Masters Theses

Date of Award

12-1991

Degree Type

Thesis

Degree Name

Master of Arts

Major

Audiology

Major Professor

James W. Thelin

Abstract

Auditory brainstem responses (ABRs) are used clinically to measure neural activity in the acoustic nerve and in the pathways of the auditory system. ABRs are commonly elicited with 100 microsecond click signals which can be presented in any of three modes: condensation polarity, rarefaction polarity or alternating (condensation and rarefaction) polarity. In normal-hearing subjects, there is little or no effect of click signal polarity (Terkildsen et al., 1973; Borg and Lofqvist, 1981; Tietze and Pantev, 1986; Beattie, 1988). However, inversion of signal polarity has been shown to significantly affect wave V of the ABR in subjects with high-frequency hearing losses (Coats and Martin, 1977; Borg and Lofqvist, 1981, 1982; Fiji, 1987; Schwartz et al., 1990). Gorga et al. (1991) used single-cycle sinusoids of various frequencies in both polarities to determine if polarity could be shown to affect wave V latencies in ABRs elicited from normal-hearing subjects. They reported significant effects of polarity on wave V latencies for low-frequency signals, with the effects decreasing as signal frequency increased. With the single-cycle sinusoids used by Gorga et al., the ABR depends upon the spectrum of the signal plus a contribution from higher-frequency regions of the cochlea. The present study utilized high-pass masking to precisely limit the regions of the cochlea contributing to the ABR, and a derived response technique which has been described by Don and Eggennont (1978, 1980) to obtain the ABR elicited from octave regions of the cochlea. Click polarity was shown to significantly affect wave V latency only when the ABR is restricted to frequencies below 500 Hz. The effect was found for all subjects; its magnitude was predictable (1.1 ms ± 0.4 ms), but its direction was not predictable. Inversion of signal polarity was shown to occasionally produce substantial changes in waveform morphology, but these changes did not occur on a predictable manner within or between subjects. The absolute wave V latency was shown to systematically decrease as the response was extended to more basal regions of the cochlea. The results obtained indicate that wave V latency decreases when the contribution of signal energy above 8000 Hz is added. While ABRs can be obtained from every region of the cochlea, the latency of wave V depends primarily upon the highest frequencies contributing to the response.

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