Q. My niece and nephew are as opposite as night and day. Jill is the quintessential risk taker, while Jared would be perfectly happy curled up under his bed—reading! Could this relate to extroversion and introversion?

A. Good guess. It could. An extreme extrovert is as different from an extreme introvert as night is from day. PET Scan studies have shown that behavioral differences may result from using different brain pathways that influence where the individual directs his/her focus—internally or externally. Work by Dr. Debra Johnson, for example, has shown that brain blood-flow rates differ in extroverts versus introverts.

Extroverts tend to have lower rates of blood flow to the brain, while introverts tend to have higher rates of blood flow to the brain (which goes along with higher rates of internal stimulation). Here are a couple more examples:

Extreme Extroversion:

• Shorter, fast-acting, and less complicated blood flow patterns involve brain areas related to sensory processing (except for smell), variety and novelty
• Dominant pathway is activated by dopamine, a brain chemical that is implicated in most, if not all, addictive behaviors. Adrenalin, amphetamines, and cocaine can increase dopamine levels)

Extreme Introversion:

• Longer, more complex blood flow patterns involve brain areas related to internal experiences (e.g., planning, pondering, remembering).
• Dominant pathway is activated by acetylcholine, a neurotransmitter that affects one’s alertness and ability to pay attention.