[Somatosensory evoked potentials in a patient with a posterior postcentral gyrus lesion--generator sources of components after N20].

We studied the distribution of the median nerve somatosensory evoked potentials (SEPs) on the scalp of a 67-year-old woman with a localized old hematoma involving the posterior part of the right postcentral gyrus identified by MRI. The patient presented with an isolated loss of discriminative sensation (stereognosis, graphesthesia and position sense) of the left extremities. With respect to the SEPs on the right hemisphere, the only components remaining were the initial negativity (N20 with latency of about 20 msec) recorded in the parieto-occipital area, which exhibited polarity inversion across the central sulcus. Since area 3b is thought to be the superficial sensory cortex, the results of sensory examination in this case suggested that the hematoma was restricted to areas 1 and 2, which are thought to be the discriminative sensory cortex, and that area 3b was unaffected. We therefore conclude that the N20 on the affected hemisphere reflected the posterior pole of a horizontal dipole generated by area 3b, and that the hematoma abolished the positivity after N20, corresponding to the positive pole of a slightly delayed vertical dipole generated by the posterior part of the postcentral gyrus (areas 1 and 2).