Robert A. Rosellini

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.

Restricted daily exposure to environmental enrichment increases the diversity of exploration

The temporal parameters of environmental enrichment have been examined in two ways. The first is by limiting the total duration of environmental exposure. The effects of this procedure are well documented for both physiology and behavior. The second method is by restricting the daily exposure to environmental enrichment while keeping the days of exposure constant. The physiological effects of this procedure have been documented, but the behavioral effects which have been observed are equivocal. The purpose of the present study was to address this issue. Male rats were individually housed at 44 days of age. They were then randomly assigned to either an enriched environment or an impoverished environment. The enriched animals (EC) were exposed to the enriched environment for 2 hours daily for 30 days while the impoverished animals (IC) were handled daily but not exposed to the enriched environment. Animals were then tested on an object exploration test. The results indicate that 2 hours of daily environmental exposure is sufficient to produce object exploration effects similar to those produced by 24 hours of daily environmental exposure.

Cross-motivational effects of inescapable shock are associative in nature.

Exposure to inescapable shock interferes with the subsequent acquisition of an appetitive operant. This deficit may be due to either associative interference or activity reduction from the inescapable shock pretreatment. The relative importance of these two factors was examined by using an appetitive response choice discrimination procedure and concurrently measuring activity. Experiment 1 demonstrated separate associative and activity effects of inescapable shock, in that the animals exposed to inescapable shock made more incorrect responses than controls and were lower in activity. Experiment 2 demonstrated that these effects resulted from the uncontrollability of the shock, not from shock exposure per se. In Experiment 3, residual effects of inescapable shock were investigated by exposing animals to discrimination reversals. On these tests, inescapably shocked animals showed performance inferior to nonshocked controls, a result indicating that the effects of inescapable shock are not completely reversed by experience with contingent reward in the discrimination task. No evidence of an activity deficit was observed during this discrimination reversal. These results strongly suggest that associative factors play a more important role than activity reduction in mediating the effects of inescapable shock, at least when these are measured in an appetitive context.

Frustration and Learned Helplessness.

This article reports the transfer of learned helplessness from one aversive motivator, shock to another, frustration. In experiment 1, animals were trained to approach food in a runway and concomitantly exposed to either escapable, inescapable, or no shock in a different situation. Extinction was conducted in the runway, and subsequently the animals were tested for hurdle-jump escape from the frustrating goal box. Inescapably shocked rats failed to learn to hurdle-jump, whereas escapably or nonshocked animals learned the frustration escape response. Experiment 2 replicated the basic finidngs of Experiment 1 and showed transfer of learned helplessness from shock to frustration when no running response had been first acquired in the runway.

Uncontrollable shock proactively increases sensitivity to response-reinforcer independence in rats.

Learned helplessness theory predicts that animals exposed to inescapable shock acquire an expectancy of response-reinforcer independence, which proactively interferes with learning of response-reinforcer dependence. The theory also predicts that this expectancy can increase sensitivity to subsequent instances of response-reinforcer independence. These experiments test the latter prediction in a paradigm that minimizes the confounding effects of shock-induced activity deficits. Rats were trained to respond for food, then given either escapable, inescapable, or no shock. Subsequently, they received two sessions of response-contingent food followed by sessions of noncontingent food deliveries. During this phase, inescapably shocked animals decreased responding faster than did controls. Experiment 2 replicated this finding with a different schedule of food delivery and a procedure that more directly minimized the possibility that the outcome is due to either direct or indirect shock-induced activity changes. These results support the prediction that uncontrollable aversive events can increase an animals sensitivity to noncontingent response-reinforcer relationships.

The conditioning of dyspneic suffocation fear. Effects of carbon dioxide concentration on behavioral freezing and analgesia.

Previous studies in our laboratory have shown that a single exposure to 100% carbon dioxide (CO2) can serve as an effective unconditioned stimulus (US) in a Pavlovian aversive-context conditioning paradigm in rats. Although the US exposure parameters employed in the initial studies were sufficient for producing a context-specific enhancement of behavioral freezing and analgesia, it had yet to be determined whether variations of these CO2 conditioning procedures would produce other conditioning effects. Thus, the purpose of the following experiment was to investigate the intensity of the US on the conditioned response (CR). The findings confirm that variations in CO2 concentrations produce changes in the CR that are consistent with principles of Pavlovian conditioning. The findings lend additional support to the tenability of a dyspneic suffocation fear theory of panic disorder, a theory that postulates that at least one type of panic attack could be a consequence of Pavlovian conditioning.

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.

Modulation of hypoalgesia by morphine and number of shock trials: Covariation of a measure of context fear and hypoalgesia

In a recent series of studies, we observed that exposure to prolonged foot shock increased hypoalgesia induced by morphine. This increase was observed only when testing was conducted in the presence of shock-associated cues, suggesting that it resulted from context-conditioned fear. However, we do not know whether the extended stressor parameters employed in that study are necessary for an observance of the effect. Therefore, in the present study, we assessed the effect of the number of shock trials (either 0, 20, 100, or 200) on the hypoalgesia observed following morphine administration. In addition, we measured activity as an independent index of context-conditioned fear, because in prior studies there had been no independent behavioral assessment of the conditioning of fear to the context. Although others have shown a covariation of conditioned fear and context-induced hypoalgesia using shock parameters and test paradigms different from our own, we sought to assess whether the same covariation would hold for conditioned fear and the hypoalgesia observed following the administration of morphine. The results showed increased hypoalgesia in all groups exposed to foot shock, demonstrating that prolonged exposure to foot shock is not necessary for an observance of this effect. In addition, the results revealed a linear relationship between number of trials of shock and hypoalgesia, but a U-shaped relationship between trials and activity. The pattern of results is considered in light of Fanselows Perceptual-Defensive-Recuperative model.

Sudden death in the laboratory rat.

Vulnerability to sudden death was produced in laboratory rats by manipulating their developmental history. Rats who were reared in isolation died suddenly when placed in a stressful swimming situation. Handling of these singly-housed rats from 25 to 100 days of age potentiated the phenomenon. However, animals who were group housed did not die even when they had been previously handled.

Pavlovian aversive context conditioning using carbon dioxide as the unconditional stimulus.

Four experiments were conducted to examine the utility of carbon dioxide (CO2) as an aversive unconditioned stimulus (US) in a Pavlovian context conditioning paradigm. Experiment 1 demonstrated that rats exposed to CO2 in a distinctive context showed elevated levels of freezing relative to controls. Experiment 2 replicated this basic effect with a modified conditioning procedure and additionally demonstrated conditioned analgesia. Experiment 3 demonstrated a positive monotonic relationship between US duration and resistance to extinction of freezing behavior as well as conditioned analgesia. Experiment 4 demonstrated extinction and an extinction-related phenomenon, renewal. These studies clearly demonstrate the utility of CO2 as a Pavlovian US.