Early auditory electric responses to fast amplitude and frequency tone glides

Using the standard far-field technique for auditory brain stem responses early electrical responses have been recorded, evoked by stimulation using brief frequency or amplitude glides of a continuous pure tone of 1 kHz at 60 dB HL. The glide duration was always 1 msec and its magnitude in the range of 50--100% relative frequency change or 5--20 dB sound level change. Ten normal-hearing subjects were studied. The responses recorded were generally dominated by a vertex positive-negative-positive wave complex in the 4--9 msec latency range. The wave form was significantly different from the AER evoked by clicks or brief tone bursts. The origin of the different response components is not clear; a cochlear origin of the first vertex-positive component is a possibility. Although responses to frequency and amplitude glides were similar in most respects there were some statistically significant differences which may be related to different auditory processing of amplitude and frequency modulation. Distinctly clearer responses were recorded to positive glides, i.e., increasing frequency or amplitude, than to negative glides. This is in contrast with psychoacoustic detection thresholds and slow cortical responses to frequency glides, which have shown essentially symmetry, but in agreement with such findings in response to amplitude glides. The findings do not agree with previously published results on VIII nerve compound action potentials in response to brief frequency glides.