The purpose of this research study is to measure current flow inside the head using magnetic resonance imaging (MRI). The data from this study will be used to map the current flow caused from the electrical stimulation inside the head. The methods develop will be used to map and better control delivery of the current for electrical stimulation to modify a psychiatric condition such as depression; or other conditions such as epilepsy, Parkinson's disease or autism.

Transcranial direct current stimulation (tDCS) and deep brain stimulation (DBS) are examples of electrical stimulation therapies that are rapidly gaining attention as means of modulating motor function, semantic processing, and executive function. Both therapies have attracted many clinical and experimental studies. tDCS has been found to have both facilitatory and inhibitory effects on the brain depending on stimulation polarity and electrode position. DBS has been thoroughly evaluated clinically for treatment of movement disorders, principally Parkinson's disease, and is extending its reach to include treatment of disorders such as focal dystonia, depression and chronic pain. While still mostly in the experimental stage, tDCS applications and acceptance are growing extremely rapidly.

Although the functional alterations associated with tDCS can be categorized without knowledge of the underlying neurophysiology, an understanding of where externally applied current actually flows in any electrical stimulation technique is crucial as a basis for understanding which brain regions, circuits, or elements are affected by these therapies, and how these changes may occur. Such knowledge will lead to a better understanding of the mechanisms underlying these therapies, and thus to more focused and effective stimulation patterns and locations. Ultimately, this will lead to more efficient and novel clinical applications.

Many studies have simulated the effects of current application in both extra- and intracranial modalities using computer simulation. Simulations will always be limited by errors in interpreting MRI data during segmentation, differences between assumed and actual electrical conductivity values, and mismatches between actual and presumed electrode locations and sizes. Thus, better methods to understand and verify current flow distributions are badly needed.

In this study a recently developed MRI-based phase imaging technique to more directly measure current densities in vivo. Unlike earlier MRI-based methods of measuring electrical current flow, the technique works without requiring subject repositioning. This methods will be validated against high-resolution subject-specific models incorporating many tissue compartments, including anisotropic white matter. Thus, a new direct measurement method against state-of-the-art modeling approaches.

Inclusion Criteria:

• right handed (as determined by the Edinburgh battery),
• English as native language.

Exclusion Criteria:

• appreciable deficits in hearing,
• appreciable problems with articulation,
• appreciable accent schizophrenia, bipolar disorder, or major depression,
• any neurological disorder associated with cognitive impairment or neuroanatomic abnormality,
• language-based learning disorder,
• any implanted metal device (precludes use of tDCS), any implanted cardiac pacemaker,
• dementia or mini-mental state exam,
• <24 estimated verbal intelligence,
• <70 active or prior history of seizure disorder, family history of seizure disorder, prescribed seizure inducing medication.

24 normal adult subjects will be assessed.