The inferior colliculus (IC) is a critical structure that integrates signals from the lower brainstem before projecting the information towards the auditory thalamus and cortex. We propose that functional zones in the IC may be created by specific combinations of inputs from brainstem sources. Aim 1 will use anatomical and physiological methods in the cat to identify functional zones in the IC based on three major excitatory inputs. We will test the hypothesis that ipsilateral projects from medial superior olive (MSO) and contralateral projects from the lateral SO (LSO) are anatomically segregated. We will show that the inputs from MSO and MSO and cochlear nucleus also define separate functional zones. Moreover, we will physiologically identify the functional zones related to MSO, LSO, and cochlear nucleus inputs. Aim 2 will identify functional zones in the IC based on the integration of excitatory and inhibitory inputs. Inhibitory inputs to the IC create inhibitory side bands and transform the response to interaural time differences (ITD). Major sources of inhibitory inputs are the dorsal nucleus of the lateral lemniscus (DNLL), the ipsilateral project from LSO, and local inhibitory IC neurons. We will test the hypothesis that the projections from the ipsilateral MSO and the contralateral DNLL have a common target in the IC. We will also show that the inputs from the contralateral DNLL and ipsilateral LSO do not converge. We will identify local circuit neurons in the IC. Finally, we will test the hypothesis that some ITD responses in IC will be altered by suppression of DNLL or inhibitory local IC inputs. In aim 3, we will identify functional cell types in the IC. We will show that the combination of K+ channels or their subcellular localization are related to cell type and GABA content in the IC. Finally, we will demonstrate that neurons with different firing patterns and whole-cell currents are related to different patterns of K+ channel and/or GABA expression.