Suppression of Word Meaning in the Unconscious Perception of Masked Words
Andreas Brocher, Jean-Pierre Koenig, and Gail Mauner
State University of New York at Buffalo, [email protected]
We present data from three language priming experiments which suggest
that the subliminal, masked presentation of a word leads to subsequent
inhibition of its salient meaning features. In all three experiments,
participants decided for letter strings presented on a computer screen
whether they were words of English. Experimental stimuli were
homonyms (e.g. BANK) which have a highly frequent (MONEY) and
infrequent (RIVER) interpretation. In the first priming experiment we show
that 250 ms after stimulus recognition only the homonym’s more frequent
meaning is retrieved from memory. In the second experiment,
homonymous prime words were presented subliminally (50 ms) and
followed by a pattern mask (#####) to interrupt its ongoing visual
processing. We found that the homonym’s more frequent but not its less
frequent reading was reliably inhibited. In the third experiment, we
included a 50 ms prime-target delay condition and found that this
condition leads to retrieval facilitation. Taken together our data extend
existing literature on perception and show that otherwise activated
semantic information is suppressed when its perception is (a) unconscious
to the participant and (b) sufficiently interrupted by another visual
stimulus.
In this paper, we present three word priming experiments whose results suggest that the
Negative Compatibility Effect (NCE, Eimer & Schlaghecken, 1998) is not restricted to low level
perceptual stimuli like arrows and squares but may be extended to the retrieval of information
about words. The NCE is a paradoxical inhibition effect that is observed in masked perceptual
priming experiments when priming effects for a target preceded by a compatible prime turn
inhibitory when the prime has been processed unconsciously. To our knowledge, this is the first
study that has explicitly tested the NCE with language stimuli. We provide evidence that
response inhibition also occurs with the unconscious retrieval of word meaning from memory.
We begin by reviewing the NCE. Schlaghecken and Eimer (2000) visually presented
their participants with double arrows for 16 ms as primes which either pointed to the left ( << )
or to the right ( >> ) on a computer screen. These arrows were not available for conscious
perception. Each stimulus was immediately followed by a mask (overlapping left and right
pointing arrows) for 100 ms which in turn was immediately followed by a target consisting of
either four left or four right pointing arrows (<<<< vs. >>>>). The authors varied the delay
between mask and target in 32 ms intervals from 0 ms to a maximum of 192 ms. Participants
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were instructed to press a left button for left pointing arrows and a right button for right pointing
arrows. They were told that speed and accuracy of their responses were essential.
Schlaghecken and Eimer found that target arrows were responded to faster and more
accurately when those stimuli pointed in the same direction as the preceding, subliminally
presented prime arrows, but only with a mask-target delay of 0 ms and 32 ms. At delays of 96 ms
or longer, there was a paradoxical inhibition effect in which responses were faster when prime
and target arrows pointed in opposite direction than in the same direction. The authors argued
that the unconscious and masked perception of visual information leads to an initial and shortlived facilitation of compatible trials which is immediately followed by a phase of inhibition
where incompatible trials are faster than compatible ones.
Because the inhibition seen in the negative compatibility effect is not under the conscious
control of participants, it may be that it is controlled by different cognitive and/or physiological
mechanisms than endogenous inhibition, which is clearly under conscious control. Evidence for
this supposition comes from studies showing that endogenous inhibition, triggered by
supraliminal stimuli that are accessible to consciousness, leads to activity in the prefrontal cortex
(Rubia et al., 2001). In contrast, inhibition triggered by unconsciously perceived stimuli that are
masked evokes brain activation in the posterior parietal cortex (Aron et al., 2003). This
neurophysiological evidence supports the contention that the inhibition resulting from negative
compatibility of targets with subliminally presented primes is due to a different cognitive
mechanism than inhibition that is more under the conscious control of the participant.
The NCE has been replicated in numerous studies (Eimer & Schlaghecken, 2002; Klapp
& Hinkley, 2002; Sumner & Bradwood, 2008) and several explanations have been discussed in
the relevant literature. Eimer and Schlaghecken (1998) have proposed that the observed
inhibition effects are due to automatic self-inhibition which originates in the motor control
system. In this account, self-inhibition is the result of two events happening in parallel. First, the
response to a specific stimulus is delayed (cf. the 100 ms mask plus the prime-target delay
ranging from 96 ms to 192 ms). Second, flow of activation from the perceptual system to the
motor control system is interrupted by an additional visual element (cf. the mask). Verleger et al.
(2004), on the other hand, argue against a separate mechanism within the motor control system
but instead suggest that the NCE occurs whenever a visual stimulus is added to the prime which
calls for a different response than the prime stimulus itself. In this account, specific attributes of
the mask may cause inverse priming (cf. the overlapping left and right pointing arrows in
Schlaghecken & Eimer, 2000) and not the prime per se. A third account is proposed by
Jaskowski (2007) who suggests that not features within the mask (i.e. the arrows) but the mask
itself causes the NCE. He suggests that inhibition occurs whenever a new object that is part of
the participant’s task at hand is introduced into the current perceptual scene immediately after a
preceding stimulus. The introduction of a new object leads participants to abort an initiated
(compatible) response.
Much experimental work on the NCE is compatible with the self-inhibition account.
Schlaghecken and Eimer (2002) and Eimer and Schlaghecken (2003) sketch a model of early
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perceptual processing and motor control which, under specific circumstances, leads to inhibitory
control of motor responses. Initially, when a subliminally presented prime is presented with a
short prime-target delay, the prime leads to rapid but weak activation of the response associated
with the prime (e.g., if the prime was >> then the compatible right arrow ( >>>> ) response
would be weakly activated). During this short inter-stimulus interval, competing incompatible
responses (e.g., the left arrow <<<< ) would be inhibited via lateral inhibitory connections. This
sequence of events would lead to a positive compatibility effect at short prime-target delays.
However, at longer intervals between prime and target but without further perceptual input for
the prime (because it has been disrupted by a mask), inhibition of the compatible response begins
to occur, presumably because it is adaptive to inhibit responses for which there is little continued
perceptual evidence. Inhibiting the weakly or partially activated compatible motor response
would also lead to releasing the incompatible motor response from inhibition. As a consequence,
the incompatible motor response has a higher level of activation than the compatible response. It
is this state of affairs that leads to the negative compatibility effect at longer inter-stimulus
intervals between prime and target.
We investigated the NCE for language stimuli using homonyms which are words with
multiple unrelated meanings. For example, BANK may refer to both a financial institution and the
side of a river. Experimental work in psycholinguistics has revealed that in the absence of any
context information only the more frequent meaning of a homonym is accessed from the mental
lexicon whereas its less frequent competitor is not considered for retrieval (Simpson, 1981;
Simpson & Burgess, 1985; Simpson & Krueger, 1991). This dominance effect makes homonyms
good candidates for testing the NCE with language specific stimuli. If the NCE is global, i.e. it
delays the retrieval of all the information associated with a specific stimulus, we should find
reliable inhibition of both the homonym’s more frequent, dominant meaning and its less
frequent, subordinate interpretation. If, however, the NCE only affects the retrieval of
information which is immediately available to the processor in conscious perception we should
only find inhibition of the homonym’s dominant but not its subordinate meaning because the less
frequent interpretation is not immediately considered for retrieval when the participant is
consciously aware of the stimulus.
In priming experiments, participants are visually presented with strings of letters for
which they make a lexical decision, i.e. they decide as quickly and accurately as possible
whether or not the presented letter string is a word of their native language. Experimental trials
typically include a prime and a target which stand in some kind of meaningful relationship to
each other (e.g. prime: GLASS – target: DRINK). It is believed that once a word is recognized (e.g.
GLASS) activation spreads from this word to its ‘neighbors’ to whom this word is linked (e.g.
DRINK or STRAW). The priming technique has therefore proven fruitful for investigating which
information about words is immediately available to the language processor (i.e. co-activated
with the recognized word) during retrieval and how the mental lexicon might therefore be
organized.
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In our first priming experiment, participants made a lexical decision to every word on the
computer screen. Our experimental stimuli were ambiguous words (e.g. BANK) which were
followed either by a target word related to their more frequent meaning (ROB) or to their less
frequent interpretation (CREEK). The delay between prime offset and target onset was 200 ms.
We replicated previous findings and found that only the dominant meaning of a homonym was
initially retrieved from memory. That is, participants were significantly faster in responding to
ROB when having seen BANK than when having responded to an unrelated word before. The
subordinate interpretation (CREEK) was neither facilitated nor inhibited by the preceding prime
word.
The second experiment was identical to the first one with the exception that ambiguous
prime words (e.g. BANK) were now presented subliminally for only 50 ms and therefore barely
available for conscious processing. They were additionally preceded and followed by a pattern
mask (######) to disguise the prime. Thus, participants only made a lexical decision to the target
words (ROB or CREEK). Results indicate that dominant meanings were no longer facilitated by
their respective homonymous prime words but instead reliably inhibited. For example,
participants took longer in deciding whether or not ROB was a word when having seen the
masked prime BANK before relative to an unrelated prime word. However, subordinate meanings
remained unaffected by their respective prime words. Participants responded equally fast to
CREEK after having seen BANK than after having seen an unrelated prime word before.
Results of Experiment 2 stand in stark contrast to those of Experiment 1. With
consciously perceived prime words, dominant but not subordinate meanings of ambiguous words
were immediately available for retrieval. However, when ambiguous words were presented
subliminally and thus not available for conscious processing, dominant meanings were
suppressed whereas their less frequent competitors were again not affected by their respective
prime words. These results suggest that the NCE which so far has only been observed with
purely perceptual stimuli may also affect word retrieval from memory. Interestingly, the NCE is
only observed for the more frequent meaning of a homonym with its less frequent competitor
being unaffected. This finding suggests that unconsciously perceived stimuli may only result in
inhibition of otherwise available, i.e. activated, information. It is not the case that all meanings of
a homonym suffer from inhibition. We may therefore conclude that (a) word meaning is
immediately available to the language processor (within 200 ms) and (b) unconsciously
perceived words lead to the inhibition of otherwise activated information.
In our third experiment, in addition to replicating the NCE observed in Experiment 2 we
were particularly interested in the question of the time course for meaning inhibition. In line with
findings from Schlaghecken and Eimer (2000), we predicted that the dominant meaning of a
homonym is initially activated even when the ambiguous word is not consciously perceived and
masked but inhibited when sufficient time passes. Indeed, Eimer and Schlaghecken (2003) have
argued that the NCE only occurs when a specific activation threshold has been reached for an
unconsciously presented stimulus. If this is the case, we should observe no inhibition effects with
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a sufficiently short delay between the subliminally presented, masked prime word and the
subsequent target word.
The design and materials of experiment three were similar to Experiment 2. However, we
added a 50 ms prime-target delay condition to the 200 ms prime-target delay condition. Half of
the participants were assigned to the former and half to the latter condition. Results confirmed
our predictions. The dominant interpretation of a homonym was only inhibited at the long primetarget delay and facilitated in the short prime-target condition. These results replicate findings
that show that a homonym’s dominant meaning is activated very quickly (even before 200 ms
post stimulus offset, cf. Experiment 1). Paul et al. (1992) report positive priming effects for the
homonym’s more frequent meaning as soon as 50 ms after the homonym’s onset. Simpson and
Burgess (1985) found the dominant interpretation being facilitated as soon as 16 ms post
stimulus offset. More importantly, the results of Experiment 3confirm Eimer and Schlaghecken’s
contention that the NCE depends on an activation threshold that has to be reached before a
specific decision (i.e. a button-press) can be inhibited. The 50 ms condition seems to tap into a
processing stage where the immediately initiated retrieval of the homonym’s dominant meaning
had not yet been cancelled by increasing inhibition.
After having replicated the dominance effect with our experimental material in
Experiment 1, we have shown in Experiments 2-3 that the NCE is not restricted to perceptual
aspects of stimuli. We tested the NCE with words that have multiple, unrelated interpretations
and found that their more frequent but not less frequent meanings are inhibited provided that the
stimuli are (a) presented subliminally and (b) masked and thus inaccessible to consciousness.
Experiment 3 additionally shows that meaning retrieval from the mental lexicon is unaffected by
inhibition at short prime-target delays. Indeed, 50 ms after a homonym has been recognized its
dominant (but not subordinate) interpretation is accessed, thus replicating the dominance effect
reported in Experiment 1.
The experiments presented in this paper were not designed to disentangle the different
approaches to the negative compatibility effect mentioned above. Note, however, that our data
can be accounted for by the model sketched in Schlaghecken and Eimer (2002; 2003). We add to
this model that (a) a look-up process in memory is instantiated as soon as a word is being
perceived (and successful as soon as 50 ms post stimulus offset) and (b) that this look-up process
feeds into the motor control system and is therefore prone to the NCE. That is, as soon as a word
is presented to the visual system, its meaning will be accessed in the mental lexicon. In terms of
homonyms, this meaning will initially only be the more frequent interpretation of the ambiguous
word. Thus, a yes-response (button-press) to the target word in line with the homonym’s
dominant interpretation will be sent to the motor control system. If the delay between prime and
target is sufficiently short, the yes-response will be executed because the inhibition component
has not yet built up sufficient activation to inhibit the response. If, however, the delay allows
activation of the inhibition component to sufficiently increase, the yes-response will be delayed.
In sum, our data suggest that the unconscious and masked perception of words with
multiple meanings results in response inhibition associated with word information (i.e. meaning)
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which is immediately available in conscious perception. The results of the experiments presented
here extend the literature on unconscious perception of visual information and show that the
NCE is not restricted to low level perceptual information but also occurs with language stimuli.
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