Mapping the internal geometry of tactile space.

Abstract

A large body of research has shown spatial distortions in the perception of tactile distances on the skin. For example, perceived tactile distance is increased on sensitive compared to less sensitive skin regions, and larger for stimuli oriented along the medio-lateral axis than the proximo-distal axis of the limbs. In this study we aimed to investigate the spatial coherence of these distortions by reconstructing the internal geometry of tactile space using multidimensional scaling (MDS). Participants made verbal estimates of the perceived distance between 2 touches applied sequentially to locations on their left hand. In Experiment 1 we constructed perceptual maps of the dorsum of the left hand, which showed a good fit to the actual configuration of stimulus locations. Critically, these maps also showed clear evidence of spatial distortion, being stretched along the medio-lateral hand axis. Experiment 2 replicated this result and showed that no such distortion is apparent on the palmar surface of the hand. These results show that distortions in perceived tactile distance can be characterized by geometrically simple and coherent deformations of tactile space. We suggest that the internal geometry of tactile space is shaped by the geometry of receptive fields in somatosensory cortex.

Reconstructing neural representations of tactile space

“…(Note that for this 108 and other tests involving ratios, the calculation of means and all statistical tests were 109 conducted on log-transformed values, which were converted back to ratios to report mean 110 values). As predicted, this result replicates the anisotropy in tactile distance perception 111 previously reported on the hand dorsum (Longo & Golubova, 2017;Longo & Haggard, 2011). 112 113 fMRI data 114 The main aim of this study was to identify the neural bases of these distorted 115 perceptual spatial configuration of the skin surface.…”

“…191 193 In the present study, we reconstructed the internal geometry of tactile space using 194 representational similarity of neural patterns between locations on the skin. Behaviorally, we 195 replicated previous reports that tactile space is stretched along the medio-lateral axis of the 196 hand dorsum (e.g., Longo & Golubova, 2017;Longo & Haggard, 2010. Critically, using a 197 novel approach that combine fMRI with MDS, we showed that similar distortions can be 198 measured directly from neural data.…”

“…In this study we investigated the neural mechanisms underlying tactile distance 62 perception by directly comparing neural and perceptual maps of the hand dorsum. We applied 63 a method we recently developed to reconstruct perceptual configurations from the pattern 64 of distance judgments using multidimensional scaling (MDS) (Longo & Golubova, 2017). MDS 65 is a method for reconstructing the latent spatial structure underlying a set of items given a 66 matrix of pairwise distances or dissimilarities between items (Cox & Cox, 2001;Shepard, 1980;67 for a similar approach applied to neurophysiological data see Sereno & Lehky, 2011).…”

“…By obtaining estimates of 88 every combination of locations, we obtained a perceptual distance matrix of the nine locations 89 for each participant. We then used MDS to construct 2-dimensional perceptual configuration 90 of the skin, as in our recent study using this paradigm (Longo & Golubova, 2017). 91 Figure 1.…”

“…These configurations are shown in Figure 2A (Individual data are shown in Figure S1 92 of the supplementary material). In order to quantify distortion of these configurations, we 93 estimated the stretch applied to an idealized square 3x3 grid that minimized the dissimilarity 94 with each configuration, as in previous studies (e.g., Longo & Golubova, 2017;Longo & 95 Morcom, 2016). Stretches were defined by multiplying the x-coordinates of a square grid 96 (reflecting location in the medio-lateral hand axis) by a stretch parameter.…”