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Poster C39, Friday, August 17, 10:30 am – 12:15 pm, Room 2000AB

Neuromodulatory effects of individualized tDCS on MEG dynamics in chronic post-stroke aphasia

Priyanka Shah-Basak1, Gayatri Sivaratnam1, Selina Teti1, Alexander Francois-Nienaber1, Tiffany Deschamps1, Jed Meltzer1;1Rotman Research Institute, Baycrest Health Sciences, Toronto, ON Canada

Research on the modulatory effects of transcranial direct current stimulation (tDCS) on neural oscillatory activity is in its infancy. Despite a large body of evidence suggesting significant behavioural gains in post-stroke aphasia with tDCS treatments, the neuromodulatory oscillatory changes in response to tDCS are largely unknown. In the current, serial tDCS-magnetoencephalography (MEG) study, we investigated language task-induced changes in oscillatory activity in individuals with aphasia after a single-session of excitatory (anodal) tDCS to left-hemisphere perilesional cortex, or inhibitory (cathodal) tDCS to the right-hemisphere homolog area. Eleven chronic stroke survivors with aphasia (mean age: 58.8±12.8 years; 7 males; years since stroke: 8.0±5.2; 3 global, 4 nonfluent, 2 fluent and 2 anomic) underwent 20-minutes of 2mA anodal- or cathodal-tDCS, or sham-tDCS during three separate sessions. Stimulation was delivered using high-definition tDCS (HD-tDCS) in a 3x1 center-surround electrode configuration. The location of the central electrode was determined individually, based on patients’ resting-state MEG abnormalities, which manifest as increased slow-wave activity in the perilesional areas. For baseline and post-tDCS language performance assessments and for training during stimulation, we used the graded repetition exercises for aphasia therapy (GREAT) battery. The difficulty level of GREAT (easy, hard) was adjusted to ensure that patients with varying levels of language impairment were equally engaged during the task. Inside the MEG, patients performed a delayed word-repetition task, prior to and after each tDCS session. This task involved reading a word and saying it after a delay. To delineate differential effects of tDCS based on lexical variables, we included low- and high-frequency words consisting of either three or one syllable(s). The oscillatory responses during word encoding were localized at the whole-brain level and compared across tDCS conditions for the low-beta (8-30Hz) and low-gamma (25-50Hz) bands. TDCS sites were localized to either the middle/superior temporal or the superior/inferior parietal areas. GREAT accuracy increased in patients receiving the hard battery (or those with less severe impairment) after anodal-tDCS (p=0.026), and there was a trend after cathodal-tDCS (p=0.15), compared to sham. No such improvement was found in patients receiving the easy battery (or those with more severe impairment). For the MEG task, accuracy and reaction time were not affected by tDCS. Overall accuracy, however, was lower for longer length (p=0.003; 13.5%) and lower frequency (p=0.005; 6.6%) words. On average, low-beta oscillatory activity was localized to bilateral posterior, temporo-occipital areas, and low-gamma activity to bilateral anterior, inferior frontal areas. After anodal-tDCS, event-related desynchronization (ERD) power was decreased in the low-beta band in the right precentral gyrus and cingulate gyrus (p<0.01). Whereas, after cathodal-tDCS ERD was increased in the low-gamma band in the left inferior frontal and superior/middle temporal areas, and bilateral middle frontal gyri (p<0.01). Our findings indicate that the hemisphere contralateral to stimulation is most affected, and the direction of oscillatory changes points to induction of left-hemispheric involvement after both types of tDCS. This study is the first to demonstrate oscillatory changes as a function of excitatory and inhibitory tDCS in post-stroke aphasia.

Topic Area: Language Disorders

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