A psychiatrist has basic training in both mental illness management as well as organic brain dysfunction. They are an MD who has completed at least an additional 4 year residency in psychiatry, and have passed their boards with the American Board of Psychiatry and Neurology (ABPN). Some will also have additional training (usually a fellowship of 1-2 years) specifically in neurologic/organic brain dysfunction. Many do not.

http://www.aadprt.org/training/default.aspx

A psychologist (in most states they must have a PhD) is not a MD. Most do not have specialized training in neurologic conditions, but instead are expert in personal counseling, adjustment issues, and psychotherapy. A neuropsychologist is one who has specialized training (usually 2 years post-doctorate) in both testing for cognitive deficits and remediation of cognitive and behavioral problems related to neurologic injury or disease.

http://www.appcn.org/mission.html#history

To make things even more complex, there are neuropsychology technicians (also called psychometrists or psychological assistants) who have a bachelor's or master's degree who are trained in how to administer (but not interpret) the vast array of psychological and neuropsychological tests. Interpretation would then by done by the neuropsychologist or psychiatrist:

http://www.napnet.org/49501.html?*se...ession*id*val*