This dissertation investigates the factors that contribute to the sentence comprehension deficits exhibited by individuals with a particular acquired language disorder, Broca’s aphasia. Individuals with Broca’s aphasia (IWBA) have no difficulty comprehending sentences that conform to canonical (S-V-O) word order. However, difficulty arises for non-canonical sentences that contain syntactic dependencies. There are several accounts that attempt to explain these deficits. The focus of these accounts differs, some focus on syntax, others on lexical level deficits, and others on reduced cognitive resources. The current dissertation offers and focuses on a more recent account of the sentence processing deficits in IWBA, the Intervener Hypothesis (IH).

The IH suggests that difficulty comprehending sentences that contain syntactic dependencies arises when comprehension requires the crossing of an intervening noun phrase (NP), or intervener. The intervener causes similarity-based interference (SBI) because it is a possible element in the syntactic dependency. This dissertation tests the IH using sentences that contain a syntactic dependency inherent to unaccusative verbs.

Chapter 2 outlines pertinent linguistic concepts. Chapter 3 provides an overview of aphasia. Chapter 4 reviews psycholinguistic literature focusing on verb processing. Chapter 5 describes the sentence comprehension deficits observed in aphasia and theoretical accounts of these deficits. Chapter 6 details the account this dissertation focuses on, the IH. Chapter 7 investigates the time-course of lexical reactivation of unaccusative verbs during sentence processing. The results revealed that IWBA exhibit a time-course of lexical reactivation similar to neurologically unimpaired age-matched control (AMC) individuals indicating that IWBA are sensitive to the unaccusative property. Chapter 8 evaluates the IH by comparing final comprehension of sentences that contain unaccusative verbs in sentence constructions that contain an intervener and ones that do not. For IWBA, chance comprehension was found for sentences that contained an intervener suggesting SBI interferes with comprehension. Chapter 9 further investigates the pattern of comprehension observed in Chapter 8 using a real-time methodology, and also includes the first semantic manipulation of an intervener. The results revealed that SBI is observed during real time sentence processing of sentences that contain an unaccusative verb and an intervener, and that animacy reduced observed SBI.

This dissertation explores how listeners extract meaning from personal and reflexive pronouns in spoken language. To be understood, words like her and herself must be linked to a prior element in the speech stream (or antecedent). This process draws on syntactic knowledge and verbal working memory processes. I present two original research studies investigating moment-to-moment sentence processing and final comprehension of sentences containing proforms in children and adults with and without language impairments.

Chapter 2 provides an overview of syntactic constraints and psycholinguistic evidence from unimpaired listeners in addition to an introduction to the language impaired populations. Chapter 3 reviews psycholinguistic evidence of similarity-based interference in verbal working memory during unimpaired language processing before presenting a recent account – the Intervener Hypothesis – that points to similarity-based interference as a source of processing and comprehension difficulty in language impaired populations.

Chapter 4 presents a series of experiments in children with typical language development (TLD) and children with a developmental language impairment – specific language impairment (SLI). The processing patterns of the two participants groups reveal qualitative differences and children with SLI demonstrate increased comprehension difficulty relative to TLD children. Chapter 5 presents a series of experiments in adults with an acquired language impairment – agrammatic Broca’s aphasia – and age-matched control participants. Results reveal increased processing and comprehension difficulty of sentences containing pronouns relative to reflexives for individuals with Broca’s aphasia.

The processing patterns demonstrated by unimpaired child and adult participants reveal a number of striking similarities. In addition, the processing patterns of children with SLI mirror those of adults with Broca’s aphasia. Although the etiology of language impairments differs in these groups, children with SLI and adults with Broca’s aphasia share many of the same language processing difficulties. As such, I evaluate the potential of a single theoretical framework (the Intervener Hypothesis) to account for the patterns observed in each group. In addition, I evaluate how processing and comprehension patterns change across the lifespan in participants with unimpaired language and the extent to which processing and comprehension patterns overlap.

Auditory sentence processing is astonishingly complex and involves the rapid processing and integration of many different forms of information. While this is seemingly effortless for neurologically unimpaired listeners, it is clear from the literature that brain damage can cause the normal language system to be disrupted in specific and testable ways. One major goal of this dissertation is to describe how the system is fractionated in aphasia by focusing on the time-course of using specific information types that appear to be involved in the unimpaired language system.

A series of sentence processing studies are presented exploring the impact of syntactic structure in neurologically unimpaired listeners and in listeners with Broca’s aphasia (Chapter 3), the impact of thematic fit and prosody in college-age adults (Chapter 4) and individuals with aphasia along with a group of age-matched healthy controls (Chapter 5).

Chapter 2 reviews research on sentence processing, and accounts of the sentence comprehension deficit in aphasia are also discussed. Chapter 3 provides evidence that similarity-based interference, which results from certain syntactic structures, contributes to the sentence comprehension deficit in aphasia. Chapter 4 examines how thematic fit/plausibility and prosody impacted syntactic structure building in college-age adults using event-related potentials (ERPs). Results revealed that the parser was able to use thematic fit/plausibility information to predict upcoming syntactic structure before the critical verb. Syntactic reanalysis was triggered at the critical verb in sentences with incongruent prosody and no plausibility cue. Chapter 5 examined how individuals with Broca’s aphasia and age-matched controls use plausibility and prosodic cues. The results from the age-matched controls were nearly identical to the college-age adults (Chapter 4). However, the group of individuals with aphasia with a less severe comprehension deficit could predict upcoming syntactic structure when provided with a plausibility cue, but without a plausibility cue had difficulty integrating prosody with syntactic structure. Those with a more severe comprehension deficit had difficulty integrating prosodic and lexical-semantic cues with syntactic structure. Thus, similarity-based interference, lexical-semantic processing and prosody are all implicated in the sentence comprehension deficit seen in individuals with Broca’s aphasia.