Conditional acceleration and external disinhibition of operant lever pressing by prereward, neutral, and reinforcing stimuli

Rats responding under a differential-reinforcement-of-low-rate schedule increased their rates of lever pressing during a 20-second click/flash stimulus that preceded the delivery of a response-independent food pellet. The increase could not be attributed to suppression of collateral behavior that has been said to mediate temporally-spaced responding. We propose that the prereward stimulus functioned as an external disinhibitor of lever pressing that had been inhibited by the constraints of the operant schedule. Support is derived from the observed disinhibitory effects of a 10-second unpaired click/flash stimulus and of unsignaled, response-independent pellets that were presented while the animals were responding under the same schedule.     

Conditioned suppression and conditioned enhancement with the same positive UCS: an effect of CS duration

Previous experiments have shown that positively reinforced operant responding is suppressed during a conditioned stimulus terminated with an electric shock (conditioned suppression). In the present experiment, the conditioned stimulus was terminated with a positive unconditioned stimulus, and it was found that the duration of the conditioned stimulus was a key factor in determining whether response suppression or response enhancement was observed during the stimulus. The lever-pressing responses of rats were maintained by a variable-interval schedule of food reinforcement. While the rats were pressing the lever, a light was occasionally turned on, its offset coincident with a brief period of access to a sucrose solution. In consecutive blocks of sessions, the light duration was 40 sec, 12 sec, or 120 sec. Results showed that the rate of lever pressing was substantially suppressed during the 12-sec stimulus, slightly suppressed during the 40-sec stimulus, and enhanced during the 120-sec stimulus.     

The control of appetitive instrumental responding does not depend on classical conditioning to the discriminative stimulus

In Experiment I, rats were exposed to a classical relationship between a clicker-light compound and response-independent food. Conditioning to the light was blocked if the clicker had previously served as a classical signal for food, but not if it had been established as a discriminative stimulus for food-reinforced lever pressing. In Experiment II, a tone-light compound served as a discriminative stimulus for lever pressing. Control by the light was blocked if the tone was independently trained as a discriminative stimulus, but not if it was trained as a classical signal for response-independent food. These results suggest that discriminative stimuli do not come to control appetitive instrumental responding by virtue of their implicit classical relationship to the instrumental reinforcer.     

Control of responding by sounds of different quality: an evolutionary analysis.

Two experiments investigated the acquisition of discriminations between two acoustic stimuli of different quality (noise bursts vs. a 2-kHz pulsed signal) when features of the everyday environment were incorporated into the experiments. In Experiment 1, rats were trained, using food, to press a lever. Throughout all sessions, 5-s trials of noise bursts (the random stimulus) were presented, after variable intertrial intervals, through a remote speaker mounted outside the experimental enclosure. The noise burst occurred randomly with respect to reinforcement of lever pressing and had no programmed relationship to the animal's behavior. When lever pressing was established, the 2-kHz signal was presented through a speaker adjacent to the response lever according to a different set of variable intertrial intervals. A response in the presence of the 2-kHz signal terminated the trial and was reinforced. The 2-kHz signal acquired control of responding within the first few trials, whereas the random stimulus exerted no control of responding. In Experiment 2, rats were trained to press the lever in the presence of the 2-kHz signal presented through the adjacent speaker on a variable intertrial interval. After 14 sessions, 5-s trials of noise bursts (random stimulus) were presented through the remote speaker on the second variable intertrial interval. The random stimulus initially elicited exploratory behavior, which then rapidly declined. Subsequently, the random stimulus exerted no or weak control of responding. The introduction of the random stimulus had no effect on responding in the presence of the adjacent stimulus. In Experiments 3 and 4 the random stimulus was presented through the adjacent speaker, and the stimulus correlated with reinforcement was presented through the remote speaker. In both experiments, there was persistent control of responding by the random stimulus and slow development of control by the stimulus correlated with reinforcement. In Experiment 5, both stimuli were presented through the adjacent speaker. There was persistent control of responding by the random stimulus.     

Effect of signaled reinforcement on response variability

Three experiments examined the effect of signaling reinforcement on rats' lever pressing on contingencies that reinforced variable responding to extend the exploration of signaled reinforcement to a schedule that has previously not been examined in this respect. In Experiment 1, rats responding on a lag-8 variability schedule with signaled reinforcement displayed greater levels of variability (U values) than rats on the same schedule lacking a reinforcement signal. In Experiment 2, rats responding on a differential reinforcement of least frequent responses schedule also displayed greater operant variability with a signal for reinforcement compared with rats without a reinforcement signal. In Experiment 3, a reinforcement signal decreased the variability of a response sequence when there was no variability requirement. These results offer empirical corroboration that operant variability responds to manipulations in the same manner as do other forms of operant response and that a reinforcement signal facilitates the emission of the required operant.     

The effect of rearing environments on the contrafreeloading phenomenon in rats

Eight naive rats were reared in enriched or impoverished environments for 39 days after weaning and then lived in operant chambers, in which they could obtain food pellets freely or by lever pressing, for 25 or 30 days. The animals raised in an impoverished environment acquired the bar-press response quickly when placed in the operant chambers and maintained a preference for obtaining food via bar pressing. Animals raised in an enriched environment did not learn to lever press, as demonstrated by low levels of responding and the lack of bar pressing when free food was subsequently removed. It was concluded that restricting animals' postweaning environments facilitated learning in a choice situation, probably because of increased activity levels. The results are interpreted in relation to previous studies on rearing environments and on contrafreeloading.     

Is induction produced by upcoming food-pellet reinforcement the outcome of an increase in overall activity or in operant responding?

Researchers have demonstrated that rats reliably increase their rates of pressing a lever for 1% liquid-sucrose reinforcement if they will soon have the opportunity to press a lever for food-pellet reinforcement. In the present experiments, the authors investigated if this increase in response rates occurred because the upcoming food pellets produced an increase in all behaviors (i.e., general arousal) or an increase in only the specific operant response (i.e., lever pressing). The results of Experiments 1 and 2 showed that the appearance of induction in rats' lever pressing for 1% sucrose reinforcement when food-pellet reinforcement was upcoming did not coincide with increases in the frequency of running in a wheel or making a nonreinforced nose-poke response. On the other hand, in Experiment 3, the authors found the appearance of induction coincided with increase nonreinforced lever presses on an adjacent lever. These results shed doubt on the idea that induction is a result of a general increase in all activity, and suggest instead that the increase in responding that occurs during induction is limited to the operant response.     

Overshadowing of a context aversion by a novel incentive in operant conditioning

Two experiments examined the processes underlying the suppression of instrumental behaviours by lithium in rats, as reported by Meachum (1988 and this issue). Experiment 1 examined whether presenting a novel sucrose solution prior to lithium chloride administration would overshadow aversion learning to either the stimuli of the operant chamber or to familiar food pellets. After lever pressing had been established, and in the absence of responding, animals received free deliveries of a novel sucrose solution, familiar food pellets, or both, or they were exposed to only the cues of the operant chamber, prior to lithium injections. Lever pressing for food pellets was then assessed. It was found that the animals receiving the novel sucrose, either alone or with the familiar food pellets, pressed more for pellets than either the group receiving only food pellets or the group exposed to only the context. In addition, there was no appreciable difference in the response rates between the context-only group and the group that received the familiar food pellets. These outcomes were interpreted in terms of the novel sucrose overshadowing aversion learning to the context. Experiment 2 investigated whether in fact aversive contextual conditioning could be obtained using the present parameters. This was accomplished by directly manipulating the contexts. In this experiment animals were trained to lever press in two distinctive contexts. Subsequently, one context was paired with the novel sucrose, and the second was experienced in the absence of reinforcement prior to toxicosis. During a subsequent non-reinforced test it was found that responding in the context paired with the novel sucrose was considerably higher than responding in the context that was experienced alone. These findings stand in contrast to the taste-mediated contextual potentiation observed when a consumatory response is used to assess aversive contextual conditioning.     

Effects of food deprivation and reinforcement magnitude on conditioned suppression

In Experiment I, the responding of rats lever pressing on a variable-interval schedule for sucrose solution was partially suppressed by a variable duration conditioned stimulus followed by shock. When food deprivation was increased, response rates during and before the conditioned stimulus increased monotonically. Varying the concentration of sucrose across blocks of sessions or from session to session in a semi-random sequence had little effect on response rates either before or during the conditioned stimulus. With a fixed sequence of increasing concentrations across a five-session block, increased concentration produced much more rapid increases in response rates before than during the conditioned stimulus. In Experiment II, rats were presented with the same sequence of increasing concentrations across a five-session block. When tested at 80% body weight, response rates increased rapidly as concentration increased, but at 100%, body-weight rates increased only slightly. The effect of a change in body weight in Experiment II thus mimicked the effect of the conditioned stimulus in the latter part of Experiment I. These findings support the view that the effect of a pre-aversive conditioned stimulus is similar to that of a change in food deprivation, but unlike that of a change in reinforcement magnitude.     

The effect of type of behavior on behavior change caused by type of upcoming food substance

Previous research suggests that rats will decrease their consumption of a low-valued substance if a high-valued one will soon be available (anticipatory contrast), but will increase their rate of operant responding for a low-valued substance if a high-valued one will soon be available (positive induction). The present experiments tested whether rats would increase their operant rate of licking or lever pressing for 1% liquid-sucrose reinforcement when 32% sucrose reinforcement was upcoming in the same session. Results indicated that upcoming 32% sucrose increased rates of lever pressing for 1% sucrose, but did not produce similar increases in rates of licking. In fact, upcoming 32% sucrose significantly reduced lick rates in Experiment 2. The present results suggest that the different changes in behavior may be linked to the specific response that the subjects must engage in to obtain the reward (i.e., licking vs. lever pressing), and not to the function of the behavior (i.e., consummatory vs. operant) or to how frequently the substances are available (i.e., continuously vs. intermittently).     

Techniques for establishing schedules with wheel running as reinforcement in rats.

In three experiments, access to wheel running was contingent on lever pressing. In each experiment, the duration of access to running was reduced gradually to 4, 5, or 6 s, and the schedule parameters were expanded gradually. The sessions lasted 2 hr. In Experiment 1, a fixed-ratio 20 schedule controlled a typical break-and-run pattern of lever pressing that was maintained throughout the session for 3 rats. In Experiment 2, a fixed-interval schedule of 6 min maintained lever pressing throughout the session for 3 rats, and for 1 rat, the rate of lever pressing was positively accelerated between reinforcements. In Experiment 3, a variable-ratio schedule of 20 or 35 was in effect and maintained lever pressing at a very stable pace throughout the session for 2 of 3 rats; for 1 rat, lever pressing was maintained at an irregular rate. When the session duration was extended to successive 24-hr periods, with food and water accessible in Experiment 3, lever pressing settled into a periodic pattern occurring at a high rate at approximately the same time each day. In each experiment, the rats that developed the highest local rates of running during wheel access also maintained the most stable and highest rates of lever pressing.     

Variable-interval schedules of timeout from avoidance

Rats were trained on concurrent schedules in which pressing one lever postponed shock and pressing the other occasionally produced a 2-min timeout during which the shock-postponement schedule was suspended and its correlated stimuli were removed. Throughout, the shock-postponement schedule maintained proficient levels of avoidance. Nevertheless, in Experiment 1 responding on the timeout lever was established rapidly, was maintained at stable levels on variable-interval schedules, was extinguished by withholding timeout, was reestablished when timeout was reintroduced, and was brought under discriminative control with a multiple variable-interval extinction schedule of timeout. These results are in contrast with Verhave's (1962) conclusion that timeout is an ineffective reinforcer when presented to rats on intermittent schedules. In Experiment 2 the consequence of responding on the timeout lever was altered so that the shock-postponement schedule remained in effect even though the stimulus conditions associated with timeout were produced for 2 min. Responding extinguished, indicating that suspension of the shock-postponement schedule, not stimulus change, was the source of reinforcement. By establishing the reinforcing efficacy of timeout with standard variable-interval schedules, these experiments illustrate a procedure for studying negative reinforcement in the same way as positive reinforcement.     

Effects Of Fixed-interval Schedule And Reinforcer Duration On Responding Reinforced By The Opportunity To Run

Two experiments investigated the effects of schedule value and reinforcer duration on responding for the opportunity to run on fixed-interval (FI) schedules in rats. In the first experiment, 8 male Wistar rats were exposed to FI 15-s, 30-s, and 60-s schedules of wheel-running reinforcement. The operant was lever pressing, and the consequence was the opportunity to run for 60 s. In the second experiment, 8 male Long-Evans rats were exposed to reinforcer durations of 15 s, 30 s, and 90 s. The schedule of reinforcement was an FI 60-s schedule. Results showed that postreinforcement pause and wheel-running rates varied systematically with reinforcer duration but not schedule value. Local lever-pressing rates decreased with reinforcer duration. Overall lever-pressing rates decreased with reinforcer duration but increased with schedule value. Although the reinforcer-duration effect is consistent with previous research, the lack a schedule effect appears to be the result of long post-reinforcement pauses following wheel-running reinforcement that render the manipulation of the interval requirement ineffective.     

Compounding of discriminative stimuli correlated with chained and multiple schedules

In studies of stimulus compounding (1) the stimuli are presented randomly, (2) primary reinforcement is correlated with each stimulus, (3) a specific response is emitted during each stimulus, and (4) the response is necessary to produce the reinforcer. The present experiments assessed the importance of these procedures by (1) presenting light and tone stimuli in fixed order, (2) removing reinforcement (food) during one stimulus, (3) preventing the response (lever pressing) from being emitted, and (4) eliminating the contingency between lever pressing and food. These variables were presented in various combinations within the context of chained and multiple schedules. When the stimuli were combined in the schedule component correlated with each stimulus, the frequency of lever pressing increased in most instances (additive summation). This suggests that the effect of combining stimuli was not closely tied to the specific procedures used in previous experiments. However, presenting the stimuli in a fixed order did have an effect: the level of responding to the compound was generally greatest when the stimuli were combined in the component correlated with the higher frequency of lever pressing to the single stimulus. Additive summation failed to occur consistently when response-independent food was correlated with each stimulus, and when both lever pressing and food were eliminated during one stimulus.     

Suppression of random-ratio and acceleration of temporally spaced responding by the same prereward stimulus in monkeys

A 1-min tone and light signal that preceded two free pellets of food suppressed the random-ratio responding of four rhesus monkeys, but accelerated the same subjects' responding on a differential-reinforcement-of-low-rate schedule in separate sessions. Both schedule-specific interactions occurred during the first presentations of the signal that previously had been paired with food outside the operant sessions. Thus, neither effect was adventitiously produced. In two subjects, both the direction and magnitude of the prereward change in differential-reinforcement-of-low-rate responding appeared related to baseline response rates: the more rapid the baseline responding, the less was the acceleration during the signal. Suppression and acceleration did not appear as dichotomous effects with separate parameters, but as related effects at least partly determined by the characteristics of the baseline operant performance.     

Marking effects in instrumental performance on DRH schedules

Three experiments investigated the effect of presenting a brief stimulus after a response sequence on the rate of lever-pressing by rats on differential reinforcement of high rate (DRH) schedules. In Experiment 1 enhanced responding was produced by a visual stimulus presented during a 500-msec delay of reinforcement compared to a condition in which no stimulus was presented. In Experiment 2 rats responded on a multiple DRH DRH schedule in which the DRH contingency was reinforced on a 50% schedule in each component. Equivalent levels of responding occurred in the components when reinforcement was signalled in one component and when the signal was presented following the non-reinforced schedules in the other components. A further group of rats received the stimulus presented after non-reinforced schedules in one component but not at all in the other component; responding was enhanced in the former component relative to the latter component. In Experiment 3 brief stimuli presented after the completion of DRH components on a second-order VR (DRH) schedule elevated response rates irrespective of whether the signal was presented paired or unpaired with reinforcement. The present data support the view that a brief signal may serve to mark a response sequence in memory and facilitate instrumental performance.     

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.     

Renewal of operant performance formerly eliminated by omission or noncontingency training upon return to the acquisition context

Renewal of operant performance formerly eliminated by omission or noncontingency training was explored in two experiments with rats. When pressing a lever was trained with food reinforcement in one context (A) and then eliminated in a second context (B), responding was renewed by returning the rats to the original context (A). This ABA renewal effect was demonstrated in Experiment 1 when the elimination training was an omission procedure (delivery of food for withholding responding) and in Experiment 2 when it was a noncontingency procedure (delivery of food irrespective of responding). Because omission training (differential reinforcement of other behavior) and noncontingency training have been used in applied settings as effective procedures to reduce undesired human behaviors, the clinical implications of our findings for the relapse of undesirable behavior were discussed.     

Further observations on overt “mediating” behavior and the discrimination of time

When the lever-pressing behavior of five rats was maintained by a DRL schedule (reinforcement was scheduled only when a specified waiting time between successive responses was exceeded), collateral behavior developed that apparently served a mediating function. In two cases this behavior did not arise until the experimental environment included pieces of wood that the rats started to nibble. When collateral behavior first appeared, it was always accompanied by an increase in responses spaced far enough apart to earn reinforcement. If collateral behavior was prevented, the number of reinforced responses always decreased. Extinction of lever pressing extinguished the collateral behavior. Adding a limited-hold contingency to the schedule did not extinguish collateral behavior. It appears that the rat can better space its responses appropriately when concurrently performing some overt collateral activity. The amount of this activity apparently comes to serve as a discriminative stimulus. To assume the existence of internal events that serve as discriminative stimuli in temporal discriminations is, at least under some circumstances, unnecessary.     

Positive conditioned suppression: conditioned suppression using positive reinforcers as the unconditioned stimuli

Research has revealed the phenomenon of conditioned suppression in which the rate of responding is reduced during a stimulus that is paired with noncontingent shock. The present study replicated this procedure, but used noncontingent positive reinforcers instead of the aversive shock. The lever-pressing responses of rats were reinforced with food or water. While the rats were responding, a stimulus was occasionally presented and paired with the delivery of a noncontingent positive reinforcer, which was either food, water, or brain stimulation for different rats. The result was a reduction in the rate of responding during the conditioned stimulus. This finding shows that conditioned suppression occurs during a signal for reinforcing as well as aversive stimuli.