Transcranial Alternating Current Stimulation (tACS) in Aphasia

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. Transcranial direct current stimulation (tDCS) is one of the most widely used such technique. While tDCS has had relative success in chronic aphasia (>6 months after stroke), it has not been efficacious during subacute stages (<3 months after stroke). But enhancing language recovery early after stroke is desirable because of its potential impact on long-term language outcomes and quality-of-life. The current study will investigate the efficacy of high-definition tACS (HD-tACS) to help restore neural oscillatory activity in aphasia. TACS differs from 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. By exogenously tuning the perilesional oscillations with tACS, the investigators hope to up-regulate communication across these areas and other connected areas to improve language outcome. 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 two frequencies (alpha/10 Hz and low-gamma/40 Hz) combined with language tasks and electroencephalography (EEG) to evaluate subsequent behavioral and neurophysiological changes. Investigators plan to recruit 50 participants: 25 stroke survivors with aphasia at lease 1 month after stroke, and 25 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 at 10 Hz or 40 Hz, or sham-tACS. All participants will receive all three stimulation types during separate visits. 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. EEG will 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.

In the within-subject design healthy participants and stroke survivors will receive tACS stimulation at 10 Hz, 40 Hz, and sham settings in a randomized order with >48 hours washout period in between stimulation sessions.

Participant, care provider, investigator and outcomes assessor are all blinded. One study team member will be designated as unblinded.

High-Definition-tACS will be delivered via a battery operated alternating current stimulator (Soterix) using two 3x1 center-surround montages. Targets of stimulation will be localized based on the 10-10 International EEG system with center electrodes placed at a frontal and a temporoparietal site. The current is turned on and increased in a ramplike fashion over approximately 30 seconds. Participants will undergo tACS with 10-Hz stimulation for 20-minutes with 1 milliampere (mA) peak-to-peak intensity. Stimulation will be maintained no longer than 20 minutes.

High-Definition-tACS will be delivered via a battery operated alternating current stimulator (Soterix) using two 3x1 center-surround montages. Targets of stimulation will be localized based on the 10-10 International EEG system with center electrodes placed at a frontal and a temporoparietal site. The current is turned on and increased in a ramplike fashion over approximately 30 seconds. Participants will undergo tACS with 40-Hz stimulation for 20-minutes with 1 milliampere (mA) peak-to-peak intensity. Stimulation will be maintained no longer than 20 minutes.

High-Definition-tACS will be delivered via a battery operated alternating current stimulator (Soterix) using two 3x1 center-surround montages. Targets of stimulation will be localized based on the 10-10 International EEG system with center electrodes placed at a frontal and a temporoparietal site. The current is turned on and increased in a ramplike fashion for 10 to 30 seconds and then ramped down. In this way, the participants experience the same initial sensations (mild tingling) as the active tACS groups.

Improvement on noun and verb retrieval performance as determined by increases in accuracy and decreases in reaction time.

tACS responsiveness depending on language impairment types, stroke lesion and language lateralization characteristics will be explored.

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

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