Increases In Reward Magnitude Research Articles

Shared and distinct reward neural mechanisms among patients with schizophrenia, major depressive disorder, and bipolar disorder: an effort-based functional imaging study.

Unwillingness to exert effort for rewards has been found in patients with schizophrenia (SCZ), major depressive disorder (MDD), and bipolar disorder (BD), but the underlying shared and distinct reward neural mechanisms remain unclear. This study aimed to compare the neural correlates of such impairments across different diagnoses. The neural responses in an effort-expenditure for reward task (EEfRT) were assessed in 20 SCZ patients, 23 MDD patients, 17 BD patients, and 30 healthy controls (HC). The results found shared activation in the cingulate gyrus, the medial frontal gyrus, and the middle frontal gyrus during the EEfRT administration. Compared to HC, SCZ patients exhibited stronger variations of functional connectivity between the right caudate and the left amygdala, the left hippocampus and the left putamen, with increase in reward magnitude. In MDD patients, an enhanced activation compared to HC in the right superior temporal gyrus was found with the increase of reward magnitude. The variations of functional connectivity between the caudate and the right cingulate gyrus, the left postcentral gyrus and the left inferior parietal lobule with increase in reward magnitude were weaker than that found in HC. In BD patients, the degree of activation in the left precuneus was increased, but that in the left dorsolateral prefrontal cortex was decreased with increase in reward probability compared to HC. These findings demonstrate both shared and distinct reward neural mechanisms associated with EEfRT in patients with SCZ, MDD, and BD, implicating potential intervention targets to alleviate amotivation in these clinical disorders.

Neural substrates of the impaired effort expenditure decision making in schizophrenia.

Unwillingness to expend more effort to pursue high value rewards has been associated with motivational anhedonia in schizophrenia (SCZ) and abnormal dopamine activity in the nucleus accumbens (NAcc). The authors hypothesized that dysfunction of the NAcc and the associated forebrain regions are involved in the impaired effort expenditure decision-making of SCZ. A 2 (reward magnitude: low vs. high) × 3 (probability: 20% vs. 50% vs. 80%) event-related fMRI design in the effort-expenditure for reward task (EEfRT) was used to examine the neural response of 23 SCZ patients and 23 demographically matched control participants when the participants made effort expenditure decisions to pursue uncertain rewards. SCZ patients were significantly less likely to expend high level of effort in the medium (50%) and high (80%) probability conditions than healthy controls. The neural response in the NAcc, the posterior cingulate gyrus and the left medial frontal gyrus in SCZ patients were weaker than healthy controls and did not linearly increase with an increase in reward magnitude and probability. Moreover, NAcc activity was positively correlated with the willingness to expend high-level effort and concrete consummatory pleasure experience. NAcc and posterior cingulate dysfunctions in SCZ patients may be involved in their impaired effort expenditure decision-making. (PsycINFO Database Record

How sequential changes in reward magnitude modulate cognitive flexibility: Evidence from voluntary task switching.

There is much evidence that the prospect of reward modulates cognitive control in terms of more stable behavior. Increases in expected reward magnitude, however, have been suggested to increase flexible behavior as evidenced by reduced switch costs. In a series of experiments, the authors provide evidence that this increased cognitive flexibility following increases in reward magnitude also promotes deliberate task switching. A modified task switching paradigm with forced- and free-choice trials and varying reward prospects was used. In Experiments 1-3 the prospect of a reward increase as compared to unchanged high reward increased voluntary switching rate (VSR). Experiment 4 showed that the prospect of a reward decrease did not alter VSR as compared to unchanged low reward. Experiment 5 used a standard voluntary task switching procedure and confirmed VSR effects found in Experiments 1-4. These findings are strong evidence for a mechanism that biases the cognitive system either toward stability or flexibility depending on changing reward expectation. Results are discussed within the framework of the adaptive gain theory.

The role of the nucleus accumbens core in impulsive choice, timing, and reward processing.

The present series of experiments aimed to pinpoint the source of nucleus accumbens core (AcbC) effects on delay discounting. Rats were trained with an impulsive choice procedure between an adjusting smaller sooner reward and a fixed larger later reward. The AcbC-lesioned rats produced appropriate choice behavior when the reward magnitude was equal. An increase in reward magnitude resulted in a failure to increase preference for the larger later reward in the AcbC-lesioned rats, whereas a decrease in the larger later reward duration resulted in normal alterations in choice behavior in AcbC-lesioned rats. Subsequent experiments with a peak timing (Experiments 2 and 3) and a behavioral contrast (Experiment 4) indicated that the AcbC-lesioned rats suffered from decreased incentive motivation during changes in reward magnitude (Experiments 2 and 4) and when expected rewards were omitted (Experiments 2 and 3), but displayed intact anticipatory timing of reward delays (Experiments 2 and 3). The results indicate that the nucleus accumbens core is critical for determining the incentive value of rewards, but does not participate in the timing of reward delays.

Midazolam administered to rats induces anterograde amnesia for changes in reward magnitude.

L. P. Crespi (1942) showed that rats trained to run an alley for a large food reward slowed down when shifted to a small reward. This effect is usually interpreted as an aversive emotional response to reward reduction (A. Amsel, 1958). Benzodiazepines attenuate the behavioral effects of reward reduction (cf. C. F. Flaherty, 1990), but the emphasis has been on their anxiolytic, not memory-impairing, effects. Researchers trained rats (175-200 g) to run an alley for food until asymptote was reached. Reward magnitude was then either decreased (Experiment 1) or increased (Experiment 2). The benzodiazepine midazolam (1 mg/kg ip), injected immediately prior to a decrease or increase in reward magnitude, impaired the later retention of both changes in a manner consistent with anterograde amnesia. The findings suggest that the memory-impairing effects of benzodiazepines may, at least in part, influence the response to reward reduction.

Preference for Predictable Small Rewards over Unpredictable Larger Rewards

Numerous prior studies have reported that rats, pigeons, and humans prefer predictable over unpredictable rewards of equal frequency and magnitude. A frustration-theory analysis of this preference suggests that it obtains because the unpredictable partial reinforcement procedure is aversive whereas the predictable discrimination procedure loses its aversiveness. The preference, on such an analysis, arises due to the tendency to avoid the unpredictable of two alternatives. Since frustration varies as a function of magnitude of reward, the avoidance tendency should increase with increases in reward magnitude in the unpredictable alternative. One group of rats in the present study showed a clear preference for seven versus five 45-mg Noyes Pellets. A second group showed the oft reported preference for five pellets predictable versus five pellets unpredictable. A third group of rats showed a preference for a five-pellet predictable reward over a seven-pellet unpredictable reward. The results of this experiment provide evidence for a frustration-theory analysis of the preference for predictable reward.

The effects of schedules of reinforcement and gradual or abrupt increases in reward magnitude on resistance to extinction

During an initial phase of acquisition training, rats were given 41 trials (1 trial/day). The continuous-gradual group received one 45-mg food pellet on each trial during this phase, as did the continuous-abrupt group. Two partial groups, the partial-gradual and partial-abrupt groups, were maintained on a 50% partial reinforcement schedule during this initial phase, with one food pellet as reward on reinforcement trials. In a subsequent 11-day acquisition phase, the continuous-abrupt and partial-abrupt groups were shifted immediately to a 12-pellet reward magnitude administered according to the reinforcement schedule received during Phase 1. The continuous-gradual and partial-gradual groups also continued on their respective reinforcement schedules but experienced gradual increases in reward magnitude up to 12 pellets. Finally, all subjects were given 20 trials of extinction training. The extinction results showed that with partial reinforcement training, gradual increases in magnitude of reward produced greater resistance to extinction than did abrupt increases in reward amount. The theoretical significance of these data was discussed.

Positive contrast following gradual and abrupt increases in reward magnitude using delay of reinforcement

In Experiment 1, rats were given a 1-pellet reward for 48 preshift trials. During a subsequent 20-trial postshift phase, one group was shifted to a 12-pellet reward on Trial 1, a second was shifted on Trial 11, and a third was given 1 more pellet each trial and then 12 pellets for the last 10 trials. The speeds of all three groups increased to a level above that of a control group given a 12-pellet food reward throughout training (positive contrast). In experiment 2, rats were shifted from 1 to 12 pellets either gradually or abruptly following either abbreviated training (9 trials) or extended training (20 trials). One group of control subjects received 12 pellets throughout training. The results revealed a positive contrast effect for gradually shifted subjects following extended training but not following abbreviated training. The abrupt shift procedure produced positive contrast following abbreviated training but only a marginal effect following extended training. These results indicate that, contingent upon the amount of preshift training, either gradual or abrupt reward increases may produce positive contrast.

Incentive shift in the rat following training to resist punishment

An incentive shift paradigm was used to test for the similarity of fear and frustration. In Experiment 1, rats trained to resist electric shock punishment showed neither a negative contrast effect nor any performance decrement when reward was shifted from 10 to one pellet. Experiment 2 replicated the basic findings of Experiment 1, but also showed that punishment training did not influence the magnitude of performance shift for animals receiving increases in reward magnitude. Finally, Experiment 3 additionally found that rats sensitized to punishment showed an increase in negative contrast effect. These results support the hypothesized functional similarity between conditioned fear and conditioned frustration with learned persistence or sensitivity to one generalizing to the other as suggested by Amsel's (1972) theory of persistence.

Positive and negative contrast effects using delayed reinforcement

In the first experiments, four groups of rats received either 2 pellets (L) or 22 pellets (H) in two successive phases in a straight runway, with a 20 sec delay of reinforcement. The sequences of reward magnitudes were HL, HH, LH, and LL. Significant positive (LH vs HH) and negative (HL vs LL) contrast effects were obtained. A second experiment, utilizing only an increase in reward magnitude (1–8 pellets) and an unshifted control group, both receiving delayed reinforcement, confirmed the positive contrast effect observed in the first experiment. The positive contrast effect was discussed in terms of the possible inhibition generated by the use of delayed reinforcement.

Effects of an increment in monetary incentive magnitude on instrumental responding and repeated increases in reward magnitude in humans

Human Ss performed at a reliably higher level for a tone previously paired with an increase in monetery incentive magnitude and consequent positive incentive contrast effects than did individuals for a tone previously associated with the larger control groups’s quantity of incentive. In Experiment 2, upward incentive contrast effects which resulted from a single increment in magnitude of monetery reinforcement were eliminated in individuals who experienced the increase in amount of money after an upshift in reward amount. These data are consistent with the notion that positive incentive contrast effects in human Ss are due to some form of appetitive motivational state.

Negative and Positive Behavioural Contrast in the Same Animals

THE reactions of animals to situations that suddenly become more attractive or less attractive have long seemed potentially useful for the evaluation of drugs and other treatments assumed to affect emotional responsivity1. Crespi2,3 reported that rats shifted to a larger (or smaller) amount of reward ran faster (or slower) to the goal in a runway than control rats who were accustomed to the larger (or smaller) amount. But it has usually been difficult to find reliable overshooting or “positive contrast” effects to an increase in reward magnitude, and even the more reliable negative contrast effect (undershooting to a decrease in reward magnitude) has proved troublesome when separate experimental and control groups are used4. Furthermore, the transient character of the effects, even when obtained, makes them inconvenient for the chronic testing of various treatments, such as antidepressant drugs or brain lesions. Finally, recent demonstrations of “behavioural contrast” have depended on somewhat complex multiple schedule situations, not necessarily related to that originally used by Crespi5–7. Here we report a simple technique for the repeated study of both positive and negative effects in the same animals, with results that are stable over weeks of daily testing.

Amobarbital sodium and instrumental performance changes following an increase in reward magnitude

Four groups of hungry rats (N = 12) received 54 straight runway trials at a constant magnitude reward of either 15 or one food pellet, followed by 42 trials in which all groups received 15 pellets reward. Within each reward condition one group received a daily injection of amobarbital sodium before running and one group received a saline injection. The rate of performance increase following the reward shift was not affected by the drug, in contrast to prior observations that the rate of performance decrease following a reward decrement was attenuated by the drug. These data indicate that amobarbital does not degrade S’s ability to detect changes in reward stimuli, and support the position that the prior observation resulted because the drug reduces the emotional consequences of reward decrements.