1. A method of determining the average frequency of a sample of a band-limited EEG waveform in the presence of an intermittent interfering unwanted alpha rhythm waveform of relatively greater amplitude than the components of the wanted waveform and of a defined frequency range within said band limits, comprising sensing a subject's brain waveform, limiting said waveform to frequencies within said band limits, detecting in a sample of said waveform at least every alternate zero crossing point in the waveform, counting the events defined by adjacent alternate crossing points, identifying events having durations respectively within and outside a range corresponding to the duration of a cycle of a waveform within the defined frequency range of the unwanted waveform, determining the number and the sum of the durations of the events having durations outside said range during said sample, and calculating from said number and said sum the average frequency of these latter events.