Exclusive ipsilateral representation of sequential tactile differences challenges contralateral dominance

Tactile perception is traditionally attributed to contralateral somatosensory (S1) processing, yet the functional role of ipsilateral S1 in human tactile discrimination remains unclear. Using fMRI during a sequential vibrotactile discrimination task, we examined how frequency information is represented when participants judged which of two successive stimuli delivered to the same fingertip was higher in frequency. Contralateral S1 showed robust activation, and ipsilateral S1 showed suppression during unilateral stimulation; however, linear mixed-effects analyses revealed reliable frequency-dependent modulation in ipsilateral S1, which was absent when no comparison was required. Multivariate representational analyses further demonstrated that fMRI activity patterns representing the differences between successive stimuli were strengthened under memory demands, particularly within ipsilateral S1 and parietal cortices. Furthermore, the representational separability predicted individual discrimination accuracy. The exclusive representation in the ipsilateral S1 was not hand-specific; that is, the results were consistent for both hands, indicating a bilateral and symmetric encoding scheme. These surprising findings demonstrate that the ipsilateral, not contralateral, S1 contributes to tactile discrimination, challenging classic contralateral models of somatosensory processing. These unprecedented findings highlight interhemispheric coordination as a key mechanism underlying perceptual decisions.