This study proposes to use transcranial direct current stimulation (tDCS) in patients with chronic strokes and aphasia in order to characterize and enhance the mechanisms of language recovery following injury. Prior to enrollment subjects will undergo six "site-finding' sessions involving various placements of positively charged (anodal), negatively charged (cathodal), and sham stimulation over the damaged and intact hemispheres of the brain, along with standard tests of language. Subjects who are found to experience a transient improvement in language ability in this initial experiment will participate in an incomplete cross-over design study to determine if 10 sessions of tDCS stimulation lead to prolonged language benefit when delivered to the hemisphere and at the polarity shown previously to respond best to stimulation.

Transcranial Direct Current Stimulation (tDCS) uses a small (typically 1-2 mA) electrical current which is passed through the scalp and is delivered for up to 20 minutes. Due to the safety and tolerability of tDCS, as well as its demonstrated capacity to temporarily enhance cognitive, motor and affective functions of the brain, there has been an explosion of interest in the technique since the first reports in 2000. tDCS has been used safely in a wide variety of domains including studies of the visual system, working memory, verbal fluency and motor function. tDCS has also been used safely to enhance brain function in subjects with a variety of brain pathologies including stroke, depression and Alzheimers Disease. In this study, the investigators hope to address whether tDCS can be used to enhance language function in individuals with post-stroke aphasia.

This two part experiment will address 1) Can tDCS lead to transient improvement in language ability in individuals with stroke and chronic aphasia? and 2) Does repeated tDCS lead to prolonged improved language ability in individuals with stroke and chronic aphasia?

In Experiment 1, which will be referred to as the 'site-finding' portion of this protocol, participants will undergo anodal, cathodal, and sham stimulation of the intact or injured hemisphere to determine what area of the brain responds best to tDCS. There will be six separate sessions which are spaced apart by at least 2 days. Tests of language will be used to characterize changes in performance that are induced in each condition.

In Experiment 2, subjects who had participated in Experiment 1 and were found to experience a transient improvement in language ability after receiving tDCS will participate. Subjects will be stimulated using the best montage from Experiment 1. This is an incomplete cross-over design study to determine if 10 sessions of tDCS stimulation delivered to the brain will lead to improvement in language. Improvement will be operationally defined as performance on one or more language measures that is significantly better than baseline and sham performance (i.e. non-stimulation conditions). Half of our subjects will be randomized to a group receiving 10 sessions (divided in to two five-day periods) of real tDCS. The other half of our subjects will be randomized to undergo sham stimulation before undergoing optional real tDCS stimulation. All subjects will engage in language rehearsal during stimulation. Tests of language will be used to characterize changes in performance that are induced by real and sham stimulation. Follow-up language testing will be performed at 2 weeks and 2 months after stimulation to determine whether tDCS can have prolonged effects on language.

Inclusion Criteria:

• 1. Presence of aphasia attributable to one or more unilateral left hemispheric stroke(s)
• 2. The most recent stroke must have occurred 6 months prior to inclusion in the study
• 3. Must speak English as a native language
• 4. Must be able to understand the nature of the study, and give informed consent

Exclusion Criteria:

• 1. Subjects with bilateral strokes.
• 2. History of neurologic illness(es) other than stroke
• 3. Current unstable medical illness(es)
• 4. Epilepsy or Seizure(s) within the last six months
• 5. Subjects with metallic objects in the face or head other than dental apparatus such as braces, fillings, and implants.
• 6. Subjects with Pacemakers or ICDs
• 7. Diagnosed psychiatric disorders
• 8. Pregnancy
• 9. Current alcohol or drug abuse

Participants will be blinded to which tDCS group (real or sham) they are initially enrolled in; however, since only the sham group crosses over this will signal to subject that they are now getting real stimulation.