Ipsilateral somatosensory cortex contains task-relevant tactile representations despite contralateral activation dominance

Abstract

Tactile perception has long been thought to be processed primarily in the contralateral somatosensory (S1), yet the functional role of ipsilateral S1 in human tactile discrimination remains unclear. We designed an fMRI experiment to investigate the neural representation of sequential vibrotactile discrimination as participants judged which of two successive stimuli delivered to the fingertip was higher in frequency. Contralateral S1 showed robust activation during unilateral stimulation, whereas ipsilateral S1 exhibited overall suppression. Interestingly, we found reliable frequency-dependent modulation in ipsilateral S1, which was absent in a control condition requiring no comparison between stimuli. Multivariate representational analyses further demonstrated that the difference in the neural representations between successive stimuli was strengthened under memory demands, particularly within ipsilateral S1 and parietal cortices. The ipsilateral representation was observed for stimulation of either hand, suggesting a bilateral and symmetric encoding mechanism rather than a hand-specific effect. Together, these findings demonstrate that the ipsilateral S1 contributes to tactile discrimination despite its overall suppression during unilateral stimulation, challenging classic contralateral models of somatosensory processing. Our results highlight interhemispheric coordination as a key mechanism underlying perceptual decisions.