A method for producing the coherent frequency: a steady-state auditory evoked response in the electroencephalogram.

Transient and steady-state auditory evoked responses in the electroencephalogram are used to study the effect on the brain of graded changes in the concentration of general anesthetics. A method is described using modern signal processing techniques to improve the analysis of steady-state auditory evoked responses (SSAER). The SSAER was obtained using headphones to give 100-200 auditory click stimuli from 6.5 Hz.to 50.5Hz in 1-Hz steps. The resulting electroencephalogram signals were filtered and subject to Fourier analysis, after which a series of coherence indexes were derived based on waves with significant power in the fundamental but with minimal harmonic content. These were plotted against the range of stimuli and fitted with a third-order polynomial. The frequency at which the maximum coherence index was achieved (highest possible value = 1) was derived from polynomial interpolation. The repeatability of the method was examined in 10 awake subjects using runs of ascending then descending stimulating frequencies. The mean maximum coherence index was at 38 Hz, with the 95% confidence interval of 37.3 Hz-38.9 Hz. There was no difference between ascending and descending sweeps. The method provides an automatic analysis of the SSAER that obviates the need to make subjective decisions about which is the dominant wave, a major problem in the analysis of the transient auditory evoked responses.