Ambivalent Dopamine

Many of animal behaviors can be considered as the end product of the motivation to maximize benefits and minimize costs to the animal. Through intensive investigation based on psychopharmacological manipulations , dopamine in the mammalian nucleus accumbens (NAcc) has been implicated to play a key role in multiple aspects of moti-vational control of behaviors. However, the exact mechanism by which dopamine produces its behavioral effect is still debated A central issue in this debate concerns the role of NAcc dopamine along the two main axes of motivational control, sali-ence and valence. Valence view supports the role of dopamine in mediating appetitive value of the primary reward or conditioned stimuli as well as in reinforcement learning. On the contrary, salience view emphasizes its role in evoking conditioned motivation or " wanting " in the presence of stimuli predicting primary reward, which can engage animals in exerting efforts to obtain predicted reward. Neurophysiological investigations on the activity of dopamine neurons and their actions in the striatum can provide important clues. For example, activity of dopamine neurons or their reflection in the striatum that are similar for appetitive (e.g., juice or monetary gain) and aversive (e.g., air puff or monetary loss) events would suggest a common motivational component or salience of different stimuli that might have opposite valence. In contrast, modulation of dopamine and striatal activity in the opposite directions by such stimuli would imply a role in encoding motivational valence. A recent study reported both types of modulation in the activity of dopamine neurons in the primate brain and has raised a possibility that distinct populations of dopamine neurons might be involved in representing salience and valence separately (Matsumoto and Hikosaka, 2009). What is known about the role of the dopamine system in the human brain in coordinating our behaviors in response to aversive events such as monetary losses? Although many previous studies using functional magnetic resonance imaging (fMRI) have characterized the activation in the ventral striatum, including the NAcc, in relation to the evaluation of anticipated and experienced reward and punishment in various contexts, such studies have provided only indirect clues regarding the role of dopamine signals in the striatum, since dopamine neurons constitute only a subset of neural inputs to the striatum. The fMRI study by Carter et al. (2009) offers an important insight into the role of dopamine signals in human decision making. The authors of this study measured …