90 Biological Bases of Personality
If the stimulus attribute had been intensity, these kinds of re-
sults might be compatible with Eysenck’s theory of increased
sensitivity of introverts to low-intensity stimuli. But the evo-
lutionary type of explanation offered by Stelmack for the
greater survival significance of low-frequency sounds is not
convincing.
Recent studies have focused on the P300 EP component,
many using the “odd-ball” paradigm in which the participant
listens with eyes closed to a sequence of tones in which one
tone is presented frequently and another one (the oddball)
rarely. The rare tone is the signal for some task. These are
usually vigilance tasks on which extraverts’ performances
and EP reactions are expected to decline more rapidly than
those for introverts. However, when the task is made less
montonous or response requirements are high, the differ-
ences may disappear or even be reversed with larger EP am-
plitudes in extraverts (Stenberg, 1994).
The intensity of the stimulus is another factor in the I-E
difference. Brocke, Tasche, and Beauducel (1997) found that
introverts showed larger P3 reactions to a 40-db stimulus,
whereas extraverts showed a larger amplitude of P3 in re-
sponse to a 60-db stimulus. Introverts’ EP amplitudes de-
creased going from 40 db to 60 db, whereas extraverts
increased going from the less intense to the more intense
stimulus. These effects were a function of the impulsivity
component rather than the sociability component of the E
scale used in the study. The results of studies that vary the
experimental conditions suggest that attention and inhibition
may be the basic mechanisms governing the nature of the
relationship between E and cortical EPs. Responses at
the brain-stem level are probably less susceptible to these
mechanisms, and Eysenck’s theory does involve the brain
stem and other points along the reticulocortical arousal
system in I and E.
Stelmack and Wilson (1982) found that extraverts had
longer latencies for the EP subcortical wave V (inferior
colliculus) for stimulus intensity levels up to but not includ-
ing 90 db. The direction of the finding was confirmed in a
second experiment (Stelmack, Campbell, & Bell, 1993) and
in a study by Bullock and Gilliland (1993). Different doses of
caffeine and levels of task demand were used in the latter
study, but the differences between extraverts and introverts
held across all levels of caffeine and task demand. The results
support Eysenck’s theory more strongly than those using cor-
tical EPs, which seem more susceptible to stimulus, task, and
background arousal factors. A study by Pivik, Stelmack, and
Bylsma (1988), however, suggested that Eysenck’s arousal-
inhibition hypothesis may not be broad enough. These re-
searchers measured the excitability of a spinal motoneuronal
reflex in the leg and found that extraverts showed reduced
motoneuronal excitability as measured by reflex recovery
functions. These results show that the inhibitory properties of
the nervous system related to E may extend well below the
reticulocortical level.
Another line of EP research is based on Gray’s (1982,
1987) model of personality. Gray proposed that impulsivity, a
dimension close to extraversion, is related to sensitivity to
signals (conditioned stimuli) of reward whereas anxiety,
close to neuroticism, is related to sensitivity to signals of pun-
ishment. This model suggests that the learned biological
significance of stimuli, in addition to the intensity of stimula-
tion, governs the strength of reaction to them.
Bartussek, Diedrich, Naumann, and Collet’s (1993) results
supported the theory by showing a stronger EP response (P2,
N2) of extraverts than introverts to tones associated with
reward (winning money) but no differences in tones associ-
ated with punishment (losing money). In a later experiment,
however, extraverts showed larger P3 EP amplitudes to stim-
uli associated with bothreward and punishment compared to
neutral stimuli (Bartussek, Becker, Diedrich, Naumann, &
Maier, 1996).
DePascalis and his colleagues also presented findings sup-
porting Gray’s theory. In one study they used a questionnaire
scale developed more directly from Gray’s theory measuring
the approach tendency (DePascalis, Fiore, & Sparita, 1996).
Although they found no effect for E itself, the participants
scoring high on the approach scale had higher EP (P6) ampli-
tudes in response to stimuli (words) associated with winning
than to those associated with losing, and the reverse was true
for low-approach motive subjects.
Eysenck’s and Gray’s theories have also been tested using
peripheral autonomic measures of activity like the electroder-
mal activity (EDA), or skin conductance (SC), heart rate
(HR), and blood pressure (BP). These are only indirect mea-
sures of cortical activity and reactivity because they occur in
the autonomic nervous system (ANS) and are controlled by
limbic system centers, which in Eysenck’s model are associ-
ated more closely with neuroticism than with E. The results in
relation to E are similar to those obtained with more direct
cortical measures. Reviews by Smith (1983) and Stelmack
(1990) showed mixed and inconclusive findings relating tonic
EDA arousal to E, but some evidence of stronger SC re-
sponses of introverts than extraverts in response to low-to
moderate-intensity stimuli and stronger responses of ex-
traverts in response to high-intensity stimulation. Tonic
(base-level) measures of HR (Myrtek, 1984) and BP
(Koehler, Scherbaum, Richter, & Boettcher, 1993) are unre-
lated to E. Young children rated as shy and inhibited had
higher and less variable HRs, and a high HR at 21 months is
the same behavior pattern at 48 months (Kagan, Reznick, &