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Anaesthetic-induced spatial inhomogeneities of the electrencephalogram(EEG) using "high density" electrode mapping have not previously been reported. We measured the scalp EEG with a dense electrode (128-channel) montage during the course of light general anaesthesia with xenon and then propofol in normal human subjects. EEG was measured during induction and recovery of general anaesthesia in five normal subjects, and we obtained analysable data from three of these subjects. EEG topographies were plotted on a realistic head surface. Scalp fields were spatially de-blurred using a realistic head model and projected onto an averaged cortical surface Both xenon and propofol elicited large increases in midline frontal theta-band EEG power. Propofol reliably elicited orbitofrontal delta activity. Xenon, but not propofol, caused large increases in delta over the posterior cortex. Increased gamma power was observed for both anaesthetic agents at midline electrodes over the posterior cortex, but not anteriorly. Anaesthesia-induced delta and theta waves were differentially distributed along the anterior-posterior axis of the brain in a manner that corresponds well to the anatomy of putative neuronal generators. The distribution of anaesthetic-induced changes in fast gamma-band power seems to reflect functional differences between the posterior and anterior aspects of the cerebral cortex. These preliminary observations were consistent within our small sample, indicating that larger studies of anaesthetic effects using high-density recordings are warranted.

High-density EEG Mapping during General Anaesthesia with Xenon and Propofol: A Pilot Study