Physiological studies and neural mechanisms of echo suppression in the inferior colliculus of the cat

Abstract

Neurons in the inferior colliculus (IC) are sensitive to stimulus direction in free field and to individual directional cues, such as interaural disparities in time (ITD) and level (ILD). Our single-unit recordings in the IC of anesthetized cats explored neural mechanisms in the IC that might mediate echo suppression, using free-field and virtual space (VS) stimuli. Click pairs were used to simulate a sound source (lead) followed by a single reflection (lag) at delays of 1–100 ms. In both free field and VS, correlates of psychophysical echo suppression were found in the responses of most IC neurons. In many neurons, the suppression was directionally dependent in a way that could be linearly predicted from the response to the lead, with the most effective lead locations giving maximum suppression. Some neurons showed a decoupling between echo suppression and the directional response to the lead, suggesting that excitation and suppression are mediated by different neural mechanisms. Further decoupling was achieved by selectively holding constant some directional cues such as ITD and ILD in the VS stimuli. Results suggest that neural suppression in the IC depends on specific directional cues and is therefore a likely correlate of echo suppression in spatial hearing. [Work supported by NIH.]