There are three major hypotheses regarding the lateralization of emotion in the brainthe right-hemisphere hypothesis (RHH), the valence hypothesis, and the approachwithdrawal hypothesis. The approach-withdrawal hypothesis, which is the most widely accepted, states that emotions that elicit approach behaviors are lateralized to the left hemisphere, while emotions that elicit withdrawal behaviors are lateralized to the right hemisphere. In line with this hypothesis, it has been found that persons with depression show left frontal hypoactivity and right frontal hyperactivity. This hemispheric asymmetry appears not to influence mood but rather emotional reactions to affective stimuli. That is, a person with such an asymmetry does not show a predominant negative mood, but rather heightened negative reactions to occurrences in the environment. The asymmetry may also be a biological marker of depression, with research evidence that it is found in remitted depressives and in infants of depressed mothers. Currently, research in this area focuses on identifying the mechanism underlying the link between the asymmetry and depression.

Difficulty shifting attention away from negative stimuli once engaged is a well-established cognitive bias observed in depression. This study attempted to determine whether this impaired disengagement of attention is lateralized in the brain. Thirty depressed and 30 control participants performed an attention disengagement task wherein the valence of the stimulus and the visual field was presented. The depressed group had longer reaction times than the control group, indicative of the typical cognitive and psychomotor slowing seen in depression. The effect of visual field presentation on the ability to disengage attention however was different for controls as compared to the patients. In controls, a distinct right hemisphere advantage was seen for disengaging attention which is in line with research that has identified right hemisphere structures as the seat of behavioural inhibition. In the depressed group, however, this right hemisphere advantage was not observed.

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