Journal of Experimental Psychology: General

Journal of Experimental Psychology: General The Perceptual Homunculus: The Perception of the Relative Proportions of the Human Body Sally A. Linkenauger, Hong Yu Wong, Michael Geuss, Jeanine K. Stefanucci, Kathleen C. McCulloch, Heinrich H. Bülthoff, Betty J. Mohler, and Dennis R. Proffitt Online First Publication, December 15, 2014. http://dx.doi.org/10.1037/xge0000028

CITATION Linkenauger, S. A., Wong, H. Y., Geuss, M., Stefanucci, J. K., McCulloch, K. C., Bülthoff, H. H., Mohler, B. J., & Proffitt, D. R. (2014, December 15). The Perceptual Homunculus: The Perception of the Relative Proportions of the Human Body. Journal of Experimental Psychology: General. Advance online publication. http://dx.doi.org/10.1037/xge0000028

Journal of Experimental Psychology: General 2014, Vol. 143, No. 6, 000

© 2014 American Psychological Association 0096-3445/14/$12.00 http://dx.doi.org/10.1037/xge0000028

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The Perceptual Homunculus: The Perception of the Relative Proportions of the Human Body Sally A. Linkenauger

Hong Yu Wong

Lancaster University

University of Tübingen

Michael Geuss and Jeanine K. Stefanucci

Kathleen C. McCulloch

University of Utah

Lancaster University

Heinrich H. Bülthoff

Betty J. Mohler

Max Planck Institute for Biological Cybernetics, Tübingen, Germany and Korea University

Max Planck Institute for Biological Cybernetics, Tübingen, Germany

Dennis R. Proffitt University of Virginia Given that observing one’s body is ubiquitous in experience, it is natural to assume that people accurately perceive the relative sizes of their body parts. This assumption is mistaken. In a series of studies, we show that there are dramatic systematic distortions in the perception of bodily proportions, as assessed by visual estimation tasks, where participants were asked to compare the lengths of two body parts. These distortions are not evident when participants estimate the extent of a body part relative to a noncorporeal object or when asked to estimate noncorporal objects that are the same length as their body parts. Our results reveal a radical asymmetry in the perception of corporeal and noncorporeal relative size estimates. Our findings also suggest that people visually perceive the relative size of their body parts as a function of each part’s relative tactile sensitivity and physical size. Keywords: body perception, visual perception, somatosensation, proprioception

of our hands relative to the length of our torso. For example, when looking in mirrors, the relative length of our hands compared to our legs seems obvious. Consequently, it is reasonable to assume that we perceive the relationship between the sizes of our body parts in an accurate manner. In fact, models of proprioception have often assumed accurate metric information about the body (Soechting, 1982; van Beers, Sittig, & van der Gon, 1998). However, the information underlying the tactile perception of the body does not relay the relationship between body parts with respect to their actual physical sizes, but rather the relationships between their tactile sensitivity. The need for certain body parts to engage in more precise and complex actions than others requires those body parts to have a higher degree of tactile sensitivity. Consequently, these body parts have higher densities of smaller tactile receptive fields and more extensive neural representation in the somatosensory cortex. This distribution of tactile sensitivity comes at a cost to tactile size constancy, because the same sized object stimulates a different number of receptive fields depending on its position on the body, which leads to differences in tactile size perception across different body parts (this has been popularly referred to as Weber’s illusion (Weber, 1834)). Put simply, objects feel larger on more sensitive body parts than on less sensitive body parts (Anstis, 1964; Goudge, 1918; Weber, 1834; Weinstein, 1968).

When we look at the proportions of our bodies, we have an abundance of visual information specifying our body’s proportions. We refer, for example, to the information indicating the length of our arms relative to the length our legs or the length

Sally A. Linkenauger, Department of Psychology, Lancaster University; Hong Yu Wong, Center for Integrative Neuroscience, University of Tübingen; Michael Geuss and Jeanine K. Stefanucci, Department of Psychology, University of Utah; Kathleen C. McCulloch, Department of Psychology, Lancaster University; Heinrich H. Bülthoff, Max Planck Institute for Biological Cybernetics, Tübingen, Germany and Department of Brain and Cognitive Engineering, Korea University; Betty J. Mohler, Max Planck Institute for Biological Cybernetics, Tübingen, Germany; Dennis R. Proffitt, Department of Psychology, University of Virginia. Sally A. Linkenauger’s research was supported through the Alexander von Humboldt foundation. Part of Heinrich H. Bülthoff’s research was supported by the Brain Korea 21 PLUS Program through the National Research Foundation of Korea funded by the Ministry of Education. Correspondence concerning this article should be addressed to Sally Ann Linkenauger, Lecturer, Flyde College, Lancaster University, Bailrigg, Lancaster, United Kingdom, LA1 4YF or to Heinrich H. Bülthoff, Director, Max Planck Institute for Biological Cybernetics, Spemannstrasse 38, Tübingen, Germany, 72076. E-mail: [email protected] or [email protected] 1


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LINKENAUGER ET AL.

Some recent evidence has also suggested that the differences in the shape of these receptive fields across a body part can influence tactile size perception. On the back of the mammalian hand, skin receptors are ovular and span lengthwise along the hand, and hence, an object placed across the width of the hand stimulates more receptive fields than when placed across the length of the hand (Pons, Wall, Garraghty, Cusick, & Kaas, 1987). In parallel to Weber’s illusion, it has been found that objects of the same size feel larger when placed across hand width rather than the length of the hand (Longo & Haggard, 2011). However, this anisotropy only occurs on the back of the hand; on the palm, receptive fields are roughly circular. Consequently, no difference in tactile size perception as a result of stimulus orientation has been found on the palm (Longo & Haggard, 2011). Interestingly, Longo and Haggard (2010) had individuals make estimates of their hand size and shape by indicating the position of various parts of their unseen hand (positioned underneath a opaque board) using a pointing task. It was found that people’s estimates of their hand size and shape were drastically distorted in a manner consistent with the shape of their sensory receptive fields, with hand width being exaggerated with respect to hand length (Longo & Haggard, 2010). In parallel, these distortions were mainly found for the back of the hand; when individuals made these estimations with their palm facing upward, the size of the distortions decreased significantly, and in some cases, even disappeared (Longo & Haggard, 2012a). Even when estimating seen hand parts using a visual matching task, these distortions persist, albeit to a lesser magnitude (Longo & Haggard, 2012b). Hence, these Weber’s illusion-based distortions appear to influence not only tactile size perception but the perceived size and shape of the hand as well. Interestingly, the degree to which individuals experience a change in tactile size (Weber’s illusion) is a fraction (30%) of what would be expected if perceived tactile size was derived solely from differences in tactile receptive field size (340%; Green, 1982; Taylor-Clarke, Jacobsen, & Haggard, 2004). Subsequently, the perceptual system must have in place some compensatory mechanism that decreases the tactile size discrepancies across body parts to achieve an ecologically adequate tactile size constancy. One account for how tactile constancy is achieved proposes that tactile information is rescaled to an object-centered space, which is based on more spatially reliable sensory modalities, mainly vision (Röder, Rösler, & Spence, 2004; Taylor-Clarke, Jacobsen, & Haggard, 2004). In other words, the felt tactile location of the object is remapped to a visual representation of the body, thus minimizing most of Weber’s illusion. We will refer to this account as “tactilevisual remapping.” Some evidence suggests that tactile-visual remapping is the compensatory mechanism. Manipulations of the visual size of body parts can influence tactile size perceptions on those body parts (Taylor-Clarke, Jacobsen, & Haggard, 2004). Looking at a body part can enhance tactile acuity in that body part, even in individuals with neural deficits, resulting in somatosensory impairment (Kennett, Taylor-Clarke, & Haggard, 2001; Serino, Farnè, Rinaldesi, Haggard, & La`davas, 2007). In addition, reaction times (RTs) to tactile stimuli are faster when viewing the body part on which the tactile stimulus is presented, but only when the reaction to the stimulus requires a spatial judgment (Press, TaylorClarke, Kennett, & Haggard, 2004). Areas of the somatosensory cortex appear to become more active when an object is perceived tactilely and visually concurrently (Schaefer, Flor, Heinze, &

Rotte, 2006), suggesting that both visual and tactile information inform the perception of tactile size. If tactile-visual remapping occurs, then one would expect that with sufficient visual information available, individuals would be reasonably accurate in judging the relative proportions of the body, because this is presumably required for tactile size constancy. An alternative account to tactile-visual remapping is that the perceiver could experience that a less sensitive body part is disproportionately larger than the more sensitive body part to a magnitude that counteracts the majority of Weber’s illusion; we will refer to this as “reverse distortion.” If the limb is larger, then the object residing on it must be relatively larger as well. Correspondingly, it has been shown that the haptic perception of size increases when using the rubber hand illusion to make the hand feel larger (Haggard & Jundi, 2009). Interestingly, exposure to a visually minified hand size and magnified forearm size virtually eliminated Weber’s illusion on those two body parts (TaylorClarke, Jacobsen, & Haggard, 2004). Drawing from this, it is possible that the perception of the relationship between body part sizes is already distorted, and experimentally inducing more visual distortion of the relative sizes of body parts merely decreases the remainder of Weber’s illusion that is left. If this is the case, then the perception of the relative proportions of the body should be distorted as a function of their actual size and sensitivity. Some evidence supports this possibility. Decreasing sensory input from a certain limb via anesthetic leads to shrinkage in the body part’s representation in the somatosensory cortex and increases in the perception of its size (Gandevia & Phegan, 1999). Chronic pain decreases the somatosensory representation of the afflicted body part and increases its apparent size (Gandevia & Phegan, 1999; Moseley, 2005). Amputees typically report that their phantom limb is large (Flor, 2002), presumably because the limb has limited sensory input. As a result, in these atypical cases, there appears to be an inverse relationship between the tactile sensitivity of a body part and its perceived size. Reverse distortion explanation would also predict that in everyday situations humans would have a distorted perception of their bodily dimensions. First, less sensitive body parts should be overestimated more than more sensitive body parts. Second, given an equal (or similar) degree of sensitivity, physically larger body parts should be distorted less than smaller body parts due to the consequence that they are already physically larger. Anecdotal experiences suggest that individuals do seem to misperceive their bodily dimensions. For example, the average lengths of the human foot and forearm are approximately the same (Dreyfuss & Tilley, 1993). Yet, the majority of people will persist in believing that their forearm is longer until they directly compare them. Other bodily relationships that are typically met with skepticism are the equalities between average arm span and the average height as well as average hip span and average shin length (Dreyfuss & Tilley, 1993). In fact, most individuals have to be taught to correctly draw human body proportions (Fairbanks & Fairbanks, 2005), because otherwise the drawings are wrought with several systematic distortions (Fuentes, Longo, & Haggard, 2013; Kahill, 1984). In the event that these distortions in perceived body proportions do occur, it is important to determine whether these distortions are limited to the comparison of one body part to another, or whether these distortions generalize to comparisons between the sizes of


THE PERCEPTUAL HOMUNCULUS

body parts and noncorporeal objects. For example, my foot may seem smaller than my forearm, but does it also seem smaller than a stick that is its same length? If these distortions dynamically arise from systematic asymmetries in tactile signals from relative parts of the body, then it stands to reason that these distortions would only be present when comparing body parts relative to one another, because no such tactile mapping exists between a body part and a noncorporeal object. However, if these distortions were a result of a static transformation of overall body size, then one would expect that individuals would misperceive the relationship between physical object size and body part size. It is also entirely plausible that comparisons between body parts and comparisons between the body and external objects tap into the same or different bodily representations, which could also explain both potential outcomes. All in all, it is still unknown how humans perceive their relative body part size under normal circumstances when looking at their bodies. In other words, how proportionally accurate is our perceptual homunculus? To assess this, we had people estimate the length of various body parts by using their other body parts or a noncorporeal object as a metric. In this way, we could determine whether the perception of body parts’ sizes were distorted in respect to each other versus globally as assessed by a noncorporeal object.

Experiment 1: Body Estimates With Hand or Dowel To assess the accuracy with which people perceive the relative sizes of their body, we had people estimate various body part lengths by using their hand length or a noncorporeal object as a metric. According to the visual-tactile remapping account, individuals should perceive the relative proportions of their bodies accurately. Conversely, the reverse distortion account would predict distortions in the perception of bodily proportions with respect to their physical size and tactile sensitivity.

Method Participants. Twenty-eight undergraduate students (14 female) from the University of Virginia participated for course credit. All participants had normal or corrected-to-normal vision and no visible morphological abnormalities. Procedure. Participants’ hand lengths were measured using a tape measure, but they were told these would be used for a later experiment. Participants were randomly assigned to either the hand group or dowel group. The hand group was told they would make several estimates of their bodies using their dominant hand as a measuring device. They were instructed to estimate how many hand lengths (defined by the palm-wrist intersection to the longest finger tip) could fit into certain body parts. They were encouraged to use fractional units and to be as accurate as possible. Participants estimated the length of their leg (defined by their hip bone and their heel’s bottom), their torso (defined by the top of their shoulder and their hip bone), their arm length (on an outstretched arm, defined by the bone protrusion on their shoulder to the longest fingertip), their head (from chin bottom to the top of their head), their entire body (top of head to heel’s bottom), and their foot (from the back of the heel to the tip of the longest toe). In the dowel group, participants held a 55 cm (l) by 3 cm (w) wooden dowel. Black tape was wrapped around the dowel so that the length from the black tape to one end of the dowel was the same length

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as the participant’s hand. They made the same six estimates (one per body part), but used the length between the black tape and the dowel end as their unit of measure. Body part estimation order was randomized. Following all estimates, the actual dimensions of participants’ body parts were measured.

Results and Discussion Individuals’ estimates were transformed into centimeters by multiplying hand length by the body part estimate. Accuracy ratios were then created by dividing their estimate by the actual length of the body part. Consequently, if participants’ estimates were perfectly accurate, then that should result in an accuracy ratio of 1. Accuracy ratios over 1 signaled overestimation, and accuracy ratios under 1 signaled underestimation. A repeated-measures analysis of variance (RMANOVA) was conducted with body part as the repeated measures variable, measuring device as a between-subjects variable, and accuracy ratios as the dependent variable. The analysis revealed a significant effect of measuring device with dowel ratios being significantly smaller, M ⫽ 1.07, SE ⫽ 0.07, than hand ratios, M ⫽ 1.38, SE ⫽ 0.07, F(1, 26) ⫽ 9.22, p ⬍ .01, ␩p2 ⫽ 0.26; see Figure 1. There was also a significant effect of body part, with some body parts being more overestimated than others, F(5, 130) ⫽ 13.66, p ⬍ .01, ␩p2 ⫽ 0.34. There was also a significant interaction between measuring device and body part, F(5, 130) ⫽ 4.52, p⬍.01, ␩p2 ⫽ 0.15, suggesting that the difference between body parts was driven by the hand condition rather than the dowel condition. The largest overestimation was for the torso and the smallest was for the foot; see Table 1. These results support the reverse distortion hypothesis, as body parts were quite distorted with respect to each other, but only when comparing corporal lengths. The pattern of distortions between the relative body parts also adheres to the reverse distortion hypothesis with less sensitive body parts being overestimated more than more sensitive body parts.

Figure 1. Accuracy ratios of the body estimates across the difference measurement device conditions. Error bars represent ⫾1 SE of the mean.

LINKENAUGER ET AL.

4 Table 1 Means of the Accuracy Ratios for Each Experiment

Experiment 1 Body part


Foot Head Arm Leg Torso Full body

C 0.98 (.06) 1.18 (.08) 1.53 (.13) 1.42 (.14) 1.76 (.18) 1.41 (.13)

2 N 0.94 (.05) 1.14 (.06) 1.23 (.07) 0.92 (.06) 1.26 (.08) 0.93 (.05)

N .95 (.04) 1.05 (.04) 1.04 (.06) 1.07 (.06) 1.33 (.09) 1.16 (.08)

3 C a

1.12 (.03) 1.52 (.10) 1.34 (.05) 1.27 (.11) 1.70 (.14) 1.47 (.14)

4

5

6

N

C

N

C

N

C

N

0.99 (.05) 1.39 (.10) 1.19 (.07) 0.95 (.05) 1.36 (.08) 1.02 (.06)

0.82 (.04) 0.78 (.05) 0.75 (.03) 0.83 (.05) 0.90 (.09) 0.85 (.04)

0.96 (.05) 1.04 (.06) 0.89 (.03) 0.95 (.09) 1.09 (.06) 1.05 (.08)

1.24 (.06) 1.22 (.05) 1.24 (.08) 1.27 (.07) 1.26 (.07) 1.26 (.13)

1.04 (.05) 0.95 (.04) 1.12 (.05) 1.04 (.05) 1.02 (.05) 1.12 (.03)

1.05 (.07) 1.29 (.06) 1.38 (.06) 1.16 (.06) 1.84 (.13) 1.35 (.09)

0.80 (.04) 0.87 (.04) 1.04 (.05) 0.91 (.05) 1.24 (.08) 1.10 (.08)

Note. The C indicates the data for the corporeal metric condition, and the N indicates the data for the noncorporeal metric condition. Standard errors are in parentheses below the means. a For Experiment 3, this value is actually derived for the hand using the foot as a metric rather than vice versa in the other experiments.

Experiment 2: Body Part Estimates With a Drawn Hand To investigate whether the differences in overestimation between the hand and the dowel were due to object shape rather than a corporeal versus noncorporeal metric, participants estimated their body parts using a paper hand as a metric.

Methods Participants. Fifteen (six female) participants recruited from the university community around Tübingen, Germany were paid 8€ per hour. All participants had normal or corrected-to-normal vision and had no visible morphological abnormalities. Procedure. Participants were presented with a blank sheet of paper on top of which they were instructed to place their dominant hand. Their hand was then traced onto the paper, with the tips of their fingers at the top end and the intersection of their wrist and palm completing the drawing at the bottom. The procedure was the same as Experiment 1, except participants used the length of their drawn hand as a metric.

greater, M ⫽ 1.38, SE ⫽ 0.06, than both dowel estimates, M ⫽ 1.07, SE ⫽ 0.06, and drawn hand estimates, M ⫽ 1.10, SE ⫽ 0.06, ps ⬍ .01; see Figure 2. Dowel estimates and drawn hand estimates did not significantly differ from each other, p ⫽ .71. These findings suggest that the results in Experiment 1 were not a function of the differences in the shape of the metric.

Experiment 3: Body Part Estimates With Foot or Dowel To determine whether the effects observed in Experiment 1 are not specific to using one’s hand as a metric, participants used their foot or foot-sized dowel as a metric. We expect that if these effects are attributable to the relative proportions of all body parts, then we should obtain similar results when participants use another sensitive body part as a metric.

Results and Discussion An RMANOVA was conducted with body part length as a repeated measures variable and accuracy ratios as the dependent variable. Body part was significant, F(5, 70) ⫽ 5.73, p ⬍ .01, ␩p2 ⫽ 0.29; see Figure 2. The largest overestimation was for the torso and the smallest was for the foot; see Table 1. To compare the drawn hand, actual hand and dowel estimates, we combined the data from this experiment to the data from Experiment 1; although, it is important to note that Experiment 1 and Experiment 2 were two different samples. We compared the data from this experiment and the data from Experiment 1 using an RMANOVA with body part as a repeated measures variable, measuring device (hand, dowel, or drawn hand) as the betweensubjects variable, and accuracy ratios as the dependent variable. As in Experiment 1, body part was significant, F(5, 200) ⫽ 17.81, p ⬍ .01, ␩p2 ⫽ 0.31, and the interaction between measuring device and body part was also significant, F(10, 200) ⫽ 3.82, p ⬍ .01, ␩p2 ⫽ 0.16. Measuring device was also significant, F(2, 40) ⫽ 7.39, p ⬍ .01, ␩p2 ⫽ 0.27. Fisher’s least significant difference (LSD) post hoc comparisons showed that hand estimates were significantly

Figure 2. Accuracy ratios of the body estimates across the difference measurement device conditions. Error bars represent ⫾1 SE of the mean.



Methods Participants. Twenty-eight participants (14 female) from the University of Virginia participated for course credit. All participants had normal or corrected-to-normal vision and had no visible morphological abnormalities. Procedure. Participants were asked to remove their shoes, and their foot length was measured. Participants were randomly assigned to either the foot metric or dowel metric condition. The procedure was identical to Experiment 1, except instead of using their hands or a hand-sized dowel as the metric, participants used their foot or a foot-sized dowel as a metric to estimate the length of their body parts. Participants also estimated their hand with their foot metric.

Results An RMANOVA was conducted with body part length as a repeated measures variable, accuracy ratios as the dependent variable, and measuring device as a between-subjects variable. Measuring device was significant. The foot estimates, M ⫽ 1.40, SE ⫽ 0.06, were more overestimated than the foot-sized dowel, M ⫽ 1.15, SE ⫽ 0.06, F(1, 26) ⫽ 8.07, p ⬍ .01, ␩p2 ⫽ 0.24. Body part was also significant with the torso being most overestimated and the hand being least, F(5, 130) ⫽ 14.03, p ⬍ .01, ␩p2 ⫽ 0.35, see Table 1. There was no significant interaction between body part and measuring device, p ⫽ .11.

Experiment 4: Body Part Estimates With Forearm or Dowel We also conducted another experiment to assess the effect of using a body part with low tactile sensitivity as a metric to assess if these effects are modulated by high versus low tactile sensitivity. If these distortions are due to relative sensitivity, then individuals should underestimate when using a corporeal metric with a low level of tactile sensitivity instead of overestimate (as they did using a highly sensitive corporeal metric).

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SE ⫽ 0.82, F(1, 19) ⫽ 10.53, p ⬍ .01, ␩p2 ⫽ 0.36; see Figure 3. Body part was significant, F(5, 95) ⫽ 2.76, p ⫽ .02, ␩p2 ⫽ 0.13, with the arm being underestimated the most and the torso being underestimated the least; see Table 1. There was no significant interaction between measuring device and body part, p ⫽ .76. When considering much of the whole arm that does not include the forearm is the hand, which appears to be underestimated the most in all of these cases, this pattern is almost identical to what we observed in the previous studies and equally as compelling. In the previous experiments, the hand was part of the arm, so when estimating the arm, the effect of the hand was always canceled out (i.e., the hand part of the arm was always one hand), so the arm distortions were likely driven by the forearm and upper arm. In this experiment, this canceling out of hand sensitivity was unlikely to be present; however, the canceling out of the low sensitivity of the forearm likely was present. Hence, the arm was least underestimated probably due to the high sensitivity of the hand and the lack of the influence of the forearm.

Experiment 5: Body Part Estimates on a Cylinder With Hand or Dowel We conducted this experiment to assess whether the perception of the metric (i.e., hand) or the perception of body parts was responsible for the observed overestimations. Participants estimated length using their hand or a hand-sized dowel. The actual length of the to-be-estimated body part was measured prior to the experiment and then presented as a vertical cylinder rather than a body part. If these distortions are specific to the relative perception of body parts, then we should not expect a difference in the distortions across body part lengths when they are not presented on the body.

Methods Participants. Twenty-one (11 female) participants were recruited from the university community around Tübingen, Germany and were paid 8€ per hour. All participants had normal or corrected-to-normal vision and had no visible morphological abnormalities. Procedure. The procedure was nearly identical to Experiment 1, except that individuals estimated their body with their forearm (as defined by the palm and wrist intersection the forearm and upper arm intersection; the inside of the forearm) or a dowel that corresponded to the length of their forearm.

Results and Discussion An RMANOVA was conducted with body part length as a repeated measures variable, measuring device (forearm or dowel) as a between-subjects variable, and accuracy ratios as the dependent variable. Measuring device was significant, with individuals estimating with the forearm underestimating more, M ⫽ 0.82, SE ⫽ 0.04, than individuals estimating with the dowel, M ⫽ 0.99,

Figure 3. Accuracy ratios for each body part for both the forearm and dowel conditions. Error bars represent ⫾1 SE of the mean.

LINKENAUGER ET AL.

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Methods Participants. Twenty-six (10 female) participants recruited from the university community around Tübingen, Germany were paid at a rate of 8€ per hour. All participants had normal or corrected-to-normal vision and had no visible morphological abnormalities. Stimuli and Apparatus. Using poster board, we created a cylinder that was approximately 2.5 m in height and approximately 8 cm in diameter. The cylinder was positioned on a stand to secure it vertically. A 2 cm thick green line was drawn around the bottom of the cylinder. Another green line was drawn on a small piece of poster board, which was tightly wrapped around the cylinder so that it could be moved up and down the length of the cylinder. Therefore, one could manipulate the vertical distance between the two lines on the cylinder to display different lengths. Procedure. Participants were randomly assigned to either the hand or the dowel condition. Prior to the beginning of the experiment, the dimensions of participants’ bodies were measured (the same dimensions as in Experiment 1: head, leg, arm, hand, torso, full body, and foot length). They were told the measurements were to aid in determining correct tracking positions for a later virtual reality experiment. The experiment was conducted in a virtual reality laboratory, which had infrared motion capture cameras mounted on the ceiling, infrared tracking objects (shoes, belt, and gloves with trackers attached), as well as a head-mounted display in an open box on the floor. Participants were positioned in front of the cylinder. Participants were instructed to estimate the length between the two green lines on the cylinder. Participants were presented with six lengths in random order, and each length corresponded to the length of their previously measured body part. Participants in the hand group estimated the lengths on the cylinder using their hand length as a metric, and participants in the dowel group used the hand-sized dowel as the metric.

Results An RMANOVA was conducted with body part length as a repeated measures variable, measuring device as a betweensubjects variable, and accuracy ratios as the dependent variable. Body part was not significant, p ⫽ .38; see Table 1. However, there was a significant effect of measuring device with dowel estimates being less overestimated, M ⫽ 1.05, SE ⫽ 0.05, than hand estimates, M ⫽ 1.25, SE ⫽ 0.05, F(1, 24) ⫽ 7.63, p ⫽ .01, ␩p2 ⫽ 0.24; see Figure 4. These results suggest that the differences between the different body parts found in Experiments 1, 3 and 4 were not due to differences in the ability to estimate objects of different lengths.

Experiment 6: Body Part Estimates With Hand or Dowel in a Mirror In this experiment, we addressed whether the observed body misperception was a result of a first-person perspective on the body. Participants were instructed to look at a reflection of their body parts and hands in a full-length mirror while making their estimates. We also made sure to record and store raw values of actual bodily dimensions in addition to ratios in order to assess

Figure 4. Accuracy ratios of cylinder estimates when either using the hand or a dowel as a metric. Error bars represent ⫾1 SE of the mean.

their predictive power with respect to the reverse distortion hypothesis.

Methods Participants. Twenty-four (11 female) participants were recruited from the university community around Tübingen, Germany and were paid 8€ per hour. All participants had normal or corrected-to-normal vision and had no visible morphological abnormalities. Procedure. The procedure was the same as Experiment 1 except that participants were instructed to use their reflection in a mirror to make their estimates. A full-length mirror was positioned against a wall. Participants were instructed to stand at a distance in front of the mirror so that their entire body was visible. However, all participants were instructed to use the reflection of their hand (or the hand-sized dowel) and the reflection of their body parts to make their estimates.

Results An RMANOVA was conducted with body part length as a repeated measures variable, measuring device as a betweensubjects variable, and accuracy ratios as the dependent variable. Once again, measuring device was significant with dowel estimates, M ⫽ 1.00, SE ⫽ 0.05, and less overestimated than hand estimates, M ⫽ 1.38, SE ⫽ 0.04, F(1, 22) ⫽ 36.13, p ⬍ .01, ␩p2 ⫽ 0.62. There was also a significant interaction between body part and measuring device, F(1, 22) ⫽ 2.48, p ⫽ .04, ␩p2 ⫽ 0.10. Body part was also significant, F(5, 110) ⫽ 22.52, p ⬍ .01, ␩p2 ⫽ 0.51, with the torso being most overestimated and the foot being least; see Table 1. All of the combined experimental data is shown in Figure 5. According to the reverse distortion hypothesis, the perceived amount of distortion should be a function of the difference in physical size between body parts and the difference between the



Figure 5. Ratios of overestimation for Experiments 1– 6 collapsed across body part (or body part length in the case of the cylinder). Error bars represent ⫾1 SE of the mean. The images above the bars represent each condition for each experiment.

tactile sensitivity of the body parts. Theoretically, one could calculate a predicted outcome for our data based on this assumption. The product of sensitivity of one body part (S1) and the physical size of a body part (P1) multiplied by the perceptual distortion (D1) should be roughly equal to the product of the sensitivity (Sh), physical size (Ph), and degree of distortion (Dh) associated with the hand, so that S1P1D1 ⬵ SkPkDk. Hence, D1/Dk ⬵ SkPk/S1P1 and because we had individuals estimate D1/Dk, the distortion of the body part relative the distortion of the hand, we can predict what this relative distortion should be using the differences in tactile sensitivity across the body part and the hand and the relationship between the physical sizes of the hand and the body part. In order to do this, we approximated the relative tactile sensitivity between body parts using Weinstein (1968; two-point discrimination threshold for males, p. 202) to make a prediction about what individuals’ distortions should be for each body part relative to the hand (SkS1). We calculated the sensitivity of the hand by taking the mean of the finger and palm threshold. We also calculated participants’ actual body size ratios by dividing the actual length hand by the actual length of the body part (). Note that we did not include the full body in this analysis due to the variability in the sensitivity across the body. Hence, if the reverse distortion hypothesis is the case, then product of the sensitivity ratios and the actual body size ratios should predict participants’ estimates over and above either of these variables in isolation. In order to assess this, we used a linear mixed modeling to assess the relative influence of these variables (Baayen, 2008; Jaeger, 2008). In this analysis, we analyzed the data in the hand

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condition rather than in the dowel condition as our predictions pertained to the difference between the body part and the hand. In a null model, only the random effects of participants and body part were entered. The Akaike information criterion (AIC; a measure of model quality, when taking into account fit and complexity with lower being better; Akaike, 1973) for the null model was 38.53. In a second model, we added only the sensitivity ratio to the null model. In a third model, we added only the size ratio to the null model. In the fourth model, we added both the size and sensitivity to the null model. In the fifth model, we added the product of the size and sensitivity to the model as an interaction. The single addition of sensitivity did not improve model fit, ␹(1)2 ⫽ 1.73, p ⫽ .19, AIC ⫽ 43.14, nor did the singular inclusion of body size ratios, ␹(1)2 ⫽ 0.35, p ⫽ .55, AIC ⫽ 39.26. Interestingly, adding both sensitivity ratios and size ratios did significantly improve model fit, ␹(2)2 ⫽ 16.16, p ⬍ .01, AIC ⫽ 34.33. The fifth model significantly improved the fit of the model over both the null model, ␹(3)2 ⫽ 30.99, p ⬍ .01, and the fourth model, ␹(2)2 ⫽ 14.81, p ⬍ .01, AIC ⫽ 24.97. In this fifth model, neither size ratios, p ⫽ .44, nor sensitivity ratios, p ⫽ .43, were significant independently as fixed factors. However, the interaction between the size and sensitivity ratios was significant, p ⬍ .01; see Figure 6, suggesting that the improved of the fit of this model was a function of the product of the size and sensitivity ratios. To test the robustness of these results across different hand part sensitivities, we reconducted this analysis using only the finger sensitivity threshold and again using only the palm sensitivity threshold instead of the combination between the two. For the

Figure 6. Participants’ overestimations plotted as a function of the predicted estimates, which was composed of the product of the relative sensitivity and physical size of each respective body part. The solid black line is the linear regression line, and the shaded area around the line represents a 95% confidence region. The dashed black line depicts what the regression line would have looked like if our predictions perfectly fit participants’ overestimations.


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finger sensitivity analyses, the null model (AIC ⫽ 38.53) and the model including only the size ratios (AIC ⫽ 39.26) remained the same as in the previous analysis. The single addition of sensitivity did not improve model fit, ␹(1)2 ⫽ 1.2439, p ⫽ .26, AIC ⫽ 44.34, nor did the singular inclusion of body size ratios, ␹(1)2 ⫽ 0.35, p ⫽ .55. Adding both sensitivity ratios and size ratios did significantly improve model fit, ␹(2)2 ⫽ 17.282, p ⬍ .01, AIC ⫽ 34.77. The model with the interaction between size and sensitivity ratios significantly improved the fit of the model over both the null model, ␹(3)2 ⫽ 22.08, p ⬍ .01, and the model with the additive effect of size and sensitivity ratios, ␹(2)2 ⫽ 4.804, p ⫽ .03, AIC ⫽ 31.62. In the interaction model, neither size ratios, p ⫽ .32, nor sensitivity ratios, p ⫽ .42, were significant independently as fixed factors. However, the interaction between the size and sensitivity ratios was significant, p ⬍ .01. The results of this analysis using only the finger sensitivity mirror the results of the analysis using a combination of palm and finger sensitivity. In the next analysis, we used only the sensitivity of the palm to calculate the sensitivity ratios. Once again, the null model (AIC ⫽ 38.53) and the model including only the size ratios (AIC ⫽ 39.26) remained the same as in the previous analysis. The single addition of sensitivity did not improve model fit, ␹(1)2 ⫽ 1.2437, p ⫽ .26, AIC ⫽ 42.15, nor did the singular inclusion of body size ratios, ␹(1)2 ⫽ 0.35, p ⫽ .55. The additive effect of sensitivity ratios and size ratios did significantly improve model fit, ␹(2)2 ⫽ 17.284, p ⬍ .01, AIC ⫽ 32.57. The interaction model significantly improved the fit of the model over both the null model, ␹(3)2 ⫽ 22.08, p ⬍ .01, and the additive size and sensitivity model, ␹(2)2 ⫽ 4.800, p ⫽ .03, AIC ⫽ 27.22. In this interaction model, neither size ratios, p ⫽ .32, nor sensitivity ratios, p ⫽ .42, were significant independently as fixed factors. However, the interaction between the size and sensitivity ratios was significant, p ⬍ .01. The results of this analysis also mirror the results of the analyses using only the finger sensitivity and a combination of finger and palm sensitivity. To test which sensitivity area of the hand (palm, finger, or combination) provided the best model fit, we calculated the relative likelihood of each interaction model using their respective AICs (palm ⫽ 27.22, finger ⫽ 31.62, combination ⫽ 24.97). The relative likelihood of the combination model was 0.73, meaning that the combination model would come out as the best model 73% of the time. The relative likelihood of the palm model was 0.32, meaning that the palm model would come out as the best model 32% of the time, while the relative likelihood of the finger tip model was only 0.04, meaning the finger model would come out as the best model only 4% of the time. This analysis suggests that while the combination model was the most explanatory model which provided the best fit and least amount of information lost, the palm model was still relevant since it did provide some significant explanatory power and had a high likelihood of being the best model. However, the finger model had the least amount of explanatory power and had a very low possibility of coming out as the best model. As a result, we are justified in discounting the finger model and concluding that the data is best explained by either using a combination of the finger and palm sensitivities or by using the palm sensitivity. In order to determine whether there was a similar relationship in the dowel condition, we used a similar analysis to determine whether relative sensitivity and/or relative physical body size

ratios could predict participants’ estimates with a noncorporeal object. In a null model, only the random effects of participants and body part were entered, and the AIC for the null model was ⫺10.52. We then tested the null model with the data from the noncorporeal condition against the same models as we did for the corporeal condition. Interestingly, none of the alternative models improved upon the null model (the sensitivity ratio model, AIC ⫽ ⫺6.06; the physical body size ratios, AIC ⫽ ⫺8.10; combined sensitivity and size model, AIC ⫽ ⫺3.26; combined sensitivity and size model with the interaction, AIC ⫽ 0.74. This suggests that the dowel estimates do not appear to vary by our predictions based on the product of relative size and sensitivity as in the hand condition.

Discussion These experiments reveal large and systematic distortions in the relative size of perceived overall body proportions when viewing one’s own body. Not only do these distortions appear when assessing one’s body from a first-person perspective, they also persist when viewing one’s body from a third-person perspective. These distortions were most pronounced when comparing one body part to another, and these distortions are not present when viewing the same body lengths on a noncorporeal cylinder. In terms of perceiving relative body proportions, it seems that individuals appear to err drastically in a manner that is consistently differs across different body parts. These results could be interpreted as support for “reverse distortion,” which we had previously introduced as a potential mechanism to compensate for tactile size perception distortions resulting from differences in receptive field density (Weber’s illusion). Specifically, on more sensitive areas of the body, tactile receptive fields are smaller and denser. Hence, an object of the same size will stimulate more receptive fields on more sensitive body parts, leading to an increase in the perception of object size. However, the differences in tactile size perception across body parts are much smaller than one would expect if perception were based solely on the size of the receptive fields; e.g., an object on your hand should feel 3.5 times larger than it does on your back, but rather, you only experience that the object as 1.3 times larger. Hence, it is likely that a compensatory mechanism is in place to decrease the influence of receptive field size on size perception to achieve an acceptable degree of tactile size constancy across body parts (Röder, Rösler, & Spence, 2004; Taylor-Clarke, Jacobsen, & Haggard, 2004). In parallel, experiments have shown that if a body part is made to feel and/or appear larger (e.g., via the rubber hand illusion), than the object residing on that body part feels larger as well (Heed et al., 2011; Haggard & Jundi, 2009). As a result, we hypothesize that one potential compensatory mechanism is that of reverse distortion. In reverse distortion, the perceptual system distorts the experience of a body part’s size to a magnitude that compensates for its differences in tactile receptive field size. This hypothesis results in two different predictions: (a) less sensitive body parts will be distorted and appear larger in comparison to more sensitive body parts, and (b) given an equal degree of sensitivity, body parts that are already physically larger will need to be distorted less than body parts that are smaller due to the notion that they are already physically larger.



Indeed, we did find that the perceived length of each body part appeared to vary inversely with that body part’s sensitivity. For every experiment (except for Experiment 5 when body lengths were presented on the cylinder), the foot was the least overestimated and the torso was the most overestimated. When the forearm was used as a metric, being one of the least sensitive of the body parts (Weinstein, 1968), body parts were then underestimated except for the torso, which is also in line with a reverse distortion account. In a similar vein, estimates when comparing two body parts were much more distorted than estimates when comparing a body part and a noncorporeal object. In addition, the systematic differences in the distortions across body part lengths were not evident when those same lengths were presented on a vertical cylinder rather than on the body, suggesting that these differences are driven by the perception of the relative proportions of the body, not by the lengths themselves. That said, there was not a perfect 1-to-1 correlation between body part sensitivity and body part overestimation. For example, the leg was consistently less overestimated than the arm, even though it is roughly as tactilely sensitive or even less sensitive than the arm. However, considering that the perceptual distortions should be a function of physical size and tactile sensitivity, this finding is not surprising and/or contradictory to the reverse distortion hypothesis. The leg is already physically much larger than the arm (25% larger; (Dreyfuss & Tilley, 1993). As a result, the leg requires less perceptual size distortion because it is already physically larger, which is what we found. The same case can be made for the drastic overestimation of the torso in comparison to other tactilely insensitive body parts. The torso is half the physical length of the leg and 2/3 the length of the arm (Dreyfuss & Tilley, 1993); hence, it is much physically shorter than both the leg and arm with an equal or even lesser degree of sensitivity (Dreyfuss & Tilley, 1993; Weinstein, 1968). Consequently, it stands to reason that the torso should be the most affected by reverse distortion. As shown by our linear mixed effects model, the product of body part’s relative sensitivity and relative size fits the data quite well, above and beyond either sensitivity or physical size in isolation. Hence, these claims appear to be supported. For corporeal comparisons, our model did not perfectly predict participants’ estimates, as evidenced by the slope of the regression line in comparison to the dashed line depicting perfect prediction; see Figure 6. Our model consistently predicted larger distortions than were evident in the data. Yet, recall that our model was built under the assumption that S1P1D1 ⬵ SkPkDk. Hence, in our predictions, we assumed that reverse distortions were of a magnitude that completely eliminated Weber’s illusion (i.e., the hand being equal to the other body part). In reality, this is not the case, because individuals do experience Weber’s illusion, albeit at a decreased magnitude (30% instead of 340% in some cases; Green, 1982; Taylor-Clarke, Jacobsen, & Haggard, 2004). Therefore, in reality, the reverse distortions should not be of such a magnitude to completely control for Weber’s illusion (as in our model), but should be slightly smaller. Interestingly, we find that exact pattern in our data, whereby individuals overestimate with respect to the product of body parts’ relative sizes and sensitivities, but not to a magnitude that completely eliminates Weber’s illusion. Additionally, in these studies, we found that relative body length distortions were present only when comparing body parts relative to each other. The distortions drastically decreased and, in some

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cases, disappeared when comparing a body part to a noncorporeal object. This finding also supports the reverse distortion hypothesis. If body parts were scaled relative to their physical size and tactile sensitivity, it would be difficult to understand how this would translate to noncorporeal objects to which no tactile sensitivity and/or somatosensory cortical architecture is available. Hence, these distortions should be specific to the relationships between body parts, and our data supports this notion. When using a body part as a metric, the differences in the overestimation between body parts was much higher than when using a dowel as evidenced by the interactions between body part and metric corporality. When using a noncorporeal object as a metric, this pattern of distortions disappeared and/or drastically lessened, suggesting that these distortions are specific to the relative proportions of the body. Additionally, the product of relative physical size and relative sensitivity did not predict participants’ overestimations in the dowel condition in the linear mixed effects model. However, the product of relative size and sensitivity strongly predicted participants’ estimates in the hand condition. As a result, we can tentatively conclude that these reverse distortion effects appear to be isolated to the comparison of corporal objects rather than noncorporeal. The finding that these distortions persist when viewing one’s body in a mirror was one which we honestly did not anticipate. The visual information specifying bodily proportions was sufficient for individuals to perform accurately in this task; however, the same systematic distortions in the relative proportions of the body were just as pronounced when viewing one’s own body in a mirror. In parallel to this finding, large visual distortions in the perception of hand shape have also been reported, even in the event that individuals have full visual information about the relative dimensions of their hands (Longo & Haggard, 2012b). Although it is somewhat disturbing to think that even our visual perceptions of our body are distorted, it also begs the question as to why our perceptions of our relative bodily proportions would need to be accurate. It seems reasonable to assume that the main reason to have access to accurate perceptions of bodily proportions would be to perform actions successfully. Nonetheless, actions can be performed in the absence of any knowledge of bodily extents in a trial and error learning process (Gibson, 1979). Through visual motor experience, individuals can use visual feedback from their movements to map motor commands directly to visual stimuli without having to consider the dimensions of the body. Consider using a joystick to control a mechanical arm. A user can learn to grasp objects of different sizes and at several different distances without having any knowledge about the relative dimensions of the mechanical arm and hand. The user merely needs to calibrate her movements of the joystick (motor command) to the movements of the mechanical arm (visual feedback). These results support this interpretation by implying that it is unlikely that the motor system utilizes the perception of bodily proportions to engage in perceptual-motor activities. However, it is also possible that the bodily representation that we assessed is not the one utilized by the motor system, since many have shown that individuals have several bodily representations (Dijkerman & de Hann, 2007; Haggard & Wolpert, 2005). Although these findings add support for the reverse distortion hypothesis, it is important to note that because we did not directly manipulate tactile sensitivity directly, we cannot make any strong


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conclusions in terms of the origin and causes of these distortions. Both behavioral and neuroimaging research has shown clearly that individuals have many representations of their own bodies (Kaas, Nelson, Sur, Lin, & Merzenich, 1979). Hence, this qualification is always evident in this field of research. Through the paradigm that we employed in these experiments, we quantified a very distorted body representation that persists even when viewing one’s body in a mirror, and the distortions across different body parts appear to vary with respect to physical size and tactile sensitivity. These distortions were primarily apparent when comparing relative body parts than when comparing a body part to a noncorporeal object. Even so, it could be the case that these distortions serve a very different purpose. Future research is needed to explore the origin of this distorted body representation. Interestingly, previous research that had individuals draw their body parts yielded a different pattern of distortions. When asked to draw stick figures of their bodies using a provided head figure as a reference, individuals show large-scale distortions, including a large exaggeration of body width, underestimation of body height, and underestimates of their forearms and shins (Fuentes, Longo, & Haggard, 2013). This finding is quite interesting because when individuals actually look at their own forearms in comparison to other body parts, they actually overestimate their forearm lengths (i.e., consider our data as well as the foot vs. forearm comparison). In addition, we found that people drastically overestimate the length of their body (relative to the head) rather than underestimate it. As a result, this suggests that individuals may be tapping into different body representations when drawing their body versus when looking at their body. Body perception has been used as a diagnostic tool in many clinical disorders, including but not limited to paralysis, neglect, and eating disorders (anorexia, bulimia, body dysmorphic disorder). Scientists typically assume that these patients have a distorted view of their body size (Hollander, Neville, Frenkel, Josephson, & Liebowitz, 1992; Kaplan, Rossell, Enticott, & Castle, 2013; Rosen, Reiter, & Orosan, 1995; Veale et al., 1996); however, the current work shows that all people have a distorted view of the relative size of their body parts. The pattern of distortion in healthy individuals should be taken into account before drawing inferences based on distortions in the perceptions of the body in clinical populations because some of these body perception distortions are common to everyone. Additionally, the methodology employed in these studies could possibly provide a more sensitive diagnostic measure, since these distortions were robust across individuals. Also, if our results are indeed a function of tactile receptive field density, it is possible that the atypical distortions in these populations are due to atypical tactile sensitivity. Future research will investigate this possibility by employing the paradigm introduced in these studies with clinical populations. Tactile constancy may be achieved via a distorted perception of bodily proportions. These results support that interpretation. They do not necessarily rule out alternative explanations for the achievement of tactile constancy, such as tactile-visual remapping (Röder, Rösler, & Spence, 2004; Taylor-Clarke, Jacobsen, & Haggard, 2004). However, these results do make such an account less likely than reverse distortion. A tactile-visual remapping account is reliant on an accurate representation of bodily proportions. At present, there is no evidence for an accurate representation of bodily proportions, as shown in this work when viewing one’s bodily

Figure 7. Scaled illustrations based on the experimental data of the perceived proportions of the body (right) versus the actual proportions of the body (left).

dimensions, and in that of others, when drawing one’s own body’s proportions (Fuentes, Longo, & Haggard, 2013). Most importantly, these experiments reveal large systematic distortions in individuals’ perceptions of their relative bodily proportions; see Figure 7, something that had previously been assumed to be accurate (Soechting, 1982; van Beers, Sittig, & van der Gon, 1998).

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Received April 16, 2014 Revision received September 24, 2014 Accepted September 25, 2014 䡲