This study will assess the effects of transcranial alternating current stimulation (tACS) on language recovery after stroke as well as healthy language functions.

Aphasia is a debilitating disorder, typically resulting from damage to the left hemisphere, that can impair a range of communication abilities, including language production and comprehension, reading, and writing. Approximately 180,000 new cases of aphasia are identified per year, and approximately 1 million or 1 in 250 are living with aphasia in the United States (NIH-NIDCD, 2015). Treatments are limited and provide modest benefits at best. The current emphasis in aphasia rehabilitation is to formulate intensive speech and language therapies and augment therapeutic benefits by providing brain stimulation concurrent with therapies.

The current study will investigate the efficacy of high-definition tACS (HD-tACS) to help restore neural oscillatory activity in aphasia. TACS differs from widely used transcranial direct current stimulation (tDCS) in that sinusoidal, alternating currents are delivered rather than constant currents. TACS can manipulate the ongoing oscillatory neuronal activity and potentially increase functional synchronization (or connectivity) between targeted areas. This feature of tACS is quite attractive, given the new body of evidence suggesting that language impairments stem from diminished functional connectivity and disruptions in the language network due to stroke. The selection of tACS frequencies in this study is guided by our preliminary work examining pathological neural oscillations found near stroke-lesioned areas (or perilesional) in aphasia and by the involvement of specific frequencies during a verbal short-term memory task. By exogenously tuning the neural oscillations with tACS, the investigators hope to up-regulate communication across regions within the language network and other connected areas to improve outcomes. If successful, tACS will be a powerful and novel treatment approach with reverberating positive impact on long-term recovery.

The study will employ HD-tACS in a within-subject and sham-controlled design, using frequencies ranging from theta to low-gamma (4-40 Hz) combined with language tasks. Magnetoencephalography (MEG) or electroencephalography (EEG) will be used to determine tACS frequencies and to evaluate behavioral and neurophysiological changes in response to tACS. Investigators plan to recruit 200 participants: 100 stroke survivors with aphasia and 100 healthy controls.

Participants will complete language testing that covers a broad range of language functions, medical history, and MRI. Eligible participants will undergo active tACS or sham-tACS over 3-4 sessions. The tACS administrator and participants will be blinded to the stimulation type. The order of stimulation type will be counterbalanced across participants. Washout period between visits will be at least 48 hours to minimize potential carryover effects. MEG will be collected prior to tACS sessions during a language task to determine tACS frequency. EEG may be acquired before and after tACS during periods of rest (resting-state) and during language tasks. Participants will complete a questionnaire at the end of stimulation visits to assess potential side effects of tACS. Total time enrolled in the study is expected to be 2-3 weeks, which may be longer depending on participant's availability.

Inclusion Criteria:

Healthy Controls

• 18 years of age or older
• Fluent in English
• No history of neurological or psychiatric disorders

Stroke Patients

• Diagnosed with post-stroke aphasia by referring physician/neuropsychologist
• Consent date >=1 months after stroke onset
• Right-handed
• Fluent in English
• 18 years of age or older

Exclusion Criteria:

• Severe cognitive, auditory or visual impairments that would preclude cognitive and language testing
• Presence of major untreated or unstable psychiatric disease
• A chronic medical condition that is not treated or is unstable
• The presence of cardiac stimulators or pacemakers
• Any metal implants in the skull
• Contraindications to MRI or tACS
• History of seizures
• History of dyslexia or other developmental learning disabilities