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Release: March 29, 1999

Brain activity differs in introverts and extroverts, UI study shows

IOWA CITY, Iowa -- A University of Iowa study adds to growing evidence that being shy or outgoing may be all in your head. Investigators looking at cerebral blood flow and personality found more conclusive signs of different brain activity in introverts and extroverts.

This is the first study to reveal the connections between activity of the thalamus and introversion and extroversion, said Debra L. Johnson, Ph.D., UI assistant research scientist in psychology and the study's lead investigator. "We found more evidence that people might be shy or outgoing because of the way their brains are structured."

Previous studies have shown that introversion and extroversion are based on variations in brain function, but those studies did not describe all the locations found in this study. The UI researchers examined 18 healthy individuals using positron emission tomography (PET) scans, which can provide a high-resolution image of the entire head.

The PET scans revealed that introverts have more activity in the frontal lobes of the brain and anterior, or front, thalamus. These areas are activated when a person's brain takes on internal processing such as remembering, problem solving and planning. Extroverts exhibit more activity in the anterior cingulate gyrus, temporal lobes and posterior thalamus. These areas are typically thought to be more involved in sensory processing such as listening, watching or driving.

The differences in cognitive style and sensory-processing relate to the qualities associated with introversion and extroversion. True introverts are quiet, inwardly focused and reclusive. Extroverts are gregarious, socially active and sensation seeking.

"Introverts get more of their stimulation internally, whereas extroverts seek outside sources," Johnson said. "Extremely introverted and extroverted personalities are two ends of a continuum, with most people falling somewhere in between."

Johnson added, "The implication is that one personality trait introversion or extroversion - isn't right or wrong. These variations in brain activity suggest that a lot of our individual differences have an underlying biological cause."

The subjects, 10 men and eight women, first took personality tests to determine the extent to which they were introverts or extroverts. The researchers later had the subjects lie down with their eyes closed while the PET scan measured brain activity.

"Lying quietly allows the mind to be free and do what it naturally does," Johnson explained. "When a part of the brain becomes active, there is increased blood flow to that region, which shows up on the PET scan."

The findings were published in the February issue of the American Journal of Psychiatry.