Cellular Mechanisms Underlying Sound Localization

Univ Of Maryland, College Park, College Park MD

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Abstract

The microsecond temporal precision shown by auditory systems is essential for location and decoding of sound. This grant examines the neural circuitsthat encode and process temporal information in the barn owl and other birds in order to identify shared, convergent features of temporal coding circuits. Temporal information is processed in the cochlear nucleus magnocellulads, which projects to the nucleus laminaris (NL), where interaural time differences (ITDs) are first computed. Since precise temporal coding in the inputs to NL is critical to the detection of ITDs, we will determine how phase-locking changes during circuit development. These studies will be carded out in parallel with studies of features associated with temporal coding such as the regulation of myelination, the expression of K+ channel and NMDAR subtypes. What is needed for good ITD detection? ITD coding neurons generally possess bipolar morphology in both birds and mammals, with inputs from each ear segregated onto dendritic trees. Since this segregation improves ITD coding in modeling studies, we will use in vitro analyses to look for the predicted dendritic non- linearities in coincidence detector neurons. The ham owl's sound localization is more accurate than a chicken's. We will use in vivo recordings to determine if this is due to computational power (i.e. more neurons devoted to a particular ITD computation) and/or to specific improvements in the responses of individual neurons. ITD detection circuits are made up of delay lines and coincidence detectors. Delay lines are more accessible in the barn owl than most other animals because their organization in NL is regular. We will measure click-evoked delays to determine the exact nature of the map of interaural delays, and we will describe delay line projections in high and low best frequency regions of several bird species. The similarity in physiological responses in the cochlear nucleus angularis and mammalian cochlear nucleus suggests convergent evolution of cells specialized for encoding relevant features of the auditory stimulus. These hypotheses will be tested in a study that correlates NA cell morphology with response types, and a study that measures synaptic transformations within NA.