Spatial organization of receptive fields in the auditory midbrain of awake mouse.

Efficient encoding of sensory information can be implemented by heterogeneous response properties of neurons within sensory pathways. In the auditory system, neurons in the main auditory midbrain nucleus, the inferior colliculus (IC), show heterogeneous response properties to various types of acoustic stimuli including behaviorally relevant sounds. The receptive fields of these neurons, and their spatial organization, may reveal mechanisms that underlie response heterogeneity in the IC. The mouse is becoming an increasingly popular system for auditory studies and although some studies have examined spectral characteristics in the IC, most of these have been conducted in anesthetized animals. There were two goals of this study. The first goal was to examine the frequency representation of awake mouse IC in fine spatial resolution. The second goal was to determine whether there is a spatial organization of excitatory frequency tuning curves in the IC of awake mice. We achieved these goals by histologically reconstructing locations of single and multiunit recordings throughout the IC in a mouse strain with normal hearing (CBA/CaJ). We found that the tonotopic progression is discontinuous in mouse IC, and we found that there is no clear spatial organization of frequency tuning curve types. Rather, there is heterogeneity of receptive fields in the bulk of the IC such that frequency tuning characteristics and hence the structure of excitatory and inhibitory inputs does not depend on location in the IC. This heterogeneity likely provides a mechanism for efficient encoding of auditory stimuli throughout the extent of the mouse IC.