The Reinforcement Sensitivity Theory proposes that a neurobiological system, the Behavioral Activation System, defines individual differences on the subject's sensitivity and reactivity to appetitive stimuli associated with mesocorticolimbic structures, while this system does not mediate aversive stimulus processing. However, Jeffrey A. Gray's model also predicts the system's antagonism between this appetitive system and another aversive stimulus sensitive system, the Behavioral Inhibitory System/Fight-Flight-Freeze System, mostly associated with limbic structures. Therefore, direct modulation of brain activation during appetitive stimulus processing should be expected from the Behavioral Activation System, while inverse modulation during aversive stimulus processing may be expected to reflect the system's antagonism. Using the Sensitivity to Reward scale of the SPSR questionnaire to assess individual differences in the activity of the reward system, we present different behavioral and neuroimaging data to illustrate our view. The first experiment was based on a simple letter-judgment task while viewing erotic and aversive pictures selected from the International Affective Picture System. A second experiment employed a task performed by participants to detect infrequent aversive (i.e., stop) signals when responding to reward. The results from these studies were consistent with the idea that Behavioral Activation System-related personality traits mediate the brain activation associated with appetitive stimulus processing in reward-related areas, while it also showed its antagonism to aversive systems through a negative mediation on the limbic cortex activation. To conclude, sensitivity to reward may be understood as a form of impulsivity related to both better appetitive learning and poorer aversive learning.