The inferior colliculus is a major site of convergence of inputs from many neural centers. Through studies of neurons in some of the main auditory structures, the PI and his group have identified transformations in the representation of ITDs that are best explained by the convergence of inputs. Two transformations are (1) sharpened tuning to ITDs and (2) increased numbers of neurons with properties intermediate between those recorded at lower levels. ITD functions in the superior olivary complex are too broad for an efficient population code. The sharpened ITD functions of the IC, however, approach the sharpness of those in the auditory thalamus. The emergence of intermediate neurons in the IC provides insights into the neural representations of ITDs. The ITD functions of suche intermediate neurons appear to have asymmetrical """"""""saw-tooth"""""""" shapes and a consequent shift in the centroid of the response relative to inputs from the SOC. This shape and shifted centroid is purported to arise from inhibitory inputs that converge with partially overlapping, excitatory inputs from the SOC. Neurons sensitive to different ITDs would then be created by the same excitatory input converging with inhibitory inputs with different degrees of overlap and strength of inhibition. In this way, a place code could be refined from a representation creased using a coarse set of delay lines, in contrast to the Jeffress model that implies that a place code is created in one step with fine-grain delay lines. The proposed experiments will examine the effects of inactivating the sources of inhibitory inputs on the sharpening and shifting of ITD functions in IC neurons. The PI will also deposit an anterograde tracer into the inactivation site, and a retrograde tracer in the IC. The anatomical experiments, combined with the recording and inactivation studies, will provide a comprehensive approach to ITD processing in the IC and the role of converging inhibitory inputs.
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