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.     

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.     

The nature of the initial coping response and the learned helplessness effect

The availability of an effective coping response has been shown to attenuate the deleterious behavioral and physiological consequences of inescapable electric shock. In the current study, two groups of rats could escape tailshock by turning a wheel. When short-latency responses that appeared to be elicited by shock onset were permitted to terminate shock, rats subsequently failed to learn to escape in a shuttlebox and did not differ from rats which received an equivalent amount of inescapable shock. However, when a relatively long-latency response was required and short-latency responses were not allowed to affect shock, rats subsequently readily learned to escape in the shuttlebox. The implications of these results for explanations of the manner in which prior exposure to shock influences subsequent escape learning were discussed.     

Successive reversals of a discriminated preference for signaled tailshock

Three rats received unmodifiable tailshock at random intervals in a shuttlebox. In a continuous-choice situation, Ss could choose between an auditory signal immediately preceding or immediately following the tailshock. Over repeated daily 3-h sessions, each S acquired a spatial discrimination indicating a strong preference for the signal preceding tailshock. This preference continued undiminished through two successive reversals of the position associated with signaled shock. This demonstration precludes explanations of the preference-for-signaled-shock phenomenon based upon primary reinforcement value or acquired value of the signal, position preferences, and overt modification of the aversiveness of the reinforcer through such means as postural adjustments. An explanation of recent failures to obtain the preference-for-signaled-shock effect is offered.     

Discriminated avoidance learning and reversal by goldfish in a shuttlebox using a linear presentation procedure

Goldfish, trained in a shuttlebox with cues and shock controlled by a linear presentation procedure, learned to control prevailing cue states and shock, and thus to discriminatively avoid shock presentation. The linear presentation procedure, adapted for the shuttlebox from the sequence of cue presentation occurring in the Y-maze, utilized three cues on each trial, a trial-start (TS) cue, a shock-paired (S?) cue, and an unpaired (S+) cue. At trial onset (TS cue), the goldfish had 10 sec to respond. The first response produced, via response-contingent programming, either the shock-paired (S?) or unpaired (S+) cue, and subsequent responses produced alternation of S+ and S?. The shock was omitted only if the S+ cue state prevailed 10 sec after trial onset. AU other cue states (TS and S?) were paired with shock. Goldfish learned to respond and control the prevailing cue state so that the S+ cue prevailed at 10 sec posttrial onset for a variety of different color-cue combinations and also learned to reverse their originally learned cue preference when the color cues were reversed. The linear presentation procedure represents an alternative discrimination learning procedure that appears to be free of the interpretational problems encountered in training goldfish in the shuttlebox apparatus with other one- and two-stimulus procedures.     

Further avoidance studies with goldfish: Brightness discrimination and transposition

In two experiments using the shuttlebox apparatus and negative (shock) reinforcement, goldfish learned to discriminate between same color signals differing only in brightness and to choose the brightness level paired with shock-omission to avoid shock. Experiment 1 compared two signal presentation procedures, one presenting only one signal at a time but allowing for successive alternation of signals within a trial (SUC) and the other presenting both signals simultaneously (SIM). SIM performance was superior to SUC in acquisition and showed more positive transfer with signal color changed (CC), more negative transfer with brightness-level—shock pairing reversed (R), and, although attenuated, more negative transfer with both signal color changed and shock pairing reversed (CC + R). Further, SIM relearning following reversal was faster than SUC. In Experiment 2, using the SIM procedure, eight transfer groups were tested for transposition with one of their original acquisition signals replaced in transfer by a signal bearing either the same or the opposite brightness relationship to the retained signal. Four transfer groups showed positive and four showed negative transfer effects, but all eight groups showed transfer performances consistent with a transpositional expectation. The present two experiments alone do not elucidate the mechanisms accounting for either the SIM superiority over SUC in Experiment 1 or the transposition effect in Experiment 2. These experiments show only that the transposition phenomenon can be supported by negative reinforcement procedures, which has not been demonstrated before. Now that techniques exist for producing negatively reinforced transposition, work investigating the underlying mechanisms and comparing them with the mechanisms of positively reinforced transposition can begin.     

Categorical color coding in goldfish

Goldfish, trained in the shuttlebox apparatus to avoid shock, acquired a color discrimination between two colors (red/green) and were tested in transfer with a new set of colors (yellow/blue). Transfer color shock-pairing was either consistent with (red=yellow, blue=green) or opposite to (red≠ yellow, green≠blue) categorical color coding seen in pigeons. Groups with transfer shock-pairing consistent with categorical color coding showed positive transfer, and groups with transfer shock-pairing opposite to categorical color coding showed negative transfer, similar to an attenuated reversal learning effect. These results indicate that goldfish, like pigeons, code different colors as behavioral equivalents even though they can easily learn to discriminate between them. As with pigeons, the finding of the categorical color coding phenomenon changes the conclusions drawn from earlier goldfish conditional-discrimination transfer studies using only signal color changes between acquisition and transfer testing, from evidence of concept learning to evidence for categorical color coding, on the grounds of parsimony. It is important to note that this finding affects only the explanation of conditional-discrimination transfer effects, and the fact remains that both pigeons and goldfish can learn to conditionally discriminate—pigeons for positive reinforcement, and goldfish to avoid shock.     

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.     

Stimulus control of immunization against chronic learned helplessness

Two experiments investigated the effectiveness of multiple (five) sessions of signaled eseapable-shock pretraining in preventing (immunizing against) the shack-escape impairment produced by an equal number of sessions of signaled inescapable shock. In Experiment 1, rats were exposed to 50 pairings per session of a white-noise stimulus with escapable shock during the immunization phase. Subsequently, they were exposed to 50 pairings per session of a different (houselight) stimulus with inescapable shock. Shock-escape performance in a shuttlebox test with constant illumination revealed no evidence of immunization relative to the performance of rats given five prior sessions of light-signaled inescapable shock only. Experiment 2 was identical in all respects to Experiment 1, except that both the escapable- and the inescapable-shock phases for animals in the immunization treatment group involved the same stimulus (houseüght) as a shock signal. Under these circumstances, the prior escapable-shock training significantly reduced the shuttle-box escape deficit engendered by chronic exposure to signaled inescapable shock; performance in the shuttle-box was not reliably different from that of rats exposed to signaled escapable shock alone. These findings suggest that, under chronic conditions, the development of stimulus control using Pavlovian conditioning procedures may serve to modulate the normally prophylactic influence on later shock-escape acquisition of serial exposure to escapable and inescapable shocks.     

Preexposure to situational cues produces a direct relationship between two-way avoidance learning and shock intensity

In Experiment 1, four groups of subjects (n = 16 each) were exposed to the situational stimuli of a shuttlebox apparatus for 4 h. Subsequently, 200 two-way avoidance trials were administered (100/day) with either .3- or 1.6-mA shock and with either small or large reward (presence or absence of visual stimuli following the response). Avoidance performance was directly related to shock intensity on both days and to magnitude of reward on the 2nd day. In Experiment 2, four groups of subjects (n = 24 each) were given 4 h of exposure either to the situational stimuli of the shuttlebox or to a neutral box. Then, 10 two-way avoidance trials were given with 1.6-mA shock. Subsequently, subjects were allowed to escape from one of the shuttlebox compartments to an adjacent safe box. Following preexposure to situational stimuli, avoidance performance was superior whereas escape-from-fear performance was inferior. This latter finding demonstrated that less fear of situational cues was present during avoidance training in the preexposed condition. All of these results support the effective reinforcement theory, an extension of two-factor theory, which emphasizes the importance for avoidance learning of the amount of fear of situational cues present following a response.     

Conditions that potentiate the effects of electroconvulsive shock administered 24 hours after avoidance training

Three experiments assessed the conditions that potentiate effects of an electroconvulsive shock (ECS) administered 24 h after avoidance training. Stimuli present immediately prior to the ECS were systematically varied. In Experiment 1, which employed a passive avoidance task, the primary determinant of whether the ECS disrupted retention was whether the situational cues present at the time of ECS delivery were those associated with the initial training experience: ECS disrupted performance only when it was administered in the original training apparatus, regardless of whether or not a footshock was presented immediately prior to ECS. In Experiment 2, which employed an active, shuttlebox avoidance task, both the situational cues from the training apparatus and a footshock were necessary to potentiate the disruptive effects of the ECS. Experiment 3 revealed that ECS effects on performance of the active avoidance task can also be potentiated by a combination of apparatus cues and the warning signal used in initial training. These results are interpreted as indicating that informational functions of stimuli present when an ECS is administered are important determinants of the effects of the ECS.     

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.     

Location preference and flavor aversion reinforced by amphetamine in rats

To associate the identical drug state with both a location and a flavor, rats were given intraperitoneal amphetamine injections and then confined for 20 min in one side of a shuttlebox with access to a flavored solution; on control trials without injections, they were confined for 20 min in the opposite side with a different flavor. In the first experiment, the rats were placed in the shuttlebox immediately after injections; in the second experiment, they were placed in the shuttlebox 20 min after injections. Subsequent free-choice tests in both experiments revealed an increased choice of the side of the shuttlebox associated with amphetamine but also an aversion to the flavored solution associated with the drug.     

Serial order anticipation learning in rats: Memory for multiple hedonic events and their order

Many characteristics of a series of discrete independent hedonic events may be remembered by rats in terms of, for example, how many events were rewarded and how many were nonre-warded. Such memory for multiple hedonic events, which has been shown to be a potent factor controlling instrumental responding, was examined here in five investigations employing serial anticipation learning in a runway. It was found that the ability of rats to remember the hedonic events reward and nonreward is highly developed, accurate, and quite resistant to forgetting and interference. Rats not only remembered a rewarded event and a nonrewarded event, but they also remembered the order in which the two events occurred. Rats remembered how many nonrewarded events there had been accurately enough to suggest that they were using some form of a counting mechanism. Rats exhibited little forgetting of eight prior discrete hedonic events, one rewarded followed by seven nonrewarded, even when these occurred over an interval of 20 min and involved considerable potential interference. In the serial learning situation employed here, marked primacy effects were obtained, earlier nonrewarded trials in a series being better anticipated than later ones. The primacy effect was found to depend upon the type of series employed. By assuming that stimulus generalizations occur between the multiple hedonic events remembered by rats, all anticipatory learning obtained here could be explained in considerable detail.     

Avoidance differences between rats and gerbils

Gerbils and rats learned equally well to discriminate the lighted, safe arm from the unsafe arm during Y-maze avoidance trials. Gerbils, however, were inferior to rats in initiating this response in time to avoid shock. Two subsequent experiments on passive avoidance did not support the interpretation of these data based on a greater incidence of shock-induced activity suppression in gerbils. In both experiments, gerbils required more shocks than rats to learn a staying response, indicating a pronounced locomotor response bias in gerbils that is not compatible with the required passive avoidance response. A fourth experiment, using a shuttlebox, found that the relative active avoidance performance by these species depends upon whether intertriai responses are permitted and punished. When they are, gerbils are inferior to rats, since their high level of locomotor responding is not compatible with the behavior required, i.e., staying during the intertrial interval and running during the CS-US interval. On the other hand, gerbils are not inferior when intertrial responses are prohibited, since their locomotor bias is not punished and is compatible with the required avoidance response.     

Avoidance,classical, and pseudoconditioning as a function of species-specific defense reactions in high- and low-avoider rat strains

Rats of the Australian High Avoider (AHA) and Australian Low Avoider (ALA) strains and their reciprocal crosses were exposed to 50 trials of one of three shuttlebox procedures. The avoidance group received pairings of a tone and shock. If the animals shuttled during the tone, they avoided the shock. If they waited until the shock came on, they could then escape it. The classical group received pairings of the tone and a brief inescapable shock. If they shuttled during the tone, the tone ceased and they immediately received the shock. If they did not shuttle, they received the brief shock at the termination of the tone. The pseudoconditioning group received the tone and the shock explicitly unpaired. If they shuttled during either the tone or the shock, the stimulus was terminated. There was no acquisition of anticipatory responding under the pseudoconditioning procedure. All groups evidenced an increase in anticipatory responding over trials under the classical procedure. The AHAs acquired the response faster and reached a higher asymptote than did the ALAs. Performance of the two reciprocal crosses fell in between. A similar pattern was observed under the avoidance procedure, albeit at slightly higher response levels. Subsequent studies established that the AHAs acquired a one-way avoidance response quickly, but were impaired on a passive avoidance task, whereas the reverse was the case for the ALAs. The reciprocal crosses were proficient at both tasks. These results suggest that shuttlebox avoidance is largely accounted for by classical conditioning of the predominant defensive response. When that response is compatible with performance on the task, acquisition is rapid (AHAs), and when it is not, acquisition is slow (ALAs).     

Generality of the failure-to-escape (helplessness) phenomenon in rats

A series of four experiments investigated a number of parameters reported to produce “helplessness” in rats. Consistent differences in escape behavior were not found between inescapably shocked and restrained rats when a FR 1 shuttling response was used. Escape latencies also did not differ between groups when a reduced shock intensity was employed during escape training in FR 2 procedure or when an increased FR 3 response was employed during escape training. Findings are discussed in terms of the robustness of the failure-to-escape phenomenon from which “helplessness” in the rat is inferred.     

Escape acquisition following inescapable shock in the rat

Male rats which had received approximately 21 min of pulsed, inescapable tail shock during a 6-h session in a wheel-turn chamber were markedly deficient in acquisition of an FR 2 crossing escape response in a shuttlebox when first tested 22 or 70 h later (Experiments 1 and 2). Rats which had received identical amounts and patterns of escapable/avoidable shock, however, were not deficient (Experiment 1). Preventing wheel-turn responses during the inescapable shocks prevented the occurrence of the subsequent escape deficit, whereas reducing the feedback provided for the first crossing response of the FR 2 requirement enhanced the deficit (Experiment 3). These data can be best explained by the learned helplessness hypothesis and indicate that the types of responses available and made during the inescapable shocks are more important than previously indicated.     

Contextual generality of roosttime restlessness in captive American robins (Turdus migratorius)

A series of experiments with captive American robins examined factors affecting roosttime restlessness, that is, the burst of locomotor activity that appears at dusk and is thought to be a correlate of the communal roosting habit of this species. First, the restlessness occurred earlier on cloudy days than on sunny ones, suggesting the importance of illumination level in initiating the activity burst. Second, in studies on social effects, the restlessness was greater when robins were totally isolated than when they had visual-auditory access to conspecifics; indeed, when a small flock of robins was housed together, restlessness at dusk was suppressed completely. Third, the birds were more restless when they could see a human observer than when they could not, implying an interaction of tearfulness with the roosting tendency. Finally, restlessness occurred in a large flight cage as well as in small cages, as long as the birds were not maintained in a flock. In sum, roosttime restlessness appears to be a robust phenomenon that occurs under a wide range of laboratory conditions, but which also can be substantially influenced by numerous environmental variables. Further experimental examination of these variables may advance present understanding of the survival value of communal roosting in this species.     

Autocontingencies: Factors underlying control of operant baselines by compound tone/shock/no-shock contingencies

Previous research has shown that postshock acceleration of baseline responding, which normally results from exposure to a shock/no-shock autocontingency, is eliminated when a suppressive tone-shock contingency is simultaneously presented (Davis, Memmott, & Hurwitz, 1975). Three experiments were performed to explore this inability to produce joint suppressive/ accelerative control by compound tone-shock and shock/no-shock contingencies. Progressively degrading the tone-shock contingency in Experiment 1 maintained conditioned suppression and resulted in asymptotic levels of postshock acceleration in all degraded groups. Evidence for accelerative control by the autocontingency was also recorded in a control group that received a totally reliable tone-shock relation. Experiments 2 and 3 pursued this latter finding, which is in direct conflict with our earlier results. The appearance of joint suppressive/ accelerative control by tone-shock and shock/no-shock contingencies appears to be related to the number of shock trials given per session; moreover, relatively small differences in trial density (e.g., three trials per 22.5 min vs. three trials per 30 min) are critical to establishing joint autocontingency control. The importance of shock rate is discussed with regard to the relative waiting time hypothesis, an alternative model of Pavlovian control, as well as to previously reported conditioning failures involving compound suppressive/accelerative procedures.