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.     

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.     

Inhibitory conditioning resulting from a reduction in the magnitude of reinforcement

Five experiments, all employing conditioned suppression in rats, studied inhibitory conditioning to a stimulus signalling a reduction in shock intensity. Experimental subjects were conditioned to a tone signalling a 1·0 mA shock and to a tone-light compound signalling a 0·4 mA shock. On a summation test in which it alleviated the suppression maintained by a third stimulus also associated with the 1·0 mA shock, the light was established as a conditioned inhibitor. Retardation tests gave ambiguous results: the light was relatively slow to condition when paired, either alone or in conjunction with another stimulus, with the 0·4 mA shock, but the difference from a novel stimulus control group was not significant. Two final experiments found no evidence at all of inhibition on a summation test in which the light was presented in conjunction with a stimulus that had itself been associated with the 0·4 mA shock. The results of these experiments have implications for the question of what animals learn during the course of inhibitory conditioning.     

Acceleration and suppression of rats' responding to avoid foot shock and tail shock

Signalled response-independent shocks were superimposed on rats' wheel-turn responding to avoid shock administered to their feet through a grid floor or to their tails through fixed electrodes. In Experiment I, a tone paired with response-independent foot shock increased responding in three of four rats; a tone paired with tail shock increased responding in only one of four rats and suppressed responding in two rats. In Experiment II, a tone presented randomly with respect to response-independent shock had no reliable effect on responding to avoid foot shock or tail shock. In Experiment III, tail shock and foot shock were compared in a within-subject design while the temporal pattern of responding during conditioned stimuli was recorded. Responding during the conditioned stimulus preceding foot shock was characterized by initial suppression of responding at tone onset, followed by increased responding just before response-independent shock. Responding was suppressed throughout the conditioned stimulus preceding tail shock. Foot shock elicited bursts of responding, but tail shock did not.     

Contextual Control of Latent Inhibition by the Reinforcer

Three experiments examined the contextual control of latent inhibition (LI) by the unconditioned stimulus (US) using a within-subjects conditioned suppression procedure with rats. The effect of reducing the context change produced by the introduction of the shock US was investigated by presenting this US during preexposure to the conditioned stimulus (CS). Although limited CS preexposure in the absence of the US had no impact on subsequent conditioning, preexposure in the presence of the shock retarded both excitatory and inhibitory conditioning. We conclude that the introduction of the US during the conditioning phase of a normal LI experiment can produce a contextual change that reduces the observed magnitude of LI.     

Stimulus and temporal cues in classical conditioning

In 2 experiments, separate groups of rats were given stimulus conditioning, temporal conditioning, untreated control and (in Experiment 2) learned irrelevance control procedures, followed by a compound with both stimulus and temporal cues. Stimulus conditioning consisted of a random 15-s duration conditioned stimulus (CS) followed by food; temporal conditioning consisted of food-food intervals of fixed 90 s (Experiment 1) or fixed 75 + random 15 s (Experiment 2). The stimulus group abruptly increased responding after CS onset, and the temporal group gradually increased responding over the food-food interval. When the food-food interval was fixed 90 s, the temporal cue exerted stronger control in the compound, whereas when the food-food interval was fixed 75 + random 15 s, the stimulus cue exerted stronger control. The strength of conditioning, temporal gradients of responding, and cue competition effects appear to reflect simultaneous timing of multiple intervals.     

Thresholds for conditioned suppression using X-rays as the pre-aversive stimulus

Four male, 12-week-old Sprague-Dawley rats were used to determine the threshold for behavioral response to X-irradiation using the conditioned suppression technique. They were maintained at 80 per cent body weight and initially trained to stable performance on a VI 1 min schedule with 16 per cent sucrose solution as reinforcement. After a stable baseline was obtained, animals were placed in the instrumental conditioning box beneath the X-ray machine for a half-hour session each day. While subjects were actively pressing the lever for reinforcement, 15-sec X-ray exposures of 0.5 R/sec were administered, followed immediately by electric shock. After all animals had exhibited conditioned suppression, the dose-rate was decreased in subsequent sessions in an effort to establish threshold. The results indicate that X-rays at a dose-rate as low as 0.004 R/sec can be an effective pre-aversive stimulus for the rat.     

Pavlovian conditioning of shock-elicited aggression: a discrimination procedure

Two auditory stimuli, separated by a fixed intertrial interval, were alternately presented to two rats in a closed environment. The positive conditioned stimulus (CS+) terminated with the offset of a 2-mA, 0.75-sec shock. The negative conditioned stimulus (CS-) terminated without shock. The incidence of the "stereotyped fighting posture" was recorded during the CS+, the CS-, the intertrial interval, and shock. The results showed an increase in the percentage of conditioned responses during the CS+, and a decrease during both the CS- and the intertrial interval, when the duration of the conditioned stimuli and the intertrial interval was 16 sec. Appropriate changes in the incidence of aggression during the two stimuli were obtained following the reversal of the stimulus functions. During the acquisition and reversal phases there was a between-session decrement and a within-session improvement in the incidence of aggression during the CS+, defined as warm-up. The presentation of free shocks before the conditioning sessions was effective in reducing the warm-up only when the interval between shocks was 64 sec. These data were interpreted as demonstrating classical conditioning of shock-elicited aggression, with little chance of non-associative factors contributing to the measurement of the conditioned response.     

Integration of cardiac responses to serial stimuli after Pavlovian conditioning in rats

The expression of cardiac responses to sequences of two sounds was studied in restrained rats following discriminative trace or delay conditioning. Stimuli paired with a tail shock 10 sec later (CS1) elicited conditioned bradycardia. Unpaired or neutral stimuli (CS0) elicited mostly tachycardia. Rats did not learn to suppress responding to nonreinforced sequences with an interval of 6 sec between sounds. Responses to the second stimulus were significantly augmented following a CS1 stimulus, but not following a CS0 stimulus. Real-time summation of simple responses provided a more complete and quantitative prediction of dual responses than did resetting or facilitation. These results extend the time range over which summation may be observed from less than 2 sec to at least 16 sec. They appear to be inconsistent with models involving competition between unitary representations of stimuli in short-term memory and suggest the existence of multiple stimulus traces with independent time courses.     

Varying temporal location of a conditioned stimulus in heart rate conditioning ofMacaca mulatta

The several functions that a stimulus can assume were investigated in a Pavlovian conditioning procedure. The subjects were six rhesus monkeys; the response under observation was heart rate. The conditioning began with a temporal separation of zero between a signal and a regularly repeating electric shock; the signal was then moved to a series of earlier locations in the inter-shock interval. After six sessions at each location, two sessions followed in which only the shock was delivered periodically. The findings included: (1) A two-phased conditioned cardiac rate response seen at the first location became more multiphasic and irregular during longer intervals between signal and shock; (2) the location where the conditioned response peaked became increasingly variable as the signal was moved back, but this variability maintained a constant proportion to the signal-shock interval; and, (3) heart rate during a presignal period, and during a comparable period in shock only sessions, was generally deceleratory early in training and acceleratory thereafter. Sessions with the signal showed heart rate in the presignal period to have become acceleratory earlier in training than sessions with shock only. The data pertain to stimulus control over heart rate as a function of: (A) the temporal proximity of a signal to an aversive stimulus; and, (B) the presence or absence of the signal. The use of appropriate response units in cardiac conditioning is also discussed.     

The potentiation effect during serial conditioning

The development of suppression in rats to a target conditioned stimulus (CS) was compared in trace and serial conditioning procedures. The interval between the end of the target CS and the shock unconditioned stimulus (US) was filled by a second CS in the serial, but not the trace, procedure. In five experiments the serial procedure produced superior conditioning. This potentiation effect, however, depended critically upon the level of conditioning to the stimulus interpolated between the target CS and the US. When conditioning to the interpolated CS was either reduced by giving independent nonreinforced trials with this CS alone or enhanced by independent reinforced trials, the potentiation effect was abolished. In addition, the insertion of a trace interval between the target and interpolated CSs reduced the effect. However, the magnitude of conditioning to the target CS was unaffected by post-conditioning changes in the conditioned strength of the interpolated CS. These findings are discussed in terms of the contribution of both the association between the CSs themselves, which is inherent in the serial procedure, and that between the target CS and the US.     

Associative transfer and stimulus selection in classical conditioning of the rabbit's nictitating membrane response to serial compound CSs

Four experiments were conducted to determine whether in conditioning to a serial compound, CS1-CS2-UCS, there are (a) associative mechanisms operating to extend conditioning beyond the bounds of a CS-UCS contiguity gradient and (b) stimulus selection processes acting to attenuate the potency of CS-UCS contiguity. In Experiments 1 and 2, the CS2-UCS interval was held at .35 sec while the CS1-UCS interval was varied across groups from .75 to 2.75 sec. CS1 test trials revealed substantial CR acquisition at all CS1-UCS intervals. Moreover, Experiment 2 indicated that when the contribution of cross-modal generalization from CS2 to CS1 was factored out, there still remained a substantial level of conditioning, which Experiment 3 indicated was attributable to an associative mechanism like higher-order or sensory conditioning. The observation of CR acquisition at CS1-UCS intervals of 4.75, 8.75, and 18.75 sec in Experiment 4 suggested that serial compound training yields conditioning to CSs located well beyond the single CS contiguity gradient for the rabbit's nictitating membrane response. Experiments 1 and 2 also indicated the presence of stimulus selection processes because, at the shorter CS1-UCS intervals (.75 and 1.25 sec), the levels of test-trial responding to CS2 fell below those observed to the less contiguous CS1.     

Cross-modal transfer of conditioned suppression in rats: Effects of US intensity and extinction of the initial conditioning task

In conditioned suppression of water licking behavior by rats, we obtained data indicating general transfer of fear conditioning. A series of experiments resulted in two major findings. First, pairing of a neutral stimulus with a shock in the initial conditioning task facilitated acquisition of subsequent fear conditioning to another neutral stimulus, if the conditioned fear of the initial task was extinguished prior to the second task and if equally strong shocks were employed in both conditioning tasks. Second, omission of the extinction treatment or employment of weaker shocks in the initial task resulted in retardation, rather than facilitation, of the second conditioning task. An application to human clinical settings is discussed.     

Conditioned suppression of behavior maintained by intracranial stimulation as a function of stimulation intensity.

Conditioned suppression was demonstrated in two experiments with rats lever pressing on a fixed-ration 1 schedule for lateral hypothalamic intracranaial stimulation (ICS)'n Experiment I, conditioned suppression of responding for low-intensity ICS was obtained with a moderate intensity of foot shock, In Experiment II, low and high intensities of ICS were alternated within the same session and the same animal The suppression that was exhibited with low intensity ICS was minimal or absent with high-intensity stimulation, despite the pairing of foot shock with each warning stimulus. Conditioned suppression was a function of ICS intensity, and was independent of response rates. The inverse relationship between ICS intensity and degree os suppression is consistent with a motivational analysis of conditioned suppression. Previous reports of resistance to suppression of behaviors maintained by ICS may now be attributed to the use of high-intensity stimulation.     

Attenuation of latent inhibition by post-acquisition reminder

Using lick suppression by water-deprived rats as an associative index, white noise-footshock pairings resulted in less manifest conditioning when repeated non-reinforced presentations of the white noise preceded conditioning than when no stimulus pre-exposure was given, i.e., latent inhibition was observed. However, the latent inhibition deficit was reduced in animals who received as a reminder treatment shock-alone presentations in another context during the retention interval. Animals conditioned without prior stimulus pre-exposure and those exposed to the white noise and shock unpaired during the conditioning phase of the study showed no change in lick suppression as a result of the reminder treatment. These results suggest that the behavioural deficit produced by non-reinforced pre-exposure to the to-be-conditioned stimulus arises at least in part from a reversible retrieval failure rather than a lack of acquisition.     

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.     

No sex difference in contextual control over the expression of latent inhibition and extinction in Pavlovian fear conditioning in rats

Three experiments with Wistar rats searched for a sex difference in contextual control over the expression of latent inhibition and extinction. Experiment 1 used a latent inhibition procedure; Experiments 2 and 3 employed an extinction preparation. All experiments used a shock as the unconditioned stimulus, a tone as the conditioned stimulus, and suppression of food magazine visits as the measure of conditioned responding to the tone. Each experiment revealed a reliable context effect on conditioned responding to the tone; after conditioning in a separate context, conditioned responding in the former latent inhibition or extinction context was attenuated relative to conditioned responding in a control context. There was no sex difference in the magnitude of this effect. These results are discussed in the framework of sex differences in the hippocampus and of the putative role of this structure in various instances of contextual learning.     

Biological vs experiential factors in phobic conditioning

The present study was performed as a test of phobic conditioning being a case of prepared learning (Seligman, 1971). Skin conductance responses were conditioned in three groups of subjects to either slides of snakes and spiders; electric outlets; or geometric shapes, as conditioned stimuli (CSs) with shock to the forearm as the unconditioned stimulus (UCS). The purpose of the experiment was to compare electrodermal conditioning to potentially phobic CSs, i.e. snakes and spiders, with conditioning to fear-relevant, but non-phobic, CSs, i.e. electric outlets.Each subject saw two pictures, either a snake or a spider; or two different slides of electric outlets; or two different geometric shapes. Only one of the two cues (the CS + ) was immediately followed by the shock-UCS during the acquisition phase. Thus, a differential paradigm was used. There were 4 habituation, 12 acquisition, and 16 extinction trials. The duration of the pictures was 8 sec with an intertrial interval of between 35–45 sec.The results showed reliable effects of conditioning in all three groups during the acquisition phase. Furthermore, a significant interaction during extinction is reported, indicating responses to potentially phobic CSs to be more resistant to extinction than responses to the other two classes of stimuli. It is concluded that the present results favor an interpretation of phobic conditioning in terms of biologically prepared learning.     

Conditioned inhibition of fear: Evidence for a competing response mechanism

In two experiments, inhibitory conditioning was attempted by presenting a discrete CS in a neutral stimulus environment shortly following the termination of either shock (Experiment 1) or a second discrete CS which had been paired in a forward manner with shock (Experiment 2). Evidence of successful inhibitory conditioning was mixed in Experiment 1, where the properties of the CS were assessed within an escape-from-fear procedure. Postresponse presentations of the CS enhanced performance, whereas the presentation of the CS prior to responding did not have the expected degrading effect on performance. In Experiment 2, the inhibitory properties of the CS were assessed by combining this stimulus with an excitatory CS and presenting the compound to rats engaged in a water-reinforced licking response. Less response suppression was found in reaction to this compound relative to three separate comparison conditions, thus witnessing the success of the inhibitory-conditioning procedure used. The common assumption that inhibitory conditioning results from the nonreinforcement of a CS in a situation where reinforcement is expected, i.e., one which contains previously reinforced cues, is not supported by these data, for no previously reinforced cues were simultaneously presented with the CS during inhibitory training. The data are in agreement with a conditioned antagonistic-response interpretation of inhibitory conditioning.     

Effects of real vs pretend shock on semantic differential judgments and the GSR in some classical conditioning paradigms

In two experiments subjects were shown different color 1-sec lights, one just before the end of a 4-sec UCS and the other coming 10 sec after UCS termination and 25 sec before the next UCS. For some groups the UCS was strong electric shock; for others it was a tone which subjects were instructed to imagine was a strong shock. The primary data were changes in semantic differential ratings of the lights before and after 12 “conditioning” trials. There were no differences among the groups. In a third experiment a 6-sec delay CS was used for GSR conditioning, some subjects receiving actual shock and others pretend shock. Only the former showed conditioning. The results are discussed in terms of a model of experimental demand.