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.     

Frequency of reinforcement as a parameter of conditioned suppression

Two pigeons were trained on a multiple VI-1; VI-4 min schedule for food reinforcement until a stable differential rate of response was established. Each component of the multiple schedule was in effect for 10 min and was separated from the other by 1 min time out. An Estes-Skinner conditioned suppression procedure was superimposed on each component of the schedule. The relative magnitude of the suppression behavior was measured during its acquisition and extinction, and at CS durations of 100, 200, and 300 sec. The initial magnitude of the suppression behavior was less severe on the VI-1 baseline than on the VI-4, and it extinguished more rapidly on the VI-1. As the relative duration of the CS was increased, the suppression behavior became less severe on both baselines, but the initial differential magnitude in the suppression remained intact.     

Classical skin conductance response conditioning: effects of intermittent reinforcement and information about schedule contingencies.

Skin conductance responses were differentially conditioned using reinforcement schedules of 25%, 50%, 75%, and 100%, manipulated between subjects. Half of the subjects were informed about schedule contingencies, and half were uninformed. The interstimulus interval was 6 sec. Discrimination of first-interval responses (1.0-3.5 sec after conditioned stimulus [CS] onset) by informed subjects did not vary with the ratio variable, but that by uninformed subjects improved with increasing reinforcement ratio because of diminished response levels to the nonreinforced CS (CS-). Discrimination of second-interval responses (3.6-7.0 sec after CS onset) improved as a function of increasing reinforcement ratio because of elevated response levels to the reinforced CS (CS+), but the effect was not persistent across trials in informed subjects. Performance in the first and second intervals did not reflect sequential increments and decrements as a function of reinforced and nonreinforced trials. Third-interval responses (7.1-9.9 sec after CS on nonreinforced trials) were not affected by schedule manipulations, but unconditioned responses diminished with increasing reinforcement ratio. Information about schedule contingencies led to superior discrimination of first-, second-, and third-interval responses and to suppression of unconditioned responses.     

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.     

Temporal properties of responding during stimuli that precede response-independent food

Pigeons' keypecks were reinforced with grain on the average of once per minute by schedules that maintained low response rates and by schedules that maintained high response rates. During these schedules, a fixed-duration conditioned stimulus (CS) ranging from 7.5 to 120 sec in duration across conditions terminated with response-independent food. Response rates during the CS were inversely related to CS duration. The rates and the temporal patterns of responding during the shortest CS were similar whether the ongoing schedule maintained high response rates or low response rates. As CS duration increased, the rate and pattern of responding during the CS converged on the rate and pattern of responding maintained by the baseline schedule. These data indicate that changes in responding during stimuli that signal response-independent reinforcement are not homogeneous throughout the CS; that response measures, such as “suppression ratios”, which presume homogeneity may mislead us; and that conditioned suppression and conditioned enhancement may be better talked about in terms of species-specific approach and avoidance than in terms of emotional states.     

Effects of partial reinforcement on conditioning,conditional probabilities,asymptotic performance,and extinction of the rabbit’s nictitating membrane response

Classical conditioning and extinction of the rabbit nictitating membrane response was investigated under 100 per cent, and two 50 per cent reinforcement conditions (50% equated total trials, and 50% equated reinforced trials) at average intertriai intervals (ITIs) of 30 and 60 sec. It was found that: (1) partial reinforcement reduced the rate of conditioning, but all groups eventually attained asymptomatic performance levels near 100 per cent; (2) first-order and second-order conditional probabilities following reinforced and nonreinforced trials were virtually identical; and (3) no partial reinforcement extinction effect (PREE) was observed.     

Habituation of conditioned suppression

Habituation of a conditioned emotional response was investigated using a procedure which eliminated contaminating temporal discriminations. Three rats were trained to bar press on a random interval 60 sec schedule of milk reinforcement and variable duration tone-shock pairings were superimposed upon this baseline. Very little recovery from conditioned suppression was found over 60 sessions of testing and no systematic differences were found after a month's “vacation” from the procedure. Analysis of responding within the CS period showed uniform suppression. The data are discussed in terms of stimulus predictability.     

A conditioned reinforcer maintained by temporal association with the termination of shock

Two experiments were conducted to determine whether a stimulus can be established as a positive conditioned reinforcer by associating it with the termination of shock, but without training the animal to make any response in its presence. In the first, six rats were conditioned to press a bar to terminate shock on a variable ratio schedule; white noise was then substituted as the immediate consequence, with the shock terminating 30 sec after the last press in its presence. It was found that the rate of pressing in the absence of noise depended on the contingency between the pressing and the noise. The second experiment sought to determine whether the difference in rates before and after the onset of the noise was due to the reinforcement of prior responding by the onset of the noise or to the suppression of subsequent responding by differential reinforcement of competing behavior. Six more rats were trained in the same manner, but with shock terminating 30 sec after the onset of the noise, regardless of what the animal did in its presence. Again the rate was higher before the onset of the noise, indicating that pressing was indeed maintained by the noise as a conditioned reinforcer.     

Positive conditioned suppression: effects of CS duration

During a brief conditioned stimulus (15 or 30 sec) that terminated with the response-independent delivery of banana pellets, operant responding reinforced by other food pellets according to a variable-interval schedule of reinforcement was suppressed in the squirrel monkey. Conditioned stimuli of longer duration (1, 2, and 3 min) did not reliably affect the rate of operant performance. Brief conditioned stimuli generated homogeneous response patterns of nearly complete suppression. Increasing the CS duration did not enhance responding, as previously reported, but led to alternate bursting and pausing, which suggested a loss of control by the conditioned stimulus. The results suggest that the magnitude of "positive" or "negative" conditioned suppression reflects the strength of the classical conditioning process.     

Immediate and delayed punishment of human behavior by loss of reinforcement

Young adult humans pressed a key to obtain money. When responding was punished by presentation of a stimulus signifying that money was lost, response frequencies decreased and response latencies increased. Since these changes did not increase relative earnings, the aversive properties of loss of reinforcement were manifested independently or reinforcement gain. When loss punishment was delayed for either 10, 20, or 40 sec the extent of suppression was found to vary inversely with the response-punishment interval. Subsequent manipulations indicated that the effectiveness of delayed punishment was increased when the response also produced immediate conditioned punishment, i.e., a stimulus paired with the delayed loss stimulus. Instructions about the response-punishment contingency had similar effects. The findings were consistent with animal studies of delayed shock punishment, insofar as a similar delay-of-punishment gradient was observed, and with studies of delayed positive reinforcement, insofar as mediation through conditioned punishment (or instructions) increased the effectiveness of delayed punishment.     

Conditioned suppression or facilitation as a function of the behavioral baseline

Rats were exposed to a multiple schedule of reinforcement. During one component, a bar-press was followed by reinforcement only if it occurred between 15 and 20 sec after the previous response. This differential-reinforcement-of-low-rate (DRL) schedule produced a typical slow rate of responding. During the other component, reinforcement followed the first response to be emitted during limited periods of time which occurred at fixed intervals. These fixed-interval schedules with a limited hold produced higher response rates, described as `interval' or `ratio-like' behavior. Responding during the DRL component increased in frequency during a tone which ended with an unavoidable shock of low intensity, but decreased during the tone when the shock intensity was raised. The `interval' and `ratio-like' responding decreased in frequency during the tone at all shock intensities. Initial acceleration of the DRL responding appeared to be due to adventitious punishment of collateral behavior which was observed between the bar-presses. The more severe conditioned suppression during the fixed-interval components might be the result of the lower probability of reinforcement after any single response.     

Facilitation of food-reinforced responding by a signal for response-independent food

Five pigeons whose key pecking was maintained by 4-sec access to grain on a variable-interval 2-min schedule received Pavlovian differential conditioning trials superimposed upon the instrumental baseline. The conditioned stimuli were changes in the stimulus on the key from white to red, or to a white horizontal line against a dark background. The positive conditioned stimulus was 20 sec long, and was followed immediately by 8-sec access to grain. The negative conditioned stimulus, also 20 sec long, was never paired with response-independent food. All pigeons responded more rapidly in the presence of the positive conditioned stimulus than in the presence of the negative one. The positive conditioned stimulus produced an increase in response rate over the pre-conditioned stimulus period. The negative conditioned stimulus had no marked effect upon response rate. When the roles of the positive and negative stimuli were reversed, and the duration of the response-independent reinforcement was reduced to 4 sec, the new positive conditioned stimulus came to facilitate responding, and the new negative conditioned stimulus no longer produced facilitation. A second discrimination reversal produced similar outcomes. When a third reversal was initiated, and the duration of response-independent reinforcement was reduced to 2 sec, the difference between the effects of the positive and negative stimuli diminished.     

Maintained generalization testing of conditioned suppression

This study concerns the use of a multiple stimulus discrimination procedure for producing data on the generalization of conditioned suppression. Four rats were maintained on a variable interval schedule of milk reinforcement in the presence of five stimuli varying in auditory click rate. When response rates were stable, electric shock was regularly paired with the termination of one of the click stimuli. For two rats the shock was paired with the slowest click rate, and for two rats shock was paired with the fastest click rate. The VI schedule remained in effect. Plots of the relative rates of response to each of the five stimuli yielded concave gradients for both animals suppressed at the slowest click rate, and flat gradients with a sharp drop at the warning stimulus for both animals suppressed at the fastest click rate. When the warning stimuli were reversed for both pairs of subjects, both gradient forms were reproduced. The present procedure was contrasted with procedures used by other investigators.     

The discriminative control of free-operant avoidance despite exposure to shock during the stimulus correlated with nonreinforcement

Four rats were trained in darkness on a free-operant avoidance procedure in which shocks occurred randomly, but lever presses could reduce their frequency. Discrimination training followed, during which responses in light continued to reduce shock frequency, but responses in darkness had no effect. During each cycle, the light period was 4 min, while darkness lasted only until a 20-sec interval had elapsed without a response. This no-response requirement was increased to 40 sec for three animals and eventually to 60 sec for two of them. Discriminative control developed, despite a greater shock density in the dark, with response rate and number of responses per shock maintained or increasing during light and decreasing to very low values in darkness. Two animals were later exposed to a procedure in which shock density was unaffected by responding either in light or darkness. A 60-sec no-response requirement was continued in the dark. Discriminative control persisted through 42 sessions for one animal and required 45 sessions to approach extinction for the other animal. The role of the light as a potential conditioned reinforcer of other behavior in the dark was implicated in the development and persistence of discriminative control. These data support shock-frequency reduction as reinforcement for avoidance behavior.     

UCS duration and conditioned suppression: Acquisition and extinction between-groups and terminal performance within-subjects

Two experiments demonstrated a positive monotonic relation between UCS duration and conditioned suppression ranging from no suppression at 0.05 sec to nearly complete suppression at 3.0 sec. In Exp 1, 5 acquisition groups were run at 0.5 mA. No suppression occurred at the shortest duration, approximately 50% suppression by Session 3 in two intermediate groups (0.2 and 0.5 sec) and nearly complete suppression by Session 3 at the two longest durations (1.0 and 3.0 sec). The relation between UCS duration and recovery of responding during extinction followed a parallel pattern. In Exp 2, terminal performance was studied within-subjects and the same relation was replicated.     

Some notes on conditioned suppression and reinforcement schedules

Two pigeons were trained on an FR 150 schedule of reinforcement. An Estes-Skinner conditioned suppression procedure was then superimposed on this performance at varied intervals. If the CS occurred during the early stages of the ratio run, complete suppression resulted. If the CS occurred during the later stages of the run, the birds continued to respond until the reinforcement was obtained, which was then followed by complete suppression.     

Effects of conditioned reinforcement frequency in an intermittent free-feeding situation,

Key-pecking intermittently produced a set of brief exteroceptive stimulus changes under two-component multiple schedules of conditioned reinforcement. Throughout the study, free access to grain was concurrently provided on an intermittent basis via a variable-interval tape. Free food presentations scheduled by the tape were delivered if no peck had been emitted for 6 sec, and the brief stimulus changes produced by responding under the multiple schedules were those which accompanied food presentation. The second component of each multiple schedule was always associated with a 1-min, variable-interval schedule of conditioned reinforcement. The schedule associated with the first component was systematically varied and conditioned reinforcement was either absent (extinction) or programmed on a 1-, 3-, 6-, or 12-min variable-interval schedule. Under these conditions, rate of responding in the manipulated component decreased monotonically with a decrease in the frequency of conditioned reinforcement. In addition, contrast effects were often obtained in the constant, second component. These results are similar to those obtained with similar multiple schedules of primary reinforcement.     

The effect of acceleration on food-reinforced DRL and FR

Performance on DRL 10 sec and FR 5 was studied after exposure to acceleration. After four rats, two on each of the above schedules, had stabilized they were exposed to 5 hr of acceleration at 5 G immediately before daily experimental sessions. Food intake was also studied in rats given access to food daily in their home cages and exposed to acceleration immediately before the free-feeding session. Weight gain of free-feeding animals and reinforcement intake of experimental animals dropped after acceleration. Over-all response rate on the FR was depressed markedly by acceleration but local response rates did not appear to be affected. IRT distributions of DRL sessions after acceleration were markedly shifted toward the long intervals. A sequential plot of IRTs on acceleration days showed an altered, but relatively stable, temporal patterning of responses followed by an abrupt return to the normal baseline toward the end of the session.     

Differential acquisition,extinction, and reinstatement of conditioned suppression in mice

In animals, the reappearance of conditioned fear responses after extinction has been primarily investigated using single-cue conditioning paradigms. However, a differential paradigm can overcome several of the disadvantages associated with a single-cue procedure. In the present study, the reinstatement phenomenon was assessed in mice using a differential conditioned suppression paradigm. In a first phase, one conditioned stimulus (CS + ) was consistently paired with an unconditioned stimulus (US; footshock) while another CS (CS–) was not, resulting in selective suppression of previously trained instrumental behaviour during the CS + . After the extinction phase, half of the animals (reinstatement group) were presented with unsignalled USs, while the other half were not (control group). A differential return of conditioned responding was observed in the reinstatement group, but not in the control group. The implications of these findings for future conditioning research are discussed.