The broad goal of the proposed research program is to advance our understanding of sound localization. The noteworthy similarities and differences between sound location processing in birds and mammals clearly show that there are many ways of localizing sounds and separating sources from background noise;these different strategies can provide inspiration for the development of new technologies such as cochlear implants. We propose a set of experiments to establish the nature of the interaural time difference (ITD) map, its mechanistic dependence on conduction velocity and afferent convergence, and its emergence during development. We focus on barn owls because they are capable of great accuracy in detecting ITD and are thus a model for studies of sound localization. Sensitivity to ITD first appears in nucleus laminaris (NL), a brainstem auditory nucleus with a similar function to the mammalian medial superior olive. In this application, four aims focus on ITD circuits in NL. We begin with studies of how delay lines create maps of ITD. We will measure conduction delays in NL, and develop a linear model of the delay lines to determine how conduction velocities can account for the map of ITDs. We will then use our investigation of the adult map of ITD to determine how ITD coding develops in young barn owls, animals whose head grows extensively after hatching. In our third aim, we will examine coincidence detection mechanisms that underlie sensitivity to ITDs. Recent intracellular recordings from NL neurons in vivo show that tonal stimuli induce an oscillatory membrane potential in the NL cell. These membrane oscillations must originate from phase-locked synaptic inputs from NM fibers, although how presynaptic, synaptic and postsynaptic properties affect the formation of this potential remains unclear. We will combine morphological and theoretical analyses to test the hypothesis that large numbers of phase locked synaptic inputs are necessary for the formation of the membrane potential oscillations in NL. The fourth and last aim of this application uses a comparative approach to experimentally address the current controversy between map-like place codes in birds vs. rate-based population codes in mammals. In the broader context, understanding the mechanisms and evolution of ITD coding reveals common computational solutions that can guide technological advances.
The noteworthy similarities and the differences between sound location processing in birds and mammals shows there are many ways of localizing sounds and separating sources from background noise;these different strategies can inspire the development of new technologies such as cochlear implants. Current binaural implants work independently, so that the timing of pulses is not synchronized or coordinated between the ears, and much of the fine-grained temporal structure required for effective ITD processing is not preserved. In the long term, it may be beneficial to develop bionic devices that can extract ITD and ILD information at high precision over a wide frequency range, and recode it in the manner appropriate for each individual.
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