The primary auditory cortex (AI) of mammals shows several superimposed functional organizations when explored with simple signals such as pure tones. Basic spatial organizations have been described now for stimulus frequency, bandwidth, spectral envelope, frequency modulation, latency, response threshold, and binaural interaction. The consequences of these organizations for the cortical representation of complex signals, in particular elemental speech signals, is of special interest since similar principles may provide the basis for the perception and categorization of speech in humans. The representational principles will be explored with elemental speech signals in AI of speech squirrel in AI of squirrel (Saimiri sciureus) naive to the sounds and animals that have acquired behavioral affinity to the signals. The behavioral relevance of the studied complex signals will be established by engaging the animals in a psychophysical task of signal discrimination and generalized classification. The task will be performed under varying stimulus and environmental conditions such as by using different stimulus intensities, fundamental frequencies, and to utilize generalized classification schemes that operate independent from signal level and background conditions, thus approaching human discrimination and classification abilities. Determining the cortical encoding of elemental speech sounds on conjunction with reverse-correlation analysis of spectro-temporal receptive fields in naive and highly trained animals will illuminate basic attributes of complex signal representations as well the refinement of coding attributes with learning to discriminate and classify the signals. Recording of cortical activity from animals while they are being trained to discriminate speech sounds will let us follow the time course of plastic changes and establish physiological correlates of perceptual discrimination and detection thresholds. The emergence of refined spatial-temporal patterns of cortical activity by learning-induced plasticity and their relationship to perceptual capacities of the animals provides a basic cellular model of speech sound representation.
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