FIGURE 3.

DA value signals encode price and modify the maximal price rats will pay for positive or negative reinforcement. Positive reinforcement: DA (DA) concentration (mean ± SEM) evoked by a reward predictive cue and delivery of a 45 mg sugar pellet decreased across the first five prices in a within-session behavioral economics-based task. In this task, the unit-price (responses/mg sugar) increased across fixed epochs of time (A). Optogenetic stimulation alters price sensitivity in a representative rat. Cumulative response records from one animal responding in the behavioral economic task under baseline conditions (light orange), and those in which DA release is amplified at cue presentation (dark orange) and at reward delivery (purple) (B). Changes in value were assessed using demand curves which measure changes in consumption in response to changes in unit-price. We formally extracted a dependent measure of value (i.e., α) which, represents the rate at which demand curve decay. Demand decays at a faster rate when the animal becomes more sensitive to price. As the animal is willing to pay less for the commodity, we would interpret the resulting increase in α as a decrease in value (C). The same data from the cumulative records in panel (B) are replotted in the form of demand curves to illustrate the optogenetic-induced shifts in value (D). Negative reinforcement: The concentration of DA evoked by a warning signal that predicted the opportunity to avoid decreased with the price to avoid. Inset: Representative avoidance trial shows that DA concentration began increasing in anticipation of warning signal presentation (E). The concentration of DA release events during the safety period decreased with price in trials in which the rat successfully avoided electrical foot shock (F). Optogenetic activation of VTA DA neurons at the warning signal made animals more sensitive to price, consistent with a negative reward prediction error (G). In contrast, optically stimulating DA neurons at successful avoidance made animals less sensitive to price, consistent with a positive reward prediction error (H). Republished from Schelp et al. (2017) and Pultorak et al. (2018). ^∗p < 0.05.