Similar patterns have been found in children less than a year old. In one study
with 10-month-old infants, smiling was associated with higher left hemisphere
activity, whereas crying was associated with higher right hemisphere activity
(Fox & Davidson, 1988). In other experiments, infants showed increases in left
hemisphere activity when their mothers reached down to pick them up (Fox &
Davidson, 1987), when they heard laughter (Davidson & Fox, 1982), and
when they tasted something sweet (Fox & Davidson, 1986). In all cases, the
children experiencing positive emotions had relatively more activity in their lefthemisphere than in the right. Because the infants had not yet reached their first
birthday, researchers argue that the association between cerebral asymmetry
and emotion is something we are born with rather than the result of learning.
Individual Differences in Cerebral Asymmetry
Additional research has taken the association between cerebral asymmetryand emotion one step further. Most people typically have higher activation
in one hemisphere than in the other, even when in a relatively nonemotional
resting state. However, which hemisphere displays the higher activity level is
not the same for everyone. Some people tend to have higher activity in the
left hemisphere when resting, whereas others tend to have more right hemi-
sphere activity. Moreover, like other individual differences, differences in ce-rebral asymmetry tend to be fairly stable over time. If you show a higher level
of activity in one hemisphere over the other today, you probably will show the
same pattern when taking an EEG test next week or even next year.
This observation leads to another intriguing question. Because left and
right hemispheric activity is associated with positive and negative moods, canwe use EEG data to predict moods? The answer appears to be “Yes.”
Participants in one study were identified as having either higher left hemi-
sphere or higher right hemisphere activity when resting (Davidson &
Tomarken, 1989). These individuals then watched films designed to elicit cer-
tain emotions, such as happiness or fear. As expected, people with a higher
level of left hemisphere activity were more responsive to the positive mood
films, whereas participants with higher right hemisphere activity levels reacted
more to the films that produced negative moods.
Again, similar patterns can be found in infants. Ten-month-old babies in
one study were identified as having either higher left hemisphere or higherright hemisphere activity when resting (Davidson & Fox, 1989). The babies
were then divided into those who cried and those who did not cry when
separated from their mothers. As expected, the criers tended to have higher
right hemisphere activity, whereas the noncriers were those with higher left
hemisphere activity.
How can we account for these findings? Initially researchers explained the
results in terms of thresholds for positive and negative mood (Davidson &
Tomarken, 1989). They speculated that people with higher right hemisphere ac-
tivity require a less intense negative event to experience fear or sadness. A minordisappointment or a rude remark might be enough to push them over the
threshold into a negative emotional state. On the other hand, people who gener-
ally have a higher level of left hemisphere activity may require a less intenseAssessment: Brain Electrical Activity and Cerebral Asymmetry 245