Dopamine prediction error responses update demand.

According to the law of demand, the quantity demanded for a reward is dependent upon the price; as the price of the reward increases, the quantity demanded decreases (1). Conversely, when the price decreases, the quantity demanded increases. This relationship can be observed at any local bar during “happy hour,” a limited time during which the price of beer is discounted. As happy hour draws to a close, patrons quickly consume pints of beer and order “one more round.” The discounted price increases the quantity demanded of beer. The relationship between price and the quantity demanded is formalized using demand curves. Fluctuations in price cause movement along a demand curve, whereas changes in factors such as subjective value or income shift demand. These shifts result in new demand curves (Fig. 1). Understanding the neural mechanisms that participate in this economic tradeoff has implications in neuroscience, economics, and policy making. In PNAS, Schelp et al. (2) demonstrate that dopamine teaching signals, phasic activity that codes for the differences between received and predicted values, cause changes in the behavioral manifestations of economic demand. Consumers determine demand by calculating the values of rewards they desire, and weighing those values against their income and externally controlled prices. How does the nervous system mediate this transformation from desire to demand? It is likely that neural substrates contributing to this transformation can be found in the reward system. Dopamine neurons respond to rewards and are an ideal candidate to influence consumer demand. These neurons emit action potentials in phasic bursts that code for reward prediction errors, the differences between received and predicted rewards (3, 4), as follows: