The purpose of this research study is to examine the effects of transcranial Direct Current Stimulation (tDCS) on verbal retrieval and cognition and sensorimotor control and to determine if tDCS can be used as a way to improve retrieval, sensory, and motor abilities in individuals with Parkinson's disease (PD).

Objective: Based on models of semantic memory retrieval developed, under which the pre-supplementary motor area (preSMA) of the dorsomedial frontal lobes play an important role, the study will examine the treatment of cognitive deficits in PD with HD tDCS applied to the preSMA to improve function in neural circuits supporting verbal retrieval. In addition, as the preSMA is implicated in motor planning and sequencing, the study also will explore whether HD tDCS applied to the preSMA will have an effect on speech and gait.

Specific Aims:

1. Examine the therapeutic effects on verbal retrieval function by modulating the preSMA using HD tDCS.
2. Examine the therapeutic effects on speech production and processing by modulating the preSMA using HD tDCS
3. Examine the therapeutic effects on gait and balance by modulating the preSMA using HD tDCS

Hypotheses:

1. : Active HD tDCS (1 ma anodal HD tDCS to the preSMA compared to sham) will improve verbal fluency (i.e, phonemic and category fluency on Controlled Word Association Test) as primary outcome measures.
2. : Active HD tDCS (1 ma anodal HD tDCS to the preSMA compared to sham) will improve speech sequencing.
3. : Active HD tDCS (1 ma anodal HD tDCS to the preSMA compared to sham) will improve general motor sequencing

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.

De-identified individual participant data (IPD) that underlie the results reported in published articles, including baseline demographic and clinical characteristics required to replicate the findings, will be made available.

Beginning 6 months and ending 36 months following publication.

Access will be provided via email request to the corresponding author or through the designated repository required by the publishing journal.

Participants, assessors, and technicians interacting with participants will be blind to assigned conditions. The software for the transcranial direct current system allows for maintaining blinds when uploading and running protocols.