The behavioral approach system (BAS) reflects the propensity to respond to signals of reward, including stimuli associated with safety and goal-oriented attack (e.g., anger). Hypomania/mania has been posited to involve increased BAS activity. In contrast, depression has been posited to involve decreased BAS activity. Building on past research, which suggests that increased left frontal cortical activity is a neurophysiological index of BAS activity, the present research tested the hypotheses that proneness toward hypomania/mania symptoms would be related to increased relative left frontal activity and that proneness toward depression symptoms would be related to decreased relative left frontal activity in response to an anger-evoking event. Results from 67 individuals who had completed a measure of proneness toward these affective symptoms and were exposed to an anger-evoking event supported the hypotheses.Prominent motivation theories share the assumption that two systems underlie much behavior. One system manages appetitive, incentive motivation and approach behavior. It has been called a behavioral activation system (Fowles, 1980(Fowles, , 1988, behavioral approach system (BAS; Gray, 1982Gray, , 1987Gray, , 1990Gray, , 1994aGray, , 1994b, and behavioral facilitation system (Depue & Collins, 1999;Depue & Iacono, 1989;Depue, Krauss, & Spoont, 1987). The other system, the behavioral inhibition system (BIS), inhibits ongoing behavior (Gray, 1982(Gray, , 1987(Gray, , 1990(Gray, , 1994b).Gray 's (1982, 1987, 1994a, 1994b) theory has served as the basis for most of the other theories. Hence, we focus on it. Gray has posited that these fundamental motivation systems are present in the mammalian central nervous system and that each responds to separate classes of reinforcing events with particular types of behavior. Moreover, each is mediated by a separate set of interacting brain structures that process particular types of information. The BIS responds to conditioned stimuli associated with punishment, conditioned stimuli associated with the omission or termination of reward (frustrative nonreward), and novel stimuli. It causes the interruption of ongoing behavior, increases arousal, which prepares the organism for the next behavior, and increases attention toward stimuli, which would increase information gain. The neural structures posited to be involved in the BIS are the septohippocampal system (composed of the septal area, entorhinal cortex, dentate gyrus, hippocampus, and subicular area), the prefrontal cortex, and the monoaminergic pathways that ascend from the mesencephalon to innervate the septohippocampal system.The BAS is posited to respond to stimuli associated with reward and relieving nonpunishment (safety). It also responds to stimuli associated with skilled escape and predatory aggression. The neural structures involved in the BAS are the basal ganglia (the dorsal and ventral striatum, and dorsal and ventral pallidum), the dopaminergic fibers that ascend from the mesencephalon (substantia nigra and nu...