Integration of Parallel Processing Streams in the Inferior Colliculus of the Barn Owl
<p>The study of the neural mechanisms underlying sensation has demonstrated than an intimate relationship exists between behavioral function and neural structure. One common structural theme in sensory processing is the use of parallel processing streams. Parallel processing occurs in virtuall...
| Summary: | <p>The study of the neural mechanisms underlying sensation has demonstrated than an intimate relationship exists between behavioral function and neural structure. One common structural theme in sensory processing is the use of parallel processing streams. Parallel processing occurs in virtually every sensory modality and in almost every species studied to date. This work describes the integration of parallel auditory processing streams for time, intensity and frequency in the inferior colliculus of the barn owl, Tyto alba, which leads to the formation of a centrally synthesized map of auditory space.</p>
<p>Interaural time differences (ITDs) provide the primary cue for localization in the horizontal plane. ITDs are extracted from the firing patterns of auditory nerve fibers by a coincidence detection circuit located in the owl's brainstem. Interaural level differences, which are used by the owl for vertical plane localization, are computed by an anatomically distinct and parallel circuit. The brainstem and midbrain structures that process time and level differences are independent up to the level of the inferior colliculus. In the lateral shell division of the central nucleus of the inferior colliculus, these two processing streams are combined by single neurons that exhibit spatially restricted auditory receptive fields.</p>
<p>The first part of this thesis characterizes the detailed physiological properties of lateral shell neurons and describes a model of hierarchical integration within the shell that underlies the synthesis of space-specific neurons located in the topographic auditory space map in the external nucleus of the inferior colliculus (ICx). The second part examines the integration of the parallel, narrow band frequency channels arising at the level of the brain stem coincidence detector, nucleus laminaris. The output of single coincidence detector neurons, which encode interaural phase difference, does not unambiguously signal horizontal location. Multiple phase ambiguous narrow band frequency channels are integrated in the lateral shell to eliminate phase ambiguity. Experiments presented here describe the relationship between signal bandwidth and phase ambiguity in an attempt to elaborate the neural circuitry underlying the integration of parallel, narrow band, interaural phase difference channels.</p>
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