International Journal of Comparative Psychology

Two experiments investigated the effects of outcome devaluation induced by conditioned taste aversion on the performance of the components of a heterogeneous chain of instrumental actions. In Experiment 1, thirsty rats were trained to perform two actions, R1 and R2, i.e., chain pulling and lever pressing counterbalanced, in sequence to gain access to a sucrose outcome in a single session; i.e., R1→R2→O. Immediately after this session or after a delay the rats were injected with lithium chloride and given an extinction test on the two actions the next day. Although the immediate and delayed groups did not differ in the incidence of R1 on test, the immediate group reduced their performance of R2 relative to the delayed group. Experiment 2 assessed the effect of incentive learning after outcome devaluation. All rats were given an injection of lithium chloride immediately after training on the heterogeneous chain for sucrose reward after which half of the rats were reexposed to the sucrose whereas the remainder were reexposed to water prior to the extinction test. Although reexposure had no effect on the test performance of the R2 component in the chain, it significantly reduced performance of R1. These data are consistent with previous claims that responses proximal to outcome delivery are influenced by a Pavlovian incentive process whereas those more distal are controlled by the experienced incentive value of the outcome; i.e., by an instrumental incentive process.

Contrast refers to a comparison between two conditions of reward such that the relation between them is magnified (relative to an appropriate control condition). It is an effect that is opposite in direction from generalization (which is a form of averaging). Three kinds of contrast have received substantial empirical attention: Incentive contrast in which a sudden change in reward (either an increase or a decrease) results in an overreaction to the change, relative to a control condition; anticipatory contrast in which an anticipated improvement in reward results in less consumption of an initial reward; and differential or behavioral contrast in which a change in reward associated with one stimulus results in a change in behavior associated with a second stimulus in the opposite direction. Here I discuss a fourth kind of contrast that I call within-trial contrast. In this form of contrast a discriminative stimulus is preferred when it follows a less appetitive event (effort, delay, or the absence of reward). A model of this kind of contrast is proposed that is based on a presumed change in the hedonic state of the organism between the end of the less appetitive event and the reward (or stimulus signaling the reward). It is distinguished from an account based on the relative reduction in delay to reinforcement. Finally, I suggest that a general form of this within-trial contrast may help to explain various complex human social phenomena including cognitive dissonance, justification of effort, the effect of extrinsic rewards on behavior that is maintained by intrinsic rewards, and learned industriousness.

Rats in a vehicle treated control condition when shifted from 4% to 32% sucrose displayed successive positive contrast by responding at a significantly higher lick rate in a 5 min trial than rats maintained on 32% sucrose throughout the experiment. In contrast, rats treated with an escalating dose regimen of D-amphetamine (1-10 mg/kg) over a 4 day interval failed to display successive positive contrast. Withdrawal from drug treatment had no effect on lick rate or response latency in rats maintained on 32% sucrose. These data are consistent with many previous reports that withdrawal from a binge-like regimen of psychostimulant drug administration disrupts responding for natural reward stimuli. These findings support the use of psychostimulant withdrawal as a model of drug-induced dysphoria and suggest that incentive contrast is a particularly sensitive measure of these changes in motivation and emotion.

In four experiments (three in operant chambers, one in a runway) with food-deprived rats, we sought to obtain instrumental successive negative contrast (iSNC) and consummatory successive negative contrast (cSNC) following shifts in the value of liquid rewards. Despite finding robust cSNC in each of the four experiments, there was no indication of iSNC in any of the measured instrumental responses (pressing a lever, licking an empty spout, or time to traverse a runway). Consistent with the literature, these results might be taken to suggest that iSNC cannot be obtained following a downshift in liquid reward value. However, behaviors observed in the downshifted rats suggests that the absence of iSNC might be due to the occurrence of competing responses or nonoptimal test conditions. Thus, the failure to observe iSNC in rats that show cSNC is interpreted as a failure of performance.

A within subjects simultaneous contrast experiment evaluated nucleus accumbens (NAc) neural responses to a low (0.05 M) and high (0.5 M) concentration of sucrose in 6 rats. During continuous trials, rats were given repeated brief intraoral infusions of the low and then the high concentration of sucrose while electrophysiological activity of NAc neurons and oromotor behavior (EMG) were measured. Following the continuous phase of testing, the two concentrations were infused in an alternating manner. The results showed that rats reduced oromotor behavior when infused with the low concentration of sucrose when alternated with the high concentration (i.e., during alternating trials) relative to the infusion during the continuous low condition (negative contrast). Rats also increased oromotor behavior for the high concentration when presented during alternating relative to continuous trials (positive contrast). Of 137 NAc neurons, 35 exhibited brief inhibitions or excitations to tastant delivery during baseline testing that were correlated with oromotor output. Some NAc neuronal activity reflected negative or positive contrast effects while other neurons encoded alternating testing in general and still other neurons encoded sucrose concentration. These data demonstrate that neuronal activity in the NAc is altered in coincidence with the expression of contrast in consummatory behavior.

Rats received training in the consummatory successive negative contrast (cSNC) situation in which access to 32% sucrose solution during ten daily trials is followed by a downshift to 4% sucrose. Separate groups were exposed to the hot plate test for pain sensitivity immediately after either the first or the second downshift trial (Trials 11 and 12, respectively). Rats exhibited hypoalgesia after Trial 12 downshift, but not after Trial 11. These results suggest that cSNC induces the release of endogenous opioids that cause hypoalgesia, but only after some experience with the downshifted solution. This interpretation is supported by experiments demonstrating that opioid agonists reduce cSNC, whereas opioid antagonists enhance it.

Two experiments examined multiple schedule behavioral contrast in a group foraging paradigm. Groups of five rats foraged simultaneously in a large open field apparatus with two feeding stations. Food pellets were delivered at each of the feeding stations on multiple Variable Time schedules. As predicted by both the matching law and the ideal free distribution, the relative distribution of behavior between the two feeding stations roughly matched the relative rate of food delivery at the feeding stations. These differences were reflected in both the behavior of individual animals and in the behavior of the group. Positive behavioral contrast was found in Experiment 1, evidenced by an increase in the frequency of response in one component produced by a decreased rate of food delivery in the other component. Negative behavioral contrast was found in Experiment 2, evidenced by a decreased frequency of response in one component produced by an increased rate of food delivery in the other component. Interestingly, there was virtually no correlation between the behavior of an individual animal and the number of pellets consumed by that animal. The present results support other attempts to compare the matching law to the ideal free distribution. The data also show that behavioral contrast is predicted by both models and in fact occurs in ways consistent with both models.

A negative incentive contrast experiment was conducted to determine whether responses to a reward reduction facilitate adaptive decisions by bumble bee ( Bombus impatiens ) foragers. In the acquisition phase of the experiment subjects were trained on blue targets (artificial flowers) that contained 50% (weight percent) or 20% sucrose solution and in the test phase subjects were given a choice between familiar targets and yellow targets that had either an economic profitability lower than, equal to, or higher than blue targets, where all targets contained 20% sucrose solution. Subjects trained to a low reward concentration showed a consistent preference for blue targets in the test phase of the experiment, while subjects that experienced a reward reduction exhibited a temporary disruption of consummatory behavior and developed preferences that reflected the profitability of targets. These results support a functional interpretation of responses to a reward reduction: incentive contrast induces foragers to visit alternative sources of nectar and, thereby, facilitates economical decisions. The choice behavior observed over the test phase suggests that associative processes then direct the formation of flower choices.

Dopamine neurons in the ventral tegmental area (VTA) - nucleus accumbens (NAC) pathway track both absolute and relative properties of reward. The present study used 6-hydroxydopamine lesions of the VTA to test the obligate role of this nucleus in morphine- and cocaine-induced suppression of conditioned stimulus (CS) intake and in chlordiazepoxide- and morphine-induced appetite stimulating effects. The results showed that an 80% reduction in accumbens DA fully prevented drug-induced appetite stimulating effects, augmented a latent inhibition-like effect, but failed to disrupt druginduced suppression of CS intake. These data demonstrate that, while the VTA is essential for responding to the reward-enhancing effects of chlordiazepoxide and morphine, it does not contribute to cocaine- or morphine-induced devaluation of the lesser saccharin reward cue.