Date of Award

Summer 2022

Document Type

Open Access Dissertation

Department

Communication Sciences and Disorders

First Advisor

Julius Fridriksson

Abstract

Identifying effective and efficient rehabilitation tools is crucial to improve language outcomes for persons living with chronic aphasia. Speech entrainment has proved to be particularly successful in improving speech output in nonfluent aphasia. It is hypothesized that, for patients with aberrant oscillatory synchronization between anterior and posterior language regions of the left hemisphere, speech entrainment may act as an external gaiting mechanism to bolster an impaired efference copy and improve synchrony between these regions. Theoretical and empirical evidence supports this idea that speech production relies on anterior-posterior connectivity in the left hemisphere.

Transcranial alternating current stimulation (tACS) delivers low, periodically-alternating currents to improve functional connectivity between targeted brain regions through the amplification and entrainment of endogenous oscillations. Previous work suggests that in-phase tACS (alternating current with 0° relative phase difference) improves behavioral performance while anti-phase stimulation (current delivered with 180° phase difference) results in impaired behavioral performance secondary to impeded network synchronization.

The goals of the present investigation were to determine: 1) if HD-tACS boosts behavioral outcomes (as measured by speech fluency and timing in a speech entrainment task) and 2) the extent to which stroke-induced damage predicts HD-tACS-induced effects on fluency. This was a preliminary and proof-of-concept study in which high definition transcranial alternating current stimulation (HD-tACS) was paired with a speech entrainment paradigm in a cohort of persons with nonfluent aphasia. 1 mA of HD-tACS at a theta frequency (7 Hz) was applied to anterior and posterior regions of the left hemisphere across three stimulation conditions: 1) in-phase stimulation, 2) anti-phase stimulation, 3) sham.

Group level analyses failed to support these hypotheses. Although not statistically significant, the primary behavioral outcome measure (the proportion of correct script words) and secondary measures such as the number of total correct words demonstrated a higher median for the in-phase condition while the number of speech errors was higher during the ‘anti-phase’ condition. Spectral-temporal analysis used mel-frequency cepstral coefficients in a dynamic time warping algorithm to examine the temporal distance between the AV model and patient productions during the task. Results suggest that patients’ speech was better entrained (as evidenced by a smaller distance between the model and participant) during the in-phase stimulation condition as compared to sham.

Retrospective neuroimaging data suggest that patients who demonstrated better behavioral performance during the in-phase stimulation, had greater preservation of the inferior temporal gyrus (z = 4.26) and poorer coherence as measured by rsfMRI between anterior and posterior regions (inferior frontal gyrus, pars opercularis to middle temporal gyrus; z = -2.51).

The current study relied on a network approach and, for the first time, introduced alternating electrical current stimulation to synchronize anterior-posterior language regions in the left hemisphere. Preliminary data are encouraging and suggest that tACS may improve speech output for some speakers with nonfluent aphasia during a speech entrainment task. Data also suggest improved temporal alignment (entrainment) for some participants with aphasia during the in-phase stimulation condition. These pilot data contribute to a growing body of research that applies noninvasive brain stimulation as an adjuvant to speech-language therapies and further inform how external modulation may facilitate neural plasticity in stroke survivors.

Rights

© 2022, Lynsey McGrath Keator

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