The Behavioral Subproject will determine the classification and evaluate critical features of both natural and synthesized communication signals in nonhuman listeners, and explore the ontogenetic development of sensitivity to these signals. Stimuli will be selected or abstracted from monkey communication calls and from human speech, and critical features of those stimuli will be determined behaviorally by using detection, discrimination, classification and lateralization procedures that have been developed and refined in this laboratory. An overall hypothesis of this work is that, while the mammalian auditory system has generalized mechanisms that can classify communication signals, specialized processing mechanisms also exist in the auditory system that process conspecific communication signals more efficiently than do the generalized processors. We further suggest that exposure to a normal neonatal auditory environment is essential for species-typical postnatal perception of these signals. Experiments will be conducted in three major areas. In the first, the ability of nonhuman primates will be determined for detection of changes in the amplitude and phase relationships of components of complex sounds. These experiments will use computer generated signals consisting of multiple harmonics with an amplitude spectrum resembling those of vowel sounds and will help identify those acoustic features which promote the discriminatory of such speech-like sounds. The premise of these experiments is that many communicatively relevant signals of human and nonhuman primates share certain properties and are processed by similar mechanisms in both species. In the second set of experiments, the discrimination and classification abilities of nonhuman primates will be evaluated using both conspecific and heterospecific communication signals. These experiments will test the hypothesis that mechanisms have evolved which permit each species to process its own communication signals more efficiently than those of other species. These studies also will utilize computer-synthesized versions of the natural calls to isolate those features of the signals which are critical for discrimination and classification. The third set of experiments will explore the effects of limiting the auditory environment of neonatal chinchillas on the subsequent discriminability and classification of auditory events. These experiments will evaluate the hypothesis that while mammalian species share common mechanisms for processing communication signals, exposure to a normal auditory environment is essential to validate the development of such mechanisms. In sum, these experiments will contribute to our understanding of the conditions which enhance the efficient processing and perception of communication signals in mammals.
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