Neocortical Mechanisms of Categorical Speech Perception

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DESCRIPTION (provided by applicant): The basic mechanisms underlying comprehension of spoken language are unknown. For example, how does the brain extract the most fundamental linguistic elements-consonants and vowels, or the distinctive features which compose them-from a complex and highly variable acoustic signal? An investigation of the cortical representation of speech sounds during categorical perception can likely shed light on this fundamental question. Categorical perception occurs when a change in a variable such as phonemic contrast along a continuum is perceived, not as a gradual function but rather as a discrete category change (for instance, from "pa" to "ba"). Previous research has implicated the superior temporal cortex and also motor cortex in the processing of phonemes. However, how those areas actually represent (i.e. code) phonemes is undetermined, mainly due to limitations of non-invasive recording techniques. Electrocorticography (ECoG), the direct application of electrodes to the brain surface for recording cortical activity, is a promising approach since it can provide high spatial and temporal resolution. In this study, I propose to examine the mechanisms of categorical speech processing by utilizing direct high-density ECoG recordings from the human neocortex during neurosurgical procedures. The principal focus of the mentored K99 phase will be to elucidate the emergent representation of phonemes in the superior temporal gyrus that underlies categorical perception. Training in high-density ECoG, advanced signal processing and source reconstruction methods, and direct electrocortical stimulation will be acquired to achieve this aim. These approaches will then be applied during the independent R00 phase to investigate the role of the motor cortex in speech perception- specifically, 1) to determine its functional organization during phoneme perception and production, 2) to determine its functional connectivity with auditory speech cortices, and 3) to determine its essential and causal role in categorical speech perception. PUBLIC HEALTH RELEVANCE: The aim of this research is to reveal the fundamental mechanisms that underlie comprehension of spoken language. An understanding of how speech is coded in the brain has significant implications for the development of new diagnostic and rehabilitative strategies for language disorders (e.g. aphasia, dyslexia, autism, et alia). Abnormal perception of phonemes is a central component to language disability in all of these conditions.