Running Head: Asymmetrical Body Perception

Asymmetrical Body Perception 1

Running head: ASYMMETRICAL BODY PERCEPTION

Asymmetrical Body Perception: A Possible Role for Neural Body Representations

Sally A. Linkenauger*1, Jessica K. Witt2, Jonathan Z. Bakdash1, Jeanine K. Stefanucci3, & Dennis R. Proffitt1 1University of Virginia 2Purdue University

3The College of William and Mary

Word Count: 3,897

Abstract Asymmetrical Body Perception 2

Perception of our body is not only related to the physical appearance of the body but also to the neural representation of the body in somatosensory cortex. In right-handed people, the area of the somatosensory cortex devoted for the right hand is larger than the left hand, but for left- handed people there is symmetry. Taking advantage of these naturally occurring neural differences, we examined perceived arm length in right-handed and left-handed participants.

When looking at each arm and hand individually, right-handed participants perceived their right arms and hands to be longer than their left arms and hand, while left-handed participants perceived both arms accurately. These experiments reveal a possible relationship between implicit body maps in the brain and conscious perception of the body.

Asymmetrical Body Perception: Neural Representations Relate to Body Perception Asymmetrical Body Perception 3

When we look at our body, what we see is likely influenced by the neural representation of the body in the cortex and is not solely due to the way the body appears physically. In right- handed individuals, the neural representations of the body are asymmetric, in that there is typically more cortical area and higher neural activation associated with the right side of the body than the left; whereas left-handed individuals usually have symmetrical cortical body representations (ref, ref, ref). Extending this finding, the current studies assessed whether asymmetries in cortical representation would also be related to the perceived size of the associated body part. Obtaining variability in the size of cortical areas was achieved by taking advantage of naturally occurring individual differences due to handedness. If the amount of neural representation influences the perceived size of the body, then right-handed people should perceive their right arm longer than their left arm, and left-handed people should perceive both arms as the same. For right-handed participants, we found these anticipated asymmetries in perceived arm length and hand size as well as in anticipated reachability and graspability. In contrast, perceived arm length and anticipated reachability were symmetrical for left-handed participants, paralleling the symmetries in their cortical representations. These findings provide compelling evidence for the hypothesis that neural bodily representations are reflected in how we visually perceive our body.

Neuroimaging studies have uncovered hemispheric asymmetries in cortical areas associated with body representation in right-handed people but not in left-handed people. Right- handed individuals have a greater cortical surface area in their left sensory cortex and higher activation their left primary motor and sensory cortices for contralateral movements, while left- handed individuals appear to have a near symmetrical surface area and activation (Amunts et al.,

1996; Zilles et al., 1997; Kawashima, et al. 1997; Kim et al., 1993). Electroencephalography Asymmetrical Body Perception 4

(EEG) studies have reported that right-handers show greater neural activation and a larger hand area in the left somatosensory cortex than in the right when engaging in a motor task (Jung et al.,

2003, Buchner, Ludwig, Waberski, Wilmes, & Ferbert, 1995). Hemispheric asymmetries have also been revealed in areas of the parietal lobe associated with visuomotor processing. The left

(but not right) inferior parietal lobule has been implicated in the updating of the body representation in right-handed individuals (Devlin et al., 2002). Right-handed patients with optic ataxia due to damage to the left parietal lobe show deficits in reaching to stimuli in the right visual field and deficits in reaching with the right hand, whereas similar damage to the right hemisphere results in only a deficit in reaching to the left visual field, suggesting an asymmetry in the body representation between the two hemispheres (Perenin & Vighetto, 1988).

Paralleling the neuroimaging data, right-handed people tend to be much more reliant on their dominant hand than left-handed people. For example, when performing a grasping task, right-handed people use their right hand for 90% of the grasps, while left-handed people used both hands equally (Gonzalez, Whitwell, Morrissey, Ganel, & Goodale, 2007). In addition, right-handed people evaluate that their ability to reach and grasp a hand tool as more impaired than left handed people if the tool’s handle is oriented towards the non-dominant hand

(Linkenauger, Witt, Stefanucci, Bakdash & Proffitt, in press). In general, hand preference surveys indicate that left-handed people are typically less “left-handed” than right-handed are right-handed (Tan, 1998).

Taken together, there is compelling evidence for structural and functional cortical symmetry differences between right- and left-handed people. In the current experiments, we assessed whether these differences would be reflected in the visually perceived size of arms and hands. Asymmetrical Body Perception 5

Experiment 1: Handedness and Arm Length Perception

If the cortical representations of the arm affect perceived arm length, then right-handed people should perceive their left and right arms to be different lengths, because right-handed people have a larger representation of their right arm than their left. However, because left- handed people have a near equal sized cortical representation their left and right arms, the extents of their right and left arms should appear similar. To test this notion, right- and left-handed participants indicated the perceived length of both their left and right arms.

Method

Participants. Fifteen right-handed (7 female) and 15 left-handed (7 female) University of

Virginia students participated. Handedness was assessed through self-report, which was consistent with the hand that signed the consent form. All participants had normal or corrected- to-normal vision.

Design and Procedure. Participants were instructed to stand and extend their right or left arm to be perpendicular to their body and to place the fingers of their non-extended hand on the protrusion of their shoulder defined by the intersection of the clavicle and the humerus. Then, participants made estimates of the lengths of their arms as defined by the distance from the protrusions on their shoulders to the end of their finger tips on their extended hands. Participants estimated arm length by instructing a researcher standing perpendicular to the extended arm to adjust a blank measuring tape horizontally so that the extent on the tape matched the perceived length of the participant’s arm. Participants could refer to the extent of their arms continuously while making the estimate (see Figure 1). Next, participants’ grip strength was assessed for each Asymmetrical Body Perception 6 hand using a dynamometer. Then, participants estimated the length of their other arm; arm order was counterbalanced. Lastly, the actual extents of the participants’ arms were measured.

Results. Arm length estimates were assessed by calculating a ratio of perceived arm length divided by actual arm length. A repeated-measures analysis of variance (ANOVA) with arm as a within-participants factor was used to compare the arm length ratios between right- and left-handed participants. No main effect of arm was found, F(1, 28) = 1.37, prep = .67; however, as predicted, an significant interaction was found between arm and handedness, F(1, 28) = 4.74,

2 prep = .90, ηp = .15. Separate paired-samples t-tests for right- and left-handed participants revealed that right-handed participants underestimated the length of their left arm more than the length of their right arm, t(14) = 2.12, prep = .91, always one-tailed. However, we did not find this difference for left-handed individuals, t(14) = -.82, prep = .70 (see Figure 2A). One-sample t- tests were used to compare the ratios to 1 (perfect accuracy) and revealed that right-handed participants were accurate in their perception of their right arm, t(14) = -.85, prep= .50, but

2 underestimated their left arm, t(1,14) = -2.38, prep= .94, ηp = .29. Left-handed participants were accurate in estimating both their right and left arms, prep = .75, prep = .50; respectively.

Variability in the difference between the left and right arm estimates did not differ by handedness, prep = .52, suggesting that these differences are not due to increases in variability of the spectrum of left-handedness.

Given that sensory feedback is generally greater for body parts that engage in more activity (Pantev, Engelien, Candia, & Elbert, 2001; Hamilton & Pascual-Leone, 1998), requiring a larger somatosensory area, we hypothesized that perceived arm length would be related to the arm’s strength. Relative strength ratios were created by dividing participants’ grip strength measurements of the right hand by that of the left hand1. Ratio scores greater than 1 indicate Asymmetrical Body Perception 7 increased strength with the right-hand compared to the left-hand, and ratio scores less than 1 indicate higher relative strength with left-hand. Relative hand strength significantly correlated with relative arm length (perceived length of right arm/perceived length of left arm), r = .44, prep= .91 (see Figure 2B). Asymmetries in hand strength were positively related to asymmetries in perceived arm length.

Experiment 2: Handedness and Perceived Reachability

With perceptual differences often come behavioral consequences. Given that right- handed people perceive their right arms as being longer than their left arms, they should also anticipate that they can reach farther with their right arm than their left. In this experiment, right- and left-handed participants estimated how far they could reach with their left and right arms.

Method

Participants. Fifteen right-handed (11 female) and 15 left-handed (8 female) University of Virginia students participated. Handedness was assessed through self-report and by the

Edinburgh Handedness survey (Oldfield, 1971), right-handed participants (M = 91.3, SD =

12.11); left-handed participants (M = -70.7, SD = 28.22). All participants had normal or corrected-to-normal vision.

Stimuli and Apparatus. Participants were seated on a chair at a uniformly colored table that measured 91.5 cm by 91.5 cm and was 74.5 cm tall. Participants’ shirt backs were clamped to the back of the chair using binder clips to prevent forward lean. Participant sat at a close but comfortable distance to the table.

Design and Procedure. Participants estimated their reach with their right and left arms.

From the opposite side of the table from the participant, the experimenter moved a white plastic Asymmetrical Body Perception 8 chip toward the participants. The participants informed the experimenter when they thought the chip was close enough that they could just grasp the chip with a specific arm without moving their shoulders from the back of the chair. The binder clips served as a constant reminder of this constraint. Participants were encouraged to instruct the experimenter to make adjusts to the position of the chip. Participants made 3 estimates of reachability with a specific arm that were contralateral (starting 30º from center moving towards center, away from the reaching arm), ipsilateral (30º from center, towards the reaching arm), and central, and then made the 3 same estimates of reachability using the other arm. This order was counterbalanced. Between right and left arm reachability estimates, participants performed the grip strength task as described in

Experiment 1. At no time during the reachability estimates were participants allowed to reach over the table. At the end of the estimates, participants’ actual reaching abilities were assessed for each arm in each direction.

Results

Accuracy of reachability was determined by dividing perceived reachability by actual reachability. As in other reaching studies (Rochat & Wraga, 1996), participants overestimated their reaching ability. To compare reachability for right- and left-handed participants, a 2 (left vs. right hand) by 3 (direction: contralateral, ipsilateral, and center) by 2 (right vs. left- handedness) repeated measures ANOVA was performed. Hand and direction were both specified as within-participants factors, whereas handedness was a between-participants factor.

The most interesting finding was a hand by handedness interaction, F(1, 28) = 14.14, prep > .99,

2 ηp = 0.34 (see Figure 3). No other main effects were significant, except for direction, F(2,27) =

27.72, prep >.99, which revealed that participants overestimated their contralateral (M =1.22, SD Asymmetrical Body Perception 9

=.19) and center (M = 1.19, SD = .16) reaches more than their ipsilateral reach (M =1.09, SD

=.13).

Separate 2 (arm) by 3 (reaching direction) repeated-measures ANOVAs for right- and left-handed participants showed that for right-handers, main effects were found for arm, F(1, 14)

2 = 21.53, prep = .99, ηp = 0.59. Right-handed participants overestimated their reach more with their right arm (M = 1.17, SD = 0.15) than with their left (M = 1.08, SD = 0.14). For left-handed participants, no main effect of arm was found, F(1, 14) = 1.86, prep = 0.72. For both groups,

2 there was a main effect for direction, Fs (2, 28) > 10.71, preps = .99, ηps > 0.43, with similar patterns as was found in the 3-way ANOVA. These results show that in addition to perceiving their right arm as longer, right-handers also thought they could reach farther with their right arm; whereas, left-handed people did not perceive any difference between their arms.2

Experiment 3: Right-handedness, Hand Size and Graspability

Because the hand area in the somatosensory cortex is asymmetrical in right-handed individuals, we also expected a difference in the perceived size of the right and left hands. We had right-handed participants estimate the size of their right and left hands as well as indicate their maximum grasping abilities with their right and left hands.

Method

Participants. Fifteen right-handed (6 female) University of Virginia students participated to fulfill a research requirement for course credit. Handedness was assessed through self-report and the Edinburgh Handedness survey (Oldfield, 1971) (M = 87.35, SD = 22.87). All participants had normal or corrected-to-normal vision. Asymmetrical Body Perception 10

Stimuli and Apparatus. Participants were seated on a chair at the table described in

Experiment 2. Block size estimates were made on a Dell laptop computer whose screen measures 33.5 cm diagonal. Sixteen square blocks of different sizes (ranging from 4 cm to 24 cm) were constructed from foam board that was 1.5 cm thick. Each block had two parallel black lines (3 cm in length) marked on two opposing sides of the block to indicate where the participants should anticipate placing their fingers when grasping.

Design and Procedure. Participants sat at the table and were told that they were going to estimate whether they thought they could grasp the block in front of them. Participants were told that the grasp that they had to anticipate entailed placing their thumb on one of the black lines on the edge of the block and then extending their hand across the entire block to place any one of their other fingers on the black line on the other side of the block. A successful grasp was defined as being able to lift the block completely off the table. When participants indicated that they understood the instructions, they were asked to close their eyes as the experimenter placed one of the 16 blocks in the center of the table in front of the participant with the black lines on the block perpendicular to the participant. The participant opened their eyes and indicated whether they thought they could successfully grasp the block with a specific hand. Then participants estimated the width of the block by using the arrow keys on the laptop to move two circular white dots (.30cm in diameter) on a black background to be the same distance away from each other as the distance between the two black lines on the blocks (the width of the block). The laptop computer was placed on a stool beside the participant’s hand that was not being used for grasp estimates. Participants estimated their grasping ability with all 16 blocks with one hand and then all 16 blocks with the other hand for a total of 32 trials. Blocks were presented in random order and hand order was counterbalanced. Asymmetrical Body Perception 11

After making grasping ability judgments, participants were told to estimate the length and width of their hand. Hand length was defined as the extent between the crease at the bottom of their palm and the longest fingertip. Hand width was defined as the extent between the intersection of the pinky and palm to the intersection of the index finger and palm. The experimenter stood in front of participants while participants looked at the palm of their hand, and the experimenter extended a blank tape measure perpendicularly to the extent they were estimating (horizontally for length and vertically for width; as prevention from using a landmark matching heuristic). Participants indicated to the experimenter when they thought that the extent on the tape measure matched the length (or width) of their hand. Participants were encouraged to make fine adjustments. After participants estimated the length and width of one hand, their grip strength for both hands was assessed. Participants then estimated the length and width of their other hand. Hand order was counterbalanced. After making the estimates, the actual length and width of each hand was measured as well as actual maximum graspability for each hand.

Results and Discussion

Graspability accuracy was determined for each participant’s left and right hand by dividing perceived graspability by actual graspability. Right and left hand graspability accuracies were compared using a paired-samples t-test which indicated that participants overestimated their grasping ability more with their right hand (M = 1.12, SD =.13) than with

2 their left (M = 1.07, SD = .15), t(14) = 2.14, prep = .93, ηp = .25. Similar to reachability, participants overestimated their graspability with both their right and left hands, t(1, 14) = 3.60,

2 2 prep = .98, ηp = .48; t(14) = 1.85, prep =.89, ηp = .20, respectively (see Figure 4A).

In order to determine hand size accuracy values, perceived hand area was determined by multiplying the perceived width and length for each hand. The perceived area was then divided Asymmetrical Body Perception 12 by the actual area to arrive at a measure of accuracy. One participant’s data was removed from the analysis due to his admission of misunderstanding the instructions. Participants underestimated the size of their left hand (M = .91, SD = .19) more than the right hand (M = .99,

2 SD = .18), t(1,13) = 3.03, prep =.97, ηp = .41. As found with arm length perception, participants were accurate in perceiving the size of their right hand (M = .99, SD = .18), prep = .29, while

2 participants underestimated the size of their left (M = .91, SD = .19), t(13) = 1.82; prep = .89, ηp

=.20 (see Figure 4B).

Strength ratios were constructed as in previous experiments. Hand size ratios were calculated by dividing the perceived right hand area by the perceived left hand area. Graspability ratios were calculated by dividing perceived graspability with the right hand by perceived graspability with the left hand. Although graspability ratios were not significantly related to the strength ratios, r = .22, prep= .71, hand size symmetry ratios were positively related to strength ratios, r = .54, prep = .93 (see Figure 4). This finding suggests that the stronger the right hand is in comparison to the left, the larger the right hand is perceived relative to the left. As found with reaching and arm length perception, the right hand is perceived to be larger and more capable of grasping than the left.

General Discussion

Although functional and structural differences have been documented between cortical hemispheres, this is the first demonstration that these differences may have perceptual consequences. Conscious perception of the dimension of arms and hands appear to be consistent the size of their neural bodily representations. Currently, we cannot determine whether these perceptual effects are the result, the cause, or just coincidental with the neurological discrepancies between right- and left-handers. Neuroimaging research has shown that training in Asymmetrical Body Perception 13 complex actions with a specific body part can increase the size of the representation of that body part in the motor and somatosensory cortices (Elbert, Pantev, Wienbruch, Rockstroh, & Taub,

1995; Hamilton & Pascual-Leone, 1998; Nelles, Jentzen, Jueptner, Muller, & Diener, 2001).

Similarly, increasing the use of a certain effector over an extended period may also affect the perception of the effector in addition to the size of its area on the somatosensory cortex.

However, because many neural and perceptual differences accompany handedness that cannot be attributed to experience (McManus, 2002), it is possible that these perceptual effects are the result of innate differences between right- and left-handed people rather than a byproduct of experience. In addition, these affects could arise from a reciprocal relationship between experience-driven and innate differences.

Just as handedness is considered to have an adaptive function (Wilson, 1998), we believe that these perceptual effects are adaptive as well. Right-handers perceive their right, and more functional arm, as being longer than their left arm. An exaggerated perception of one arm, which coincides with the anticipation of greater reaching capabilities with that arm, may suggest a mechanism for why right-handers tend to use their right arm more often, even when reaching awkwardly to the left side (Gonzalez et al., 2007).

The current findings are nicely situated in the context of other research that demonstrates effects of functionality on perceived distance. For instance, targets just beyond reach look closer when a participant intends to reach to them with a tool compared to when no tool is held (Witt,

Proffitt, & Epstein, 2005). Tools placed in an orientation that make them easier to grasp appear closer than when their orientation makes them more difficult to grasp (Linkenauger et al., in press). Golf holes and softballs look bigger to athletes who are playing well (Witt, Linkenauger,

Bakdash, & Proffitt, 2008; Witt & Proffitt, 2005). Here, we demonstrated differences in the Asymmetrical Body Perception 14 perceived length of the effector itself. Thus, the perceptual differences documented here could reflect the functionality of the arm as well as the neurological representation of the arm.

Future studies could look at whether neurological and functional differences affect the perception of other body parts. For instance, perhaps soccer players who favor their right leg perceive it to be longer than their left. These results can also be explored within training and developmental paradigms. If perceptual differences are present early in life or in training, then perhaps special care to engage the opposite hand will help those developing new skills to reduce their right biases, especially in situations where ambidexterity is advantageous.

In summary, right-handed people perceived their right arm to be longer and their right hand to be larger than their left counterparts. This perceptual bias extended into their perceived action capabilities such that right-handed people judged that they could reach farther with their right arm and grasp larger objects with their right hand compared with their left arms and hands.

In contrast, left-handed people perceived their arms to be equally long and to be able to reach equally far. Similarly for left-handers, there is no difference between perceived hand size and the maximum size of objects that can be grasped with each hand. These results suggest that the body representation in the somatosensory cortex may extend beyond tactile sensations and bodily awareness and may influence visual perception of the body as well. Asymmetrical Body Perception 15

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Author Note

Sally A. Linkenauger, Jonathan Z. Bakdash, Dennis R. Proffitt; Department of

Psychology; University of Virginia. Jessica K. Witt; Department of Psychological Sciences,

Purdue University. Jeanine K. Stefanucci; Department of Psychology; The College of William and Mary.

We wish to thank William Epstein, Judy Deloache, and Daniel Willingham for their comments and revisions on earlier versions of the manuscript. This research has been supported by NIH Grant RO1MH075781to the fourth and fifth authors.

Address correspondence to Sally Linkenauger, 102 Gilmer Hall, Charlottesville, VA

22904; email: [email protected]. Asymmetrical Body Perception 19

Footnotes

1 Due to a coding error, grip symmetry scores for 3 right-handers and 3 left-handed were unavailable and hence, not included in the correlation.

2 Due to a coding area, a substantial portion of the grip strength data was not recorded, and as a result, we have not reported the results associated with it. Asymmetrical Body Perception 20

Figure Captions

Figure 1. Photograph of example participant estimating the length of her right arm. The participant instructed the experimenter to increase or decrease the length of the visible portion of the backside of a tape measure until she perceived this length to be the same length as that of her arm. Figure 2. (A) Perceived arm length as a function of handedness. Ratios were computed of perceived arm length divided by actual arm length for each arm. Data are plotted by arm and handedness. Error bars represent 1 standard error of the mean. (B) Correlation between grip strength symmetry and symmetry in perceived arm length. Grip strength symmetry scores were computed as grip strength in right hand divided by grip strength in left hand. Symmetry in arm length was computed as perceived length of right arm divided by perceived length of left arm. Line represents linear regression of the data. Figure 3. (A) Perceived maximum reach distance divided by actual maximum reach distance as a function of arm and reaching direction for right-handed participants. Error bars represent 95% within-subjects confidence intervals. (B) Perceived maximum reach distance divided by actual maximum reach distance as a function of arm and reaching direction for left- handed participants. Error bars represent 95% within-subjects confidence intervals. Figure 4. (A) Ratio scores of perceived hand size divided by actual hand size for the each hand for right-handed participants. Error bars represent 95% within-subjects confidence intervals. (B) Ratio scores of perceived maximum grasp size divided by actual maximum grasp size for the each hand for right-handed participants. Error bars represent 95% within-subjects confidence intervals. (C) The relationship between grip strength symmetry and perceived hand size. Line represents linear regression of the data. (D) The relationship between grip strength symmetry and perceived maximum grasp size symmetry. Line represents linear regression of the data. Asymmetrical Body Perception 21

Figure 1. Asymmetrical Body Perception 22

Figure 2

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