. Proceedings of the National Academy of Sciences, 99(6), 4127–4131.
Ramachandran, V. S., & Hubbard, E. M. (2001a). Neural cross wiring and synesthesia. Journal of Vision, 1(3),
67–67. doi:10.1167/1.3.67
Ramachandran, V. S., & Hubbard, E. M. (2001b). Psychophysical investigations into the neural basis of
synesthesia. Proceedings of the Royal Society B: Biological Sciences, 268, 979–983.
Ramachandran, V. S., & Hubbard, E. M. (2001c). Synaesthesia–a window into perception, thought and
language. Journal of Consciousness Studies, 8(12), 3–34.
Rey, A., Dufau, S., Massol, S., & Grainger, J. (2009). Testing computational models of letter perception with
item-level
event-related
potentials.
Cognitive
Neuropsychology,
26(1),
7–22.
doi:10.1080/09541440802176300
Rich, A. N., Bradshaw, J. L., & Mattingley, J. B. (2005). A systematic, large-scale study of synaesthesia:
implications for the role of early experience in lexical-colour associations. Cognition, 98(1), 53–84.
doi:10.1016/j.cognition.2004.11.003
Rich, A. N., & Karstoft, K.-I. (2013). Exploring the benefit of synaesthetic colours: Testing for “pop-out” in
individuals with grapheme–colour synaesthesia. Cognitive Neuropsychology, 30(2), 110–125.
doi:10.1080/02643294.2013.805686
Rich, A. N., & Mattingley, J. B. (2002). Anomalous Perception in Syneaesthesia: A Cognitive Neuroscience
Perspective. Nature Reviews Neuroscience, 3(1), 43–52. doi:10.1038/nrn702
Rich, A. N., & Mattingley, J. B. (2003). The effects of stimulus competition and voluntary attention on colourgraphemic synaesthesia. NeuroReport, 14(14), 1793–1798.
Rich, A. N., & Mattingley, J. B. (2010). Out of sight, out of mind: The attentional blink can eliminate
synaesthetic colours. Cognition, 114(3), 320–328. doi:10.1016/j.cognition.2009.10.003
Robertson, L. C. (2003). Binding, spatial attention and perceptual awareness. Nature Reviews Neuroscience,
4(2), 93–102. doi:10.1038/nrn1030
Rouw, R., & Scholte, H. S. (2007). Increased structural connectivity in grapheme-color synesthesia.
NatureNeuroscience, 10(6), 792–797. doi:10.1038/nn1906
34 / 35
Writing assignment
Jonathan van Leeuwen
Rouw, R., & Scholte, H. S. (2010). Neural Basis of Individual Differences in Synesthetic Experiences. Journal
of Neuroscience, 30(18), 6205–6213. doi:10.1523/JNEUROSCI.3444-09.2010
Saiki, J., Yoshioka, A., & Yamamoto, H. (2011). Type-based associations in grapheme-color synaesthesia
revealed by response time distribution analyses. Consciousness and Cognition, 20(4), 1548–1557.
doi:10.1016/j.concog.2011.07.005
Schmolesky, M. T., Wang, Y., Hanes, D. P., Thompson, K. G., Leutgeb, S., Schall, J. D., & Leventhal, A. G.
(1998). Signal timing across the macaque visual system. Journal of Neurophysiology, 79(6), 3272–
3278.
Simner, J., Glover, L., & Mowat, A. (2006). Linguistic Determinants of Word Colouring in Grapheme-Colour
Synaesthesia. Cognitive Neuroscience Perspective on Synaesthesia, 42, 281–289.
Simner, J., Mulvenna, C., Sagiv, N., Tsakanikos, E., Witherby, S. A., Fraser, C., Ward, J. (2006).
Synaesthesia: The prevalence of atypical cross-modal experiences. Perception, 35(8), 1024–1033.
doi:10.1068/p5469
Skelton, R., Ludwig, C., & Mohr, C. (2009). A novel, illustrated questionnaire to distinguish projector and
associator synaesthetes. Cortex, 45(6), 721–729. doi:10.1016/j.cortex.2008.02.006
Smilek, D., Callejas, A., Dixon, M. J., & Merikle, P. M. (2007). Ovals of time: Time-space associations in
synaesthesia. Consciousness and Cognition, 16(2), 507–519. doi:10.1016/j.concog.2006.06.013
Smilek, D., & Dixon, M. J. (2002). Towards a synergistic understanding of synaesthesia. Psyche, 8(01).
Retrieved from http://www.theassc.org/files/assc/2529.pdf
Smilek, D., Dixon, M. J., Cudahy, C., & Merikle, P. M. (2001). Synaesthetic photisms influence visual
perception. Journal of Cognitive Neuroscience, 13(7), 930–936.
Specht, K. (2012). Synaesthesia: cross activations, high interconnectivity, and a parietal hub. Translational
Neuroscience, 3(1), 15–21. doi:10.2478/s13380-012-0007-z
Sperling, J. M., Prvulovic, D., Linden, D. E., Singer, W., & Stirn, A. (2006). Neuronal Correlates of ColourGraphemic Synaesthesia: Afmri Study. Cortex, 42(2), 295–303.
Spruyt, A., Koch, J., Vandromme, H., Hermans, D., & Eelen, P. (2009). A time course analysis of the synesthetic
colour priming effect. Canadian Journal of Experimental Psychology/Revue Canadienne de
Psychologie Expérimentale, 63(3), 211–215. doi:10.1037/a0015299
Synesthesia. (2013, December 6). Wikipedia, the free encyclopedia. Retrieved December 6, 2013, from
http://en.wikipedia.org/w/index.php?title=Synesthesia&oldid=584819331
Thornley Head, P. D. (2006). Synaesthesia: Pitch-colour isomorphism in RGB-space? Cortex, 42(2), 164–174.
Van Leeuwen, T. M., den Ouden, H. E. M., & Hagoort, P. (2011). Effective Connectivity Determines the Nature
of Subjective Experience in Grapheme-Color Synesthesia. Journal of Neuroscience, 31(27), 9879–
9884. doi:10.1523/JNEUROSCI.0569-11.2011
Van Leeuwen, T. M., Petersson, K. M., & Hagoort, P. (2010). Synaesthetic Colour in the Brain: Beyond Colour
Areas. A Functional Magnetic Resonance Imaging Study of Synaesthetes and Matched Controls. PLoS
ONE, 5(8), e12074. doi:10.1371/journal.pone.0012074
Walsh, V., & Butler, S. R. (1996). Different ways of looking at seeing. Behavioural Brain Research, 76(1), 1–3.
Ward, J. (2013). Synesthesia. Annual Review of Psychology, 64(1), 49–75. doi:10.1146/annurev-psych-113011143840
Ward, J., Huckstep, B., & Tsakanikos, E. (2006). Sound-colour synaesthesia: To what extent does it use crossmodal mechanisms common to us all? Cortex, 42(2), 264–280.
Ward, J., Jonas, C., Dienes, Z., & Seth, A. (2009). Grapheme-colour synaesthesia improves detection of
embedded shapes, but without pre-attentive “pop-out” of synaesthetic colour. Proceedings of the
RoyalSociety B: Biological Sciences, 277(1684), 1021–1026. doi:10.1098/rspb.2009.1765
Ward, J., Li, R., Salih, S., & Sagiv, N. (2007). Varieties of grapheme-colour synaesthesia: A new theory of
phenomenological and behavioural differences. Consciousness and Cognition, 16(4), 913–931.
doi:10.1016/j.concog.2006.09.012
Ward, J., & Mattingley, J. B. (2006). Synaesthesia: an overview of contemporary findings and controversies.
Cortex, 42(2), 129–136.
35 / 35