Characteristics of very high frequency oscillations of somatosensory evoked potentials in humans with epilepsy

We previously firstly reported very high frequency oscillations (VHFOs, over 1,000 Hz) in somatosensory evoked potentials (SEP) recorded by subdural electrodes following median nerve stimulation on 6 patients. In this study, we fatherly studied the characteristics and the clinical significance of VHFOs) in SEP elicited by stimulating not only the median, but also the ulnar nerves. Conventional somatosensory evoked potentials, including higher frequency components, were recorded by subdural electrodes in 25 patients with intractable epilepsy who underwent intracranial electroencephalographic monitoring for epilepsy surgery. The location, latency, frequency, amplitude and duration of very high frequency components were analyzed. The location of VHFOs was compared with N20 for median and ulnar nerves in each patient. VHFOs were recorded in a very limited region around the central sulcus, mainly on the postcentral gyrus. All VHFOs preceded the earliest peaks of conventional SEP, with the following characteristics: frequencies ranging from 1,000 to 5,000 Hz (mostly from 1,000 to 2,500 Hz), mean amplitude 1.5 mu V, and mean duration 1.67 ms. There were no significant differences in amplitude and frequency of VHFOs among median and ulnar nerve stimulations. We hypothesize that VHFOs may be generated in the representative zone for each nerve within the primary somatosensory cortex, and hence can be utilized for more precise localization of the central sulcus.