Far-Field Electrocochleography, Adaptation

Abstract

The properties of the seven msec far-field response were investigated with regard to adaptation when series of stimuli are presented at repetition rates from 5 to 40 pps. The stimuli were 1, 2 and 4 kHz tone bursts with rise-fall times of 1 msec and they were applied as short bursts with a peak duration of 1/2 msec and long bursts with a peak duration of 8 msec. In differential recordings between the Vertex and the homolateral mastoid it is possible to measure the AP and the seven msec response at the same time so these two could be compared under identical conditions. The latencies to both potentials increased with repetition rates higher than 10 pps and long tone bursts had the strongest effect. This is in contrast to amplitude, where the seven msec response remained rather stable at repetition rates of even 40 pps with long tone bursts, while the AP exhibited a significant reduction at rates of only 10 pps. With the short type of tone burst the AP tolerated repetition rates of 20 pps. The results were independent of frequency. It is hypothesized that the difference between the behaviour of the AP and the 7 msec response should be sought in the wave-form of the basic units of the responses. In a series of averages from the same electrocochleographic response it appeared that the seven msec response is more susceptible than the AP to filtering in the low frequencies. This is taken as evidence that the unit of responses in the brainstem generators have longer durations and thus will add up to large compound potentials even if the synchronization is less precise.