Size constancy mechanisms: empirical evidence from touch

Tame, Luigi and Limbu, S. and Harlow, R. and Parikh, M. and Longo, Matthew (2022) Size constancy mechanisms: empirical evidence from touch. Vision 6 (3), p. 40. ISSN 2411-5150.

Abstract

Several studies have shown the presence of large anisotropies for tactile distance perception across several parts of the body. Tactile distance between two touches on the dorsum of the hand is perceived as larger when they are oriented mediolaterally (across the hand) than proximo-distally (along the hand). This effect can be partially explained by the characteristics of primary somatosensory cortex representations. However, this phenomenon is significantly attenuated relative to differences in acuity and cortical magnification suggesting a process of tactile size constancy. It is unknown whether the same kind of compensation takes place also when estimating the size of a continuous object. Here, we investigated whether the tactile anisotropy that typically emerges when participants have to estimate the distance between two touches is also present when a continuous object touches the skin and participants have to estimate its size. In separate blocks, participants judged which of two tactile distances or objects on the dorsum of their hand felt larger. One stimulation (first or second) was aligned with the proximo-distal axis (along the hand), the other with the mediolateral axis (across the hand). Results showed a clear anisotropy for distances between two distinct points, with across distances consistently perceived as larger than along distance, as in previous studies. Critically, however, this bias was significantly reduced or absent for judgments of the length of continuous objects. These results suggest that a tactile size constancy process is more effective when the tactile size of an object has to be approximated compared to when the distance between two touches has to be determined. The possible mechanism subserving these results is described and discussed. We suggest that a lateral inhibition mechanism, when an object touches the skin, provides information through the distribution of the inhibitory subfields of the RF about the shape of the tactile RF itself. Such process allows an effective tactile size compensatory mechanism where a good match between the physical and perceptual dimension of the object is achieved.

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