The possibility that personality type contributes to electrophysiological
differences between individuals has been explored since the late 1930s. In
particular, the personality dimension of introversion-extraversion has received
considerable attention because of its putative biological basis (Starr,
1969; Eaves & Eysenck, 1975). This theory proposes that individuals differ
appreciably in their relative preferences for attending to either the inner
world of subjectivity, with emphasis on reflective, introspective mental activity
(introverts), or to the outer world of objective events, with emphasis on
active involvement in the environment (extroverts). Eysenck (1967) suggested
that differences between introverts and extroverts are based neurophysiologitally
in the ascending reticular activating system of the brain, an area which