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Unformatted text preview: Behavioral/Systems/Cognitive Mechanisms for Adjusting Interaural Time Differences to Achieve Binaural Coincidence Detection Armin H. Seidl, Edwin W Rubel, and David M. Harris Virginia Merrill Bloedel Hearing Research Center, Department of OtolaryngologyHead and Neck Surgery, University of Washington, Seattle, Washington 98195-7923 Understanding binaural perception requires detailed analyses of the neural circuitry responsible for the computation of interaural time differences (ITDs). In the avian brainstem, this circuit consists of internal axonal delay lines innervating an array of coincidence detector neurons that encode external ITDs. Nucleus magnocellularis (NM) neurons project to the dorsal dendritic field of the ipsilateral nucleus laminaris (NL) and to the ventral field of the contralateral NL. Contralateral-projecting axons form a delay line system along a band of NL neurons. Binaural acoustic signals in the form of phase-locked action potentials from NM cells arrive at NL and establish a topographic map of sound source location along the azimuth. These pathways are assumed to represent a circuit similar to the Jeffress model of sound localization, establishing a place code along an isofrequency contour of NL. Three-dimensional measurements of axon lengths reveal major discrepancies with the current model; the temporal offset based on conduction length alone makes encoding of physiological ITDs impossible. However, axon diameter and distances between Nodes of Ranvier also influence signal propagation times along an axon. Our measurements of these parameters reveal that diameter and internode distance can compensate for the temporal offset inferred from axon lengths alone. Together with other recent studies, these unexpected results should inspire new thinking on the cellular biology, evolution, and plasticity of the circuitry underlying low-frequency sound localization in both birds and mammals. Introduction Binaural processing of acoustic signals is essential for localizing sound and extracting signals in a noisy environment. Low- frequency sounds are localized by interaural differences in the arrival times of sound created by a spatial separation of the ears [interaural time differences (ITDs)]. ITDs are thought to be com- puted by a neural mechanism similar to that proposed by Jeffress (1948), where external differences of sound arrival times at the ears are represented along internal delay lines. In the avian auditory brainstem, a modified Jeffress model is represented by nucleus magnocellularis (NM) and nucleus lami- naris (NL) (see Fig. 1 A ). Neurons in NM receive monaural input from the ipsilateral acoustic sensory epithelium of birds via the auditory nerve (AN). NM axons bifurcate and send bilateral pro- jections to neurons in NL (see Fig. 1 A ). The ipsilateral NM axon provides a simultaneous input to the dorsal dendrites of an isof- requency array of NL neurons (Hyson et al., 1994) and by traveling an extended looped trajectory putatively equalizes conduction...
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