Very high-frequency oscillations (over 1000 Hz) of somatosensory-evoked potentials directly recorded from the human brain.

The aims of this study were to record high-frequency oscillations (HFOs) associated with somatosensory-evoked potentials from subdural electrodes and to investigate their generators and clinical significance. Six patients who underwent long-term subdural electrode monitoring were studied. Somatosensory-evoked potentials were recorded directly from the subdural electrode after stimulation of the median nerve. Bandpass filter was 10 to 10,000 Hz for conventional somatosensory-evoked potential and 500 to 10,000 Hz for HFO. Three types of HFO were recorded. The first component was early HFO (407-926 Hz), which occurred before N20 peak. The second component was late HFO (408-909 Hz), which occurred after N20 peak. In addition, a novel component was recorded with a range from 1,235 to 2,632 Hz, and this component was termed very HFO. Early and late HFOs were recorded from relatively wide areas centering around the primary motor and primary sensory areas, whereas very HFO was localized around the primary sensory areas. In this study, at least three components of HFO could be identified. Only very HFO was localized around primary sensory areas, suggesting a possibility that very HFO may provide an effective method of identifying the central sulcus.

Generators of tibial nerve somatosensory evoked potential: recorded from the mesial surface of the human brain using subdural electrodes.

The generators of the initial cortical component of somatosensory evoked potentials in response to tibial nerve stimulation (Tib-somatosensory evoked potentials) are still uncertain. The purpose of this study is to localize the generators of it. A 15-year-old boy with intractable parietal lobe epilepsy was studied. Subdural electrodes were chronically implanted for presurgical evaluation of epilepsy surgery, covering the primary motor, primary sensory, and supplementary sensorimotor areas of the right leg. Tib-somatosensory evoked potentials were recorded from these areas. Highly localized prominent positive activities were recorded from electrodes on the primary motor area of the leg at 32.4 to 34.0 milliseconds. No corresponding large negative peak was recorded in any other electrodes. Weak negative activities distributed widely around the postcentral area at 33.2 to 33.6 milliseconds, accompanied by similar but positive activities in the precentral area at 32.8 to 33.2 milliseconds. There was an independent positive field on supplementary sensorimotor areas at 34.0 to 34.8 milliseconds. A small negative peak was also recorded but only from a single electrode within supplementary sensorimotor areas at 34.0 milliseconds. Our data suggest that the initial response of Tib-somatosensory evoked potentials has at least three independent generators: a radial dipole on the primary motor, a tangential dipole on the primary sensory area, and a dipole on the supplementary sensorimotor areas oriented perpendicularly to the mesial hemispheric surface.