Inferior colliculus of the house mouse

Abstract

1. Single unit responses were recorded from the dorsal cortex (DC), dorsomedial nucleus (DM) and central nucleus (CN) of the inferior colliculus (IC) of chlorprothixene anesthetized house mice. Tone and noise thresholds, spontaneous activities, frequency tuning curves, tone response patterns, spike rate-intensity functions, and tone-innoise response patterns were determined within a 40 dB sound intensity range above the units' thresholds. 2. Single unit thresholds were often lower than the average lowest behavioral thresholds, however, the envelope of the neural data resembled the behavioral curve in most of the frequency range (Fig. 2). 3. Spontaneous activity ranged from 0 to 62 spikes/s with a peak of the distribution at 1–3 spikes/s (Fig. 3). 4. About 35% of CN excitatory tuning curves were primary-like (resembling those of auditory nerve fibers). The remaining 65% were variable including broad, constant width, closed, tilted and complex shapes. The proportion of tuning curve shapes in the DC was similar to that in the CN, however the DM distribution, in which tuning curves with steep flanks and closed ones dominated, differed significantly from both (Figs. 4, 5). 5. Tonic, primary-like tone response patterns were found at least at some intensities in nearly 55% of units from all three nuclei (Fig. 8). The other response patterns observed were phasic, phasic with some tonic components, pauser, long latency, spontaneous activity inhibited, and no tone response (Figs. 7, 8). 6. With increasing sound intensity 60% of the units changed their tone response pattern, most frequently to pauser responses (Fig. 9). 7. Neurons from the three nucleus areas had significantly different shapes of rate-intensity functions (Fig. 10). Monotonic functions dominated in the CN, peaked ones in the DC and peaked and complex ones in the DM (Fig. 11). 8. Averaged spike rates of all IC units studied and averaged tendencies for rate increases or decreases changed little with increasing tone level from 10 to 40 dB above the units' tone thresholds. 9. Nine distinct tone-in-noise response patterns were seen when unit responses as a function of noise level were examined. Noise can have facilitating, enhancing, inhibitory, or little influence on the response patterns in tone intervals. In the intertone (noise only) intervals noise can have excitatory, inhibitory or no effects on the unit responses (Figs. 13, 14). Tone-in-noise response patterns depended both on tone and noise intensities and shifted frequently with changing stimulus intensity. Occurrence rates of the different tone-in-noise response patterns differed significantly between the DC and the DM or CN (Fig. 14). 10. Averaged across the different nuclei of the IC, about half of tone-in-noise responses differed from those described for the auditory nerve and ventral cochlear nucleus. 11. Tuning curve shapes correlated significantly with the forms of rate-intensity functions (primarylike tuning curves — non-peaked functions; higher order tuning curves — peaked or complex functions). There were also statistically significant relations between tone response patterns and tone-innoise response patterns. 12. Temporal response patterns of the neurons (PSTH patterns) were not related to tuning curve shapes and to forms of rate-intensity functions. 13. Results are discussed with regard to sound intensity effects on the processing of auditory information in the three IC nuclei and in comparison with studies on other mammals.