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.