Preference for signaled shock phenomenon: Direct and indirect evidence for modifiability factors in the shuttlebox

The experiments reported here investigated whether unauthorized shock modification is a factor which mediates the preference for signaled shock phenomenon (PSS) in the shuttlebox. This factor appearednot to have a critical role in PSS on the basis of earlier experiments which reported that PSS occurred in the shuttle-box even when shock was made unmodifiable by scrambling. However, the scrambling used in these experiments was not complete. In Experiments I and II reported here, no PSS occurred with completely scrambled grids; the phenomenon emerged only with unscrambled shock. Experiment III reports evidence indicating a connection between directly assessed modification and PSS with unscrambled shock, with PSS emerging only when modification had begun to occur. The pattern of results from these shuttlebox experiments parallels that found in an earlier study of PSS using a symmetrical-choice, barpress preparation.     

The US-preexposure effect in honeybees

Signaled avoidance was studied in individual honeybees that visited the laboratory regularly to take sucrose solution from a target set on the sill of an open window. During feeding, substrate vibration or airstream was used to signal a brief shock that could be avoided by breaking off contact with the food for a few seconds. Aversive conditioning of the context was measured in terms of return time (the time between successive visits). In Experiment 1, experience with unsignaled shock was found to lengthen return time—which experience with signaled shock did not—and to impair performance in subsequent avoidance training with signaled shock (the US-preexposure effect). In Experiment 2, experience with unsignaled shock given after signaled avoidance training lengthened return time but had no effect on response to the signal in a subsequent extinction test. These results closely resemble the results obtained in analogous experiments with vertebrates.     

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.     

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.     

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 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.     

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.     

Maternal holding preferences: a consequence of newborn head-turning response.

Salk observed that in an overwhelming majority of cases (80%) mothers prefer to hold their infants across the left shoulder. This preference occurs over a diversity of cultures and is not related to the hand preference of the mother. Salk suggested a fetal imprinting to the sound of the heartbeat as the mechanism responsible for this phenomenon. While such speculation is intriguing, it is not scientifically testable. 2 experiments were performed to examine a more testable hypothesis. Results of these studies suggest that the initial head-turning preference of an infant is related to the holding preference of its mother.     

Measurement of current flow through the rat under signaled and unsignaled grid-shock conditions

The present experiment was run to test the hypothesis that, when shock was signaled, rats would develop effective coping responses so as to reduce the current flow through them. A 1-sec shock was delivered through a grid floor by a fixed impedance ac shock source. The current-flow measure was taken over the last 30 of 90 trials given over 3 days and indexed by “gross skin conductance” or GSC (shock). The rat under the signaled shock condition (n=15) showed higher GSC (shock) than did the rats under the unsignaled shock condition (n=14). Thus, the result contradicted the hypothesis. There was no indication that the rats developed any preparatory response during the 5-sec signal, in terms of either GSC (signal) or posture. The results were discussed with reference to the preparatory-response hypothesis and various other possibilities.     

Counterconditioning of shock by a water reinforcer in rabbits

Three experiments studied the counterconditioning of certain properties of eyeshock in rabbits by establishing the shock as an appetitive CS for a jaw-movement response reinforced by intraoral water injections in a Pavlovian conditioning procedure. Although Experiment 1 demonstrated that such appetitive conditioning did not attenuate the unconditioned eyeblink elicited by the shock, it reduced the capacity of the shock to suppress leverpress responses reinforced by direct water injections in a signaled punishment procedure in Experiment 2. By contrast, when instrumentally reinforced licking was punished by eyeshock in Experiment 3, no such reduction in the suppressive capacities of the shock was found. The results were considered in terms of whether counterconditioning alters the response-eliciting or motivational and reinforcing properties of the shock.     

Recovery from conditioned suppression to a partially overlapping compound stimulus

In a conditioned suppression paradigm, a partially overlapping compound stimulus signaled occurrences of electric shock. That compound CS consisted of 3 min of continuous illumination of the houselight with three discrete 5-see presentations of a tone superimposed. A .5-sec electric shock was coterminous with each tone presentation. Rats that received this treatment in early conditioning sessions showed considerable recovery from conditioned suppression to the houselight as the experiment progressed. However, the effect was not reversible, and it could not be demonstrated in rats that experienced extensive prior conditioning to the houselight alone. These results are discussed in relation to a hypothesis concerning the modulation of behavioral control exerted by elements of compound stimuli and as they relate to a recent theoretical model for Pavlovian fear conditioning.     

Changeover from unsignaled to signaled avoidance with and without a feedback stimulus in rats

Two experiments are reported in which, by pressing a lever, unsignaled avoidable shocks could be changed to signaled avoidable shocks for periods of 1 min, after which unsignaled avoidance was automatically reinstated. A correlated stimulus identified the signaled schedule. Under these conditions, Ss changed over at a rate sufficient to remain in the signaled schedule more than 90% of the time. Both experiments showed that changeover performance was under the control of the correlated stimulus. Presenting the signaled schedule without the correlated stimulus did not maintain changing over. The second experiment showed that a 1-sec presentation of the correlated stimulus was sufficient to maintain changeover performance. The results were attributed to the conditioned reinforcing properties of the correlated stimulus.     

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).     

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.     

Effects of Headings on Text Summarization

A summarization task was used to study whether headings influence readers' representations of the topic structure of a text. College students (Experiments 1-3) and sixth- and eighth-graders (Experiment 3) summarized a multiple topic text that (a) included headings introducing every new subtopic, (b) included headings introducing half of the new subtopics, or (c) included no headings. In all experiments, topics were more likely to be included in a summary if they were signaled than if they were not signaled. This effect was magnified when the text was only half signaled: Signaled topics were more likely to appear in a summary if only half the text topics were signaled than if all of the topics were signaled; however, unsignaled topics were less likely to appear in a summary if half of the text topics were signaled than if none of the text topics were signaled. The findings demonstrate that readers rely heavily on headings in a task that emphasizes attention to a text's topic structure. It is suggested that previously observed signaling effects on text recall are mediated by effects on how readers represent a text's topic structure. Copyright 2001 Academic Press.     

Acquisition and extinction of facilitation in the C57BL/6J mouse

Acquisition, extinction, and transfer of facilitation were explored in a series of experiments with C57BL/6J mice. With a procedure in which an auditory target was followed by food only in the presence of a visual facilitator, Experiments 1—4 showed that the facilitator promoted magazine entries to the auditory target. This enhancement effect was eliminated by training the facilitator as a conditioned inhibitor (Experiments 1 and 3B). Enhancement was also reduced by nonreinforced presentations of the facilitator in a discrimination procedure (Experiment 1) and by simple nonreinforcement of the facilitator (Experiments 2, 3A, and 4). In contrast to the results obtained with a facilitator, simple nonreinforcement of an inhibitor, a visual cue that had signaled when an auditory target would not be reinforced, did not reduce its ability to modulate responding to that target (Experiment 4). However, both the facilitator and the inhibitor were found to transfer their modulatory effects to other targets (Experiment 4). Finally, mice demonstrated no evidence of differential responding on a biconditional discrimination procedure in which one auditory target (A1) was reinforced in the presence of one visual stimulus (L1) but not in the presence of another (L2), and a different auditory target (A2) was reinforced in L2 but not in L1 (Experiment 5). The implications of these results for analysis of the function of a facilitator are discussed.     

Conditioned flavor preferences as a function of deprivation level: Preferences or aversions?

Rats tend to prefer flavors previously consumed under low deprivation to flavors previously consumed under high deprivation (Capaldi & Myers, 1982). We attempted to distinguish among possible associative explanations by determining whether this conditioning phenomenon was based upon conditioned preferences, conditioned aversions, or both. We compared preference for flavors presented exclusively under either high or low deprivation with preference for a neutral flavor. In Experiments 1A and 1B the neutral flavor was one that had been randomly paired with both high and low deprivation, whereas in Experiments 2 and 3 the neutral flavors had not been associated with either high or low deprivation. Our results strongly suggest that this conditioning phenomenon is based upon an actual increase in preference for the flavor consumed under low deprivation rather than on any form of aversion conditioning.     

Shock intensity and warning signal effects on several measures of operant avoidance acquisition

Acquisition of both signaled and unsignaled operant avoidance learning was studied in 64 rats as a function of shock intensity, with three different warning signals used in the signaled procedure. In both signaled and unsignaled avoidance, overall response rate was a progressively increasing function of shock intensity. This was due to both an absolute and relative increase in the frequency of responses at shorter interresponse times with increasing shock intensity. Presence of an effective warning signal in the interval immediately preceding shock increased the probability of an avoidance response in this interval, decreased overall response rate, and reduced shock frequency. A buzzer signal proved most effective, followed by tone and light. However, once a warning signal occurred, the probability of an avoidance response to the signal was virtually independent of shock intensity. Also, an index of avoidance efficiency proved to be inversely related to shock intensity.     

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.     

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.