ACTIVATION OF THE HUMAN POSTERIOR PARIETAL CORTEX BY MEDIAN NERVE-STIMULATION

We recorded somatosensory evoked magnetic fields from ten healthy, rig ht-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli; exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 an d 35 ms, were explained by activation of the contralateral primary som atosensory cortex (SI) hand area. All subjects showed additional defle ctions which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemisphere s. The SIT responses were typically stronger in the left than the righ t hemisphere. All subjects had an additional source, not previously re ported in human evoked response data, in the contralateral parietal co rtex. This source was posterior and medial to the SI hand area, and ev idently in the wall of the postcentral sulcus. It was most active at 7 0-110 ms.