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Electroencephalogram (EEG) measures taken during anesthesia exhibit stereotypic changes as anesthetic depth increases.

These changes include complex patterns of frequency slowing accompanied by amplitude increases which typically peak when loss of consciousness occurs (loss of responses to verbal commands; loss of righting reflex). As anesthetic depth increases from light surgical levels to deep anesthesia, the EEG exhibits disrupted rhythmic waveforms, high amplitude burst suppression activity, and finally, very low amplitude isoelectric or 'flat line' activity. Various signal analysis approaches have been used to quantify these pattern changes and can provide an indication of loss of recall, loss of consciousness and anesthetic depth. Monitors have been developed using various algorithms for signal analysis and are commercially available, but none have as yet proven 100% accurate. This is a difficult problem and an active area of medical research.