The Lateralization of Emotion
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.
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.
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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