Hemispheric Asymmetries In The Regulation Of Aggression

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Several lines of neuropsychological research suggest differences in left and right hemisphere specialization for the processing of emotion, including anger and aggression. The left hemisphere plays a greater role in decoding linguistically conveyed emotional information, and the right hemisphere is more important in processing nonverbal emotional cues, such as prosody and facial expression of emotion. Moreover, the right hemisphere may be more highly specialized for mediating emotional responses in general and negative emotional responses such as fear and anger in particular. These conclusions are supported by studies of functional asymmetry for verbal versus nonverbal expression of affect, verbal versus nonverbal decoding of affects expressed by others, and asymmetric facial expression of affect in patients with unilateral stroke, other asymmetric neurological injuries, and transient hemisphere inactivation during Wada testing and also in normal experimental subjects.

Studies in focal lesion patients suggest an important role of hemispheric specialization in the genesis of hostile behaviors. A case control study in focal stroke patients identified lesion location in the anterior, left hemisphere as a predictor of aggressive behavior, independent of the presence of depression (Fig. 3). Among 50 patients with temporal lobe epilepsy, those

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Figure 3 Association of left hemisphere lesions with aggression. Template mapping of CT scan lesions in 18 consecutive stroke patients with violent aggressive outbursts demonstrates a preponderance of left hemisphere lesions, especially left frontal (reproduced with permission from S. Paradiso, R. G. Robinson, and S. Arndt, 1996, J.Nerv. Mental Dis. 184, 750).

Figure 3 Association of left hemisphere lesions with aggression. Template mapping of CT scan lesions in 18 consecutive stroke patients with violent aggressive outbursts demonstrates a preponderance of left hemisphere lesions, especially left frontal (reproduced with permission from S. Paradiso, R. G. Robinson, and S. Arndt, 1996, J.Nerv. Mental Dis. 184, 750).

exhibiting intermittent explosive disorder were more likely to have left amygdala and left periamygdala lesions due to encephalitis or other structural insults.

Neuropsychological and psychophysiologic studies in unselected populations of violent criminal offenders also show frequent evidence of left hemispheric dysfunction. When neuropsychological deficits are observed in studies of violent groups, they tend to involve not only frontal-executive functions but also verbal comprehension, expressive speech, and other left hemisphere language functions. These findings are consonant with those of many studies in conduct-disordered and delinquent juveniles. In addition to lower average overall IQ, these individuals frequently have a disproportionately lowered verbal IQ (language, left hemisphere) compared to performance IQ (visuospatial, right hemisphere). Psychophysiologic studies employing computerized EEG spectral analysis indicate that persistently violent behavior among psychiatric inpatients is linked to increased 8-band slow-wave activity in left frontotemporal derivations. Divided visual field, dichotic listening, skin conductance asymmetry, and lateral preference studies additionally suggest subtle abnormalities of left hemispheric function in sociopathic individuals without overt neurological lesions. In one small series of subjects examined by PET, decreased blood flow and metabolism were observed in the left temporal lobe of all four institutionalized, sexually violent offenders studied, and left frontal hypometabolism was observed in two of the four. In a SPECT study, left anterior temporal and bilateral dorsofrontal hypoperfusion were among the abnormalities distinguishing dementia patients with aggression from those without.

Based on the results of these and other laterality studies in violent individuals, different theorists have proposed that either left hemispheric or right hemispheric networks play a predominant functional role in the regulation of aggression, and that subtle developmental hemispheric abnormalities underlie repeatedly aggressive behavior in a proportion of "functional" psychopaths. A broader view is that each hemisphere performs complementary processing related to hostile behavior, and functional abnormalities of either hemisphere may produce disturbed aggressive responding through distinctive mechanisms. Left hemisphere dysfunction, implicated in a preponderance of studies, may lead to overt expression of negative affects mediated by the right hemisphere, diminished linguistic regulation over behavior, and adverse social encounters due to impaired verbal communication. Right hemisphere dysfunction may lead to improper intrahemispheric decoding and encoding of prosody, facial expressions, and other nonverbal emotional responses, overreliance on semantic processing, impaired self-awareness of physiological arousal, and a distinctive pattern of altered aggressivity.

Right hemisphere parietofrontal lesions may impair an individual's ability to interpret and produce emotional gestures, prosody, and affect and to monitor internal somatic states. This may produce an acquired "sociopathy," even in the absence of bifrontal lesions. Although the most frequently noted pattern of altered behavior after right parietal cortical lesions is one of inappropriate cheerfulness and denial of illness, irritability and aggressive outbursts may also ensue. A well-known case is that of former Supreme Court Justice William Douglas, who exhibited disturbed social judgment following a right hemisphere stroke. Although he retained sufficient linguistic and abstract reasoning ability to return to the bench, inappropriate social behaviors soon forced his retirement.

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