Cortical generators of somatosensory evoked potentials to median nerve stimulation.

In 12 patients with intractable partial seizures, chronically implanted subdural electrodes were used to define the relationship of the epileptogenic focus to cortical functional areas. Cortical somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded from these electrodes. The initial cortical positivity, postrolandic primary cortical potential (PCP), was recorded in all 12 patients with a mean latency of 22.3 +/- 1.6 msec. A potential of opposite polarity, prerolandic PCP, was defined in nine patients with a mean latency of 24.1 +/- 2.7 msec. The latency of the postrolandic PCP was 1.61 +/- 1.59 msec shorter than the prerolandic PCP (p less than 0.01, paired t test). The maximum amplitude postrolandic PCP was 2.1 times larger than the maximum prerolandic PCP (p less than 0.02, paired t test). The phase reversal of the SEPs was compared with the position of the rolandic fissure (RF) defined by electrical stimulation. This study shows that the latency and amplitude characteristics of post- and prerolandic PCPs are significantly different and give support to the concept that they are produced by different generators; and cortical SEPs are helpful in locating the RF.