By Michelle Gallagher, published on July 1, 1999 - last reviewed on June 11, 2014
Party animals and wallflowers hoping to change their social personas may have no say in the matter. A study shows that introverts and extroverts show activity in different brain structures which mirror the wildly opposing aspects of their personalities.
Debra Johnson, Ph.D., and John S. Wiebe, Ph.D., used positron emission tomography (PET) to measure cerebral blood flow—an indicator of brain activity—in individuals rated on a personality test as shy or gregarious.
Johnson, a research scientist at the University of Iowa, and Wiebe, an assistant professor of psychology at the University of Texas, asked both types to think freely while undergoing PET scans. The images they obtained clearly separated the quiet thinkers from the social butterflies. Introverts showed increased blood flow in the frontal lobes, the anterior thalamus and other structures associated with recalling events, making plans and problem-solving.
Extroverts, on the other hand, displayed more activity in the posterior thalamus and posterior insula, regions involved in interpreting sensory data.
These results highlight what the researchers consider the main difference between introverts and extroverts: inward and outward focus. Reticent people are more introspective, attentive to internal thoughts, while wilder beings are driven by sights and sounds—they crave sensory stimulation.
While this study only correlates personality and brain activity—there's no proof that one causes the other—Wiebe says the findings just go to show that "everything psychological in nature is, at some level, physiological in nature."