The overall goal of this research is to understand the auditory cortical mechanisms involved in spatial localization of complex sound. To help achieve this goal, we have implemented an earphone delivery system that mimics, at the eardrum of a cat, a sound coming from particular directions in space. The stimulus set constitutes a 'virtual acoustic space'. Using this approach, in conjunction with other stimulus paradigms, the proposed research has four specific objectives. First, we will continue our studies of directional sensitivity of single neurons in primary auditory cortex (AI), in which we map 'virtual space receptive fields' (VSRFs, i.e. aggregations of virtual-space loci effective in altering a neuron's discharge). Emphasis will be on monaural contributions to the VSRF, internal time and rate structure of the VSRF, relative contributions of interaural time, intensity and spectrum on directional sensitivity and sensitivity to simulated sound motion and stimulus complexity. We will study cortical mechanisms of the precedence effect. Second, we will study directional sensitivity of neurons in surrounding anterior (A) and posterior (P) leminiscal cortical fields, using paradigms similar to those used in AI studies. Results using simple stimuli suggest that fields A and P are good candidates for high spatial selectivity or stimulus-motion detection. Third, we will study directional sensitivity of cells in the anterior ectosylvian auditory field (AES). AES receives polysensory input and projects to the superior colliculus where auditory and visual maps have been located. Fourth, we will study the relative contributions to a neuron's directional sensitivity of direct thalamic and indirect cortico- cortical input. This will be accomplished through single-unit recording combined with reversible transmission blockade by local cortical cooling. Experiments are carried out under anesthesia, which allows necessary control of acoustic parameters, identification of cortical fields and accurate placements of microelectrodes. Acoustical calibration, stimulus synthesis and delivery, and data collection and analysis are all under computer control. There is ample behavior evidence that auditory cortex is essential to normal sound localization behavior. The experiments proposed will provide increased knowledge of the neural mechanisms involved, which is essential to the diagnosis, management, and treatment of central auditory disorders involving cerebral cortex.
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