Bilateral Symmetry of Distortions of Tactile Size Perception

Abstract

The perceived distance between touches on the limbs is generally bigger for distances oriented across the width of the limb than for distances oriented along the length of the limb. The present study aimed to investigate the coherence of such distortions of tactile size perception across different skin surfaces. We investigated distortions of tactile size perception on the dorsal and palmar surfaces of both the left and right hands as well as the forehead. Participants judged which of two tactile distances felt larger. One distance was aligned with the proximodistal axis (along the body), the other with the mediolateral axis (across the body). Clear distortions were found on all five skin surfaces, with stimuli oriented across the width of the body being perceived as farther apart than those oriented along the length of the body. Consistent with previous results, distortions were smaller on the palmar than on the dorsal hand surface. Distortion on the forehead was intermediate between the dorsal and palmar surfaces. There were clear correlations between distortion on the left and right hands, for both the dorsal and palmar skin surfaces. In contrast, within each hand, there was no significant correlation between the two skin surfaces. Distortion on the forehead was not significantly correlated with that on any of the other skin surfaces. These results provide evidence for bilaterally symmetric representations underlying tactile size perception.

Distorted body representations are robust to differences in experimental instructions

“…Moreover, distortions in hand shape were strongly correlated in both conditions, r(24) = 0.725, p < 0.0001. However, as in Experiment 1, we did not find a modulatory effect of the type of instruction on these distorted representations (t (24) These results replicate previous reports showing similar anisotropy on the hand dorsum (Canzoneri et al, 2013;Green, 1982;Le Cornu Knight, Longo, & Bremner, 2014;Longo, Ghosh, & Yahya, 2015;Longo & Haggard, 2011;Longo & Sadibolova, 2013;Miller, Longo, & Saygin, 2014) Moreover, our results show that this is a robust effect that is not modulated by the attitude that participants assume in performing the task (i.e., apparent vs objective).…”

No Correlation between Distorted Body Representations Underlying Tactile Distance Perception and Position Sense

“…In previous research, strong correlations have been found between the magnitude of distortion on the two hands and across similar conditions for both tactile size perception (Longo et al, 2015a) and position sense (Longo and Haggard, 2010, 2012a; Longo, 2014; Mattioni and Longo, 2014). Thus, it is clear that reliable individual differences exist for both perceptual abilities.…”

“…Procedures for the tactile distance task were similar to our previous studies using this paradigm (Longo and Haggard, 2011; Longo and Sadibolova, 2013; Longo et al, 2015a). Stimuli were pairs of wooden posts mounted in foamboard, separated by 2, 3, or 4 cm.…”

“…For example, as discussed above, there are clear biases to overestimate the width of the hand compared to its length, both in tactile distance perception (e.g., Green, 1982; Longo and Haggard, 2011) and in position sense (e.g., Longo and Haggard, 2010). Further, distortions are substantially larger on the hairy skin of the hand dorsum than on the glabrous skin of the palm, both for tactile distance perception (Longo and Haggard, 2011; Le Cornu Knight et al, 2014; Longo et al, 2015a,b) and position sense (Longo and Haggard, 2012a). …”

“…Here we investigated whether these individual differences are shared across abilities by measuring both in the same people. We measured anisotropy of tactile distance perception on the dorsum of the left hand using a two-alternative forced-choice (2AFC) method similar to that we have used previously (Longo and Haggard, 2011; Longo et al, 2015a). Because the method described above for producing proprioceptive maps underlying position sense focuses on the fingers, we used a revised procedure we recently reported (Longo et al, 2015b) which allows mapping the hand dorsum.…”