A method and apparatus for diagnosing and/or treating mood disorders, such as depression, is provided. Right vestibular function in subjects diagnosed with a mood disorder is diminished compared with subjects who do not have a mood disorder. Accordingly, a method and apparatus for determining right vestibular function in a subject is provided, from which a diagnosis of a mood disorder may be determined. Stimulation of the vestibular system of a subject diagnosed with a mood disorder may reduce symptoms of the mood disorder in the subject. A method and apparatus for registering a subject's response to stimulation of the vestibular system and providing therapeutic vestibular stimulation based on the subject's response is provided. A subject's response to stimulation of the vestibular system may be based on measurements of nystagmus in the subject.
