Cochlear nerve fiber responses to amplitude modulated stimuli: Variations with spontaneous rate and other response characteristics

N.P. Cooper, Donald Robertson, G.K. Yates

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    Abstract

    1. Single-fiber responses to sinusoidally amplitude-modulated (AM) tones were recorded from the cochlear nerves of anesthetized guinea pigs. Stimuli were presented at the fiber's characteristic frequency (CF) and covered the intensity range between the fiber's minimum rate threshold and 90- 100 dB SPL in 5- or 6-dB steps. The amount of modulation in each fiber's response and the average rate of the responses were quantified. The observed response modulation was compared with the modulation to be expected on the assumption that the instantaneous discharge rates varied with intensity in the same way that the average rates did (i.e., as predicted from each fiber's average-rate vs. level function).2. The difference between the observed and expected response modulation varied widely across fibers. In most fibers' the responses to a limited range of stimulus intensities (typically between 20 and 30 dB above the fiber's rate threshold) were modulated far more than expected on the basis of their average rates, with responses to stimuli either above or below this range differing progressively less from expectation. Little or no response modulation was observed above - 70 dB SPL in these fibers. Other fibers exhibited response modulation that exceeded the expected modulation by smaller amounts, but maintained this modulation to much higher sound pressure levels.3. The discrepancy between the observed and expected responses to AM stimuli also varied with the frequency of modulation (f(m)) within individual fibers. The discrepancies were least pronounced at low f(m)s (e.g., 10 Hz) but became progressively larger as f(m) was increased to between 50 and 320 Hz (subject to the inter-fiber variations described in 2, above).4. The AM response characteristics varied systematically with the fiber's spontaneous rate and other response characteristics (e.g., rate threshold, CF rate vs. level function type, and rapid adaptation characteristics). In particular, the most sensitive, high spontaneous rate fibers had responses that adapted rapidly after the onset of a stimulus, and showed the greatest enhancement of AM-related information at low-to-moderate stimulus intensities. However, these fibers appeared incapable of encoding AM-related information at high intensities, since their response rates ''saturated'' and their AM response enhancements diminished around 30 dB above threshold. In contrast, the less sensitive (i.e., higher threshold), lower spontaneous rate fibers showed less evidence of rapid adaptation near the onsets of their response, and lesser enhancements of the modulated responses predicted from their average-rate versus level functions. The responses of these low spontaneous rate fibers did not saturate fully at high intensities, however, and the fibers hence remained capable of signaling amplitude modulations over a wide dynamic range.5. The above findings are discussed in relation to previous studies. It is concluded that the AM response enhancements are most likely to be effected at the inner hair cell to cochlear nerve fiber synapse, and that these enhancements may share a common mechanism with rapid adaptation.
    Original languageEnglish
    Pages (from-to)370-386
    JournalJournal of Neurophysiology
    Volume70
    Issue number1993
    Publication statusPublished - 1993

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