SIMILAR DYNAMIC RANGE ADJUSTMENTS IN THE AUDITORY NERVE AND COCHLEAR NUCLEI OF THE DECEREBRATE CAT

Auditory-nerve fibers have dynamic ranges which are much narrower than the total operating range of the auditory system as a whole. Previous research indicates that although the dynamic ranges of auditory-nerve fibers are not adjustable, those of cochlear nucleus units may be. The goals of the present research were to determine whether dynamic range adjustment is a function of the cochlear nuclei and to explain how intensity discrimination can be performed over a wider intensity range than the dynamic range of individual auditory-nerve fibers.;Dynamic range adjustment in the cochlear nuclei appears to be predominantly determined by dynamic range adjustments which are already present in the auditory nerve. The fact that responsiveness to test tones is preserved in the presence of noise backgrounds suggests that the auditory system can perform intensity discrimination by summing the activities of all auditory-nerve fibers (the "spread theory").;Discharge rate versus stimulus level functions for best frequency tones were recorded in cat auditory nerve and cochlear nucleus units in the presence of a prolonged (15 seconds or more) background. The background was either a tone at the test frequency which was switched off during the test period or broadband noise which was presented continuously. In contrast to previous studies, I found that dynamic range adjustment occurs to approximately the same degree in the auditory nerve and cochlear nuclei. In both structures, the qualitative effect is the same regardless of a small, potentially artifactual quantitative difference. Dynamic range adjustment to tone backgrounds occurs at a rate of somewhat less than one-half (0.2 to 0.5) dB per dB increase in background level. However, this adjustment does not substantially extend the effective dynamic ranges of most units and cannot account for intensity discrimination over the full operating range of the auditory system. Noise backgrounds elicit dynamic range adjustment of somewhat less than one (0.8 to 1.0) dB per dB increase in background level, and the ability of most auditory units to respond to tone bursts is preserved in the presence of background noise at intensities which initially produced a saturated response.