Here's an interesting excerpt on how introverts and extraverts use their brain function..

One study found that introverts have more blood flow in the frontal lobes of their brain and the anterior or frontal thalamus, which are areas dealing with internal processing, such as planning and problem solving. Extraverts have more blood flow in the anterior cingulate gyrus, temporal lobes, and posterior thalamus, which are involved in sensory and emotional experience. This study and other research indicates that introversion-extraversion is related to individual differences in brain function.

Johnson, D. L., Wiebe, J. S., Gold, S. M., Andreasen, N. C. (1999). Cerebral blood flow and personality: A positron emission tomography study. American Journal of Psychiatry, 156, 252-257.