Short-latency components of evoked potentials to median nerve stimulation recorded by intracerebral electrodes in the human pre- and postcentral areas

Objective To study whether sensory afferents of the hand projected directly to the primary motor cortex (M1) as they have been well electrophysiologically described in monkeys but not in humans. Methods We recorded intracerebrally in the central areas (pre- and/or postcentral gyrus) somatosensory evoked potentials (SEPs) to median nerve stimulation in 5 (4 women, 1 man; age 14–37 years) epileptic patients during presurgical evaluation. Results The primary somatosensory cortex (S1) showed negative–positive components peaking at about 20 and 30 ms, respectively. By contrast, M1 disclosed SEPs of two types of waveforms depending on the portion of the precentral gyrus explored by the different contacts of the electrode. Here, we demonstrated, for the first time, in the medial portion of M1, shaped like an omega in the axial plane, corresponding to the motor hand area, the occurrence of a primary negative component as in S1, but of higher amplitude and peaking at about 4 ms later. In other respects, the lateral portion of M1 disclosed positive–negative components peaking at about 21 and 31 ms, respectively. Conclusions These electrophysiological findings, based on accurate spatial and temporal resolution of intracerebral recordings, suggested that somatosensory inputs from the hand projected directly to M1 in its medial portion.