Enhancement of Food-Rewarded Instrumental Responding by an Appetitive Conditioned Stimulus

The effects of extinction and an explicitly unpaired treatment on the reinforcing properties of a Pavlovian conditioned stimulus

This series of experiments examined the effects of extinction and an explicitly unpaired treatment on the ability of a conditioned stimulus (CS) to function as a reinforcer. Rats were trained to lever press for food, exposed to pairings of a noise CS and food, and, finally, tested for their willingness to lever press for the CS in the absence of the food. Experiment 1 provided a demonstration of conditioned reinforcement (using controls that were only exposed to unpaired presentations of the CS and food) and showed that it was equivalent after one or four sessions of CS-food pairings. Experiments 2 and 3 showed that, after one session of CS-food pairings, repeated presentations of the CS alone reduced its reinforcing properties; but after four sessions of CS-food pairings, repeated presentations of the CS alone had no effect on these properties. Experiment 4 showed that, after four sessions of CS-food pairings, explicitly unpaired presentations of the CS and food completely undermined conditioned reinforcement. Finally, Experiment 5 provided within-experiment evidence that, after four sessions of CS-food pairings, the reinforcing properties of the CS were disrupted by explicitly unpaired presentations of the CS and food but spared by repeated presentations of the CS alone. Together, these findings indicate that the effectiveness of extinction in undermining the reinforcing properties of a CS depends on its level of conditioning; and that, where extinction fails to disrupt these properties, they are successfully undermined by an explicitly unpaired treatment. They are discussed with respect to findings in the literature on Pavlovian-to-instrumental transfer; and the Rescorla-Wagner model, which anticipates that an explicitly unpaired treatment will be more effective than extinction in reversing the effects of conditioning.

The Relationship Between Specific Pavlovian Instrumental Transfer and Instrumental Reward Probability

Goal-directed behavior is influenced by environmental cues: in particular, cues associated with a reward can bias action choice toward actions directed to that same reward. This effect is studied experimentally as specific Pavlovian-instrumental transfer (specific PIT). We have investigated the hypothesis that cues associated to an outcome elicit specific PIT by rising the estimates of reward probability of actions associated to that same outcome. In other words, cues reduce the uncertainty on the efficacy of instrumental actions. We used a human PIT experimental paradigm to test the effects of two different instrumental contingencies: one group of participants had a 33% chance of being rewarded for each button press, while another had a 100% chance. The group trained with 33% reward probability showed a stronger PIT effect than the 100% group, in line with the hypothesis that Pavlovian cues linked to an outcome work by reducing the uncertainty of receiving it. The 100% group also showed a significant specific PIT effect, highlighting additional factors that could contribute to specific PIT beyond the instrumental training contingency. We hypothesize that the uncertainty about reward delivery due to testing in extinction might be one of these factors. These results add knowledge on how goal-directed behavior is influenced by the presence of environmental cues associated with a reward: such influence depends on the probability that we have to reach a reward, namely when there is less chance of getting a reward we are more influenced by cues associated with it, and vice versa.

How food cues can enhance and inhibit motivation to obtain and consume food

Learning may play an important role in over-eating. One example is Pavlovian-to-instrumental transfer (PIT), whereby reward cues facilitate responding to obtain that reward. Whilst there is increasing research indicating PIT for food in humans, these studies have exclusively tested PIT under instrumental extinction (i.e. when the food is no longer available), which may reduce their ecological validity. To address this, we conducted two experiments exploring PIT for food in humans when tested under instrumental reinforcement. Participants first underwent Pavlovian discrimination training with an auditory cue paired with a chocolate reward (CS+) and another auditory cue unpaired (CS-). In instrumental training participants learnt to press a button to receive the chocolate reward on a VR10 schedule. In the test phase, each CS was presented whilst participants maintained the opportunity to press the button to receive chocolate. In Experiment 1, the PIT test was implemented after up to 20 min of instrumental training (satiation) whereas in Experiment 2 it was implemented after only 4 min of instrumental training. In both experiments there was evidence for differential PIT, but the pattern differed according to the rate of responding at the time of the PIT test. In low baseline responders the CS+ facilitated both button press responding and consumption, whereas in high baseline responders the CS- suppressed responding. These findings suggest that both excitatory and inhibitory associations may be learnt during PIT training and that the expression of these associations depends on motivation levels at the time the cues are encountered. Particularly concerning is that a food-paired cue can elicit increased motivation to obtain and consume food even when the participant is highly satiated and no longer actively seeking food, as this may be one mechanism by which over-consumption is maintained.

The effects of amphetamine sensitization on conditioned inhibition during a Pavlovian-instrumental transfer task in rats

RATIONALE

Psychostimulant sensitization heightens behavioral and motivational responses to reward-associated stimuli; however, its effects on stimuli associated with reward absence are less understood.

OBJECTIVES

We examined whether amphetamine sensitization alters performance during Pavlovian-instrumental transfer (PIT) to conditioned excitors and inhibitors. We further sought to characterize the effects of amphetamine sensitization on learning versus performance by exposing rats to amphetamine prior to Pavlovian training or between training and test.

METHODS

Adult male Long-Evans rats were given conditioned inhibition (A+/AX-) and Pavlovian (B+) training, followed by variable-interval instrumental conditioning. Rats were sensitized to D-amphetamine (2 mg/kg daily injections for 7 days) or served as non-exposed controls. Rats were given a PIT test, in which they were presented with stimulus B alone or in compound with the conditioned inhibitor (BX).

RESULTS

During the PIT test, control rats significantly reduced instrumental responding on BX trials (to approximately 50 % of responding to B). Amphetamine sensitization prior to Pavlovian conditioning increased lever pressing on BX trials and reduced lever pressing on B trials compared to controls. Amphetamine sensitization between training and test increased lever pressing on B and BX trials compared to controls. No effects of sensitization were observed on conditioned food cup approach.

CONCLUSIONS

Amphetamine sensitization increases instrumental responding during PIT to a conditioned inhibitor by enhancing the excitation of conditioned stimuli and reducing the inhibition of conditioned inhibitors.

Pavlovian to instrumental transfer: a neurobehavioural perspective

Pavlovian-to-instrumental transfer (PIT) is a key concept in developing our understanding of cue-controlled behaviours. Here we have reviewed the literature on behavioural and neurobiological factors that influence PIT. Meta-analyses of the data for individual groups in PIT studies revealed that PIT is related to both the order and amounts of instrumental and Pavlovian training, and that it is critically determined by competition between instrumental and Pavlovian responses. We directly addressed the role of response competition in PIT in two experiments which showed that extensive Pavlovian conditioning produced more Pavlovian magazine visits and weaker PIT than moderate Pavlovian conditioning (Experiment 1); and that PIT lost after extensive Pavlovian conditioning was restored by Pavlovian extinction training (Experiment 2). These findings confirm that response competition is indeed an important determinant of PIT. This has significant implications for lesion and inactivation studies that assess the neurobiological substrates of PIT, as well as attempts to demonstrate PIT in the drug self-administration paradigm where the effect is yet to be reliably shown.

The effect of pre-exposure to food cues on the eating behavior of restrained and unrestrained eaters

This study investigated the effect of pre-exposure to two types of food cues (olfactory and cognitive) on food intake by restrained and unrestrained eaters. Subjects were exposed to either no cue, an olfactory cue, a cognitive cue or a combination of the two types of food cues for ten minutes prior to eating. Restrained eaters ate significantly more than did unrestrained eaters after exposure to the food cues. There was no difference in food intake when there was no pre-exposure to the cues. Although baseline subjective ratings were equivalent for both groups of subjects, after cue pre-exposure, restrained subjects, in keeping with their increased consumption, indicated a significantly greater craving, liking, and desire to eat the cued food (pizza) than did the unrestrained subjects. These findings suggest that restrained eaters are more sensitive and reactive to food cues than are unrestrained eaters. The food cues appeared to generate an appetitive urge to eat in restrained eaters.

Alternative reinforcement increases resistance to change: Pavlovian or operant contingencies?

Two multiple-schedule experiments with pigeons examined the effect of adding food reinforcement from an alternative source on the resistance of the reinforced response (target response) to the decremental effects of satiation and extinction. In Experiment 1, key pecks were reinforced by food in two components according to variable-interval schedules and, in some conditions, food was delivered according to variable-time schedules in one of the components. The rate of key pecking in a component was negatively related to the proportion of reinforcers from the alternative (variable-time) source. Resistance to satiation and extinction, in contrast, was positively related to the overall rate of reinforcement in the component. Experiment 2 was conceptually similar except that the alternative reinforcers were contingent on a specific concurrent response. Again, the rate of the target response varied as a function of its relative reinforcement, but its resistance to satiation and extinction varied directly with the overall rate of reinforcement in the component stimulus regardless of its relative reinforcement. Together the results of the two experiments suggest that the relative reinforcement of a response (the operant contingency) determines its rate, whereas the stimulus-reinforcement contingency (a Pavlovian contingency) determines its resistance to change.

Appetitive Pavlovian-instrumental interactions: effects of inter-stimulus interval and baseline reinforcement conditions

Experiment I manipulated two variables which appear to influence whether a signal for food enhances or suppresses food-rewarded instrumental performance: interstimulus interval (ISI) during classical conditioning and instrumental reinforcement schedule during testing. In two groups a 10-s conditioned stimulus (CS) and food were paired (10-s and 20- to 100-s ISI), while in a third group they were unpaired. During signalled reinforcement of lever-pressing (S+), the CS suppressed responding in both paired groups. During signalled extinction (S−), responding in the 10-s ISI group was suppressed during the CS and enhanced for 60 s after CS offset; responding in the 20- to 100-s ISI group was enhanced both during the CS and for 120 s after CS offset. Experiment II examined whether the long ISI enhancement effect would occur when the baseline response rate was lowered by satiation rather than extinction. A 20- to 100-s CS and food were paired in one group and unpaired in another. After near-satiation on a CRF schedule, CS presentations caused a reduction in responding in both groups, with no significant difference between the two groups. The results of the two experiments were interpreted in terms of an interaction between the expectancy of food generated by stimuli conditioned at short and long ISIs and the expectancy of food availability controlled by the instrumental schedule.