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Psychophysics and Neurophysiology of Stream Segregation in the Cat

Abstract

Listeners have the remarkable ability to disentangle multiple competing sound sequences and organize this mixture into distinct sound sources. A previous study in human listeners has shown that the physical separation between sounds aids in “segregating” between sound sources, whereby sounds located further apart in space are more easily segregated. Furthermore, under anesthetized conditions, animal neurophysiology has been found to parallel conditions in which humans hear one stream or multiple streams. The goal for this dissertation is to evaluate the psychophysics of spatial stream segregation and, in the same species, record neural activity in auditory cortex in the absence of anesthesia. Cats have been used extensively in auditory research due to their well-developed auditory cortex and because they have evolved accurate sound localization ability to support their nocturnal predatory behavior. We developed a novel paradigm testing the spatial resolution of stream segregation in cats to measure psychophysical performance (Chapter 2) and to uncover the spatial cues that are utilized by cats to perform this task. We then implanted chronic electrodes into primary auditory cortex to record single- and multiple-unit neural activity in awake cats (Chapter 3 and Chapter 4). Our findings show that: (1) Cats can segregate streams of broadband sounds with spatial acuity approaching that of humans. In addition, performance was consistently better for high than for low frequencies which is consistent with previous cat physiological results but contrary to human psychophysics. (2) In the absence of anesthesia, neurons in cat cortex exhibit spectral and temporal properties which are not seen in anesthetized preparations but accord with previous observations in unanesthetized marmosets. (3) Lastly, neurons in auditory cortex of awake cats that are not engaged in an overt auditory task exhibit considerably weaker stream segregation than is observed in anesthetized preparations. Although there is little evidence for stream segregation while cats are not engaged in an auditory task, it may be that segregation is influenced by selective attention or that spatial stream segregation is processed in cortical areas beyond A1. Overall, these findings provide insight into auditory mechanisms underlying stream segregation and neural properties of the unanesthetized cortex.

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