We have reported the presence of continuous gamma (30–150 Hz) activity in the human medial temporal lobe (MTL). Since the MTL is involved in learning and memory, we speculated that MTL gamma activity is related to such higher brain functions. It is thus of interest to learn how this activity changes during different states of consciousness. In this study, we recorded electrocorticographic (ECoG) activity directly from the surface of the MTL after various doses of sevoflurane anesthesia. Five epileptic patients underwent electrode placement operations in which electrodes were attached to the surfaces of the MTL and the basal temporal lobe (BTL). Immediately following the operation ECoG was recorded from each patient under four concentrations of sevoflurane anesthesia (1.5, 2.0, 2.5 and 3.0%). Fast Fourier Transform (FFT) analysis was performed on the MTL ECoGs. Under the lowest sevoflurane concentration, MTL gamma activity was observed in all patients. However, gamma activity was progressively suppressed by increased concentrations of sevoflurane, in a dose-dependent manner. Sevoflurane has been known to reduce neuronal excitability in the rat hippocampus in vitro, probably by changing GABAergic inhibition. The reduction of MTL gamma in the present study may be the result of such a mechanism. Although memory function was not tested in this study, the amount of MTL gamma activity may be related to residual memory function during anesthesia.
1Department of Psychophysiology, Tokyo Institute of Psychiatry, 2-1-8 Setagaya-ku, Kamikitazawa Tokyo 156-8585, Japan
2Departments of Anesthesiology, Tokyo Metropolitan Neurological Hospital, Tokyo 183-0042, Japan
3Departments of Neurosurgery, Tokyo Metropolitan Neurological Hospital, Tokyo 183-0042, Japan
4Corresponding Author: Sunao Uchida
Acknowledgement: This study was supported by Special Coordination Funds for Promoting Science and Technology, from the STA.
Received 15 September 1999; accepted 20 October 1999