Visual and somatosensory influence on metric biases in body representation
We do not perceive our body, and in particular our hands, accurately i.e., hands are distorted in their width and length. This phenomenon is observed in healthy individuals as well as in neurological and psychiatric disorders. What explains those distortions in the body representation? They have been long attributed to the somatotopic cerebral representation. Recently, however, it has been demonstrated that visual biases also contribute to those distortions. To understand how different degrees of tactile sensitivity and visual accessibility may contribute to distortions in our body, we used psychophysics to compare the metric representations across five body parts (hand, foot, nose, lips and dorsal portion of the neck) which vary along those two dimensions. We found that most body parts were underestimated in their dimensions. The length misestimation was predicted by their tactile acuity, supporting the influence of the cortical somatotopy on body representation. However, tactile acuity did not explain distortions observed for the width. In contrast, visual accessibility appear to mediate body distortions, as the only body part without visual accessibility, i.e., dorsal portion of the neck, was accurately perceived; while all others scale with the visual accessibility. Coherent with the multisensory nature of body representations, we argue that the perceived dimensions of body parts are estimated by integrating visual and somatosensory information, each weighted differently, based on their availability for a given body part and a given spatial dimension.