The basic mechanisms underlying speech perception are unclear. The human auditory system regularly processes highly acoustically variable inputs into invariant representations of speech- a critical function for human communication. In order to better understand this property, I propose to investigate the cortical representations of speech sounds during categorical perception (CP). CP occurs when a change in a variable such a phonemic contrast along a physical continuum is perceived, not as gradual but as an instance of discrete category. Previous functional neuroimaging research has implicated the superior temporal gyrus and sulcus of the lateral temporal cortex in phonemic processing, but not the precise means by which the cortex represents those sounds. During a one-year training program, I plan to use electrocorticography (ECoG), the direct application of electrodes to the brain surface for recording cortical activity, to examine categorical speech processing in the superior temporal gyrus. I will utilize a high-density ECoG array and signal analysis techniques during passive listening to speech stimuli in six epilepsy patients with chronic implanted subdural grids. Phonetic discrimination and identification behavioral tasks will be performed pre-operatively, and in additional healthy normal subjects, to determine boundaries for CP transitions. Phoneme stimuli will focus on three contrasts with well-known categorical psychophysical properties: /b/ vs /d/, /r/ vs /I/, and /d/ vs /t/. We hypothesize that a highly specific spatiotemporal pattern of evoked potential and/or neural oscillatory activity will reveal an emergent categorical representation of speech phonemic contrasts which closely reflects behavioral thresholds, as opposed to a continuously linear representation of acoustic parameters. An improved understanding of speech processing mechanisms has direct implications for the origin and remediation of communication disorders, including autism, dyslexia, and language learning impairment. ? ? ?
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