Our research proposes to investigate the response properties and functional organization of primary auditory cortex and neighboring areas, in the awake ferret: specifically, the study seeks to determine how timbre, i.e. the shape of the acoustic spectrum, is represented in the single-unit responses in these areas. Our study starts with tones and clicks to characterize cortical neurons and compare their response properties and the maps they form to the results we and others have already obtained in the anesthetized preparation. Nxt, we use broadband stimuli with sinusoidally modulated spectral envelopes (also called ripples), to characterize the purely temporal and purely spectral properties of cells' responses to broadband sounds. The response of cells to these ripples, together with the response to response to combinations of orthogonal ripples are analyzed by data analysis methods based on systems theory which yield auditory units response fields and fully characterize the linear part of their spectral and temporal properties. These methods are founded on the assumption that the auditory responses are linear with respect to the stimulus spectral envelope and its dynamics. Therefore, a fundamental objective of the proposed investigations is to examine the degree of response linearity by determining whether unit responses to complex sounds such as speech can be predicted from their responses to simple ripples. A second objective is to delineate the origin and extent of various types of response nonlinearities such as threshold, halt-wave rectification, saturation, and various forms of adaptation. The separability of the spectral and temporal aspects of the response fields can also be evaluated by comparing the cells response to the different types of sounds. The results that will emerge from this study will enhance our understanding of the encoding of complex acoustic spectra such as speech and music in auditory cortex. More generally, however, they will place auditory cortical processing within the larger framework of visual and other sensory processing in the brain because of the abstract nature of the stimuli and of the theoretical concepts behind them.
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