The objective of this project is to clarify mechanisms important for the perception of speech by examining electrophysiological responses in monkey primary auditory cortex (A1). People with developmental language deficits often have perceptual dysfunction in the processing of speech and other sounds. Relating these deficits to dysfunction of specific neural events requires an understanding of normal processes best accomplished by intracranial recordings not feasible in humans. Monkeys are a reasonable model because they share many features of phonetic perception present in humans. Multiunit activity, evoked potentials and current source density will be used to measure the activity in A1. These procedures yield stable indices of the synchronized neural activity required for complex sound encoding, and afford linkage with homologous responses in humans. This project will address two main problems in speech perception: (1) the relationship between the acoustic speech signal and its neural and phonetic representations, and (2) how speech is encoded in the brain when it must compete with other environmental sounds. The first problem, a key roadblock in our understanding of speech perception, is examined by defining the representation of speech when changes are made in the acoustic context of the sounds. The second problem is examined by clarifying mechanisms of auditory scene analysis, a fundamental process of speech perception that determines whether overlapping sound components are perceptually segregated into discrete sources, or merged into unified auditory images. These problems will be addressed by testing three key hypotheses relating to the neural mechanisms underlying different aspects of auditory scene analysis, and two hypotheses related to the neural encoding of consonants and vowels when presented in dynamically changing contexts. Testing these hypotheses will require that the neural representation of speech and other sounds parallel key features of human perception. Identifying the neural mechanisms in A1 involved in speech encoding will clarify normal mechanisms of language processing, and serve as a benchmark to evaluate dysfunctional mechanisms associated with abnormal language development.
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