Enhancement of food-rewarded instrumental responding by an appetitive conditioned stimulus

Four experiments are reported in which a stimulus (with a minimum duration of 60 s) signalling the delivery of “free” food was presented to rats lever-pressing for food available on a variable interval schedule. It was found that responding was enhanced in the presence of the stimulus when the baseline schedule of reinforcement was lean (Experiment I) and that the enhancement was dependent upon the pairing of the stimulus with free food (Experiments II and III). Experiment IV showed that an enhancement could be found after initial training in which stimulus-food pairings were given to subjects that were not concurrently lever pressing for food. It is argued that these results are consistent with the suggestion that an appetitive conditioned stimulus can energise appetitive instrumental behaviour.     

Facilitation of instrumental behavior by a Pavlovian appetitive conditioned stimulus

Three experiments examined appetitive Pavlovian-instrumental interactions by presenting separately trained conditioned stimuli (CSs) during reinforced instrumental responding in rabbits. Intra-oral reinforcement was used to minimize interference from peripheral responses such as magazine approach. In experiment 1, the rabbits were first trained to perform an instrumental head-raising response for sucrose reward. A conditioned jaw movement response was then established to a 2-sec CS by pairing it with sucrose; a control stimulus was unpaired with sucrose. Instrumental responding maintained by a variable-interval 40-sec schedule was enhanced during 10-sec presentations of the paired, but not the unpaired, CS. Responding on a variable-ratio 15 schedule was unaffected except on trials on which the pre-CS baseline response rate was low; in such cases the paired CS caused a long-lasting acceleration of responding. Noncontingent presentation of the sucrose reinforcer itself briefly suppressed responding but had no long-term effect. In Experiment 2, a CS that had been conditioned at a 10-sec duration produced the same pattern of effects as in the first study, indicating that facilitation resulted from CS presentation rather than from the frustrative effects of non-reinforcement of the CS. In Experiment 3 an inhibitory CS blocked facilitation by the excitatory CS but did not itself affect instrumental responding. These results support the view that Pavlovian processes play a positive role in instrumental performance and suggest that previous findings of suppression by a short-duration CS reflect peripheral interference. The dependence of facilitation on the baseline level of responding is discussed in terms of associative and motivational theories of Pavlovian mediation.     

On conditioned reinforcing effects of negative discriminative stimuli

Observing responses by pigeons were studied during sessions in which a food key and an observing key were available continuously. A variable-interval schedule and extinction alternated randomly on the food key. In one condition, food-key pecking during extinction decreased reinforcement frequency during the next variable-interval component, and in the other condition such pecking did not affect reinforcement frequency. Observing responses either changed both keylight colors from white to green (S+) or to red (S−) depending on the condition on the food key, or the observing responses never produced the S+ but produced the S− when extinction was in effect on the food key. Observing responses that produced only S− were maintained only when food-key pecking during extinction decreased reinforcement frequency in the subsequent variable-interval component. The red light conformed to conventional definitions of a negative discriminative stimulus, rendering results counter to previous findings that production of S− alone does not maintain observing. Rather than offering support for an informational account of conditioned reinforcement, the results are discussed in terms of a molar analysis to account for how stimuli acquire response-maintaining properties.     

Avoidance conditioning as a function of appetitive stimulus pretraining: Response and stimulus transfer effects

Pretrained appetitive discriminative stimuli were used as warning signals in subsequent avoidance learning. In Expt 1 identical responses were required in pretraining and in avoidance learning. An appetitive S+ facilitated avoidance learning in rats in comparison to S? or a stimulus previously uncorrelated with food. In Expt 2, the type of response in pretraining and in avoidance learning was varied. Groups with homogeneous responses in the two situations replicated Expt 1 results, whereas groups with different responses in pretraining and avoidance learning failed to show an advantage when S+ served as warning; in the heterogeneous response groups, S? was as effective as S+. Inhibitory factors in the heterogeneous groups were discussed as an explanation for these results.     

Fear as a discriminative stimulus for an appetitive instrumental response

Two experiments tested the hypothesis that anticipation of shock could be established as a discriminative stimulus for an appetitive instrumental response. In multiphase experiments, bar pressing for food was brought under the discriminative control of intermittent and gradually increasing electric shock. In a second phase, tones were estalished as either a CS+, or CS? for shock. Subsequently, the CSs were introduced on to the operant baseline. Animals trained with shock as the S^D showed an increase in responding to the CS+ and a slight decrease to the CS?. Conversely, animals trained with shock as the S^Δ showed decreased responding to the CS+ and slight increase to the CS?. These findings are seen as supportive of the Discriminative Stimulus hypothesis of learned resistance to punishment.     

Preconditioning of reinforcing properties to an exteroceptive feedback stimulus

Three groups of rats were given Pavlovian preconditioning by means of a yoking procedure and subsequently trained to avoid. One group received a light stimulus during preconditioning at the same times as the master avoiding animals received an exteroceptive feedback stimulus. During the subsequent avoidance training, this group was then given the same light stimulus as a feedback stimulus on successful avoidance trials. The group learned two-way avoidance in about the same number of trials as one-way avoidance is typically learned and substantially faster than two other control groups. These data indicate that feedback stimuli develop reinforcing properties by a Pavlovian mechanism. The data of the control groups indicate that feedback stimuli do not provide “information” about responding in the present situation.     

Signaling and affective functions of conditioned aversive stimuli in an appetitive choice discrimination: US intensity effects

Different groups of rats received Pavlovian aversive conditioning in which US-shock intensity (0.25–1.0 mA) and CS-US correlation (+, 0, ?) were factorially varied. Then, the CS was administered for each group contingent upon the reinforced response in an appetitive choice discrimination. Absolute and ratio measures of speed of running in the presence of the CS showed that, at the start of discrimination training, CS+ suppressed and CS- facilitated performance. However, later in training but prior to any evidence of choice learning, these speed effects reversed, with the magnitude of both the initial and reversed effects being a positive function of US intensity. Consistent with the reversal of speeds, associative (choice) measures showed that CS+ facilitated and CS- retarded discrimination learning relative to CSo and, within limits, these effects were also amplified by stronger USs. The findings suggest that a CS has both affective (motivational) and signaling (associative) functions and that both are influenced by US intensity; however, the CS's affective property is rapidly extinguished in the presence of a hedonicly different reinforcer, while leaving the CS's signaling property largely intact. Hence, the CS functions as a transformed signal for the new (appetitive) reinforcer and facilitates or retards learning by mediating the reinforcer's presence (CS+) or absence (CS-).     

Pigeons' withdrawal from an appetitive conditioned inhibitor under two training procedures

The approach-withdrawal behaviour of pigeons to a red keylight was measured under three conditions; a negative contingency (NC) between keylight and food reinforcement, a CI or conditioned inhibition procedure where the keylight was non-reinforced in compound with a tone which was reinforced when presented alone, and a random control procedure (RC). The keylight-food contingencies in CI and NC were identical, and keylight and food presentation frequency were the same in all conditions. Subsequently the effect of adding the red keylight to a novel CS+ during or after excitatory conditioning was examined (summation test of inhibition). The inhibitory procedures, CI and NC, generally yielded similar functions for the acquisition of withdrawal, and the results of the withdrawal and summation measures were positively correlated. The implications of the results for theories of the acquisition and behavioural action of conditioned inhibitors are discussed.     

Extinction of conditioned flavor preferences

In a series of 4 experiments, the effects of extinction on flavor preferences conditioned by mixing flavor cues with a nutrient were examined. In each experiment it was observed that rats preferred a flavor cue that had not undergone extinction to one that had. In addition, this preference was reversed in subjects trained thirsty (Experiments 1 and 2) if the associated nutrient had been devalued prior to the test or the preference for the nonextinguished cue was attenuated by nutrient devaluation in subjects trained hungry (Experiments 3 and 4). The results suggest that extinction may weaken associations between the flavor and the specific sensory properties of the nutrient and, for subjects trained hungry, between the flavor and the motivational components of the nutrient as well.     

Differential effects of two ways of devaluing the unconditioned stimulus after Pavlovian appetitive conditioning

Three experiments with rat subjects investigated the effects of two methods of devaluing a food unconditioned stimulus (US) after pairings of an auditory conditioned stimulus (CS) with that US. Experiment 1 found no effect of postconditioning pairings of the food US with lithium chloride (LiCl) on general activity to a tone CS, even though those pairings substantially reduced food consumption. Experiments 2 and 3 compared the effects on conditioned responding of postconditioning pairings of food with LiCl and with high-speed rotation. In these experiments the general activity measure was supplemented by a detailed visual analysis of the rats' behavior. Experiment 2 found that food-rotation pairings had larger effects than food-LiCl pairings on general activity responding and on two detailed behavioral measures but that food-LiCl pairings had larger effects on food consumption and on one behavioral measure. Experiment 3 replicated the findings of Experiment 2 and found that the ability of the CS to serve as a reinforcer for second-order conditioning after US devaluation was reduced more by food-LiCl pairings.     

Control of appetitive and aversive taste-reactivity responses by an auditory conditioned stimulus in a devaluation task: a FOS and behavioral analysis

Through associative learning, cues for biologically significant reinforcers such as food may gain access to mental representations of those reinforcers. Here, we used devaluation procedures, behavioral assessment of hedonic taste-reactivity responses, and measurement of immediate-early gene (IEG) expression to show that a cue for food engages behavior and brain activity related to sensory and hedonic processing of that food. Rats first received a tone paired with intraoral infusion of sucrose. Then, in the absence of the tone, the value of sucrose was reduced (Devalue group) by pairing sucrose with lithium chloride (LiCl), or maintained (Maintain group) by presenting sucrose and LiCl unpaired. Finally, taste-reactivity responses to the tone were assessed in the absence of sucrose. Devalue rats showed high levels of aversive responses and minimal appetitive responses, whereas Maintain rats exhibited substantial appetitive responding but little aversive responding. Control rats that had not received tone-sucrose pairings did not display either class of behaviors. Devalue rats showed greater FOS expression than Maintain rats in several brain regions implicated in devaluation task performance and the display of aversive responses, including the basolateral amygdala, orbitofrontal cortex, gustatory cortex (GC), and the posterior accumbens shell (ACBs), whereas the opposite pattern was found in the anterior ACBs. Both Devalue and Maintain rats showed greater FOS expression than control rats in amygdala central nucleus, GC, and both subregions of ACBs. Thus, through associative learning, auditory cues for food gained access to neural processing in several brain regions importantly involved in the processing of taste memory information.     

The effects of electroconvulsive shock on the action of a reinforcing stimulus

Three experiments sought to evaluate the effect of electroconvulsive shock on the action of a reinforcing stimulus. In all experiments behavior was maintained on a 2 min variable interval schedule for food reinforcement. Foot shock at the termination of a buzzer stimulus served as the reinforcing stimulus for conditioned suppression during the ensuing buzzer interval. Omission of foot shock at the termination of the buzzer stimulus was followed by normal responding (no conditioned suppression) during the next buzzer interval. In Exp I electroconvulsive shock followed foot shock at varying time intervals. In the first subsidiary experiment electroconvulsive shock followed an unreinforced buzzer stimulus at varying time intervals. In the second subsidiary experiment electroconvulsive shock followed foot shock at varying time intervals and an additional buzzer stimulus was sounded between the termination of foot shock and the onset of electroconvulsive shock. These three experiments demonstrated that electroconvulsive shock invariably abolished the effects of the reinforcing stimulus if it followed conditioning by no more than 10.0 sec and never had an effect if it followed conditioning by 12.5 sec or more; electroconvulsive shock was not acting as a reinforcing stimulus in this situation.     

Reinstatement of fear to an extinguished conditioned stimulus.

Four experiments are reported which demonstrate the ability of an unconditioned stimulus (UCS) presentation following extinction to partially reinstate the conditioned response. These experiments are interpreted in terms of the strengthening of an extinction-reduced UCS representation. The first two experiments address alternative interpretations in terms of sensitization, reinstating the stimulus conditions of acquisition, conditioning of background cues, and stimulus generalization. Experiment 3 suggests that reinstatement is possible with a UCS qualitatively different from that used in conditioning. Experiment 4 explores an alternative extinction procedure which especially preserves the conditioned stimulus-unconditioned stimulus association while encouraging modification of the UCS representation. The results are discussed both in terms of related empirical phenomena, such as spontaneous recovery and sensory preconditioning, and in relation to the general role of the UCS representation in conditioning.     

Effect of changing the unconditioned stimulus on appetitive blocking

In four experiments we examined the effects of changing the unconditioned stimulus (US) on appetitive blocking. In Experiments 1A, 1B, and 2 we established that substituting food for water, or water for food, in the compound stage did not attenuate blocking relative to groups that received the same US throughout conditioning. Experiment 3 showed that satiation with the US used prior to compound conditioning with a different US does not affect blocking. Experiment 4 revealed that changing the location of US delivery, as well as the quality of the US, also leaves blocking unaffected. It is suggested that these results demonstrate that blocking occurs, provided that there is no change in the affective properties of the US.     

Transfer of conditioned appetitive stimuli to conditioned aversive excitatory and inhibitory stimuli

The transfer of Pavlovian appetitive stimuli to Pavlovian aversive stimuli was examined in three experiments. In Experiment 1, rats received appetitive (Ap) conditioning designed to establish a flashing-light stimulus as either a CS+, CSo, or CS? for food, or to maintain it as a novel stimulus for US-alone subjects. Then, the stimulus was employed as a signal for weak shock in conditioned-emotional-response (CER) training. Both acquisition and extinction results showed that the ApCS+ facilitated and the ApCS? retarded aversive excitatory conditioning relative to the ApCSo and US-alone controls. Experiment 2 replicated the findings of Experiment 1 with both a moderate and a severe shock in CER training. In Experiment 3, different groups received the same appetitive conditioning as before, but to a flashing-light stimulus which was then employed as a signal for no shock in CER training. The ApCS? facilitated and the ApCS+ retarded aversive inhibitory conditioning relative to ApCSo and US-alone controls. Collectively, these findings establish that, in Pavlovian conditioning, transfer of an appetitive CS to an aversive excitor or inhibitor is facilitated by maintaining the initial conditioning contingency.     

Extinction of avoidance in rats: response availability and stimulus presentation effects

A conditioned a versive stimulus presented in prolonged, continuous fashion is said to be flooded. Using a two-way shuttle avoidance task. Polin (1959) found that animals allowed to respond to flooded stimuli extinguish faster than animals prevented from responding to discrete stimuli of equal total duration. But, as Shearman (1970) noted, the CS presentation and response availability factors were confounded. He arranged them factorially, in a 2 × 2 design, again utilizing two-way avoidance, but found no differences between any of the experimental groups, although all groups extinguished faster than a regular extinction control group, the only group for which CS termination was response-contingent. Since there is evidence from one-way avoidance studies that response prevention facilitates extinction of avoidance with CS exposure held constant (Shipley et al., 1971). we repeated the Shearman design, using a one-way avoidance paradigm, and including a group for whom the aversive CS would be presented in discrete trials at increasing intensities. A separate aspect of the present investigation concerned the measurement of a fear response as well as the avoidance response. Results of several investigations have shown (e.g. Coulter et al. 1969; Linton et al.. 1970) that avoidance behaviour can be extinguished without the extinction of fear. It was thought useful to assess the generality of this result, since in two-factor theory, fear and avoidance are related.Two supplementary experiments replicated important conditions of the initial one, while examining whether response prevention and response non-availability have similar effects (II), and the relation between the fear test and the test for extinction (III).     

Extinction of conditioned odor potentiation of startle

Several recent studies with rats (Sprague-Dawley strain) have documented that an odor previously paired with shock potentiates the acoustic startle response, a phenomenon referred to as conditioned odor potentiation of startle (OPS). A surprising finding in these studies was that OPS did not extinguish even though the odor was present throughout the 25-min test session. Therefore, the present study more fully examined extinction of OPS. The results of Experiment 1, which employed both within-subject and between-group comparisons, showed that extinction of OPS occurred in adult rats only after several days of testing. Experiment 2 used the between-group procedure and found similar results with 23-day-old rats, the youngest age that exhibits the OPS effect. Experiment 2 also showed that giving rats 15 odor-shock pairings at 16 days of age, an age where they acquire the odor-shock association but cannot express it via OPS, does not increase subsequent resistance to extinction following odor-shock pairings at 23 days of age. Taken together, the results of this study show that (1) although OPS is somewhat resistant to extinction, it does extinguish with repeated tests and (2) suggests that there are no age differences in the rate of extinction of OPS.