Brain scan studies have found no differences in stutterer's brains when they are not talking, and when they are talking fluently. But during stuttering, changes are seen in stutterers' brain activity. Left-brain areas (where speech and language are produced) that should be active during speech become inactive, while right-brain areas that should be inactive during speech become active.

Brain scans have found abnormally low activity during stuttering in the central auditory processing area, and in the area that integrates auditory and somatic (body) sensation. Stuttering seems to be related to an inability to integrate what the stutterer hears with the muscle movements he feels.

Stuttering appears to be caused by excessive amounts of the neurotransmitter dopamine in the left caudate nucleus. This is the area that translates speech into muscle movements.

We have no conscious awareness of central auditory processing and the left caudate nucleus. This explains why stuttering therapies that rely on consciously controlling your speech (and speech therapists that tell you to "try harder") are at best only temporarily effective. New stuttering therapies use drugs and computers to alter the brain areas that can't be consciously controlled.