Time dependence of binaural responses in the rat’s central nucleus of the inferior colliculus

Recordings were made from single neurons in the rat’s central nucleus of the inferior colliculus. Excitatory/inhibitory binaural interactions and interaural-level difference curves were determined for responses to 100 ms dichotic tone bursts presented to the left and right ears simultaneously. Most neurons with sustained responses to tone bursts had the same binaural response type throughout the 100 ms stimulus period. However, some neurons (39% of our sample) showed qualitatively different binaural response types during the early and late parts of the stimulus (the first 20 ms versus the last 80 ms of the tone burst). Also, for many neurons with consistent early and late binaural response patterns, the strength of binaural interaction was different during the early and late periods. For example, for neurons excited by the contralateral ear and inhibited by the ipsilateral ear during the entire 100 ms period (the most common binaural response type), the degree of inhibition was generally greater during the later part of a stimulus. This change in the strength and/or quality of binaural interaction during dichotic stimulation likely reflects a complex pattern of converging excitatory and inhibitory inputs to the inferior colliculus from lower brainstem structures as well as the time course of local synaptic events. The temporal properties of binaural interaction may influence how sound source location is represented in the central auditory system.