Listeners have a remarkable ability to perceptually segregate interleaved sequences of sounds in complex auditory environments, a process referred to as stream segregation. Previous studies of psychophysical measures of this process in humans and the cat animal model have shown that physical separation of sound sources aids in the process of segregating competing noises. This finding has also been observed in the physiological response of the anesthetized cat primary auditory cortex, wherein neurons tend to synchronize to one of two competing sound sequences differing in their source location. The goal of this dissertation is to evaluate the coding properties of the awake cat auditory cortex and explore the neural mechanisms underlying stream segregation using spatial cues. The cat animal model has been used extensively in auditory research due to their well-developed auditory cortex, and their sound localization abilities to support nocturnal hunting behavior. Here, we trained cats on a psychophysical spatial stream segregation task (Chapter 1). Then, we implanted cats with chronic neural electrodes to record single- and multi-unit activity in the primary auditory cortex. We studied spectral and temporal coding properties of primary auditory cortical neurons in awake cats (Chapter 2), and spatial stream segregation in the response of cortical neurons in off-task conditions (Chapter 3). Our findings show that 1) cats can segregate competing interleaved noise bursts with spatial acuity matching prior measurements in feline and human listeners. 2) Neurons in the primary auditory cortex of awake cats display sharp frequency tuning, temporally dynamic responses, and synchronous and non-synchronized coding of stimuli with high repetition rates—properties that have only previously been observed in the awake marmoset cortex. 3) Neurons in the auditory cortex of cats not overtly engaged in a stream segregation task show weaker segregation than is observed in anesthetized preparations. It may be that presenting stimuli with faster rates, or other mechanisms such as selective attention, are necessary to observe stream segregation in the awake cortex. Further, it may be possible that spatial stream segregation is processed in cortical areas beyond A1. Overall, these findings provide insight into the coding properties of the auditory cortex in awake cats and the neural mechanisms underlying stream segregation using spatial cues.

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.