Frequency selectivity in noise‐exposed chinchillas

Abstract

Evoked‐potential tuning curves (TCs) were recorded from chronic electrodes in the inferior colliculus of 102 chinchillas before and after acoustic overstimulation in order to relate the effects of changes in frequency selectivity to the condition of the cochlea. Pre‐ and post‐exposure measures of auditory threshold and masked thresholds (simultaneous tone‐on‐tone paradigm) were obtained at 0.5, 1, 2, 4, 8, and 11.2 kHz. The three TC variables Q 10 dB, and the low‐ and high‐frequency slopes were compared in two different groups of chinchillas: (1) animals (N = 54) with cochleas that showed distinctly normal and lesioned regions, and (2) animals (N = 48) that had near‐normal thresholds, but either had lesions that were not manifested in the threshold measures or had nearly normal sensory cell populations. In the first group, the TCs measured from normal regions of the cochlea, in general, showed no statistically (t test) significant decreases in the group averaged low‐ or high‐frequency slope, i.e., upward or downward spread of masking, for lesions located either apical or basal of the CF, respectively. In the second group, no significant differences in the TC variables were found in lesioned and nonlesioned cochleas with little or no PTS. These results show that (1) the quality of tuning is not necessarily altered in normal regions of the noise damaged cochlea, and (2) the TC cannot be considered as a reliable measure of sensory cell damage when the amount of PTS is small. [Research supported by NIOSH and USAARL.]