The US-preexposure effect in honeybees

Signaled avoidance was studied in individual honeybees that visited the laboratory regularly to take sucrose solution from a target set on the sill of an open window. During feeding, substrate vibration or airstream was used to signal a brief shock that could be avoided by breaking off contact with the food for a few seconds. Aversive conditioning of the context was measured in terms of return time (the time between successive visits). In Experiment 1, experience with unsignaled shock was found to lengthen return time—which experience with signaled shock did not—and to impair performance in subsequent avoidance training with signaled shock (the US-preexposure effect). In Experiment 2, experience with unsignaled shock given after signaled avoidance training lengthened return time but had no effect on response to the signal in a subsequent extinction test. These results closely resemble the results obtained in analogous experiments with vertebrates.     

Preference for signaled shock: A test of two hypotheses

In a test of safety signal and preparatory response explanations of the preference for signaled vs. unsignaled shock, three groups of rats were exposed to a different light-tone-shock contingency on each of the two sides of a shuttlebox. One contingency (S/P) provided both a safety signal and a warning stimulus, another (NS/NP) provided neither, and a third (S/NP) provided a safety period but no warning stimulus. Rats preferred either the S/P or the S/NP side of the shuttlebox when the alternate side provided neither safety signal nor warning stimulus. When the safety signal was available on both sides, the side without the warning stimulus was preferred. Results are interpreted as supporting the safety signal hypothesis.     

A comparison of signaled vs. unsignaled free-operant avoidance in wild and domesticated rats

Three experiments compared the performance of domesticated hooded rats and nondomesticated black rats under signaled and unsignaled free-operant leverpress avoidance. There was no difference between groups in asymptotic shock rates under unsignaled avoidance; however, the black rats avoided much more successfully when the signal was present, while the hooded rats showed little or no improvement. When a longer signal was employed (10 vs. 5 sec), the effects were essentially the same. The black rats generally had higher response rates, and this difference was most pronounced in extinction, where the hooded rats made very few responses. The domesticated rats received a disproportionately large number of shocks early in the session (warm-up) under unsignaled avoidance, but this tendency was much less pronounced for the black rats. However, both groups showed appreciable amounts of warm-up during signalled avoidance. The findings are discussed in terms of differences in levels of activation/arousal between domesticated and nondomesticated animals.     

Does shock modifiability contribute to preference for signaled shock?

The central question asked was whether differential shock modification occurs (posturally induced differences in shock contact time) under signaled and unsignaled conditions using scrambled shock. Shock modifiability was tested with two different shock sources, intensities, and scrambling units by measuring the duration of time subjects were in contact with shock. Subjects were then given a choice between the signaled and unsignaled conditions. Results showed that differential modification of shock contact time did not occur between signaled and unsignaled conditions with any shock source, intensity, or scrambler unit. In addition, subjects preferred the signaled condition. It was concluded that experiments using scrambled shock are not confounded by posturally induced differences in shock contact time.     

Can a rat count?

A 2 × 2 factorial experiment was conducted in a licking-suppression situation to test if a rat could count the number of shocks given in a 5-min session under signaled and unsignaled shock conditions. Groups F received three .7-sec grid shocks per session throughout 80 sessions, whereas Groups V received, on any day, one, two, three, four, or five shocks, with a mean of three shocks. The rats’ counting ability was assessed in terms of the post-third-shock acceleration of licking. The results of this measure were compared between Groups F and Groups V on test days in which both groups received three shocks with the identical shock sequence. There was no evidence that rats could count under either signaled or unsignaled shock conditions. The basal rate of licking was less in groups run under the unsignaled shock condition than under the signaled shock condition. The effect of fixed/variable shock frequency upon basal rate of licking was not significant. The results are discussed with reference to the optimal shock density view of Davis and Memmott (1982).     

A concurrent assessment of the positive and negative properties of a signaled shock schedule

A choice and a conditioned suppression procedure were used to assess concurrently the positive and negative properties of stimuli within a signaled shock schedule, Occasional shocks were presented to Ss responding on a variable-interval food schedule. Ss could choose whether shocks occurred alone or whether they were preceded by a 1-min signal. All Ss chose the signaled shock condition over the unsignaled one, even though food reinforced responding in the presence of the signal was suppressed. Rate of responding for food varied across stimulus conditions, with the lowest rate in the presence of the signal and the highest rate in its absence. An intermediate rate occurred under the unsignaled shock schedule. A safety analysis was applied to the data.     

The stimulus-response overshadowing phenomenon with VI versus FI schedules of reinforcement

Previous research has shown that response rates on a variable interval (VI) schedule of reinforcement decrease if a brief response-produced signal is given prior to reward. One explanation is that the signal overshadows the response because it is a better predictor of reinforcement. The S-R overshadowing effect does not occur with variable ratio (VR) schedules, however. Tarpy, Lea, and Midgley (1983) explained this fact by suggesting that the signal functions to enhance the salience of the temporal interval offset on the VI schedule (a characteristic not possessed by VR schedules), which then overshadows the response. In this experiment, the salience of the temporal interval was enhanced in another way: signaled or unsignaled reward was provided to rats responding on either a VI or fixed interval (FI) reward schedule. As predicted, rates were lowest for animals receiving signaled reinforcement on an FI schedule and highest for those receiving unsignaled reinforcement on a VI schedule.     

Differential response gradients to a Pavlovian safety signal following active avoidance training in young and adult rats

Following 300 training trials in two-way shuttle avoidance signaled by a tone (CS+), two groups each of weanling and adult rats were given Pavlovian discrimination training in which the CS+ was followed by inescapable shock, and a more intense tone (CS—) signaled no shock. An additional group at each age level received both tones paired randomly with shock or no shock. Subsequent generalization tests along the frequency dimension indicated that both pups and adults tested at the CS+ intensity showed similar gradients of frequency control. Gradients for the adults tested at the CS — intensity tended to be inverted, with least responding at CS—, a result not found in the young subjects. The results were considered in light of Pavlovian extradimensional influences on the control of avoidance behavior.     

Fear of the CS and of the context in two-way avoidance learning: Between- and within-subjects manipulations

The roles of CS fear and of context fear in signaled two-way avoidance learning were examined in two experiments in which shock intensity was manipulated either between or within subjects. For each subject, two discrete CSs, a light and a white noise, were used. For between-subjects comparisons, both CSs were paired with the same shock intensity, weak or strong. Under this condition, in which fear of the CSs and the context was greater with strong than with weak shock, avoidance performance varied inversely with shock intensity. For within-subjects comparisons, the light was paired with strong shock and the white noise with weak shock, or vice versa. In this case, context fear was constant during presentation of each CS, and avoidance performance varied directly with shock intensity. Additionally, intertrial responding was directly related to the amount of context fear. These results support effective reinforcement theory, an extension of two-factor theory, which acknowledges the contribution to avoidance learning both of CS fear and of context fear. The interchangeable effectiveness of visual and auditory stimuli as CSs is discussed with regard to stimulus specificity in avoidance learning.     

Reliability and sources of control of preference for signaled shock

Five experiments investigated the reliability of, and a possible explanation for, the preference for signaled shock (PSS) phenomenon, in which animals show a preference for signaled over unsignaled, unmodifiable shock. Experiment 1 demonstrated the generality of our previous PSS shuttlebox data. In an attempt to explain some of the studies in which a failure to obtain a PSS has been reported, Experiments 2 and 3 found that under certain conditions the PSS effect could be overridden by an innate preference for dark; however, the series of experiments in toto speaks clearly for the robustness of the phenomenon. Consistent with the recently proposed “crossover consequence hypothesis,” Experiments also revealed that shuttling behavior during manifestations of PSS is not randomly distributed in time. However, Experiments 4 and 5 found that PSS could not generally be attributed to an interaction between the acquired aversiveness of the signal and the spatially asymmetrical probability of shuttling being punished, as proposed by this hypothesis.     

Changeover from unsignaled to signaled avoidance with and without a feedback stimulus in rats

Two experiments are reported in which, by pressing a lever, unsignaled avoidable shocks could be changed to signaled avoidable shocks for periods of 1 min, after which unsignaled avoidance was automatically reinstated. A correlated stimulus identified the signaled schedule. Under these conditions, Ss changed over at a rate sufficient to remain in the signaled schedule more than 90% of the time. Both experiments showed that changeover performance was under the control of the correlated stimulus. Presenting the signaled schedule without the correlated stimulus did not maintain changing over. The second experiment showed that a 1-sec presentation of the correlated stimulus was sufficient to maintain changeover performance. The results were attributed to the conditioned reinforcing properties of the correlated stimulus.     

Signal duration and choosing between signaled and unsignaled reward

The effects of signal duration when choosing between signaled and unsignaled response-independent reinforcers were examined in two experiments. In Experiment 1, albino rats were given a choice between signaled and unsignaled food delivered on variable-time 60-sec schedules using a 20-sec signal. All subjects preferred the signaled schedule at a level comparable to that reported in an earlier study using a 5-sec signal. Experiment 2 presented six rats with a direct choice between a 5-sec and a 20-sec signal condition, and three rats with a choice between a 1.5-sec and a 5-sec signal duration. Subjects preferred the 20-sec signal over the 5-sec signal, but no pReference was found with the 1.5-sec vs. a 5-sec signal. Current theoretical views, such as delay reduction and behavioral competition, are considered.     

Nonassociative habituation,US preexposure,and backward inhibitory conditioning with signaled and unsignaled USs

In a conditioned suppression experiment, rats received a single, massed session of conditioning in which one backward conditioned inhibitory stimulus (CS-) followed shocks that were signaled by a visual cue, and a second backward CS-followed shocks that were unsignaled. Conditioning was preceded by a preexposure phase in which some groups of rats were preexposed to unsignaled shock, while others were not preexposed and remained in the experimental apparatus in the absence of shock. The groups were further distinguished by whether US preexposure and conditioning occurred in the same or different contexts, and by whether conditioning began immediately or after a 24-h rest period in the home cage. Although the conditioning itself was effective in establishing the visual cue as a conditioned excitor in the nonpreexposed groups, it was not effective in establishing the two backward cues as reliable inhibitors with either signaled or unsignaled USs. After 210 US preexposures, however, the same conditioning sessions did yield conditioned inhibition to both CS-s. A 24-h rest period in the home cage reduced the magnitude of, but did not completely abolish, the facilitative effect of US preexposure on inhibitory conditioning. Other tests demonstrated that US preexposure had retarded excitatory conditioning to the visual cue. This interference with excitatory conditioning was unchanged in magnitude after the 24-h rest period. The facilitative effect of US preexposure on backward inhibitory conditioning, and the interference effect on excitatory conditioning, were both eliminated by a change in context between US preexposure and conditioning. These observations encourage predominantly associative accounts of the effects, but allow for a small nonassociative habituation component.     

Effect of signaling extra unconditioned stimuli on autoshaping

Acquisition to a target conditioned stimulus (CS) is prevented when extra, unsignaled unconditioned stimuli (USs) are presented with sufficient frequency to remove contingency between target CS and US. Acquisition occurs, however, when the extra USs are signaled by another CS. According to the Rescorla-Wagner theory, signaling reduces contextual conditioning, which otherwise prevents acquisition. Results of Experiment 1 led to the rejection of a rival explanation derived from scalar expectancy theory by showing that acquisition does not occur when only half of the extra USs are signaled. The results of Experiment 2 were, however, contrary to the Rescorla-Wagner theory because they showed equivalent acquisition when the stimulus used to signal the extra USs was also present concurrently with the target CS. Signaling may exert its effect by converting the intertriai interval to CS?.     

Signaled reinforcement effects on fixed-interval performance of the rat

In three experiments using fixed-interval schedules with a 500-msec delay of reinforcement, rats receiving a localized light signal during the delay leverpressed more slowly than rats trained without the signal. In Experiment 1 these groups showed no differences in temporal patterning of responding, but in the remaining two experiments the signaled rats showed better patterning than the unsignaled rats. In Experiments 2 and 3 rats receiving a diffuse tone signal during the delay instead of a light pressed more rather than less rapidly than the unsignaled group. Their patterning was better than that of the unsignaled rats in one of these experiments. Several explanations for both the attenuation and enhancing effects of signaling reward are discussed.     

Short-term retention of “surprising” events following different training conditions

Terry and Wagner (1975) have suggested that short-term retention of information about an event is enhanced if the occurrence of the event is surprising. To investigate this idea, we trained two groups of pigeons in a preparatory-releaser procedure in which half the trials started with the presentation of food (the preparatory event). The preparatory food presensation was signaled by an 8-sec white keylight in the signaled, but not in the unsignaled, group. After a retention interval, varying between 2 and 32 sec, the releaser stimulus (CS_R), a red keylight, was presented for 8 sec in the absence of any reinforcement. The remaining trials were initiated by the presentation of CS_R, and the first peck occurring 8 sec after the onset of CS_R was reinforced by food. The preparatory event controlled responding to CS_R at the short retention interval, with the level of control declining systematically with increasing retention intervals. On probe test trials, the presentation of the preparatory food event was preceded by a stimulus that had previously been paired (CS+) or unpaired with food (CS?). Discriminative responding to CSr was better following CS? than following CS+ in the unsignaled, but not the signaled, group. These results suggest that the enhanced retention following surprising preparatory events reflects a generalization decrement induced by changing the signaling conditions between training and testing.     

The response-shock interval and conditioned suppression of avoidance in rats

Rats were trained to avoid unsignaled shocks with response-shock intervals of 30, 60, or 120 sec. When CSs of 60 sec duration paired with unavoidable shocks were then superimposed upon the avoidance baseline, responding decreased during the CS. Reductions in responding resulted in extra shocks which were potentially avoidable in all response-shock interval conditions, with the greatest increase in shocks in the response-shock 30-sec condition. Decreases in responding were greater when the CS was paired with a 2.0-mA unavoidable shock than with a 1.0-mA shock.     

Measurement of current flow through the rat under signaled and unsignaled grid-shock conditions

The present experiment was run to test the hypothesis that, when shock was signaled, rats would develop effective coping responses so as to reduce the current flow through them. A 1-sec shock was delivered through a grid floor by a fixed impedance ac shock source. The current-flow measure was taken over the last 30 of 90 trials given over 3 days and indexed by “gross skin conductance” or GSC (shock). The rat under the signaled shock condition (n=15) showed higher GSC (shock) than did the rats under the unsignaled shock condition (n=14). Thus, the result contradicted the hypothesis. There was no indication that the rats developed any preparatory response during the 5-sec signal, in terms of either GSC (signal) or posture. The results were discussed with reference to the preparatory-response hypothesis and various other possibilities.     

Postresponse exposure to warning signal in avoidance extinction

Delayed termination of the warning signal following extinction responses has been shown to facilitate extinction of discriminative avoidance. In order to determine the relative roles in extinction of delayed termination per se and postresponse exposure to the warning signal, which is necessarily confounded with delayed termination treatment, seven groups of rats were first trained on a one-way avoidance task in which a buzzer served as part of the warning-signal complex. Then, on nonshock extinction trials, the buzzer (a) terminated immediately with a response and was not reinstated in the postresponse interval, (b) terminated immediately with a response and was reinstated for a 5- or 10-sec period 5 or 15 sec following the response, or (c) terminated 5 or 10 sec following a response (delayed termination). Results indicated that exposure to the buzzer at postresponse intervals greater than 5 sec following responses was critically involved in reducing resistance to extinction. This finding supports a conditioned relief interpretation of the delayed warning signal termination effect and is consistent with the effect of response prevention techniques on extinction.     

Reinstatement of fear to an extinguished conditioned context

Rats were shocked in the black but not the white compartment of a shuttlebox and then exposed to the black compartment in the absence of the shock unconditioned stimulus (US) to extinguish fear responses (passive avoidance). In five experiments, rats were then shocked in a reinstatement context (distinctively different from the shuttlebox) to determine the conditions that reinstate extinguished fear responding to the black compartment. Rats shocked immediately upon exposure to the reinstatement chamber failed to show either reinstatement of avoidance of the black compartment or fear responses (freezing) when tested in the reinstatement chamber. In contrast, rats shocked 30 sec after exposure to the reinstatement chamber exhibited both reinstatement of avoidance of the black compartment and freezing responses in the reinstatement chamber (Experiment 1). Rats shocked after 30 sec of exposure to the reinstatement chamber but then exposed to that chamber in the absence of shock failed to exhibit reinstatement of the avoidance response and did not freeze when tested in the reinstatement chamber (Experiment 2). Rats exposed to a signaled shock in the reinstatement chamber and then exposed to that chamber in the absence of shock also failed to exhibit reinstatement of the avoidance response (Experiment 5). These rats showed fear responses to the signal but not to the reinstatement chamber. Finally, rats exposed for some time (20 min) to the reinstatement chamber before shock exhibited reinstatement of the avoidance response but failed to freeze when tested in the reinstatement chamber (Experiments 3 and 4). These results are discussed in terms of the contextual conditioning (Bouton, 1994) and the US representation (Rescorla, 1979) accounts of postextinction reinstatement.