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.     

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.     

A feature positive effect in conditioned suppression

A conditioned suppression experiment with rats studied the development of two discriminations involving two conditioned stimuli, A and X. In one discrimination (AX+/A?), compound presentations of A and X signaled shock and presentations of A alone signaled no-shock. In the other discrimination (A+/AX?), A alone signaled shock and AX signaled no-shock. AX+/A? discriminations were learned more rapidly than their A+/AX? counterparts. These results, which resemble the feature-positive effect of Jenkins and Sainsbury (1969, 1970), are discussed in terms of Rescorla and Wagner’s (1972) theory of conditioning and also in terms of stimulus intensity mechanisms.     

Continuity and intensity of shock in one-way avoidance learning in the rat

Six groups of rats (n = 16) differed with respect to the continuity of shock (continuous or discontinuous) and the shock intensity (.3, .8, or 1.6 mA) used during 65 one-way avoidance-conditioning trials. In general, a facilitative effect on one-way avoidance learning was obtained for continuous as opposed to discontinuous shock and for strong as opposed to weak shock. For both variables, the results are opposite to those obtained in discriminated shuttlebox-avoidance and barpress avoidance tasks. The data support an interpretation of the effect of continuity of shock which holds that discontinuous shock is, in effect, less intense than continuous shock. This interpretation allows the effects of the continuity-of-shock variable to be incorporated within the effective reinforcement theory of avoidance learning which has been proposed to account for shock-intensity effects in various avoidance tasks.     

Preexposure to situational cues and shock intensity in two-way avoidance learning

Four groups of rats (n = 16) received 65 two-way avoidance learning trials. The groups differed with respect to the amount of exposure (0 or 4 h) to the situational cues of the apparatus prior to avoidance learning and the intensity of shock (.3 or 1.6 mA) during learning. Superior avoidance performance with weak as compared to strong shock was obtained in the nonpreexposed groups. This inverse relationship between avoidance performance and shock intensity, typical of two-way avoidance learning, was eliminated in the preexposed groups. Presumably, a latent inhibition effect occurred in the strong-shock group, which resulted in a retardation of the conditioning of fear to the situational cues and a consequent improvement in performance. The results are consistent with the effective reinforcement theory, which emphasizes in aversive learning the detrimental effect of large amounts of fear remaining following a response.     

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.     

The effects of Pavlovian CSs on two food-reinforced baselines— with and without noncontingent shock

Two experiments explored the effects of Pavlovian (tone-shock) CS+, CS?, and truly random control (TRC) contingencies on two different food-reinforced instrumental baselines. One food-reinforced baseline contained noncontingent shock, while the other did not. In the first experiment, a TRC contingency was shown to produce suppression of food-reinforced responding, while a CS? contingency did not. When noncontingent shock was added to the baseline, however, the TRC stimuli failed to produce suppression, and the CS? contingency increased response rates over baseline level. In a second experiment, the effects of TRC and CS+ contingencies were compared on these same two baselines. While the CS+ produced suppression on both shock and no-shock baselines, the TRC contingency again produced suppression on only the no-shock baseline.     

Reward magnitude and shock variables (continuity and intensity) in shuttlebox-avoidance learning

In Experiment I, eight groups of rats (n = 20) were given shuttlebox-avoidance training. Two levels of shock (.3 and 1.6 mA) were combined factorially with two levels of reward (large and small) under both continuous and discontinuous (.75 sec on and 2.00 sec off) shock. Visual situational cues were absent after a shuttle response for the large-reward condition and present for the small-reward condition. Superior performance was obtained with weak rather than strong shock under both reward conditions and with large rather than small reward only under the weak-shock condition. Continuity of shock had no differential effect on performance. Experiment II allowed the conclusion that the reward effect was attributable to a reinforcement mechanism. The data were taken as support for the effective reinforcement theory, which emphasizes the importance in avoidance learning of fear conditioned to situational cues.     

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.     

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.     

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.     

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 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.     

Footshock-produced excitation and inhibition of activity in rats

Hooded rats received five 1-sec shock presentations of 0.3, 0.5, or 1.0 mA. Activity measures recorded during each 30-sec intershock interval and at various retention intervals (.10, 2.5, and 24 h) following footshock (FS) indicated that (a) 5 sec after FS activity is directly related to shock intensity, while 10–30 sec following FS activity is inversely related to shock intensity; (b) the rate of decline in activity following FS increases with successive shock presentations; (c) activity is greater 10 h after FS than at a 2.5- or 24-h retention interval; and finally (d) shock compartment confinement increased activity and resulted in a substantial alteration in the form of retention curve in the 0.3-mA group but had lesser effects upon the retention curves of the 0.5- and 1.0-mA groups. The data were interpreted as supporting the hypothesis that the “incubation effect” is a result of a decline of the activating effects of FS.     

Facilitation of licking by response-contingent electric shock

Licking behavior of rats maintained by lick-contingent water reinforcement, produced electric shocks under three conditions. Shock occurred 1 sec before, simultaneous with, or 1 sec after water reinforcement. Rates of licking rose above prepunished levels at .2-, .4-, and .6-mA intensities but were suppressed at .8 mA for all three groups. Facilitation and suppression effects were the same for the three pairing conditions. Discriminative and elicited functions of shock did not appear to account for this facilitative effect.     

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.     

Effects of stress controllability,immunization, and therapy on the subsequent defeat of colony intruders

Three experiments investigated the influence that various stress-controllability manipulations had on the defensive behaviors of rats when they were subsequently tested as intruders in previously established, aggressive colonies of conspecifics. In Experiment 1, naive subjects that had received a session of 80 shocks in a tube showed an enhanced series of defensive responses and received more bites than did a group of restrained nonshocked rats as colony intruders 24 h later. These two measures were also found to be positively correlated within each group. In Experiment 2, a group that was given 80 yoked inescapable shocks, in contrast to a group that had wheel-turn escape training and a restrained nonshocked control group, displayed more defeat and was bitten more frequently when tested as intruders on the following day. In Experiment 3, 60 trials of wheel-turn escape training were given 4 h prior to (i.e., immunization) or after (i.e., therapy) a session of 60 inescapable tube shocks. During resident-intruder testing 24 h later, both of these groups showed less defeat and received fewer bites than did an inescapably preshocked group but did not differ from a restrained nonshocked control group. These findings clearly indicate that stress controllability alters species-typical defensive responses, and their implications concerning other learned helplessness effects and interpretations are discussed.     

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.     

Functional differences between two shock-induced aggressive behaviors of mice

Five groups of male mice were given either isolation or grouped housing conditions or fight training as manipulations of social backgrounds prior to 5 days of shock-induced aggression under conditions resembling the rat paradigm for shock aggression. Three groups were given similar fight training but different levels of shock intensity during the shock trials. Shocks moderate in intensity as compared to very low intensity shocks produced more biting attacks and upright postures and maintained aggressiveness. Raising the shock intensity further produced fewer biting attacks, more upright postures, and a decline in aggressiveness. Biting attacks proved to be a more discriminating measure of differences created by different social backgrounds than the upright postures.     

Autoshaping in the rat: Interposing delays between responses and food

Rats were exposed to an autoshaping procedure in which each lever-contact or leverpress delayed trial offset and, hence, food delivery. Yoked subjects received identical trial-food pairings as did delay subjects. This procedure was studied at two delay values (2.5 and 10.0 sec) in experimentally naive rats and those which had previously received 25 sessions of autoshaping. The delay procedure retarded the acquisition of autoshaped responding in naive subjects and reduced responding for experienced subjects. Yoked subjects responded at higher levels than did delay subjects throughout training.