Somatosensory evoked high-frequency oscillations recorded directly from the human cerebral cortex

Objective: To elucidate the generator sources of high-frequency oscillation s of somatosensory evoked potentials (SEPs), we recorded somatosensory evok ed high-frequency oscillations directly from the human cerebral cortex. Subjects and methods: Seven patients, 6 with intractable partial epilepsy a nd one with a brain tumor, were studied. With chronically implanted subdura l electrodes, we recorded SEPs to median nerve stimulation in all patients, and also recorded SEPs to lip and posterior tibial nerve stimulation in on e. High-frequency oscillations were recorded using a restricted bandpass fi lter (500-2000 Hz). Results: For the median nerve oscillations, all oscillation potentials were maximum at the electrodes closest to the primary hand sensorimotor area. M ost oscillations were distributed similar to or more diffusely than P20/N20 . Some later oscillations after the peak of P20 or N20 were present: in a v ery restricted cortical area similar to P25. We investigated the phase chan ge of each oscillation potential around the central sulcus. One-third of th e oscillations showed phase reversal around the central sulcus, while later oscillations elicited in a restricted cortical area did not. High-frequenc y oscillations to posterior tibial nerve and lip stimulation were also maxi mum in the sensorimotor areas. Most of the lip oscillations showed phase re versal around the central sulcus, but most of the posterior tibial nerve os cillations did not. Conclusion: High-frequency oscillations are generated near the primary sens orimotor area. There are at least two different generator mechanisms for th e median nerve high-frequency oscillations. We suspect that most oscillatio ns are derived from the terminal segments of thalamocortical radiations or from the primary sensorimotor cortex close to the generator of P20/N20, and some later oscillations from the superficial cortex close to the generator of P25. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.