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More than Words : Lexical Processing During Sentence Comprehension in Broca's Aphasia


Though Broca's aphasia is traditionally defined as an expressive language impairment, listeners with Broca's aphasia (LWBA) typically evince sentence comprehension deficits as well. These comprehension deficits are characterized by difficulty understanding certain types of sentences that contain complex syntax. While some research proposes that the source of the comprehension disorder can be attributed to a syntactic processing delay, other research argues that syntactic processing impairments are secondary to primary lexical processing impairments. The Delayed Lexical Activation Hypothesis (DLA) suggests that a slowed lexical activation system results in lexical information "feeding" syntactic processing too slowly, leading to a mismatch between processing rate and what is required for fast-acting processing routines. A series of three studies with LWBA are presented to explore (a) if the DLA holds in the face of simple, canonically ordered sentences (Chapter 3), and (b) if lexical access delays can be mitigated through manipulations of speech input rate (Chapter 4) and/or cue based prediction (Chapter 5). Chapter 2 reviews current research with LWBA and details prior empirical evidence supporting the presence of a processing delay in both lexical and syntactic processing. Chapter 3 presents evidence of real-time lexical access during processing of syntactically simple sentences. Results showed LWBA demonstrated a pattern of protracted lexical access as compared to unimpaired controls. Chapter 4 explores if slowed input rate would combine with the purported slowed lexical activation to yield 'on-time' lexical access patterns. While a LWBA group effect of rate was not found, interesting patterns emerged when considering individual patterns of brain damage. It was found that the proportion of damage to a brain region of interest implicated in lexical, but not syntactic processing, significantly predicted the effect of rate of speech on the time-course of lexical access. Finally, Chapter 5 investigates if LWBA demonstrate predictive processing by showing 'on-time' access with contextual cues that unimpaired listeners have been shown to use - biased adjectives. LWBA were able to use semantic, not structural, cues to mitigate a lexical access delay. These results taken together support the DLA hypothesis; that a lexical access delay underlies the comprehension disorder in LWBA

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