Preference and response suppression under different correlations between shock and a positive reinforcer in rats

A lever-press response by rats was reinforced by food in two successively presented types of trial signalled by different discriminative stimuli. When responding was punished by a shock in one type of trial, a groups for which the shock always preceded the reinforcer by .5 sec (positive correlation) showed less suppression in those trials than a group which received the shock and food separated in time (negative correlation). When, in a second experiment, rats were given a choice between food alone in one end of a shuttle box and either positively or negatively correlated shock and food in the other on a concurrent schedule, the group receiving the negatively correlated shock showed a greater preference for the food alone end. On the basis of a third experiment in which a tone was substituted for the shock in the choice situation, it was argued that the effect of correlation was not simply due to the stimulus properties of the shock. A final experiment demonstrated that when shock punishment is administered during extinction of the lever-press response, the rate of extinction is slower if the shock has been previously paired with the food reinforcer. Pairing a shock with food seems to attenuate the intrinsic aversiveness of the shock through Pavlovian counterconditioning.     

Changes in feeding and foraging patterns as an antipredator defensive strategy: a laboratory simulation using aversive stimulation in a closed economy.

The effects of the risk of electric shock on the meal patterns of rats living in an operant chamber were investigated. Rats could obtain food by working on a response lever that provided reinforcement according to chained fixed-ratio continuous reinforcement schedules that allowed the animals control over meal size. Using a two-compartment operant chamber with a safe nesting area and manipulanda area with a grid floor, shock could be correlated with responding on the schedule. Shocks (less than or equal to 1.25 per hour) were scheduled to occur randomly throughout the day, independent of the rat's behavior. Shock caused a reorganization of meal patterns such that the animals took less frequent but larger meals. This pattern reduced the time the animals spent at risk without compromising caloric balance. Similar changes in feeding pattern were obtained in both hooded and albino rats. Exposure to shock in a separate chamber did not produce these behavioral modifications. The magnitude of shock-induced alterations of meal patterns was greater with chained fixed-ratio 90 continuous reinforcement than with chained fixed-ratio 10 continuous reinforcement. Additionally, the rats seemed to be able to reduce food intake but increase caloric efficiency, such that the reduced food intake did not have deleterious effects on maintenance of body weight. These behavioral modifications reduced the number of shocks received from that which would have been expected if meal pattern changes had not occurred. We suggest that this technique may provide a useful laboratory simulation of the impact that the risk of predation has on foraging behavior.     

Stimulus control of defensive burying in the rat

Rats shocked once through a wire-wrapped stationary prod mounted on the wall of a test chamber incorporated the bedding material covering the chamber floor into a defensive reaction. When tested 1 min later, they approached the prod and buried it. Evidence was provided by three separate studies of this burying response that rats had learned about the association of both the position and brightness of the prod with shock after this single conditioning trial. In Experiment 1, the amount of burying decreased if either the position or brightness of the prod had been changed prior to the test. In Experiments 2 and 3, rats were shocked through one of two prods (white or black) mounted on opposite walls of the test chamber. When the positions of the prods were unchanged for the test, almost every subject buried the prod through which it had been shocked, even when the conditioning-test interval was 24 hr; whereas, each rat directed substantial amounts of bedding material at both prods when the positions of the two prods were reversed. Thus, discriminated “avoidance” learning can be rapid, reliable, and enduring when shock is administered “by” a clearly defined stimulus object, i.e., when cue and consequence are spatially contiguous.     

Associative vs topographical accounts of the immediate shock-freezing deficit in rats: Implications for the response selection rules governing species-specific defensive reactions

Rats were placed in a novel chamber and were given a single shock, either immediately upon placement in the chamber or after a 2-m delay. During a postshock observation period, delayed shock animals froze while immediate shock rats did not (Experiments 1–5). Additionally, delayed shock animals showed an elevation in defecation, relative to unshocked controls, while immediate shock animals did not (Experiment 1). Freezing and elevated defecation were first found with a 9-s delay between placement and shock, and both responses increased linearly with increasing delays up to at least 81 s (Experiment 2). Immediate shock rats did not show escape-related behaviors when potential escape routes were available (Experiments 3–4). A 2-m preexposure to the chamber, that was separated in time from shock, did not alleviate the immediate shock deficit (Experiments 4–5). The immediate shock deficit was also apparent in a test given 24 h after shock receipt (Experiment 6). If immediate shock rats were later given delayed shock they began to freeze but showed no carryover effects of their prior shock experience (Experiment 6). It was concluded that the immediate shock deficit reflects a deficit in association formation between contextual stimuli and shock rather than a difference in defensive behavioral topography. It also appears that freezing is the dominant conditional response to shock-associated stimuli even though freezing is not unconditionally elicited by the shock itself.     

A fully automated two-way locomotor training apparatus

A fully automated two-way locomotor training apparatus is described. The device employs two bilevel chambers that are separated by a length of straight alley. The rat is dropped from the distinctively cued upper level of one chamber onto the grid floor of its lower level. To escape shock, the rat must leave this lower level, traverse the alley segment, and jump into the upper level of the opposite compartment. Although the animal is required to traverse the same alley in two directions, its terminal response always carries it into a distinctive goal region where shock never occurs. A brief experiment is described that was designed to assess the effectiveness of this device in the acquisition and extinction of a locomotor escape response with rats.     

The relative aversiveness of signalled vs unsignalled escapable and inescapable shock

In the first study, subjects escaped shock by pressing on a lever under an unsignalled condition. By pressing a different lever they changed the condition to signalled escape for three minute periods. The second study used the changeover procedure to study inescapable-unavoidable shock. Seven rats were used in each study. All subjects in both studies changed over from unsignalled to signalled conditions. Once contact with the signal condition was made, subjects responded to remain in that condition. The three different extinction conditions showed that the correlated stimulus without the signal had greater control over responding than the signal without the correlated stimulus. An analysis based upon shock and shock-free periods was presented.     

Defensive burying, nest relocation, and pup transport in lactating female rats

A wire-wrapped wooden dowel was inserted through the wall of the nest compartment of a two-compartment box. There were four conditions. Some lactating female rats were shocked by the dowel when they first touched it, and some were not, after which the dowel was either immediately withdrawn from the chamber or left in place for the duration of the ensuing 30-min test period. Three defensive behaviours were observed during the tests: the mothers buried the shock source with bedding from the floor of the chamber, they transported their pups to the adjoining “safe” chamber, and they built a new nest in the safe chamber from material salvaged from their original nest. The shocked mothers that were confronted with the shock source throughout the test period displayed significantly more of each of these three defensive behaviours than did the mothers in the other three conditions. The methods used in this study provide a simple, reliable, safe paradigm for studying maternal defensive behaviour; the results indicate that the defensive capacities of the rat extend far beyond stereotypical flight, freeze, and fight responses, which have been the focus of most research on rodent defense.     

Discriminative properties of shock-correlated reinforcement in an operant context

A two-choice discrete operant procedure was devised for the study of shock-correlated reinforcement effects in rats. In the presence of one auditory stimulus, responding on one response lever was reinforced with food; with another auditory stimulus, responding on a second lever was reinforced. It was found that discrimination performance of one group, relative to appropriate control groups, was facilitated when electric shock was correlated with reinforcement on one lever and not on the other. Further, relative discrimination levels were found to be higher on the lever correlated with the shock than on the alternate lever. The significance of the results for operant within-S studies and for a mediational theory of shock-correlated reinforcement was discussed.     

Producing either positive or negative tendencies to a stimulus associated with shock

A technique is described in which rats are pretrained with reinforcement to bar press. Each bar press was associated with a tone. This tone was later paired with one of two aspects of an electric shock, either at the onset or at the end of the shock, in a situation in which the shock is inescapable. These animals were retested in the operant situation under conditions of extinction, but with tone present as a conditioned reinforcer. The finding was that animals for which the tone was associated with shock onset extinguished quickly, whereas animals for which the tone was paired with shock termination extinguished more slowly.     

Effects on responding of mixed and multiple schedules of signalled and unsignalled response-dependent electric-shock delivery

Responding in two rats was maintained under mixed and multiple variable-interval 35-sec variable-interval 35-sec food delivery schedules. Similar rates and patterns of responding occurred in each component of the two schedules. Mixed and multiple variable-interval 65-sec variable-interval 65-sec schedules of response-dependent shock delivery were super-imposed on the mixed and multiple baseline food schedules, respectively. In one component, a 5-sec stimulus was presented on the average of once every 65 sec. Offset of the stimulus arranged that the next response would produce shock. In the other component, no stimulus was presented during the 5-sec period. The mixed schedule of signalled and unsignalled dependent shock delivery yielded similar degrees of response suppression in each component, but the multiple schedule of shock delivery revealed differential degrees of response suppression. Considerably more suppression occurred in the component not associated with the preshock stimulus, thus implicating the discriminative functions of the correlated stimulus.     

Discriminated punishment: avoidable and unavoidable shock

Warning stimuli for two punishment conditions were alternated with periods of appetitive responding by rats. In either warning stimulus, the first response produced a brief shock, terminated the stimulus, and started an interval during which the baseline appetitive schedule was in effect. Not responding resulted in stimuli of random duration, which terminated with a shock under one condition and without a shock under the other. Each subject was exposed to several shock intensities, with trials for the two conditions programmed during alternate portions of the session. In general, response frequency in the warning signal for either condition decreased with increasing intensity; however, at a given intensity, responding was more frequent in the stimulus invariably terminating with shock than in the stimulus terminating without shock when no response was made. The frequency difference was greatest at intensities intermediate between those producing minimal and maximal suppression.     

A rat chamber and electrode procedure for avoidance conditioning

A method was developed for studying the reactions of rats to aversive shock. The distinctive features were the design of the chamber and a method of restraint that allowed the use of surface electrodes to deliver the shock. Advantages of this method were: (1) accurate specification of the shock actually received by the rat; (2) elimination of all unauthorized escape or avoidance reactions; (3) elimination of the shock scramblers and floor grids required with foot-shock; and, (4) rapid acquisition of performance under various avoidance procedures and various frequencies of shock delivery.     

Choosing between predictable shock schedules: Long- versus short-duration signals

Two experiments assessed the relative aversiveness of different duration preshock signals (5 and 20 seconds) and different duration stimuli identifying shock-free periods. In the first experiment, the responding of 15 of 18 rats was maintained when it produced changes from a predictable-shock condition with a 5-second preshock signal to an identical schedule with a 20-second preshock signal; responding was not maintained when it produced the opposite changes. These results occurred with intershock intervals of both 120 seconds and 240 seconds. The second experiment assessed whether changing to the 20-second schedule was maintained by properties of the preshock signals identifying the shock periods or by properties of the stimuli identifying the shock-free periods. Four subjects were given training with the two signaled schedules in an operant chamber and then later given off-baseline preference tests in a shuttlebox. When given a choice between preshock signals, subjects chose the 5-second signals over the 20-second signals. However, when given a choice between stimuli identifying shock-free periods, subjects chose the stimulus identifying the shorter shock-free periods (i.e., the one previously correlated with the 20-second signals). These findings are discussed within the Rescorla and Wagner model of stimulus compounds and within the context of safety as a contrast phenomenon.     

The Elusiveness of Gustatory Potentiation of an Environmental Aversion

Two experiments investigated conditions that influence whether or not the presence of a flavor cue potentiates acquisition of an environmental aversion. Experiments 1 and 2 exposed rats to either a salient sucrose flavor or nonsalient water with black box-illness pairings. Half of the rats in the sucrose and water treatment groups became ill after exposure to the black chamber in their home cage. The other half of the rats in the two treatment conditions became ill in the black box. The results of each study indicated that a strong environmental aversion was established when illness was associated with exposure to the black box. The aversiveness of the black chamber was indicated by both less time spent in the black side and reduced intake in the black chamber. The presence of sucrose during conditioning did produce a stronger black box aversion than did water. Reliable potentiation of the environmental aversion, however, was seen only when the illness experience occurred in the black chamber but not when illness occurred in the home cage. A weaker aversion to the black chamber was also found in the water groups when illness occurred shortly after experience with the black box than when illness immediately followed exposure to the black chamber.

     

Conditioned and unconditioned defensive burying in the rat

When rats first encountered a mouse-trap or a flashbulb in a chamber to which they had been habituated, they buried it with bedding material from the floor of the chamber, whereas rats previously habituated to the trap or the flashbulb did not. Conversely, rats did not bury a stationary, wire-wrapped wooden prod or a length of polyethylene tubing, even on first encounter. However, almost every rat struck by the trap, shocked by the prod, exposed to an airblast from the tube, or to a flash of the bulb, buried the respective source of aversive stimulation with the bedding material, even when a comparable control object was present during conditioning and testing. Thus, the phenomenon of defensive burying is not restricted to situations in which neutral objects serve as the source of painful electric shock. Rats seem to enter the experimental environment with an already established tendency to bury some objects (unconditioned defensive burying) but not others, and they readily learn to selectively bury an object that has been the source of any one of a variety of aversive stimuli (conditioned defensive burying).     

Performance of rats under concurrent variable-interval schedules of negative reinforcement

The behavior of rats under concurrent variable-interval schedules of negative reinforcement was examined. A single one-minute variable-interval programmer determined the availability of 30-second timeouts from electric shock. These were assigned to one or the other of the two component schedules with a probability of 0 to 1.0. The response requirement for the component schedules was standing to the right or left of the center of the experimental chamber. With a six-second changeover delay, the relative time spent under one component schedule varied directly and linearly with the relative number of timeouts earned under that component schedule. The absolute number of changeovers was highest when a similar number of timeouts was earned under each component schedule, and lowest when all or nearly all timeouts were earned under one component schedule. In general, these relations are similar to those reported with concurrent variable-interval schedules of positive reinforcement.     

Vicious-circle behavior in the Y maze

Laboratory rats were given escape training in both arms of a Y maze followed by extinction in one of four conditions: regular (no shock), punished (both arms containing a shock mid-segment), choice, or forced (one arm containing a shock mid-segment, one arm shock free). Subjects in the punished and forced conditions required more extinction trials than those in the regular extinction condition. The choice condition was intermediate and not significantly different from the others. Choice subjects avoided the shock by choosing the safe alley early and consistently during extinction, but some continued to run for many trials without further punishment. It was suggested that the number of punished trials early in extinction is related to persistence of running, and the results were discussed in relation to the Mowrer-Brown and the discrimination hypotheses.     

Effects of cycle length on performance on a temporally defined avoidance schedule

Three rats were trained on a temporally defined avoidance schedule logically similar to a fixed-interval, limited-hold positive reinforcement schedule. This avoidance schedule was composed of time periods during which responses had no scheduled consequences alternating with time periods during which a response precluded shock. As with fixed-interval length and response rate on positive reinforcement schedules, an inverse relationship was obtained between the length of the no-consequence interval and response rate during the no-consequence interval. An inverse relationship was also obtained between the length of the no-consequence interval and the per cent of shocks avoided. A rate increase within the no-consequence interval, similar to that typically produced by fixed-interval positive reinforcement procedures, was displayed by one of the rats where the no-consequence interval was at intermediate values and frequency of shock was relatively high. The introduction of a discriminative stimulus correlated with the avoidance interval produced typical discriminated avoidance behavior as well as alterations in temporal patterning of responses during the no-consequence interval in the two rats exposed to this procedure. These alterations in temporal patterning disappeared when the discriminative stimulus was removed. The results were consonant with those reported in the literature involving food reinforcement and fixed-interval, limited-hold schedules.     

Reevaluation of things past

The ability of rats to reevaluate previously presented information in light of subsequently provided information was evaluated using a CER (conditioned emotional response) procedure. In Experiment 1, rats suppressed responding to a compound light + tone stimulus that was repeatedly paired with shock. Groups of rats were then presented with only one element of the compound (the tone), either presented alone or paired with shock, for 15 days. During this 15-day period, two control groups received trials with the shock alone or neither stimulus nor shock. All of the rats were then tested with the other element of the compound (the light). Rats that had received the tone paired with shock during the intervening training continued to suppress to the light, whereas rats that had received the tone alone showed rapid extinction of the CER to light. The control groups showed that this effect was not due to the number of shock presentations received. A subsequent experiment also demonstrated that these results were not due to nonspecific stimulus effects. Apparently, a subsequent change in the associative strength of one element results in a similar change to the other element of a previously established compound stimulus.     

Changes in cis(Z)-flupentixol-induced dopamine blockade produce contrast effects in rats

Two groups of rats were trained under a 45-sec variable-interval schedule. The magnitude of reinforcement used in each test session alternated daily between one and four 45-mg food pellets, the magnitude of reinforcement available at any time being signalled by lights in the operant chamber. Testing took place over two consecutive 11-day phases. During the first phase one group of rats was injected with 0.06 mg/kg of cis(Z)-flupentixol prior to testing; the other group received vehicle injections. In the second phase drug conditions were reversed. The change in drug conditions between phases produced both positive and negative successive contrast effects, consistent with the hypothesis that dopamine blockade attenuated the hedonic impact of reinforcement. Embedded within each session were two short signalled probe periods during which the reinforcement magnitude was switched to that used on the alternate days. No contrast effects were found during these brief daily probe periods.