The frequency-following response to continuous tones in humans.

Previous studies of the frequency-following response (FFR) in man suggest that it has multiple sources. Identification of these sources has been complicated by the use of tone bursts to evoke FFRs and the lack of precise methods to calculate their amplitude and latency. Tone bursts produce transient responses which confound measurements of the FFR. The use of continuous tones avoids this problem and the Fast Fourier Transform can be used to assess accurately and efficiently the presence, amplitude and phase angle of the FFR. In this study we systematically examined the frequency and intensity range over which FFRs to continuous tones could be evoked using FFRs to tone bursts for comparison. We then analyzed FFRs to continuous tones to determine the sources of this potential. FFRs to both stimuli have similar thresholds (65-90 dB SPL) and can be evoked by the same range of frequencies. Neurogenic FFRs in man occur only below 1000 Hz. The source for this potential has a latency of 8.2 +/- 0.1 ms (mean +/- SD) and is consistent with a midbrain source. At higher frequencies FFRs have a latency of less than 1 ms and are most likely cochlear microphonic. The small variation in the latency of the neurogenic FFR suggests this as a possible tool for assessing neurological disorders.