The overall goal of this research program is to gain an understanding of the neural mechanisms underlying coherent perception of sound patterns and direction. The experiments outlined in this proposal are designed to examine the effect of sound direction on neural coding of complex sound patterns. We plan to address three specific questions: (1) Are the neural processes which underlie sound localization and pattern recognition mediated by two different neural populations? (2) Do response selectivities to complex sound features vary with sound direction? (3) Suppose the response function to different sound features depends upon the sound direction, what may be the underlying basis? Single unit recordings will be made from the frog's superior olivary nucleus and tonus semicircularis, two brainstem structures in the frog nervous system which are essential for encoding complex acoustic features and sound direction. Acoustic stimuli consisting of artificially simulated natural sounds (i.e., amplitude modulated tones and noise), tape-recorded natural sounds and bursts of pure tones will be presented through a free field loudspeaker at various azimuths. Auditory responses to these stimuli under different incident angles will be quantitatively analyzed to test various hypotheses related to the above questions. The frog auditory system is chosen for this study because: (a) acoustic communication behavior of frogs is well understood,k and frog's vocal repertoire is small and that calls are stereotyped, (b) acoustic features which are essential for communication have been identified and therefore an investigation of extraction of these features by the brain would be a fertile research avenue, (c) coherent perception of complex sounds is particularly germane to frogs for whom the survival of the species depends upon the females having the ability to determine """"""""which sound comes from where"""""""", and (d) there is a large body of knowledge on the anatomy and physiology of the frog auditory system. The principles gained from this study will shed some light on the mechanisms underlying coherent perception of acoustic """"""""image"""""""" (spatial and pattern).
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