Exp Brain Res
DOI 10.1007/s00221-015-4509-0
RESEARCH ARTICLE
Use your head! Perception of action possibilities by means of an
object attached to the head
Jeffrey B. Wagman1 · Alen Hajnal2 Received: 16 September 2015 / Accepted: 18 November 2015
© Springer-Verlag Berlin Heidelberg 2015
Abstract Perceiving any environmental property requires
spontaneously assembling a smart perceptual instrument—
a task-specific measurement device assembled across
potentially independent anatomical units. Previous research
has shown that to a large degree, perception of a given
environmental property is anatomically independent. We
attempted to provide stronger evidence for this proposal by
investigating perception by an organization of anatomical
and inert components that likely requires the spontaneous
assembly of a novel smart perceptual instrument—a rod
attached to the head. Specifically, we compared cephalic
and manual perception of whether an inclined surface
affords standing on. In both conditions, perception reflected
the action capabilities of the perceiver and not the appendage used to wield the rod. Such results provide stronger
evidence for anatomical independence of perception within
a given perceptual system and highlight that flexible taskspecific detection units can be assembled across units that
span the body and inert objects.
Keywords Perception–action · Haptic perception ·
Affordances · Tool use · Handheld objects
* Jeffrey B. Wagman
[email protected]
https://about.illinoisstate.edu/jbwagma
1
Department of Psychology, Illinois State University, Campus
Box 4620, Normal, IL 61790, USA
2
Department of Psychology, University of Southern
Mississippi, 118 College Drive #5025, Hattiesburg, MS
39046, USA
Introduction
A fundamental hallmark of agency across members of the
animal kingdom is flexibility—the interchangeability of
means to achieve an end (Gibson 1994; Turvey 2013). Such
flexibility is clearly manifest in the organization of behavior. A given behavioral goal can be achieved by means of
different (organizations of) anatomical components. For
example, a person can get from point A to point B by running, hopping, crawling, or swinging, among other possibilities—each of which uses different anatomical components but can result in functionally equivalent behavioral
ends.
Given the number of components of the movement
system(s), the number of ways in which these components
can be coordinated, and the fact that this coordination must
occur online and in real time, achieving a particular behavioral goal requires spontaneously assembling task-specific
control units from potentially independent anatomical units
(Bingham 1988; Turvey 2007). Moreover, such spontaneously assembled task-specific control units can include
both organic and inert components. For example, getting
from point A to point B can also occur by means of a platform shoe, crutch, pogo stick, bicycle, or wheelchair. Each
of these tools changes an actor’s action capabilities and
creates a unique person-plus-object action system that can
nonetheless be used to achieve the (same) behavioral goal.
Importantly, flexibility is also manifest in the organization of perception (Gibson 1966). A given perceptual goal
can similarly be achieved by means of different (organizations of) anatomical components. As a person moves from
point A to point B, he or she can perceive obstacles on or
gaps in environmental surfaces by viewing those surfaces
(Cole et al. 2013; Matthis and Fajen 2014), listening to
sounds passing between or reflecting among those surfaces
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(Gordon and Rosenblum 2004), or probing those surfaces
with a hand or a foot (Hajnal et al. 2011). Similarly, each of
these means of exploring surface layout can also result in
functionally equivalent perceptual ends.
Given the number of components of the perceptual
system(s), the number of ways in which these components
can be coordinated, and the fact that this coordination must
also occur online and in real time, achieving a particular
perceptual goal requires spontaneously assembling taskspecific detection units from potentially independent anatomical units (Wagman and Hajnal 2014a). That is, doing
so requires spontaneously assembling a smart perceptual
instrument—a device that achieves an intended perceptual
goal by capitalizing on task-specific and invariant stimulation patterns (Runeson 1977). Importantly, such smart perceptual devices can similarly include both organic and inert
components. For example, perceiving obstacles or gaps can
occur by means of a wielded object such as a cane (Fitzpatrick et al. 1994) or an “enactive torch”—a handheld vibrotactile vision substitution device (Froese et al. 2012). These
tools change an actor’s perceptual capabilities, creating a
unique person-plus-object perceptual system that can nonetheless be used to achieve the (same) perceptual goal.
Along these lines, research has shown that a great deal
of flexibility is exhibited in perception of action possibilities (i.e., affordances, Gibson 1979) by means of a perceptual tool. For example, blindfolded participants perceive
the same possibilities for standing on an inclined surface
when a rod used to explore that surface is wielded by either
hand, one or both hands, different configurations of two
hands, or either foot (Wagman and Hajnal 2014a, b). In all
of these cases, perception of this affordance reflected the
action capabilities of the perceiver (i.e., the ability of the
perceiver to stand on the inclined surface) and was virtually
unaffected by the (configurations of) anatomical components used to wield the rod. Importantly, functional equivalence in this perceptual task occurred despite (a) fundamental material differences between the (inert, homogeneous,
rigid) rod and (organic, flexible, heterogeneous) the limb to
which it was attached and (b) physical, physiological, and
functional differences between (the organizations of) the
limbs used to wield the rod.
Such results provide evidence that task-specific perceptual instruments can be spontaneously assembled across
anatomical components and inert objects and that perception by means of an object attached to the body is anatomically independent. The strength of this evidence for
this claim, however, is tempered by the fact that although
the hands and the feet may not often be used to perform
this particular perceptual task, they are often used to perform other perceptual tasks. Obviously, the hands are used
in a myriad of daily perceptual tasks. Less obviously, the
feet are also used in daily perceptual tasks such as those
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Exp Brain Res
involved in maintaining upright stance, engaging in bipedal
locomotion, or operating the pedals of an automobile or
bicycle.
Therefore, stronger evidence for the anatomical independence of perception of affordances by means of a
wielded object requires demonstrating that perception
reflects the action capabilities of the perceiver and not the
anatomical components used to wield the object in a task
that is (even more) likely to require the spontaneous assembly of a novel person-plus-object perceptual system. The
experiment reported here attempts to provide such evidence. We investigated perception of affordances for standing on an inclined surface when a rod used to explore that
surface is attached to the head. In particular, we compared
perception of this affordance by means of a rod attached to
the head (cephalic perception) and by means of a rod held
in the hand (manual perception).
We expected that perception of this affordance would
reflect the action capabilities of the perceiver, but would
be unaffected by the appendage used to wield the rod. We
expected a specific pattern of results across a number of
diverse dependent measures. First, the likelihood that the
surface is perceived to afford standing on would decrease
as surface angle increases, but would be unaffected by
appendage used to wield the rod. Second, the perceptual
boundary (between inclinations that are perceived to be
“stand-on-able” and those that are not) would fall within a
range of surface angles that includes the behavioral boundary and would not differ for the two appendages. Third,
participants would be able to successfully differentiate surfaces that could be stood on from those that could not, and
this ability would not differ for the two appendages. Fourth,
confidence would be lowest (and response latency would
be longest) within a range of surface angles that includes
both the perceptual and the behavioral boundaries, but neither confidence nor response latency would be affected by
appendage used.
Method
Participants
Twenty undergraduate students (3 men; 17 women, average weight = 62.2 kg, SD = 9.1 kg) participated in fulfillment of an extra credit option in their psychology courses.
Given that the experimental procedure required standing on
inclined surface, in the interest of participant safety, it was
required that participants weigh less than 91 kg (200 lbs.)
and wear appropriate footwear (e.g., athletic shoes with a
rubber sole and no heel). Nineteen participants were righthanded; one was left-handed. Written informed consent
was obtained from all participants included in this study.
Exp Brain Res
All procedures performed in this study were in accordance with the ethical standards of the institutional and/or
national research committee and with the 1964 Helsinki
Declaration and its later amendments or comparable ethical
standards.
Materials and apparatus
A wooden surface (152 cm × 76 cm) was reinforced with
metal braces so that it was strong enough to support a participant up to 105 kg. One end of the surface was hinged to
a wooden frame (91 cm × 76 cm). The other end of the surface was supported by a metal dowel resting on metal hooks
that were screwed into vertical wooden studs (122 cm tall)
attached to each side of the frame at the following heights
from the floor (in cm)—33.50, 40.75, 50.25, 58.75, 69.00,
82.75, and 92.50. These hook positions allowed the surface
to be set to seven different angles of inclination, ranging
from 15° to 45° in increments of 5° (see Fig. 1).
Participants wore a disposable shower cap and a plastic
hard hat (ERB19762 Americana Cap Style, Woodstock,
Georgia, USA) secured with an elastic chinstrap. Participants also wore a fabric blindfold. A plastic cylinder (5 cm
outer diameter, 1.3 cm inner diameter, 7.5 cm length, 167 g
mass) was attached to the right side of the helmet (just
above the right ear) so that it was parallel to the helmet
brim. In the Head Condition, a wooden dowel (153.5 cm in
length, 1.3 cm in diameter, 95 g mass) was inserted into the
cylinder and secured with a thumb screw so that one end of
the dowel was flush with the back of the cylinder and the
other end extended in front of the participant (see Fig. 1).
In the Hand Condition, the same dowel was held in the participant’s preferred hand.
A stopwatch smartphone application was used to record
response latency. A support railing was available for participant safety at all times. A digital scale was used to measure
body weight. Portions of the Lateral Preference Inventory
(Coren 1993) were used to measure participant handedness.
Procedure
Perceptual task
Body weight and handedness of each participant were
recorded. The participant stood approximately 1 m from
the surface (set at 45° before the participant arrived), put on
the shower cap, helmet, and blindfold.
In the Head Condition, the experimenter secured the
dowel in the cylinder, and the participant stood such that
the dowel was roughly parallel to the floor. In the Hand
Condition, the participant grasped the dowel with the preferred hand and placed the distal tip of the dowel on the
floor next to the surface (see Fig. 1). In each condition, the
participant returned to these respective postures after every
trial.
The experimenter adjusted the angle of the surface and
signaled the participant to begin the trial. The participant
then initiated exploring the surface with the dowel (by
probing, scraping, tapping, see Fig. 1) and provided two
verbal responses. First, the participant reported (yes or no)
Fig. 1 Helmet apparatus (top). The Head (bottom left) and Hand (bottom right) conditions of experiment 1
13
whether he or she would be able to stand on the surface
without bending at the knees or waist or going up on the
toes1 (cf. Fitzpatrick et al. 1994). Second, the participant
rated his or her confidence in this response on a scale of 1
(“not at all confident”) to 7 (“extremely confident”). In
addition, the experimenter recorded the latency between
the signal to begin the trial and the yes or no response.
No restrictions were placed on how (or how long) the
participant explored the surface with the dowel, and no
measures were taken to prevent the participant from hearing contact between dowel and surface. Each participant performed this task in both appendage conditions in
blocked fashion. The order of conditions was counterbalanced across participants. Each angle was presented three
times within each condition, and the order in which angles
were presented was randomized within that block of trials.
Therefore, each participant completed a total of 42 trials (2
Appendage Conditions × 7 Surface Angles × 3 trials per
angle per condition). Participants did not attempt to step on
or stand on the surface until all trials were completed.
Behavioral task
After the perceptual task was completed, the participant
removed the blindfold, and the surface was set to 15°. The
participant then attempted to stand on the surface with both
heels flush with the lower edge for 5 s without bending at
the knees or waist, going up on the toes, or grasping the
railing (see footnote 1). If the participant was able to do so
successfully, he or she stepped down, the surface was set at
the next steepest angle, and the participant again attempted
to perform this task. This procedure was repeated until the
participant was unable to perform the task successfully.
The steepest surface angle that could be stood on in this
manner was deemed the behavioral boundary for that participant (cf. Regia-Corte and Wagman 2008).
Results
Probability data
A 2 (Appendage: Head vs. Hand) × 7 (Surface Angle)
repeated-measures ANOVA was conducted on percentage
of yes (i.e., “stand-on-able”) responses. There was a main
effect of Surface Angle [F (6, 114) = 86.3, p < .001,
ηp2 = .82]—yes responses decreased as angle increased.
1
These constraints were used in perceptual and behavioral task to be
consistent with previous research (e.g., Fitzpatrick et al. 1994; Wagman and Hajnal 2014a, b), to ensure that responses referred to possibilities for standing (rather than walking, climbing, crawling, etc.)
and so that the same criteria were applied across conditions and participants.
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Exp Brain Res
There was also an interaction of Appendage × Surface Angle [F (6, 114) = 2.46, p < .05, ηp2 = .15]. Followup t tests (with Bonferroni correction) showed that participants were more conservative in the Head Condition than
in the Hand Condition but only at shallow surface angles
[15°: t(19) = 2.63, p < .05, Cohen’s d = 1.18 and 20°:
t(19) = 2.44, p < .05, Cohen’s d = 0.71] (see Fig. 2, top).
The main effect of Appendage was not significant (F (1,
19) = .41, p = .53, ηp2 = .02).2
Perceptual boundaries (aggregate data)
At the level of the aggregate data, probit analysis (Finney
1971) was used to determine the perceptual boundary (the
surface angle that would have resulted in a yes response on
50 % of trials) in each condition. This analysis determined
that the perceptual boundary was 30.5° in the Head Condition (with a 95 % CI of 29.4°–31.8°) and was 29.6° in
the Hand Condition (with a 95 % CI of 27.9°–31.2°). The
overlapping confidence intervals suggest that these values
do not differ (see Fig. 2, top). In addition, the mean behavioral boundary for the participants (M = 31.0°, SD = 4.5°)
differed by an average of 1.2° from the perceptual boundaries in both conditions and fell within the 95 % CI for the
perceptual boundaries for each condition.
Perceptual boundaries (individual participant data)
Perceptual boundaries were also derived for each individual participant in each condition. The perceptual boundary
for each participant in each condition was the steepest
angle that received a response of “yes” on at least half of
the (i.e., on at least two of three) trials in that condition.3
Across participants, these values ranged from 15° to 45°
for the Hand Condition and from 20° to 45° for the Head
Condition. A t test found no significant difference between
the mean perceptual boundary (derived in this manner) in
the Hand Condition (M = 28.4°, SD = 7.6°) and in the
Head Condition (M = 30.5°, SD = 6.4°), t(18) = 1.51,
p = .15, Cohen’s d = 0.33. Moreover, neither of these values differed from the mean behavioral boundary [Hand
Condition: t(19) = 1.3, p = .20, Cohen’s d = 0.46; Head
Condition: t(18) = 0.28, p = .79, Cohen’s d = 0.09]. In
addition, we used a two one-sided test (TOST) for equivalence (Welker and Nowacki 2011) with an equivalence
margin of ±5°. A 90 % CI on the mean difference between
2
Post hoc power analyses were conducted for all nonsignificant
main effects and interactions on probability data, confidence reports,
and response latency. Assuming a medium effect size, the G*Power
program (Faul et al. 2007) estimated power to be greater than 0.8.
3
One participant reported “no” on 26 of 27 trials in the Head Condition. Therefore, a perceptual boundary could not be determined in
this manner for this participant in this condition.
Exp Brain Res
manner) in the Hand Condition (M = 30.6°, SD = 6.1°)
and in the Head Condition (M = 30.9°, SD = 10.8°),
t(19) = 0.11, p = .91, Cohen’s d = 0.03. Again, neither of
these values differed from the behavioral boundary [Hand
Condition: t(19) = .20, p = .85, Cohen’s d = 0.07; Head
Condition: t(19) = .06 p = .96, Cohen’s d = 0.02]. Again,
we used the TOST procedure to test for equivalence with
an equivalence margin of ±5°. A 90 % CI on the mean difference between perceptual boundary (derived in this manner) in the Hand and Head Conditions (−4.7° to 4.4°) fell
within this interval. By this analysis, the two conditions are
equivalent (p < .05).
Signal detection data
For each condition and for each participant, proportions of
hits and false alarms, respectively, were calculated by (a)
dividing the total number of hits by the number of trials for
which the angle was less than or equal to the perceptual
boundary and (b) dividing the total number of false alarms
by the number of trials for which the angle was greater than
the perceptual boundary.
Proportions of hits and false alarms were compared in
a 2 (Appendage: Head vs. Hand) × 2 (Response Type:
Hits vs. False Alarm) ANOVA. There was a main effect of
Response Type [F (1, 19) = 657.8, p < .001, ηp2 = .97]—
proportion of hits (M = 0.82, SD = 0.18) was greater than
proportion of false alarms (M = 0.18, SD = 0.12). Neither
the main effect of Appendage [F (1, 19) = 1.45, p = 0.24,
ηp2 = .07] nor the interaction of Appendage × Response
Type [F(1,19) = 1.98, p = .18, ηp2 = .09] was significant.
In addition, a t test on (corrected) d′ values showed no
difference in how well participants were able to differentiate surfaces that afford standing on from those that did
not in each condition (Head: M = 1.96, SD = 0.49; Hand:
M = 1.28, SD = 0.54), t(19) = 1.28, p = 0.22 Cohen’s
d = 0.25.
Confidence data
Fig. 2 Percent of yes responses (top), confidence ratings (middle),
and response latency (bottom) for the Hand and Head Conditions
perceptual boundary (derived in this manner) in the Hand
and Head Conditions (−4.5° to 0.31°) fell within this interval. By this analysis, the two conditions are equivalent
(p < .05).
As an additional check on the validity of the perceptual
boundaries derived above, probit analysis was also used to
determine the perceptual boundaries for each participant
in each condition. A t test found no significant difference
between the mean perceptual boundary (derived in this
Across participants, mean confidence ratings ranged from
4.2 to 6.3 for the Hand Condition and from 4.1 to 6.3 for the
Head Condition. Mean confidence ratings were compared
in a 2 (Appendage: Head vs. Hand) × 7 (Surface Angle)
ANOVA. There was a main effect of Surface Angle [F (6,
114) = 7.74, p < .01, ηp2 = 0.29]—confidence ratings tended
to be lowest in a range of surface angles that included the
perceptual boundary (approximately 25°–30°, see Fig. 2,
middle). Moreover, there was no difference between the
surface angle that received the lowest confidence rating in
the Hand Condition (M = 29.8°, SD = 5.7°) and the Head
Condition (M = 27.8°, SD = 7.3°), t(19) = 1.05, p = .31,
Cohen’s d = 0.31. Neither the main effect of Appendage [F
13
(1, 19) = 2.24, p = 0.15, ηp2 = 0. 11] nor the interaction of
Appendage × Surface Angle [F (6, 114) = 1.56, p = 0.18,
ηp2 = .07] was significant (see footnote 2).
Response latency
Across participants, mean response latencies ranged from
2.9 to 15.1 s for the Hand Condition and from 2.9 to 13.9 s
for the Head Condition. Mean response latencies were
compared in a 2 (Appendage: Head vs. Hand) × 7 (Surface Angle) ANOVA. There was a main effect of Surface
Angle [F (6, 114) = 4.02, p < .01, ηp2 = .18]—response
latencies tended to be longest in a range of surface angles
that included the perceptual boundary (approximately 25°–
30°, see Fig. 2, bottom). Moreover, there was no difference
between the surface angle that received the longest response
latency in the Hand Condition (M = 29.3°, SD = 7.8°) and
the Head Condition (M = 26.0°, SD = 7.9°), t(19) = 1.32,
p = .19. Cohen’s d = 0.41. Neither the main effect of
Appendage [F (1, 19) = 0.05, p = .82, ηp2 = .003] nor
the interaction of Appendage × Surface Angle [F (6,
114) = 1.15, p = .33, ηp2 = .06] was significant (see footnote 2).
General discussion
Previous research supported the hypotheses that task-specific perceptual instruments can be spontaneously assembled across anatomical components and inert objects
and that perception by means of an object attached to the
body is anatomically independent (Wagman and Hajnal
2014a, b). The experiment reported here was designed as
a stronger test of these hypotheses by investigating perception by means of a rod attached to the head—an organization of anatomical components and an inert object that is
likely to require the spontaneous assembly of a novel person-plus-object perceptual system (see Fig. 1). Specifically,
we compared perception of affordances for standing on an
inclined surface by means of a rod attached to the head and
by means of a rod held in the hand.
Despite physical, physiological, functional differences
between the hands and the head and differences in lifetime
experience perceiving surface properties by means of these
two appendages, perception reflected the action capabilities
of the perceiver and not the appendage used to wield the
object. There were no differences in perceptual boundaries (derived in three different ways) across appendage
conditions, and in none of these cases did the perceptual
boundary differ from the behavioral boundary. In addition,
there were no differences in discriminability, confidence,
or response latency across appendage conditions (see footnote 2). As expected, confidence was lowest and response
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Exp Brain Res
latency was longest within a range of angles that included
the perceptual and behavioral boundaries (see Fig. 2). The
results of the experiment reported here both corroborate
and extend the findings of Wagman and Hajnal (2014a, b)
by providing stronger evidence for the anatomical independence of perception of affordances by the haptic system
even when capabilities of that system are augmented with a
perceptual tool. In short, the results are consistent with the
description of the haptic system as a softly assembled smart
perceptual instrument (Carello et al. 1992a, b).
One limitation of the current experiment is that the conclusions rely, in part, on null effects—in particular, a pattern of both significant and null findings across a number
of dependent measures. Importantly, the predicted pattern
of results was neither arbitrary nor without theoretical
basis. Rather, the predicted pattern of results was necessary, principled, and dictated by theory. Demonstrating
anatomical independence in a given perceptual task necessarily requires showing that perceptual ability in that task is
equivalent when different anatomical components are used
to perform that task. Therefore, the prediction that perception of affordances would not be affected by appendage
was necessary. In addition, a large body of research has
shown that the mechanical energy pattern of relevance to
perception of properties both of and by means of a wielded
object is invariant across the anatomical structures used
to wield those objects (see below; Carello et al. 1992a, b;
see Carello and Wagman 2009 for a review). Therefore,
the prediction that perception of affordances would not be
affected by appendage used to wield the object was dictated by theory. Moreover, while we did not predict differences in probability, confidence, and response latency
across differences in appendage used to wield the rod, we
did predict changes in these variables for both appendages
with changes in surface angle. Therefore, the predicted pattern of significant and null results was specific.
In addition, to minimize the possibilities of Type II
errors, we conducted experiments with sufficient power to
detect an effect if one exists. As stated in footnote 1, we
estimated power for all main effects and interactions on
percent yes responses, confidence, and response latency to
exceed 80 % in the reported ANOVAs. If there are differences across appendage conditions in these variables, it is
likely that we would have detected such differences in this
experiment.
Another possible limitation of the current experiment is
that no measures were taken to prevent the participant from
hearing contact between dowel and surface. As a result,
the task may have involved the auditory system as well as
the haptic system. While this is so, previous research has
shown that participants who were able to both hear and
feel the contact between a wielded object and a surface
were no better at perceiving affordances of that surface
Exp Brain Res
than participants who could feel but not hear such contact.
Likewise, participants who could hear but not feel contact
between a wielded object and a surface were no better at
perceiving affordances of that surface than participants who
could neither feel nor hear such contact (Burton 2000). In
other words, hearing the contact between a handheld dowel
and a support surface seems neither necessary nor sufficient for perceiving affordances of that surface. Therefore,
we would expect the same pattern of results in an experiment in which measures were taken to eliminate auditory
information. Alternatively, what does seem necessary to
perceive affordances of a surface by means of a wielded
object is information about the posture of the appendage to which the object is attached. Burton and McGowan
(1997; see also Wagman and Taylor 2005) have shown that
manipulations affecting the perceived posture of the hand–
object system affect perception of affordances of surfaces
explored with that system. We might therefore expect that
such manipulations will similarly affect affordance perception in the case of the head–object system. This is clearly a
topic for future research.
A final limitation is that although there are physical,
physiological, and functional differences between the head
and the hand, in both conditions, participants made contact
with the surface by means of a wooden rod. Therefore, it
is possible that using the rod to explore the surface somehow attenuated or eliminated differences in perceptual
performance between the head and the hand. We believe
that this is unlikely given that perception of object properties is comparable across disparate anatomical components
including the hand, the foot, and the torso (Palatinus et al.
2011; Hajnal et al. 2011). However, an experiment investigating whether the anatomical independence demonstrated
here generalizes across different types of objects used to
explore the surface may also be a topic for future research.
Soft assembly and stimulation patterns supporting
perception
The results of the experiment reported here highlight that
task-specific perceptual instruments not only are spontaneously assembled for the purposes of a particular perceptual task but are also softly assembled for that task.
Soft assembly means that the components of a perceptual
instrument are flexibly and temporarily recruited for the
purposes of performing a particular perceptual task. Therefore, the instrument is assembled from dynamic properties, not anatomical components (Bingham 1988; Kugler
and Turvey 1987). As a result, the same task-specific perceptual instrument can be assembled using different anatomical components, and different task-specific perceptual
instruments can be assembled using the same anatomical
components (Carello et al. 1992a, b; Wagman and Hajnal
2014a, b). Moreover, task-specific perceptual instruments
can be assembled across the body and objects attached to
the body used as perceptual tools. Despite important differences between the rod and the appendages to which it was
attached, participants were able to create and use a personplus-object perceptual system to achieve equivalent perceptual ends. Moreover, despite important differences between
the two appendages to which the rod was attached, participants were able to use instruments constructed of different
anatomical components to achieve equivalent perceptual
ends.
A compelling argument can be made that in performing a given motor task, the excessive number of degrees
of freedom at the disposal of the movement system is an
advantage rather than a disadvantage (Latash 2012). Given
the results of this experiment and those of previous experiments (e.g., Wagman and Hajnal 2014a, b), a comparable
argument might also be made for the excessive number of
degrees of freedom available to the perceptual system(s) in
performing a given perceptual task.
To a large extent, soft assembly of perceptual instruments is what underlies the potential for anatomical independence of such instruments. However, this potential can
only be realized to the extent that the stimulation patterns of
relevance to perception are also anatomically independent.
From the ecological perspective on perception and action,
objects and events structure patterned energy distributions
such that this structure is specific to (i.e., is lawfully related
to) its source (Turvey and Shaw 1999; Turvey 2002). For
example, exploring a surface with a wielded object lawfully generates a pattern of deformation in muscular tissue
that is (potentially) informative about both the properties of
the object used to explore the surface and the properties of
the surface explored with the object (Carello et al. 1992a,
b). Moreover, this pattern is available to be detected by any
and all components of the touch system that might be used
to wield the object (e.g., hand, foot, or head). To the extent
that objects and events simultaneously structure multiple
energy distributions, the information specifying a given
object or event may be amodal or modality-independent
(Gibson 1966; Regia-Corte and Wagman 2008).
The coupling of soft assembly with the lawful structuring of energy arrays likely underlies the findings of that
perception of whether a surface can be stood on reflects the
action capabilities of the perceiver regardless of whether the
object used to explore the surface is wielded by one hand,
two hands, the foot, or the head (see Wagman and Hajnal
2014a, b). Along these lines, the results of this experiment
are consistent with the proposal that the mechanical stimulation patterns of relevance to the haptic system occur not
at the level of individual receptors or at the point of contact
with the object but rather at a much larger scale—the interconnected and nested levels of the haptic system as a whole.
13
Specifically, the results are consistent with the hypothesis that the haptic system is best described as a nested
biotensegrity structure (Turvey and Fonseca 2014). In addition, the results are consistent with the abilities of other animal species such as moose, rams, rats, and various insects to
perceive properties of surfaces by cephalic appendages such
antlers, horns, vibrissae, and antenna (see Burton 1993).
Concluding comments
Successfully performing everyday behaviors requires flexibility in perception. Perception of affordances for a given
behavior ought to reflect the action capabilities of the perceiver over the variety of contexts in which that affordance
is encountered and over the variety of means by which that
affordance might be perceived. The results of the experiment
reported here highlight this flexibility. In particular, the findings highlight that flexible task-specific detection units can be
assembled across units that span the body and inert objects.
Acknowledgments We thank Katie Jameson for help with conducting the experiments, Sarah Caputo for serving as the model in the figures, Dawn McBride for statistical consulting, and Andrew Wilson for
helpful comments on a previous draft of this manuscript.
References
Bingham GP (1988) Task-specific devices and the perceptual bottleneck.
Hum Mov Sci 7:225–264. doi:10.1016/0167-9457(88)90013-9
Burton G (1993) Non-neural extensions of haptic sensitivity. Ecol
Psychol 5:105–124. doi:10.1207/s15326969eco0502_1
Burton G (2000) The role of the sound of tapping for nonvisual judgment of gap crossability. J Exp Psychol Hum Percept Perform
26:900–916. doi:10.1037//0096-1523.26.3.900
Burton G, McGowan J (1997) Contact and posture in nonvisual judgment of gap crossability. Ecol Psychol 9:323–354. doi:10.1207/
s15326969eco0904_4
Carello C, Wagman JB (2009) Mutuality in the perception of affordances and the control of movement. In: Sternad D (ed) Progress
in motor control: a multidisciplinary perspective. Springer, New
York, pp 271–289
Carello C, Fitzpatrick P, Domaniewicz I, Chan TC, Turvey MT (1992a)
Effortful touch with minimal movement. J Exp Psychol Hum Percept Perform 18:290–302. doi:10.1037/0096-1523.18.1.290
Carello C, Fitzpatrick P, Turvey MT (1992b) Haptic probing: perceiving the length of a probe and the distance of a surface probed.
Percept Psychophys 51:580–598
Cole WG, Chan GL, Vereijken B, Adolph KE (2013) Perceiving affordances for different motor skills. Exp Brain Res 225:309–319.
doi:10.1007/s00221-012-3328-9
Coren S (1993) The lateral preference inventory for measurement
of handedness, footedness, eyedness, and earedness: norms for
young adults. Bull Psychon Soc 31:1–3. doi:10.3758/BF03334122
Faul F, Erdfelder E, Lang A-G, Buchner A (2007) G*Power 3: a flexible statistical power analysis program for the social, behavioral, and biomedical sciences. Behav Res Methods 39:175–191.
doi:10.3758/BF03193146
13
Exp Brain Res
Finney DA (1971) Probit analysis, 3rd edn. Cambridge University
Press, London
Fitzpatrick P, Carello C, Schmidt RC, Corey D (1994) Haptic and visual perception of an affordance for upright posture. Ecol Psychol
6:265–287. doi:10.1207/s15326969eco0604_2
Froese T, McGann M, Bigge W, Spiers A, Seth AK (2012) The enactive torch: a new tool for the science of perception. IEEE Trans
Haptics 5:365–375. doi:10.1109/TOH.2011.57
Gibson JJ (1966) The senses considered as perceptual systems.
Houghton Mifflin, Boston
Gibson JJ (1979) The ecological approach to visual perception.
Houghton Mifflin, Boston
Gibson EJ (1994) Has psychology a future? Psychol Sci 5:69–76.
doi:10.1111/j.1467-9280.1994.tb00633.x
Gordon MS, Rosenblum LD (2004) Perception of sound obstructing
surfaces using body-scaled judgments. Ecol Psychol 16:87–113.
doi:10.1207/s15326969eco1602_1
Hajnal A, Abdul-Malak DT, Durgin FH (2011) The perceptual experience of slope by foot and by finger. J Exp Psychol Hum Percept
Perform 37:709–719. doi:10.1037/a0019950
Kugler PN, Turvey MT (1987) Information, natural law, and the self
assembly of rhythmic movements. Erlbaum, Hillsdale
Latash ML (2012) The bliss (not the problem) of motor abundance (not redundancy). Exp Brain Res 217:1–5. doi:10.1007/
s00221-012-3000-4
Matthis JS, Fajen BR (2014) Visual control of foot placement when
walking over complex terrain. J Exp Psychol Hum Percept Perform 40:106–115. doi:10.1037/a0033101
Palatinus Z, Carello C, Turvey MT (2011) Principles of part-whole
selective perception extend to the torso. J Mot Behav 43:87–93.
doi:10.1080/00222895.2010.538767
Regia-Corte T, Wagman JB (2008) Perception of affordances for
standing on an surface depends on height of center of mass. Exp
Brain Res 191:25–35. doi:10.1007/s00221-008-1492-8
Runeson S (1977) On the possibility of “smart” perceptual mechanisms.
Scand J Psychol 18:172–179. doi:10.1111/j.1467-9450.1977.
tb00274.x
Turvey MT (2002) Perception: the ecological approach. Encyclopedia
of cognitive science. Nature Publishing Group, London
Turvey MT (2007) Action and perception at the level of synergies.
Hum Mov Sci 26:657–697. doi:10.1016/j.humov.2007.04.002
Turvey MT (2013) Ecological perspective on perception-action: What
kind of science does it entail? In: Prinz W, Beisert M, Herwig
A (eds) Action science: foundations of an emerging discipline.
MIT Press, Cambridge, pp 139–170
Turvey MT, Fonseca ST (2014) The medium of haptic perception: a
tensegrity hypothesis. J Mot Behav 46:143–187. doi:10.1080/00
222895.2013.798252
Turvey MT, Shaw RE (1999) Ecological foundations of cognition:
I. Symmetry and specificity of animal-environment systems. J
Conscious Stud 6:95–110
Wagman JB, Hajnal A (2014a) Task-specificity and anatomical independence in perception of surface properties by means of a
wielded object. J Exp Psychol Hum Percept Perform 40:2372–
2391. doi:10.1037/xhp0000014
Wagman JB, Hajnal A (2014b) Getting off on the right (or left) foot:
perceiving by means of a rod attached to the preferred or nonpreferred foot. Exp Brain Res 232:3591–3599. doi:10.1007/
s00221-014-4047-1
Wagman J, Taylor K (2005) Perceived arm posture and remote haptic perception of whether an object can be stepped over. J Mot
Behav 37:339–342
Welker E, Nowacki AS (2011) Understanding equivalence and noninferiority testing. J Gen Intern Med 26:192–196