The idea that the two cerebral hemispheres make different contributions to the development of dysphoric emotions (e.g. depression, anxiety) has attracted considerable attention in the past few years. Evidence for this idea comes from two separate research lines. The first is exemplified by the experimental work of Hugdahl,(!3) who directly manipulated hemisphere information processing (e.g. by confining visual stimuli to one visual field/hemisphere) in order to examine the differential involvement of the two hemispheres in emotional reactions. His studies indicate that fear-relevant stimuli (e.g. pictures of snakes) evoke a cardiac defence reaction when they are flashed to the right hemisphere (i.e. left visual field) of healthy subjects, but not when they are flashed to the left hemisphere (i.e. right visual field) of these subjects. Also, the two hemispheres of normal subjects apparently differ in their conditionability. That is, when visual stimuli flashed to either the right or the left side of the brain are followed by electric shocks, only the stimuli presented to the right hemisphere eventually elicit conditioned fear reactions. Similarly, studies by Wittling(1i) indicate that emotionally provocative film clips presented to the right hemisphere of normal subjects produce a higher increase in cortisol secretion than do left-hemisphere presentations of the same film clips. This suggests that cortisol reaction to emotional stimuli is under the primary control of the right hemisphere. Taken together, these findings seem to imply that the right hemisphere is more sensitive to emotional information.
A second line of research relies on direct measures of brain activity. A good example is provided by the electroencephalography ( EEG) studies of Davidson/1,5) who demonstrated that, in normal subjects, negative emotions (e.g. disgust, fear) are accompanied by a stronger right-hemisphere than left-hemisphere activation. Research findings such as these have led many authors to conclude that the right frontal areas sustain negative emotions and avoidance behaviour, while the left frontal areas are more involved in pleasant emotions and approach behaviour (for a more refined version of this theory, see Heller and Nitschke (1J>). There are good reasons to take this argument one step further and to assume that right- and left-frontal overactivation are trait-like characteristics that reflect susceptibility to avoidance-related and approach-related behaviour respectively. That is, habitual overactivation of one frontal area or underactivation of the other frontal area corresponds to certain affective styles. Germane to this issue is a study which found that young infants with a strongly activated right frontal hemisphere tend to react with crying to subsequent maternal separation. ^ The idea that asymmetries in frontal activation are linked to affective styles is further buttressed by the finding that heightened right-hemisphere activation during rest predicts the extent to which healthy adults react with fear and disgust to emotional film clips. (1,8)
Research on frontal asymmetry and affective phenomena is important because it may shed light on certain psychiatric conditions, such as poststroke depression. Depression is considered to be the most common emotional consequence of stroke, occurring in 20 to 50 per cent of stroke patients. There are robust indications that poststroke depression has a negative impact on the rehabilitation of stroke patients. (1§,> Additionally, damage to the left-frontal area is more likely than damage to any other cortical region to be associated with depression. A plausible interpretation of this association is that left-frontal lesions result in a decreased activity in this region which, in turn, would lead to a deficit of the positively valenced approach system sustained by the left-frontal areas. Such an interpretation accords well with the finding that neurologically intact patients with current unipolar depression or a history of unipolar depression exhibit reduced left-frontal EEG activity. (20)
Some authors have argued that left-frontal underactivation represents a risk factor for depression because it biases individuals in favour of a negatively valenced avoidance system. Interestingly, there is tentative evidence that left-frontal underactivation is accompanied by cognitive biases (see below) that are known to play a role in the aetiology of depression. Future research on affective brain asymmetry may clarify the neuropsychological basis of these cognitive biases. In addition, research in this domain may provide us with clinical tools for predicting treatment outcome. A study by Bruder et al.(22) nicely illustrates this point. These authors measured hemisphere activation by means of a dichotic listening task before depressive patients underwent cognitive-behavioural treatment. Cognitive therapy responders exhibited higher left-hemisphere activity (as indexed by larger right-ear advantages for verbal material) than non-responders. In fact, left-hemisphere activation was the best predictor of a favourable response to cognitive therapy.
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