“Peak-Splitting”: Intensity Effects in Cochlear Afferent Responses to Low Frequency Tones

Abstract

In 1972, Kiang and Moxon noted that a notch can sometimes be measured in the rate-intensity function of cochlear-afferent responses to tones over restricted ranges of high intensities. It has been subsequently shown that, for low-frequency tones, the notch is found in association with bimodality of period histograms (“peak splitting”) and/or with a rapid shift in response phase (Gifford and Guinan, 1983; Kiang, 1984). In the present paper we present preliminary results from a systematic study of the intensity dependence of the magnitude and phase of responses of chinchilla cochlear afferents to low-frequency tones. Low-frequency stimuli are used to facilitate precise timing of the excitation of cochlear afferents relative to basilar membrane (BM) motion (Ruggero and Rich, 1987; Ruggero et al., 1986b). Our present findings, for responses to 200 – 600 Hz tones, agree with and extend our previous results for 100 Hz tones (Ruggero and Rich, 1988): within each stimulus cycle, there appear to exist two distinct preferred times for inner hair cell (IHC) excitation of cochlear afferents. Relative to BM motion, one corresponds to displacement or velocity toward scala tympani (ST) and the other is synchronous with maximal velocity toward scala vestibuli (SV). Which of these two response phases predominates depends on cochlear location (or, equivalently, characteristic frequency: CF) and on stimulus level, but does not depend significantly on stimulus frequency.