In the current study, the investigators aim to understand the role of transcranial direct current stimulation (tDCS) in improving executive function across neuropsychiatric populations known to have deficits in this cognitive domain.

In the current study, the investigators aim to understand the role of transcranial direct current stimulation (tDCS) in improving executive function across neuropsychiatric populations known to have deficits in this cognitive domain. The investigators will select 8 relevant diagnostic categories: traumatic brain injury, major depressive disorder, bipolar disorder, schizophrenia, attention deficit hyperactivity disorder, borderline personality disorder and substance use disorder. The study will also include a cohort of healthy controls for comparison.

Inclusion Criteria

1. Male and female outpatients 18-65 years of age
2. A diagnosis of traumatic brain injury, major depressive disorder, bipolar disorder, schizophrenia, attention deficit hyperactivity disorder, borderline personality disorder and substance use disorders meeting the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria.

Exclusion Criteria

1. Contraindication to tDCS: history or epilepsy, metallic implants in the head and neck, brain stimulators, vagus nerve stimulators, shunts, pacemakers, pregnancy.

2. Active substance dependence (except for tobacco and cannabis).

3. Pregnant or nursing females.

4. Inability to participate in testing procedures.

5. Additional exclusion criteria for healthy controls:

   1. Diagnosis of psychiatric of neurological disorder
   2. Ongoing treatment with any psychotropic medications.