Temporal constraints of conscious tactile perception in the primary somatosensory cortex

The primary somatosensory cortex (S1) has long been implicated in tactile perception, yet its precise role in conscious tactile detection remains uncertain. The current study investigated the causal and time-specific involvement of S1 in tactile detection using single-pulse transcranial magnetic stimulation (spTMS). In two experiments, spTMS was applied over contralateral S1, an active control site (inferior parietal lobe; IPL), or under a sham condition at short (25 & 75 ms; Experiment 1) and longer (130 ms; Experiment 2) intervals following electrotactile stimulation of the finger. Participants performed a go/no-go detection task at sensory threshold. In Experiment 1, tactile sensitivity was significantly reduced following early S1 stimulation compared to both active control and sham conditions. However, no such effect was observed in Experiment 2, indicating a temporally limited role of S1 in conscious detection. Moreover, self-reported TMS-related distraction ratings did not account for the observed sensitivity differences, suggesting sensitivity-specific modulation by early TMS rather than general task disruption. These findings support a causal role for early S1 activity in conscious tactile detection. We propose that disruption at this early stage interferes with the initial encoding of tactile input, thereby attenuating not only immediate perceptual awareness, but also subsequent functions such as discrimination and retention. Overall, the results underscore the constrained role of S1 in conscious stimulus detection and highlight the importance of neural networks beyond S1.