Somatotopic or spatiotopic? Frame of reference for localizing thermal sensations under thermo-tactile interactions

The thermal sense is diffuse and incapable of providing precise spatial information. From a phenomenon known as thermal referral, we know that touch influences the localization of cold or warmth, leading to our perceiving illusory thermal sensations at a thermally neutral site. This study investigated the frame of reference for localizing thermal sensations under thermal referral in order to shed light on how thermal and tactile modalities coordinate to process localization information. One thermally neutral tactile stimulator and two cold (warm) stimulators were presented to different sets of three fingers of both hands. The location of the neutral tactile stimulator varied, and the strength of the referral was estimated from participants' performance in localizing the neutral tactile stimulator. By manipulating the somatotopic and spatiotopic distances between the stimulated sites, we found that the somatotopic distance--more specifically, the distance between and among the sites being defined in cortical topography--determines the strength of thermal referral. Our findings suggest that localization of thermal sensations under thermo-tactile interactions is processed with respect to the somatotopic frame of reference and that this cross-modal processing resides in early cortical areas whose organization conserves topographic information.