Using Non-invasive Brain Stimulation to Treat Word Finding Difficulty in Chronic Traumatic Brain Injury

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.