Steady-state evoked potentials in humans to continuous amplitude modulated tones

Abstract

Phase locking a computer to the modulation envelope of a continuous sinusoidally modulated amplitude modulated tone, enables an averaging technique to be used to record a periodic, steady-state evoked potential from the scalp of normal human subjects. The periodic evoked potentials had the same period as the modulation waveform. Fourier analysis was used to quantify the amplitude and phase of the first and second harmonics of the response. Responses were recorded for modulation rates from 4 to 448 Hz, for carrier frequencies from 250 Hz to 4 kHz and for sound pressure levels from 30 to 100 dB SPL. In general, the response amplitude increases with SPL. Estimates of latencies of these steady-state evoked potentials can be made by measuring the phase change of both harmonics as the modulation frequency is varied. The latencies appeared to have one of 14 discrete values from 3 ms up to 104 ms depending on the stimulus parameters. The majority of the responses had latencies from 9 to 33 ms.