Aphasia is an acquired (typically left-hemisphere) multi-modality disturbance of language that impacts around 2 million people in the USA. Aphasia impacts language production and comprehension as well as reading and writing. The ramifications of aphasia extend beyond language impairment to negatively impacting a person's social, vocational, and recreational activities. Currently, the most effective way to treat aphasia is with speech-language therapy (SLT). However, even if SLT is intensive, persons with aphasia are left with residual language delays. Recent research suggests that pairing SLT with transcranial direct current stimulation (tDCS) a non-invasive, safe, low-cost form of brain stimulation may aid language recovery in persons with aphasia. However, results from tDCS studies are inconclusive. The success of tDCS in combination with SLT could depend on the timing of tDCS since tDCS-induced effects depend on the neuronal state of the brain-networks at the time of the stimulation. In this study, the differential impact of tDCS before behavioral SLT (offline-before therapy), tDCS after SLT (offline-after therapy), and tDCS concurrently with SLT (online) on functional language recovery in persons with aphasia will be investigated. Sham tDCS (i.e., SLT alone) as a control group will also be included in the study. The investigators hypothesize that both offline and online tDCS will improve language functioning than sham tDCS.

tDCS Implementation:

2mA of direct current for 20 minutes will be delivered using a constant current stimulator via two electrodes in saline soaked sponges. To stimulate the left angular gyrus, a cathodal electrode inside a saline soaked sponge (5 x 3cm) will be placed over the CP5 according to the 10-20 international system for EEG electrode placement. The electrodes will be secured in position by a custom-built EEG cap that will be marked with the location for angular gyrus. The investigators will then confirm that the cathodal electrode is over the left angular through the use of neuronavigation. The "return" anode electrode will be placed in a saline soaked sponge (5 x 5cm) on the center of the supraorbital region.

At the beginning of the tDCS, current will slowly ram-up during the first few seconds until it reaches 2mA. The ramp-up process will acclimate the participants to tDCS-induced sensations (e.g., itching). At the end of the tDCS sessions, current will slowly ramp down in the last few seconds to 0mA. The total duration of the 2mA current will be maintained for 20 minutes. For sham stimulation, the investigators will place the electrodes in the same location as the real tDCS groups with stimulation ending after few seconds. The brief stimulation will produce tDCS-induced sensations so that participants will not be aware when the tDCS stopped delivering direct current

Speech-Language Treatment:

Standard-of-care treatment - script training will be used to provide SLT to our participants. The treatment will be delivered via a computer program, Aphasia Scripts ®, whereby a virtual therapist interactively guides the participants through treatment steps. Scripts will be five to ten sentences and developed from common templates about topics relevant to daily life (e.g., asking questions at the grocery store, ordering in a restaurant). The advantage of using a computer program instead of a human therapist is that it removes extraneous variables associated with human therapists (e.g., different levels of encouragement) and this, together with the use of script templates, ensures treatment fidelity across participants.

Duration of an individual subject's participation in the study:

Subjects will be requested to come for baseline sessions (3 to 4 hours) before the actual experiment. The baseline assessments may be done over two visits. In the baseline session the subjects' aphasia severity will be assessed. The participants will also perform baseline tasks of language and cognition in this session. Subjects will then participate in 3 weeks of tDCS and SLT (i.e., 15 sessions). The sessions will consist of 40 minutes of SLT and 20 minutes of tDCS. Subjects will be asked to return for an assessment immediately following the end of treatment and for follow-up testing after 4 and 8 weeks. Therefore, total number of visits will be 19-23 sessions (depending on how many visits are needed for the assessments) over a period of three months.

Inclusion Criteria:

• Men or women with diagnosis of fluent or non-fluent aphasia subsequent to a left-hemisphere infarct(s) that is confirmed by a MRI or CT scan
• Aphasia quotient on the WAB of 35-85
• At least 6 months post onset of aphasia (this is beyond the stage of spontaneous recovery)
• 18- 80 years of age
• Premorbidly fluent in English
• Premorbidly right-hand dominant per the Edinburg Handedness Inventory
• Visual acuity of 20/40 corrected
• Auditory acuity no worse than 30 dB HL on pure tone testing, aided in the better ear.
• Education greater than 12th grade

Exclusion Criteria:

• Any other neurological condition (other than cerebral vascular disease) that could impact language and cognition such as Alzheimer's disease, Parkinson's disease, primary progressive aphasia, and traumatic brain injury
• Active substance use
• Individuals with epilepsy