SPEECH AND LANGUAGE PROCESSING IN APHASIA

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The broad goals of this research are to identify and delineate the processes and mechanisms and the neural substrates underlying speaking and understanding by investigating speech and language processing deficits in aphasic patients. The major focus of research is on sound structure and the lexicon and the processing stages that map sound to meaning and meaning to sound. With respect to the processing stages that map sound to meaning, it is hypothesized that the deficits of aphasic patients stem from the extent to which sound structure elicits activation in the lexicon. Two potential sources of such impairment will be investigate, one focusing on activation patterns within the lexicon itself and the other on the mapping from sound structure to lexical form. The influence of lexical competition on lexical processing will be investigated by exploring the effects of phonetic category 'goodness' and its influence on the activation of its lexical competitor, the effects of lexical density and frequency on lexical decision latencies, and rhyme and cohort effects on the time course of spoken word recognition. The mapping from sound structure to the lexicon will be investigated by exploring the influence of various changes to the acoustic input on lexical access including the influence of acoustic distortion and indexical features such as voice pitch and speaker. Various methods will be used including lexical decision, semantic priming, discrimination, and eye tracking. With respect to the processes and mechanisms contributing to speech production, it is hypothesized that the speech output deficits of anterior aphasics including Broca's aphasics have as their basis impairments in articulatory implementation. To further map out the nature of this impairment, three hypotheses will be tested: 1. It is proposed that deficits will emerge in the production of those sound segments that require the complex integration of articulatory movements over short periods of time. The production of manner of articulation including stops, affricates, fricatives, and glides will be investigated; 2. It is proposed that deficits in coarticulation will emerge in larger and more complex linguistic contexts, specifically across word boundaries. The influence of vowel coarticulation on a preceding consonant and consonant articulation on a preceding vowel will be explored; 3. It isproposed that phonemic paraphasias, i.e. sound substitutionerrors, produced by anterior aphasics have a phonetic orarticulatory basis. In all of these production studies,acoustic analyses are conducted of speech productions ofboth anterior and posterior aphasic patients to infer thearticulatory states giving rise to the acoustic patterns.