Abstract: During a surgical operation, a conventional electroencephalograph provides several analog electrical signals of the brain waves of the patient, each signal indicating voltage fluctuations detected at a different area of the patient's scalp. The analog signals are converted to digital form for input to a multipurpose computer. The computer is programmed to produce respective reference traces on a video monitor indicating the voltage fluctuations detected during a preceding reference period. While the reference traces continue to be displayed, more current traces are produced on the video monitor in juxtaposed relationship to the reference traces. The current traces indicate the voltage fluctuations detected during the period immediately preceding their display. The current traces are replaced periodically with new current traces indicating more recent information. Any significant change in the patient's brain waves is detected by comparing the current traces with the reference traces.