Cross-Modal Tuning in Early Visual and Somatosensory Cortices

Conflicts between the senses shape our perception. We used functional magnetic resonance imaging to test whether exposure to spatially offset visual and tactile stimuli shifts population-level spatial tuning in early visual and somatosensory cortices. Participants fixated a marker at the center of a sketched outline of a right hand. During visual stimulation, yellow circles expanding and contracting at 4 Hz were superimposed on one fingertip on the displayed outline. Tactile stimuli were amplitude-modulated vibrations at the fingertips of the participants’ right hand, also pulsating at 4 Hz. Stimuli swept across the fingers, moving from one finger to the next every 4 s, in ascending or descending order. Within a 4 min run, visual and tactile stimuli were either presented in isolation or synchronously. Visual-tactile stimulus pairs were either always located at the same finger, or always located at adjacent fingers, with the visual stimulus shifted either towards the thumb or little finger. Population receptive field (PRF) mapping confirmed topographically organized neural populations tuned to tactile stimulation of one finger in somatosensory but not visual cortex, and vice versa for visual stimulation. Maps from unisensory stimulation agreed well with those from congruent tactile-visual stimulation. Visual-tactile spatial discrepancy resulted in a PRF shift in all participants. Shift direction was independent of sweep direction ruling out prediction of the upcoming stimulus as the source of the effect. Rather, PRFs in somatosensory cortex were shifted toward the neighboring finger, consistent with tuning for combined visual-haptic locations and vice versa in visual cortex. In sum, our results reveal cross-modal effects on population-level spatial tuning in early visual and somatosensory cortices.