Abstract
Frequency-following responses (FFRs) are known to reproduce the frequency and waveform of sinusoidal acoustic stimuli. At high stimulus levels, however, substantial harmonic distortion is present and often apparent on visual inspection of the response curves. The harmonic components are easily demonstrable by filtering or with spectral analysis. In some subjects, the harmonic distortion is so pronounced that the periods of the response correspond to the second harmonic rather than to the fundamental (first harmonic) of the stimulus frequency. On the basis of spectral analysis of scalp recorded FFRs to intense (up to 120 dB PESPL) 250 Hz tone bursts of 20 ms duration, our test subjects could be divided into three categories 1) Those in whom the first harmonic was considerably larger than the second, 2) Those with a mild predominance of first over second harmonics, and 3) Those in whom the magnitude of the second was equal to or exceeded that of the first harmonic. Results from three selected subjects, illustrating these three response patterns, are given. Harmonics of up to the fourth order could be measured, and the levels of these components as a function of stimulus level are presented. For all four harmonics, the amplitudes appear to increase gradually with stimulus intensity up to stimulus levels of 90–110 dB PESPL (60–80 dB SL) above which the increase is small, or there may even be a decrease. On comparison with corresponding data on cochlear microphonics, described by previous authors in animal studies, similarities but also important differences were noted.