Temporal discrimination of alternate days in rats

The objective was to determine whether rats could learn to time a 48-h interval. Rats (n = 6) were continuously housed in operant chambers in constant darkness. The feeding cycle consisted of unlimited access to food for 6 h, followed by 42 h without access to food (i.e., meals were available on alternate days, contingent on breaking a photobeam in the food trough). Head entries into the trough increased as a function of time prior to the meal; this increase was higher, relative to the increase that occurred at the same time of day on alternate (i.e., nonfood) days. These data suggest that rats discriminated alternate days. Next, two meals were omitted to dissociate mechanisms of a self-sustained endogenous rhythm, interval timing, and alternation. Response rate increased periodically every 24 h, which suggests that the rats anticipated alternate days by discriminating the status of the previous day as a meal or a nonmeal day.     

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.     

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.     

The effect of predictive cues on freezing in rats

Rats were given five shocks over a 5-min period and then observed for 20 min. Much more freezing was observed in animals that remained in the shock situation than in animals moved to another situation. Freezing, therefore, seems to be controlled primarily by external shock-related cues. Freezing appears to be also partly controlled by the inherent stimulus properties of the situation.     

Human-based social interaction can reward a rat’s behavior

Rats were trained to leverpress in a conventional operant test chamber; however, their behavior was rewarded solely by social interaction with a human being. This training was successful for half the subjects tested; success was confined to animals for which social interaction had occurred prior to training. Similar findings with other species are discussed. Although the effects of social interaction with a human are by no means as robust as those of food to a hungry animal, the present results suggest that the human-rat interaction may be a positive adjunct to conventional behavioral training techniques, and a possible confound where its effects are uncontrolled.     

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).     

The functional equivalence of operant behavior and foraging

Many researchers have used the controlled procedures of operant methodology to test the predictions of foraging theory on the assumption that foraging is operant behavior and operant behavior is foraging. This experiment tested the assumption that leverpressing is functionally equivalent to search effort. Four rats were exposed to two situations: a naturalistic foraging situation (a “patch”) and a standard operant chamber. Each condition represented a different prey density on the patch and a corresponding change in the probability of reinforcement in the chamber. The average number of presses per pellet was linearly related to the density of pellets on the patch. This result indicates that leverpresses were functionally equivalent to search effort.     

Autocontingencies: Rats count to three to predict safety from shock

Following training on a variable-interval food-reinforcement schedule, rats were exposed to three unsignaled shocks during each 30-min session. Although leverpressing was initially suppressed, responding was significantly accelerated following offset of the third shock, regardless of when in the session it occurred. Control sessions in which only two shocks were programmed, one early and one late, did not yield baseline acceleration. Evidence of “counting to three” was less obvious in subjects simultaneously exposed to a temporal autocontingency, that is, for which each shock also predicted a minimum 3-min safety period. The addition of a signal prior to each shock eliminated evidence of counting behavior altogether. We conclude that rats may be taught to count, but such behavior is highly unnatural and may be blocked or overshadowed by more salient sources of information.     

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.     

Influence of duration and number of inescapable shocks on intrashock activity and subsequent interference effects

A three-phase investigation of the effects of duration and number of inescapable shocks with rats was conducted. In the first phase (shock treatment), separate groups were exposed either to 64 or 128 5-sec shocks or to 32, 64, or 128 10-sec shocks. Measures of intrashock activity were found to be lower for the groups exposed to 64 or 128 10-sec shocks than for any other group. In the second phase (Test Day 1), half of each group was tested for interference with FR 1, locomotor escape-avoidance learning at either 24 or 168 h following cessation of shock treatment, using a control procedure that was designed to equate groups for exposure to test shock. The results indicated that, relative to nonshock-treated controls, at each interval only the groups previously given 64 or 128 10-sec shocks were impaired in terms of escape frequency. However, all groups given at least 64 shocks exhibited depressed intertrial responding at the 24-h, but not the 168-h, interval. In the final phase (Test Days 2–4), the control procedure for equalizing test-shock exposure was discontinued and a pattern of interference effects was observed in terms of escape-avoidance response latency that was identical to that reported for the escape frequency in Phase 2. In general, these data were viewed as indicating that duration, but not total amount of shock, was a critical determinant of behavior during inescapable shock and of the subsequent interference effect. Both effects of duration were regarded as the product of a common associative process involving the learning of immobility tendencies to shock that served to compete with later escape-avoidance responding.     

Defensive burying in rats and hamsters

Hooded rats and golden hamsters were shocked by one of two prods in a chamber with a sawdust-covered floor. Rats buried the prod through which they had been shocked, but hamsters displayed no burying behavior. Hamsters may not have buried the prod because they could not perform the required motor pattern. However, hamsters can carry and pile food pellets. Therefore, in a second experiment, rats and hamsters were shocked in a chamber with wooden blocks on the floor. Rats piled blocks around the prod through which they had been shocked, but hamsters did not. The third experiment established that, like rats, hamsters can associate a prod with shock in one trial, since they showed differential avoidance of a prod through which they had been shocked. Since hamsters are nonsocial and rats are social, these results are consistent with suggestions that burying sources of aversive stimulation evolved as an altruistic behavior.     

Influence of conspecific stress odors and shock controllability on conditioned defensive burying

In Experiment 1, rats received a session of 80 inescapable tail shocks or no shocks while restrained in a tube. During tests of conditioned defensive burying 24 h later, the bedding of the chamber contained odors from either stressed or nonstressed conspecific donor rats. Following a single prod shock, subjects that had had prior shocks or that were tested with the stress odors spent significantly less time burying the prod, made smaller piles of bedding, and displayed more freezing behavior. The combination of prior shock and stress odors during later testing enhanced these effects. In Experiment 2, a yoked group of rats that was given inescapable shocks, in contrast to a group that had wheel-turn escape training and one that was restrained but not shocked, later showed significantly less burying and more freezing when tested for defensive burying with stress odors present. In both experiments the duration of burying and the heights of piles were positively correlated, and both of these measures were negatively correlated with freezing. The demonstrated capacity of unconditioned stress odors to mediate different degrees of fear, depending upon the controllability of prior shock, is related to other studies of learned helplessness, and the predominance of freezing over burying is discussed in terms of various types of defensive strategies, stimulus-control processes, and the author’s stress-coping-fear-defense (SCFD) theory.     

Preference for information about shock duration in rats

Rats were permitted to control, by means of a changeover response, the amount of time spent in either a “differentiated” or an “undifferentiated” condition. Shock occurred in both conditions on the same variable-time schedule, half of the shocks being short (.75 sec) in duration, the other half, long (5 sec). In the differentiated (informative) condition, all short shocks were preceded by one signal and all long shocks were preceded by a discriminatively different signal. No information about shock duration was available in the undifferentiated condition, as the same signal preceded short and long shocks. A clear and consistent preference emerged for the differentiated condition, i.e., for information about shock duration. The relevance of this finding for theories which attempt to account for the preference-for-signaled-shock phenomenon was discussed.     

Effect of response-contingent vs. noncontingent shock on ducklings’ preference for novel imprinting stimuli

Individual ducklings received electrical shock in the presence of an imprinting stimulus whenever they pecked at food. Other ducklings received an identical series of shocks in the presence of an imprinting stimulus, but for them shock delivery was independent of their pecking behavior. In a subsequent session, the use of shock was discontinued and all birds were afforded the opportunity to approach either the imprinting stimulus (i.e., the stimulus previously present during shock) or a novel imprinting stimulus that was simultaneously presented. Ducklings that were shocked when they pecked at food either exhibited no preference or they preferred the original imprinting stimulus. In contrast, birds for whom shock was independent of their feeding behavior preferred the novel stimulus. These findings imply that the delivery of shock in the presence of an imprinting stimulus can endow the stimulus with conditioned aversive properties. They also imply that the stimulus will acquire little or no aversiveness if shock delivery is contingent upon a specific response such as pecking.     

A remote control training system for rat navigation in complicated environment

INTRODUCTION Electrical stimulation in the animal brain is an indispensable means to investigate brain mechanisms in many experimental setups. However, the cable connections between the stimulation instruments and the animal brain always limit the animal’s freedom of movement and thereby cause entwisting and breaking of the cables, especially in freely moving animal ex-periments. In order to solve this problem, telemetry systems have been designed to deliver the stimulationsignals as sub…     

The preference of rats for free or response-produced food

Three experiments were performed to investigate the preference of rats for either free or response-produced food. Rats were trained to free feed in an operant chamber and then to leverpress for food. Subsequently, they were given a choice between continuing to leverpress for food on CRF, FR 2, and FR 10 schedules or free feeding. In three experiments the independent variable was the method of free-food presentation: A fixed free-food dish was used in the first experiment, a movable dish in the second, and a large flat dish in the third experiment. The results of the three experiments were very similar, with most of the rats showing a preference for the free food. This preference increased further when more than one response was required to produce a food pellet. These results contradict any conclusion that rats have a generalized tendency to prefer response-produced to free food but provide no answer to the question of why rats do on occasion respond for food in the presence of free food.     

Conditioned fear assessed by freezing and by the suppression of three different baselines

Rats received either forward or backward pairings of an auditory CS and shock. They were then tested for conditioned suppression to the CS while barpressing for food, licking a sucrose solution, or being spontaneously active. Behavior was simultaneously observed using a time-sampling method. In each case, forward-conditioned animals exhibited more freezing than controls, and freezing was reliably correlated with suppression of the baseline. These results suggest that the different loss-of-baseline measures of aversive conditioning reflect the amount of defensive behavior evoked by the CS. They also suggest the utility of freezing as an index of conditioning. Freezing assayed by the time-sampling method was comparable to the more conventional indices of conditioning in sensitivity to the effects of conditioning.     

Time horizon and choice by pigeons in a prey-selection task

In two operant simulations of prey selection, the choice behavior of pigeons and the predictions of a model of the effect of session length on optimal prey selection were compared. All but one of the predictions were confirmed. In the first experiment, a higher proportion of “bad” items (comparatively long delays to food) was accepted in 10-min sessions in one operant chamber than in the first 10 min of 20-min sessions in a different chamber. The proportion of bad items accepted increased toward the end of the 20-min sessions. In the second experiment, the birds were exposed both to 20-min sessions in one distinctively marked operant chamber and to sessions of variable length, with a mean of 20 min, in another. A higher proportion of bad items was accepted in the variable-session chamber than in the fixed-session chamber at the beginning of the 20-min sessions. This difference decreased toward the end of the sessions but did not reverse as expected. In both experiments, an independent test analogous to the peak procedure was administered to establish whether the birds discriminated among the different session lengths.     

Inescapable shock interferes with the acquisition of an appetitive operant

This article reports the reinforcer generality of the interference effect resulting from exposure to inescapable shock. In Experiment 1, rats that received inescapable shock showed weak interference with the acquisition of an appetitive operant compared to animals exposed either to escapable or no shock. In Experiment 2, the response-reinforcer contingency was degraded by introducing a 1-sec delay of reinforcement on the appetitive task. Inescapable shock produced much stronger interference with the acquisition of the operant response than in Experiment 1. The results demonstrate reinforcer generality of the debilitating effects produced by inescapable shock.