Evidence for interhemispheric processing of inputs from the hands in humanS2 and PV

In the present investigation, we identified cortical areas involved in the integration of bimanual inputs in human somatosensory cortex. Using functio nal magnetic resonance imaging (fMRI) and magnetoencephalography (MEG), we compared the responses to unilateral versus bilateral stimulation in anteri or parietal cortex and areas in the Sylvian fissure of the contralateral he misphere. The extent of fMRI activation on the upper bank of the Sylvian fi ssure, in the second somatosensory (S2) and the parietal ventral (PV) areas , was significantly larger for bilateral stimulation than for unilateral st imulation. Using MEG, we were able to describe the latency of response in S 1 and S2/PV to unilateral and bilateral stimulation. The MEG response had t hree components under both stimulus conditions. An early peak in S1 at 40 m s, a middle peak in S2/PV at 80-160 ms, and three late peaks in S2/PV at 25 0-420 ms. There was an increase in magnetic field strength in S2/PV to bila teral stimulation at 300-400 ms post stimulus. The fMRI results indicate th at, as in monkeys, S2/PV receives inputs from both the contralateral and ip silateral hand. The MEG data suggest that information is processed serially from S1 to S2. The very late response in S2/PV indicates that extensive in trahemispheric processing occurs before information is transferred to the o pposite hemisphere. The neural substrate for the increased activation and f ield strength at long latencies during bilateral stimulation can be account ed for in three ways. Under bilateral stimulus conditions, more neurons may be active, neuronal firing rate may increase, and/or neural activity may b e more synchronous.