Cerebellar Transcranial Direct Current Stimulation and Aphasia Treatment

People with post-stroke aphasia are left with some degree of chronic deficit for which current rehabilitative treatments are variably effective. This study investigates the behavioral and neural effects of multiple consecutive cerebellar tDCS sessions coupled with computerized naming therapy in stroke survivors with aphasia.

The investigators will study the effect of right cerebellar transcranial direct current stimulation (tDCS) stimulation during language therapy for naming in individuals with chronic aphasia (\>6 months post stroke). Naming difficulties are a persistent and common symptom in aphasia after left-hemisphere (LH) stroke. Although the interventions to improve naming can have benefits, a massive number of treatment sessions is usually required to show gains, particularly in individuals with chronic, large LH stroke. tDCS is a promising adjunct to traditional language therapy. tDCS is a safe, non-invasive, non-painful electrical stimulation of the brain which modulates cortical excitability by application of weak electrical currents in the form of direct current brain polarization. It is usually administered via saline-soaked surface sponge electrodes attached to the scalp and connected to a direct current stimulator with low intensities. Prior investigators have mainly focused on the role of LH in language recovery, wherein the electrode is placed in the left frontal or temporal region. However, in individuals with large lesions involving key language areas like the frontal and temporal cortex, it may be difficult to find viable tissue to stimulate in the LH. This study uses a novel electrode placement for chronic stroke patients with aphasia with large LH lesions. Targeting the intact right cerebellum allows for the possibility of identifying a single target that can be used across groups of people with aphasia with varying lesion sites and size in the LH. Evidence from functional neuroimaging and clinical studies indicate that the right cerebellum is important for both language and cognitive functions.

The investigators will utilize a randomized, double-blind, sham controlled, within-subject crossover trial design. A random subset (half) of participants will be assigned to the "anode" group (Group Anode) and other half will be assigned to the 'cathode' group (Group Cathode). Participants will take part in 2 intervention periods of 15 computerized naming training sessions (3-5 sessions per week), with either tDCS + naming therapy or sham+ naming therapy, separated by 2 months. Detailed language evaluation will take place before, immediately after, 2 weeks and 2 months post-intervention for each condition. The investigators will test the hypothesis that anodal tDCS or cathodal tDCS over the cerebellum and computerized naming therapy is associated with greater gains in accuracy in naming pictures, compared to sham combined with the same computerized naming therapy.

Eligibility Criteria:
Inclusion Criteria:

* Participants must have sustained a left hemisphere stroke.
* Participants must be fluent speakers of English by self-report.
* Participants must be capable of giving informed consent or indicating another to provide informed consent.
* Participants must be age 18 or older.
* Participants must be premorbidly right handed.
* Participants must be at least 6 months post stroke.
* Participants must have an aphasia diagnosis as confirmed by the Boston Diagnostic Aphasia Examination (BDAE) Short Form.
* Participants must achieve at least 65% accuracy on screening task (comparable to treatment task) on 1 of 3 attempts

Exclusion Criteria:

* Participants with lesion involving the right cerebellum
* Previous neurological or psychiatric disease.
* Seizures during the previous 12 months.
* Uncorrected visual loss or hearing loss by self-report.
* Use of medications that lower the seizure threshold (e.g., methylphenidate, amphetamine salts).
* Use of N-methyl-D-aspartate receptor (NMDA) antagonists (e.g., memantine).
* \> 80% (140 out of 175) correct responses on the Philadelphia Naming Test at baseline.
* History of brain surgery or any metal in the head.
* Scalp sensitivity (per participant report).