Lateralization of Emotion
  Paul Achuff

 Although a slice of the human brain across a sagittal plane 
reveals evident symmetry, the right and left hemispheres are not 
entirely symmetrical. Asymmetries, both functional and anatomical, 
exist throughout the neocortex and cortical substructures. Staunch 
research into functional hemispheric asymmetry began nearly 135 
years ago with Pierre Paul Broca's declaration that the left 
hemisphere is predominately responsible for language related 
behavior. With the discovery of lateralized hemispheric 
functioning in language established, the study of emotional 
lateralization began. In 1881, Jules Bernard Luys published a 
paper proposing the existence of an "emotion" center in the right 
hemisphere complementary to the "intellectual" centers in the left 
hemisphere (Harrington, 1995). His theory arose from the distinct 
and constant personality differences he saw in patients suffering 
from hemiplegia. Patients suffering from right hemiplegia were 
more passive and apathetic, whereas patients suffering from left 
hemiplegia were more emotionally volatile. Luys believed the 
discrepancy in affect was the result of the destruction of the 
normal inhibiting center for emotion in the right hemisphere since 
motor control occurs contralaterally. While historic studies of 
emotion have implicated the right hemisphere as the seat of 
emotion, this idea is not entirely true. To speak of the right 
hemisphere alone as being responsible for emotion clearly 
oversimplifies hemispheric asymmetry. Current evidence indicates 
that emotion may be lateralized to both hemispheres. 
 The study of emotional behavior generally focuses on patients 
with unilateral brain lesions. A lesion to the left hemisphere 
generally results in feelings of anxiety, despair, or anger, 
whereas a lesion to the right hemisphere can result in emotional 
placidity or indifference toward failure. Moreover, depression may 
impair right hemisphere functioning, and anxiousness may impair 
left hemisphere functioning (Tucker, 1987). Furthermore, damage to 
the left hemisphere is more likely to result in a catastrophic 
depressive reaction than is analogous damage to the right 
hemisphere (Goldstein, 1939). While these findings clearly suggest 
that management of emotion is asymmetrical, the precise 
lateralization of function is not known. This paper will examine 
two proposed theories of emotional asymmetry before attempting to 
reconcile the theories.

Two general theories of emotional asymmetry have been proposed. 
The right hemisphere hypothesis posits that the right hemisphere 
is dominant over the left hemisphere for all forms of emotional 
expression and perception. The valence hypothesis states that 
hemispheric asymmetry for expression and perception of emotions 
depends on emotional valence; the right hemisphere is dominant for 
negative emotions and the left hemisphere is dominant for positive 
emotions (Hellige, 1993). Both hypotheses have received empirical 
support.
 Behavioral evidence supports the theory that the right hemisphere 
is dominant for expressing and perceiving emotion, irrespective of 
valence. For instance, the left side of the face is more 
emotionally expressive than the right side. In a study examining 
possible asymmetries in the expression of emotion, full face 
photographs and their mirror reversals were split down the midline 
(Sackheim et al., 1978). Composite pictures were put together 
using two right sides and two left sides. The intensity of the 
emotional expression evident in a series of pictures was rated by 
participants. Left side composites were deemed more emotionally 
expressive than right side composites. Since the majority of 
facial muscles are controlled by contralateral projections, 
researches concluded that the right hemisphere is greatly involved 
in the production of emotional expression. Moreover, the ability 
to correctly identify positive and negative emotions shown in 
faces is disrupted more by right hemisphere injury than left 
hemisphere injury (Bowers et al., 1985). Additionally, disruptions 
of prosody and intonation of speech resulting from right 
hemisphere injury occur regardless of whether the emotion being 
expressed is positive or negative (Hellige, 1993). 
 Dichotic listening is another behavioral technique designed to 
study laterality of function. Unlike the retina, which sends nasal 
input contralaterally and lateral input ipsilaterally, each ear 
sends information from all its receptors to both hemispheres. 
However, Kimura (1967 367) proposed that when sounds are presented 
dichotically, the ipsilateral pathways from both ears are 
inhibited.  Thus, auditory stimuli presented to both ears are 
projected contralaterally. 
 Carmon and Nachshon (1973) used nine emotion eliciting sounds in 
their experiment: the laughing, crying, and shrieking of a man, 
woman, and a child. After listening to a dichotic pair, 
participants then indicated what sound they heard by pointing to 
an illustration depicting the characteristics of that sound. 
Overall, 19 of the 25 subjects were more accurate in identifying 
the stimulus presented to the left ear.
 Studies have supported the idea that right hemisphere damage 
interferes with the perception of emotion more than equivalent 
left hemisphere damage. In one recent study, patients with right 
hemisphere damage showed significant impairment during 
discrimination and identification tasks involving emotional words 
than on analogous tasks involving nonemotional words. Controls 
with left hemisphere damage did not show the same disassociation 
(Borod et al., 1992 LB RB 383). Another study required patients 
with right hemisphere damage to discriminate between pairs of 
identical sentences differing only in spoken intonation. When the 
patients had to identify whether the sentences were different, 
they performed more poorly than the controls. However, when 
identifying the emotion conveyed by a story, their performance was 
equal to the controls. These findings suggest that while patients 
do display difficulty in perceiving emotion, patients have not 
entirely lost the ability to comprehend different emotions (Tucker 
et al., 1977).
 The emotional valence theory has been supported by studies 
examining the effects of emotional experience on 
electrophysiological responses recorded using an EEG. In one 
experiment (Davidson, et al., 1979 HA), EEG responses were 
recorded from electrodes placed over the frontal and parietal 
areas of both hemispheres while participants watched videotape 
segments designed to elicit positive or negative emotion. In the 
frontal lobes, the left hemisphere was more active during the 
segment perceived by the participants as positive and more right 
hemisphere activation was present during the segment perceived as 
negative. Davidson (1992) reported similar findings using movies 
that elicited happiness or disgust. No asymmetry was found at the 
parietal locations.
 Evidence supporting the valence theory has also been obtained in 
10 month old infants. An actress spontaneously generated a happy 
or sad face while the infants sat on their mothers' laps. An EEG 
measured cortical activity during the video segments. Happy 
segments elicited more activity in the left frontal region than 
over the right frontal region (Davidson and Fox, 1982). Moreover, 
greater activity in the left frontal cortex was recorded when 
infants produced spontaneous positive facial expressions and 
greater activation of the right frontal cortex when negative 
facial expressions were produced (Fox, 1991). Fox and Davidson 
(1986) recorded EEG responses from two day old infants who were 
given sugar water and citric acid. Greater activity of the left 
hemisphere was evident after the infants tasted sugar water than 
after they tasted citric acid. These findings are consistent with 
the hypothesis that the left hemisphere is involved in perceiving 
positive emotions and the right hemisphere is involved in 
perceiving negative emotions.
 In another study, patients with uncontrollable emotional 
outbursts accompanying epileptic seizures were examined (Sackheim 
et al., 1982). Of the 91 patients presenting with uncontrollable 
outbursts of laughter, left side hyperexcitability was twice as 
likely as right side hyperexcitability. 
 While no hypothesis of hemispheric asymmetry is consistently 
supported, there is apparent evidence for lateralization of 
production and perception of emotion. The struggle now is finding 
a way to reconcile the findings.   
 One proposal is that the right hemisphere is dominant for 
emotional experiences and the level of activation of the right 
hemisphere determines whether the emotional experience is positive 
or negative. Overactivation of the right hemisphere is associated 
with negative emotional experiences while underactivation is 
correlated with positive emotional experiences (Hellige, 1993). 
Since both hemispheres function as somewhat of a functional unit, 
change in one hemisphere will result in a converse change in the 
complimentary hemisphere. Thus, left hemisphere activation merely 
may be the result of right hemisphere activation, or it may 
function to regulate the intensity of the emotional reaction in 
the dominant right hemisphere (Hellige, 1993).  
 In another theory, right hemisphere domination and valence 
related asymmetry may coexist. According to this theory, anterior 
portions of both hemispheres are differentially specialized with 
regard to experiencing emotions. As noted earlier, the left 
frontal region is more active during positive emotion and the 
right frontal cortex is more active during negative emotion. 
However, the right hemisphere may be dominant for the perception 
of emotion, irrespective of valence (Davidson, 1992). 
 There is considerable evidence that the right hemisphere is more 
involved in the perception of emotion than is the left. There is 
also considerable evidence that the left hemisphere does indeed 
have a role in expressing and experiencing emotion. However, the 
extent to which either hemisphere is involved in emotion is still 
unclear. While many theories subsist, the laterality of emotion is 
far from determined.

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