Treatment of Cognitive and Sensorimotor Deficits in Parkinson's Disease With High Definition Transcranial Direct Current Stimulation

Inclusion Criteria: * Diagnosed with PD having a verbal fluency deficit based on neuropsychological test (T-score \< 40 or a T-score \<-1.0 SD below the average T-score): Semantic Object Retrieval Test (SORT), COWAT (both letter and category fluency), Boston Naming Test (BNT), Rey Auditory Verbal Learning Test (RAVLT) * 50 - 90 years old * Capable of understanding and signing an informed consent (able to answer consent comprehension questions) * Fluent in speaking and reading English Exclusion Criteria: A potentially study-confounding, tDCS-contraindicated, or EEG-contraindicated psychological, neuropsychiatric, neurological, or other medical issues: * Montreal Cognitve Assessment (MOCA) score \<23, unless an accompanying study partner/caregiver is with them and we can obtain the written consent of both the participant and the participant's accompanying study partner/caregiver (i.e., the participant's spouse, adult child, parent, or adult sibling); * history of seizures; * unexplained episodes of loss of consciousness (as these may be related to brain alterations or epilepsy); * suffering from severe or frequent headaches; * unstable or uncontrolled neuropsychiatric illness; * severe traumatic brain injury (based on the Ohio State TBI Identification; Method); * brain tumor; stroke; present drug abuse/misuse; * brain tumor; * serious or life-threatening diseases, including congestive heart failure, chronic obstructive pulmonary disease, or active malignancy; * Huntington's disease; * stroke; * cranial implants or skull defects that affect tDCS administration; * implanted brain medical devices, including, deep brain stimulators (DBS); * implanted pacemakers; * any electrically, magnetically, or mechanically activated implants; * cardiac, neural, or medication implants; * vascular clips or other electrically sensitive support systems in the brain; * damaged skin at the sites of stimulation (the device should only be used on healthy, intact skin, as wounds may alter resistance to current); * skin conditions such as dermatitis, psoriasis, or eczema; * pregnant women; * diagnosis of just dysarthria; Use of medications that interact with or potentially interact with tDCS or EEG effects: * anti-convulsants; * carbamazepine; * sulpiride; * pergolide; * lorazepam; * rivastigmine; * dextromethorphan; * D-cycloserine; * flunarizine; * ropinirole; * citalopram; * stimulants; Additionally, non-English speakers will be excluded because not all of the screening forms, questionnaires, and tests are available in languages other than English.