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.     

Inescapable shock and food-competition dominance in rats

Inescapable electric shock disrupts escape-avoidance learning in another apparatus. This study demonstrates a deficit in a nonlearning task in which no aversive stimulus occurs. In Experiment 1, inescapable shock lowered rats’ dominance in a food-competition situation relative to restrained controls. In Experiment 2, inescapable shock lowered rats dominance in the same food-competition situation relative to a group that received the equivalent amount of escapable shock, demonstrating that the inescapability of the shock caused at least part of the decrement observed in Experiment 1. Experiment 3 does not find that inescapable shock caused a significant difference in food consumed or running time when the rats were tested alone, showing it unlikely that the dominance effects were caused by decreased hunger or reduced running following inescapable shock.     

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.     

Control over noxious events and choice

Three experiments are reported assessing whether rats prefer controllable over uncontrollable aversive shock. In Experiment 1, subjects chose between escapable and inescapable shock while relative shock duration varied parametrically. In Experiment 2, subjects again chose between escapable and inescapable shock, but duration was held constant and equal. The final experiment gave subjects a choice between avoidable and unavoidable shock under several signaling conditions. Choice behavior proved sensitive to relative shock duration and to predictability of shock but not to controllability of shock.     

The nature of the activity deficit produced by inescapable shock

Two experiments investigated the nature and etiology of the reduced activity in the presence of shock produced by prior exposure to inescapable shock. Previous experiments have demonstrated this deficit in the presence of gridshock. However, gridshock hurts less if movement across the grids is reduced. It is thus unclear whether the inescapable-shock-produced deficit represents a facilitation of learning to reduce movement across the grids in order to alleviate pain or is an “unconditioned” reduction in movement in response to shock. The first experiment tested these possibilities by examining the effects of inescapable shock on subsequent movement during shock delivered via fixed tail electrodes to freely moving subjects. Inescapably shocked subjects still moved less in response to shock than did escapably shocked and restrained control subjects. Experiment 2 examined the possibility that this deficit occurs because unconditioned movement in response to shock during pretreatment diminishes after a few seconds, the reduction then being adventitiously reinforced by shortly ensuing shock termination. Activity during inescapable shock was closely monitored by ultrasonic motion detection. Although activity did decrease across trial blocks, the required within-trial patterns did not occur. Shock-elicited activity did not diminish after a few seconds of shock, but remained unchanged across the 5-sec shock presentations.     

Activity during prior shock determines subsequent shock-elicited fighting in the rat

Two experiments investigated the relationship between activity during shock and the magnitude of subsequent impairment of shock-elicited fighting in the rat. Different levels of intra-shock activity were engendered in two ways. In Experiment 1, differing temporal forms of inescapable shock were employed to produce markedly different levels of activity. In Experiment 2, a passive-escape procedure was used to explicitly reinforce nonmovement during shock relative to a yoked, inescapable shock control. Results indicated that relative to the performance of subjects not previously shocked, fighting impairment was produced only by those prior treatments that promoted reduced intrashock activity. Since one of the prior shock treatments involved inescapable shock but the other did not, these findings may be viewed as strong support for the notion that behavior during shock, rather than uncontrollability, is the critical determinant of the observed impairment effects. There was some suggestion in both studies that shock treatments that resulted in sustained or increased intrashock activity tended to produce augmentation of fighting. Both inhibitory and facilitative effects of prior shock exposure are discussed in terms of an interacting response theory of shock treatment effects.     

Interference with avoidance behavior as a function of qualitative properties of inescapable shocks

Temporal form (continuous vs. pulsating) and shock source (alternating current vs. direct current) were factorially combined to produce four shock treatments. The effects of inescapable presentations of these stimuli on subsequent avoidance response acquisition were measured in dogs (Experiment 1) and in rats (Experiment 2) and revealed an interaction of shock variables. Initially, all groups that received ac shock showed impaired performance for the pulsating and continuous shock conditions; groups that received dc continuous shock were also impaired, while those that received dc pulsating shock were not. While this pattern of interference persisted for dogs, it was transient in rats, with only the ac continuous-shock group continuing to be impaired. Mean avoidance performance were positively related to mean activity levels during inescapable shocks for the dc shock groups but not for the ac shock groups.     

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.     

Role of shock intensity in the learned helplessness paradigm

Although there have been many studies of the interference effect produced by exposure to inescapable shock, little is known about the role of shock intensity. This experiment factorially manipulated four levels of shock intensity during exposure to inescapable shock and three levels of intensity during the test for interference. Interference occurred at each training shock intensity when training and test shocks were similar. Interference was not obtained when training intensity was high but testing intensity low or medium or when training intensity was low or medium and test intensity was high. These findings pose problems for learned helplessness, learned inactivity, competing motor response, and catecholamine depletion hypotheses of the interference effect in the rat.     

Influence of postpartum shock controllability on subsequent maternal behavior in rats

Responses of mother rats were observed 24 h before and 24 and 72 h after exposure to one of three 8-day postpartum treatments: shock escape training, yoked inescapable shock, or restrained with no shock. In contrast to those in the other two groups, the dams given inescapable shock showed slower speed to approach the nest, shorter durations of being on the nest, and lower frequency and shorter total duration of oral contact with their pups. These dams also retrieved their pups less frequently, but this measure, as well as the frequency of leaving the nest, did not result in significant differences between groups. Since the traditional interpretations of the learned-helplessness effect were not entirely able to account for these findings, the observed uncontrollable-stress-produced changes in maternal behavior were examined from an ethological perspective.     

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.     

Conditioned fear and neophobia following inescapable shock

Lick-suppression tests were used in seven experiments to assess the transsituational transfer of fear in the learned helplessness paradigm. Two sources of fear combined to suppress test drinking in inescapably shocked rats. A situational odor was strongly associated with shock pretreatments and mediated the transfer of conditioned fear during testing. Fear of the pretreatment odor was greater following inescapable shock than after escapable shock or restraint. This conditioned suppression was retained for at least 72 h after pretreatment. Neophobia was enhanced as a second, nonassociative reaction to inescapable shock. Unconditioned fear was augmented by a novel odor in the test context, but otherwise was weak and dissipated within 72 h. However, neophobia was necessary for differential conditioned suppression in inescapably shocked rats. The pretreatment odor elicited fear only when tested in a novel context. Initial habituation to the test apparatus reduced conditioned fear. These data provide additional evidence for odor-mediated transfer of helplessness. Conditioned fear and neophobia are discussed in relation to recent anxiety interpretations of the phenomenon.     

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.     

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.     

Inescapable shock interferes with the acquisition of a low-activity response in an appetitive context

This study investigates the effects of exposure to inescapable shock on the acquisition of a low-activity appetitive response using a trial procedure. Inescapable shock was found to interfere with the acquisition of a nose-poke response to obtain food as compared with animals exposed to either escapable shock or no shock. In addition, general activity levels were measured separately during the trial and the intertrial interval during the appetitive test. Inescapably shocked animals were less active during the trial component than were either the escapably shocked or the nonshocked animals. However, no differential levels of activity were observed during the intertriai interval component of the appetitive test. The relevance of these findings for both the learned helplessness and the learned inactivity hypotheses is discussed.     

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.     

The effect of controllable and uncontrollable neonatal preshocks on adult escape/avoidance behavior in the guinea pig (Cavia porcellus)

Neonatal guinea pigs were given escapable, inescapable, or no shock and were later tested as adults on a signaled escape/avoidance task. During the neonatal period, the animals that could escape shock learned to do so quickly and steadily increased their overall level of activity, while those that could not, displayed a consistent decline in activity. Furthermore, during adult escape/avoidance sessions, guinea pigs, that could control neonatal shock were superior to those that lacked such control. These findings extend the generality of the interference effect to the guinea pig and highlight the influence of early control of aversive events on this phenomenon.     

A potential shock-reducing contingency in the backshock technique: Implications for learned helplessness

Electric shocks were delivered to rats through a subcutaneously implanted back electrode. Experiment 1 evaluated the relationship between number of paws grounded and total power dissipated in the rat. In Experiment 2, the threshold of shock-induced vocalization, a putative index of aversiveness, was found to be positively correlated with the number of paws grounded. These findings suggest that when the backshock technique is used, the aversiveness of shock potentially can be modified by the posture adopted by the experimental animal. Caution should be exercised, therefore, in attributing deficits in escape behavior following inescapable shock administered with back electrodes to learned helplessness.     

The effects of order of shock durations on helplessness in rats

Whereas rats exposed to a series of progressively decreasing shock durations show deficits in shuttle-escape performance 24 h later, the same number and intensity of shocks in the reverse (increasing) order of durations does not produce the “learned helplessness” effect (Balleine & Job, 1991). We conducted two experiments to establish the generality of this shock-duration order effect on other measures of distress and helplessness in rats. In Experiment 1, rats exposed to decreasing durations of inescapable shock showed reduced consumption of quinine-adulterated water (finickiness), whereas increasing durations produced no finickiness. By contrast, increasing shock durations produced greater conditioned fear to the shock context than did decreasing shock durations in Experiment 2. The differential effects of shock-duration order on finickiness and fear are explicated in terms of the specificity of fear conditioning during exposure to increasing versus decreasing series of shock duration orders.     

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).