Anisotropies of tactile distance perception on the face

Abstract

The distances between pairs of tactile stimuli oriented across the width of the hand dorsum are perceived as about 40% larger than equivalent distances oriented along the hand length. Clear anisotropies of varying magnitudes have been found on different sites on the limbs and less consistently on other parts of the body, with anisotropies on the center of the forehead, but not on the belly. Reported anisotropies on the center of the forehead, however, might reflect an artefact of categorical perception from the face midline, which might be comparable to the expansion of tactile distance perception observed for stimuli presented across joint boundaries. To test whether tactile anisotropy is indeed a general characteristic of the tactile representation of the face, we assessed the perceived distance between pairs of touches on the cheeks and three locations on the forehead: left, right, and center. Consistent with previous results, a clear anisotropy was apparent on the center of the forehead. Importantly, similar anisotropies were also evident on the left and right sides of the forehead and both cheeks. These results provide evidence that anisotropy of perceived tactile distance is not a specific feature of tactile organization at the limbs but it also exists for the face, and further suggest that the spatial distortions found for tactile distances that extend across multiple body parts are not present for stimuli that extend across the body midline.

Perception of Tactile Distance on the Back

“…In contrast, our results indicate the presence of a reversed anisotropy (with along distances feeling bigger than across ones) on the lower back, consistent with the recent report of Plaisier et al (2020). Qualitatively similar anisotropies of tactile distance have been reported on a range of body parts beside the hand dorsum, including the palm (Fiori & Longo, 2018;Longo, 2020;Longo et al, 2015), the forearm (Green, 1982;Le Cornu Knight et al, 2014), the thigh (Green, 1982;Tosi & Romano, 2020), the shin (Stone et al, 2018), the face (Fiori & Longo, 2018;Longo et al, 2015Longo et al, , 2020, and the upper back (this study). While several studies have failed to find any anisotropy at all on the belly (Green, 1982;Longo et al, 2019;Marks et al, 1982), to our knowledge, the lower back is the first body part in which a reversed anisotropy is present.…”

“…It is not clear whether the body midline functions as a categorical boundary in this way, though studies have found localization biases in the direction of the spine (Cholewiak et al, 2004;van Erp, 2005). One recent study compared tactile distance judgements for stimuli crossing the face midline on the forehead compared to the left or right side of the forehead, finding no evidence for any categorical effect of the face midline (Longo et al, 2020). It is possible, however, that the presence of the spine may make the midline more salient on the back.…”

“…While the majority of studies measuring anisotropy of tactile distance perception have measured perception on the hand, a number of studies have extended these findings to other parts of the body. In addition to the hand, there is evidence for tactile distance anisotropy on the forearm (Green, 1982;Le Cornu Knight et al, 2014), the thigh (Green, 1982;Tosi & Romano, 2020), the shin (Stone et al, 2018), and the face (Fiori & Longo, 2018;Longo et al, 2015Longo et al, , 2020. Intriguingly, across each of these body parts, the direction of anisotropy is for distances oriented with body width to be judged as larger than those oriented with body length or height.…”

Intact Organization of Tactile Space Perception in Isolated Focal Dystonia

“…However, its efficacy for the treatment of FHD has been investigated in only two patients thus far. 11,12 Therefore, we prospectively ---------------------------------------------------------…”

“…1A). 4,[7][8][9][10][11] Recently, a paradigm has been developed that allows testing of tactile spatial perception across 8 orientations to provide a more accurate estimation of RF size, shape, and orientation. 7 Indeed, perceptual distortions of tactile space match cortical tactile space organization in the primary somatosensory cortex.…”