Responses of peripheral auditory neurons to two-tone stimuli during development: II. Factors related to neural responsiveness.

In an accompanying paper (Fitzakerley et al., 1994), it was demonstrated that there is a significant developmental correlation between the appearance of tuning and two-tone suppression. However, it was also found that some sharply tuned neurons meeting minimal adult standards did not exhibit suppression. Therefore, in order to investigate other factors that may be related to the demonstration of suppression in peripheral auditory neurons, the relationship of two-tone suppression with various parameters related to neural responsiveness was studied in perinatal kittens. A positive correlation was made between the observance of suppression and driven discharge rate, characteristic frequency (CF), and threshold, all properties which change significantly during the final stages of cochlear differentiation. In older animals, suppression was also observed in higher percentages of neurons having low spontaneous rates (< 1 spk/s). Suppression was evoked less often by test tones placed below CF than above CF in neurons recorded from younger animals, and was generally produced by a narrower range of test tone intensities than those recorded from adults. As a result, the conventional description of suppression as observed in peripheral auditory neurons in mature animals must be extended to include these factors when responses of immature neurons are considered.