Frequency response areas of neurons in the mouse inferior colliculus. III. Time-domain responses: Constancy, dynamics, and precision in relation to spectral resolution, and perception in the time domain.

The auditory midbrain (central nucleus of inferior colliculus, ICC) receives multiple brainstem projections and recodes auditory information for perception in higher centers. Many neural response characteristics are represented in gradients (maps) in the three-dimensional ICC space. Map overlap suggests that neurons, depending on their ICC location, encode information in several domains simultaneously by different aspects of their responses. Thus, interdependence of coding, e.g. in spectral and temporal domains, seems to be a general ICC principle. Studies on covariation of response properties and possible impact on sound perception are, however, rare. Here, we evaluated tone-evoked single neuron activity from the mouse ICC and compared shapes of excitatory frequency-response areas (including strength and shape of inhibition within and around the excitatory area; classes I, II, III) with types of temporal response patterns and first-spike response latencies. Analyses showed covariation of sharpness of frequency tuning with constancy and precision of responding to tone onsets. Highest precision (first-spike latency jitter < 1 ms) and stable phasic responses throughout frequency-response areas were the quality mainly of class III neurons with broad frequency tuning, least influenced by inhibition. Class II neurons with narrow frequency tuning and dominating inhibitory influence were unsuitable for time domain coding with high precision. The ICC center seems specialized rather for high spectral resolution (class II presence), lateral parts for constantly precise responding to sound onsets (class III presence). Further, the variation of tone-response latencies in the frequency-response areas of individual neurons with phasic, tonic, phasic-tonic, or pauser responses gave rise to the definition of a core area, which represented a time window of about 20 ms from tone onset for tone-onset responding of the whole ICC. This time window corresponds to the roughly 20 ms shortest time interval that was found critical in several auditory perceptual tasks in humans and mice.