Weber's illusion and body shape: Anisotropy of tactile size perception on the hand.

Abstract

The perceived distance between touches on a single skin surface is larger on regions of high tactile sensitivity than those with lower acuity, an effect known as Weber's illusion. This illusion suggests that tactile size perception involves a representation of the perceived size of body parts preserving characteristics of the somatosensory homunculus. Here, we investigated how body shape is coded within this representation by comparing tactile distances presented in different orientations on the hand. Participants judged which of two tactile distances on the dorsum of their left hand felt larger. One distance was aligned with the proximodistal axis (along the hand), the other with the mediolateral axis (across the hand). Across distances were consistently perceived as larger than along ones. A second experiment showed that this effect is specific to the hairy skin of the hand dorsum and does not occur on glabrous skin of the palm. A third experiment demonstrated that this bias reflects orientation on the hand surface, rather than an eye- or torso-centered reference frame. These results mirror known orientational anisotropies of both tactile acuity and of tactile receptive fields (RFs) of cortical neurons. We suggest that the dorsum of the hand is implicitly represented as wider than it actually is and that the shape of tactile RFs may partly explain distortions of mental body representations.

Categorical perception of tactile distance

“…All PSEs were reliably less than 0 (indicating significant mediolaterally biased anisotropies; all p's < .04, all t's > 2.23). This replicates Longo and Haggard's (2011) finding that tactile distances are perceived as longer across than along the hand, and show that this anisotropy extends down the forearm (see also Green, 1982 Consistent with previous findings , there was also a Surface effect, F (1, 25) = 9.56, p < .01, η p 2 = .28, with larger anisotropies on the dorsal than the ventral surface. Finally, there was a significant interaction between Body Part and Surface, F (2,50) = 4.08, p < .05, η p 2 = .14.…”

The role of visual similarity and memory in body model distortions

“…Interestingly, distortions of hand shape measured in localization tasks matched those found in tactile size perception of the hand (Linkenauger et al, 2015;Longo & Haggard, 2011;Weber, 1834Weber, /1996. Hand distortions measured in localization tasks were consistent with anisotropies characterizing the hand's tactile acuity and receptive field geometry , 2011.…”

“…However, multiple studies indicate the presence of systematic distortions in the perception of bodily proportions (Linkenauger et al, 2015;, 2011Saulton, Dodds, Bülthoff, & Rosa, 2015). Those distortions were demonstrated in visual estimations tasks (Linkenauger et al, 2015;Longo & Haggard, 2012) as well as in tactile and localization tasks , 2011.…”

“…Those distortions were demonstrated in visual estimations tasks (Linkenauger et al, 2015;Longo & Haggard, 2012) as well as in tactile and localization tasks , 2011. In this study, we are particularly interested in better understanding the origin of the distortions measured in localization tasks .…”

Mapping the internal geometry of tactile space.

“…For example, Longo and Haggard (2011) asked participants to make forced-choice judgments about which of two tactile distances felt bigger, applying one in the medio-lateral axis of the hand dorsum (across the hand) and one in the proximo-distal axis (along the hand). There was a large bias to perceived distances oriented along the medio-lateral axis as larger.…”