Simultaneous early processing of sensory input in human primary (SI) and secondary (SII) somatosensory cortices

The anatomic connectivity of the somatosensory system supports the simultaneous participation of widely separated cortical areas in the early processing of sensory input. We recorded evoked neuromagnetic responses noninvasively from human primary (SI) and secondary (SII) somatosensory cortices to unilateral median nerve stimulation. Brief current pulses were applied repetitively to the median nerve at the wrist at 2 Hz for 800-1,500 trials. A single pulse was omitted from the train at random intervals (15% of omissions). We observed synchronized neuronal population activity in contralateral SII area 20-30 ms after stimulation, coincident in time with the first responses generated in SI. Both contra- and ipsilateral SII areas showed prominent activity at 50-60 ms with an average delay of 13 ms for ipsilateral compared with contralateral responses. The refractory behavior of the early SII responses to the omissions differed from those observed at similar to 100 ms, indicative of distinct neuronal assemblies responding at each latency. These results indicate that SII and/or associated cortices in parietal operculum, often viewed as higher-order processing areas for somatosensory perception, are coactivated with SI during the early processing of intermittent somatosensory input.