Date of Award

12-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Speech and Hearing Science

Track

Speech-Language Science

Research Advisor

Devin M. Casenhiser, PhD

Committee

Jessica S. F. Hay, PhD; Eun Jin Paek, PhD; Kevin Reilly, PhD; Tim Saltuklaroglu, PhD

Keywords

Agrammatism;Aphasia;Cortical Tracking;EEG;Language;Neural Synchrony

Abstract

Purpose. People with aphasia following stroke commonly exhibit marked difficulties in sentence comprehension, referred to as receptive agrammatism. However, theories of receptive agrammatism differ in their assumptions regarding the underlying mechanisms of impairment. Representation-based theories ascribe receptive agrammatism to a loss of grammatical knowledge. Processing-based theories suggest that individuals with receptive agrammatism have intact grammatical knowledge but may possess insufficient neurocognitive resources to apply this knowledge within cognitively demanding contexts. In this study, we used electroencephalography (EEG) to investigate whether these perspectives may be disambiguated by the neural activity profiles of people with aphasia during comprehension. For this purpose, we examined aphasia-related changes in cortical “tracking” of phrases—which has been suggested as a neural correlate of successfully encoded grammatical content. By considering cortical tracking alongside subject-specific performance on a set of clinical assessments, we aimed to identify which cognitive-linguistic mechanisms best explain the patterns of impaired sentence comprehension presented by individuals with receptive agrammatism.

Methods. Nine individuals with post-stroke aphasia and nine age- and education-matched neurologically healthy controls participated in this study. All were native English speakers. Language abilities were assessed using the Quick Aphasia Battery, and working memory was assessed using a modified listening span task. Following assessment, we recorded EEG as participants listened to sentences that varied in their syntactic and semantic complexity. After each sentence, participants were asked to identify pictures that matched the sentence they had just heard. Cortical tracking was quantified via Mutual Information (MI) between EEG responses and the syntactic features of the sentences. We evaluated how cortical tracking differed between groups and across levels of linguistic complexity, as well as whether these differences predicted participant performance.

Results. Both groups tracked syntactic structure similarly and showed delayed cortical responses for trials where they responded incorrectly. Both groups also showed increased tracking for semantically reversible sentences. However, people with aphasia performed worse on these items than controls, and there was no clear relationship between cortical tracking and task performance. Further investigation revealed that scores on offline clinical assessments of sentence processing were significant predictors of cortical tracking, whereas performance on the sentence-picture matching task was best explained by age, overall severity, and decreases in theta band (4-7Hz) power.

Conclusion. Our findings suggested that people with aphasia may be subtly impaired in their syntactic abilities. Our EEG findings also suggested that patients presented with delayed syntactic processing speeds and increased reliance on phonological working memory when sentences were semantically reversible. Moreover, individuals with aphasia may have possessed insufficient neural resources for post-interpretive task completion, when task-demands were high and sentence content was complex. We interpret these findings as being consistent with processing-based accounts of receptive agrammatism.

Declaration of Authorship

Declaration of Authorship is included in the supplemental files.

ORCID

0000-0002-1703-3840

DOI

10.21007/etd.cghs.2023.0636

Additional Files
2023-021-Rafferty-DOA.pdf (159 kB)
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