Right hemisphere dominance is usually very clear for negative emotions, but becomes less clear when some positive emotions are taken into account. To explain the right hemisphere’s differential involvement in positive and negative emotions, some authors have assumed separate hemispheric specializations for these two categories of emotions (Borod 2000; Murphy et al 2003; Davidson et al 2004). In other words, both hemispheres process emotion, but each hemisphere is specialized for particular types of emotion, particularly in the lateral frontal cortex (Wager 2003). In one formulation, the left hemisphere controls positive emotions and the right hemisphere controls negative emotions (Sackeim et al 1978; Robinson and Starkstein 1989; Davidson 1992; Gur et al 1994). A disproportional number of patients who have suffered trauma to the left frontal lobe, especially to the lateral prefrontal cortex or basal ganglia, become depressed (Paradiso et al 1999; Narushima et al 2003). Patients with right frontal damage, however, are more likely to show signs of inappropriate cheerfulness and mania (Starkstein et al 1989). Further, individuals with affective disorders have shown abnormal laterality patterns that may be suggestive
