The purpose of this study is to learn more about how brain stimulation affects word finding problems in people who have a traumatic brain injury (TBI). The type of brain stimulation used is called transcranial direct current stimulation (tDCS). tDCS delivers low levels of electric current to the brain and high definition tDCS (HD-tDCS) delivers the current with multiple electrodes on the scalp. This current is delivered with HD-tDCS to parts of the brain that may help with remembering things. The investigators hope that this can help to improve word finding and memory problems in people with TBI.

The investigators plan to recruit English-speaking participants aged 18-85 years with a history of chronic TBI (> 1 year since injury prior to enrollment), all of whom have problems with cognition. The participants will be randomized to receive (1) active first followed by sham HD-tDCS condition, or (2) sham first followed by active condition in order to assess the efficacy of HD-tDCS on improving verbal retrieval function. The randomization will be double-blinded to the participants and the research personnel who administer the procedures. The study therefore adopts a double-blind randomized cross-over design. The proposed study will measure response to HD-tDCS treatment over the pre-supplementary motor area (preSMA) region when compared to sham with verbal retrieval function (verbal fluency, naming, verbal learning) as the primary outcome. Secondary outcome measures include cognitive performance other than verbal retrieval function and electroencephalography (EEG) measures. The participants will receive two phases of 10 sessions of active stimulation (1 mA anodal HD-tDCS targeting preSMA for 20 min) or sham across 2 weeks. All participants will be blinded to their condition. EEG and neuropsychological tasks will be completed at baseline, immediate follow-up after session 10, and a 2-month follow-up. The participants will also undergo an MRI session at baseline. Those participants randomized into the active or sham group will have the opportunity to return after 2 months and receive sham (if active first) or active (if sham first) treatment and will undergo the EEG and neuropsychological tests again immediately following the last HD-tDCS session and at a 2-month follow-up.

Inclusion Criteria:

• Age between 18 and 85
• Fluent in speaking and reading English
• Able to provide informed consent
• Has a TBI at least one year prior to enrollment and not related to military experience
• Has a confirmation of verbal retrieval difficulties as measured by the Verbal Retrieval Difficulty Interview questions

Exclusion Criteria:

• Lifetime major or active neurologic conditions (e.g., stroke, epilepsy, brain tumor, dementia, seizure occurrence less than one year ago)
• Lifetime major or active cardiovascular conditions (e.g., cardiac arrythmia, heart failure, heart attack)
• Current substance use disorder
• Lifetime major psychiatric disorders (e.g., schizophrenia, bipolar disorder)
• Severe depression at the time of enrollment (BDI-II >= 29) or psychiatric ER visits or hospitalization less than 6 months ago prior to enrollment
• Current sensory (e.g., blind, deaf) or physical (e.g., severe motor weakness) impairment that interferes with testing
• Contraindications for tDCS or MRI
• The person cannot be left alone for 8+ hours.
• Not verbally communicative.
• Currently undergoing and not wishing to discontinue speech and cognitive therapy during study participation.
• Incapable of understanding the consent or unable to consent for oneself.
• Unable to travel to BIDMC's Berenson-Allen Center
• Pregnancy

Individual participant data (IPD) will be shared in a de-identified format in accordance with NIH data-sharing policies and institutional guidelines.

Data will be available after publication of primary results or 12 months following study completion, whichever occurs first. The data will be accessible for at least 5 years post-publication.

Qualified researchers may request access by submitting a data request with a description of the intended use.

Participants, assessors, and technicians interacting with participants will be blind to assigned conditions.