Presentation

Priyanka Shah-Basak^1,2, Samantha Hudson¹, Priya Balasubramanian³, Anna Freiberg¹, Brian Schmit³, Jeffrey Binder¹; ¹Language Imaging Lab, Department of Neurology, Medical College of Wisconsin, Milwaukee USA, ²Cognition and Brain Stimulation Lab, Department of Neurology, Medical College of Wisconsin, Milwaukee USA, ³Integrative Neural Engineering & Rehabilitation Lab, Marquette University, Milwaukee USA

Recent advances underscore the role of functional network-level impairments in post-stroke aphasia. Stroke induces disruptions in the lesion zone and in functionally connected brain regions far from the lesion. A fundamental mechanism proposed for interregional communication (connectivity) is coordinated or synchronized oscillatory activity between distant brain regions. Disruptions in this synchronized activity can impede relevant information transfer across the language network and contribute to language impairments in aphasia. Our current work focuses on examining whether synchronized oscillatory activity can be manipulated in the language network using high-definition transcranial alternating current stimulation (tACS), and whether it is possible to restore normal synchronization to induce language recovery in post-stroke aphasia. One mode of oscillatory communication is via synchronization of phases (systematic phase differences between two or more anatomically distinct regions), which is hypothesized as a mechanism by which distant task-relevant brain regions integrate information across a network. For example, theta phase synchronization between prefrontal and temporal areas has been shown to support resource monitoring and storage in visual working memory. tACS was shown to directly interact with ongoing, task-relevant oscillatory activity and to influence the phase synchronized activity between stimulated regions. The behavioral effects depend on tACS phase and frequency, but these have not been systematically evaluated in the context of higher-level language processes. We evaluated the ability of tACS to modulate picture naming performance in 4 older healthy individuals (age range 61 to 69; 2 women) and 2 stroke survivors with aphasia (men; age 56 and 44 years). We tested our tACS setup with sham, in-phase (0-degree phase difference) and anti-phase (180-degree difference) modes using theta (4-7Hz) and alpha (8-12Hz) frequencies, targeting inferior frontal and parietal regions. During and immediately after tACS, participants engaged in a picture naming task including control trials with scrambled pictures, with concurrent collection of EEG. TACS was applied at 1mA peak-to-peak amplitude using two 3x1 center-surround montage configurations placed over left frontal and parietal regions. To assess the phase and frequency-specific effects of tACS, we computed imaginary coherence, a measure of phase synchronization, between stimulated regions on the post-tACS EEG data. We hypothesized improvement in naming accuracy and/or reaction times and increased synchronization with in-phase tACS, and diminished performance and reduced synchronization with anti-phase tACS, compared to sham-tACS. No major side effects were reported with tACS. Participants could not distinguish sham-tACS. In-phase alpha-tACS increased alpha synchronization, and anti-phase theta-tACS decreased theta synchronization between the targeted frontoparietal regions compared to sham-tACS. Naming accuracy in response to real pictures increased in 2 healthy controls and the stroke patient receiving anti-phase theta-tACS. Two healthy controls and the stroke patient who received in-phase alpha-tACS showed increased accuracy. Increase in accuracy after anti-phase theta-tACS was unexpected. Suppression of frontoparietal theta synchronization with anti-phase tACS may have enhanced synchronization with another, more pertinent region(s) in the network. Our preliminary results indicate the feasibility of tACS to manipulate synchronized oscillatory activity underlying language processes. The relationship between tACS phase and frequency may be non-linear, depending on the ongoing activity in the language network.

Topic Areas: Disorders: Acquired, Language Therapy