Rule-based learning impairment in rats with lesions to the dorsal striatum.

The present study examined the effects of lesions to the dorsal striatum (DS) in Sprague-Dawley rats, when tested on the acquisition and successive shifts in the position of a goal arm in an eight-arm radial maze. In the procedure we used, rats had to retrieve the location of one baited arm among the eight arms of the maze after it had just been presented as a sample during a forced trial. After attainment of a fixed learning criterion, rats were submitted to five successive shifts in the goal location. Results showed that DS rats were able to learn the position of the goal arm during the acquisition phase as efficiently as sham-operated rats. In contrast, when the position of the goal arm was shifted, although DS rats were able to learn its new position, they made significantly more errors and required more sessions to reach criterion than sham-operated rats. These results suggested that both groups did not solve the task using the same behavioral strategy. The analysis of responses made suggested that sham-operated rats solved the task using the pairing rule between the forced and the free run (matching-to-sample rule), while DS rats solved the task using only visuo-spatial processing. These data therefore suggest that the dorsal striatum plays an important role in rule-learning ability.     

Olfactory discrimination, reversal learning, and stimulus control in rats.

Rats were trained to discriminate lights, tones, or odors and then given a series of discrimination reversals. Only rats trained with odors showed positive transfer on the first reversal and acquisition of a reversal set. Other experiments demonstrated that rats preferentially attend to odors when presented in compound with lights or tones; that odors exert more discriminative control than tones in tests using compound stimuli of competing sign; and that after pretraining on the positive stimulus, acquisition of an odor but not a light discrimination occurs with virtually no errors. These results demonstrate the importance of stimulus modality in the establishment of stimulus control and the need for more careful analysis of stimulus factors in cross-species comparisons of learning ability.     

Effects of overtraining on reversal learning by rats in concurrent and single discriminations

Two experiments examined the whole-partial effect of overtraining in concurrent discriminations and assessed the effect against single discrimination training in rats. In Experiment 1 overtraining facilitated reversal in Group W, in which rats were given concurrent training on two simultaneous discrimination tasks (A and B) in original learning before both tasks were reversed. By contrast, overtraining delayed reversal in Group P, in which animals were given the same training as in Group W in original learning, but only Task A was reversed. After overtraining, Group W reversed more rapidly than Group P. Without overtraining, Group P reversed more rapidly than both Group W and Group C, in which animals were trained only on Task A before this task was reversed. Experiment 2 investigated the effects of overtraining on the reversal of a successive discrimination in Groups W, P, and C. In addition, animals in a further group, Group S, received the same concurrent training as Group W before Task B was omitted and Task A reversed. Overtraining facilitated reversal in Groups W, C, and S but delayed it in Group P. After overtraining, Group W reversed more rapidly than Groups P, S, and C. Both Groups S and C also reversed more rapidly than did Group P. Without overtraining, Group P reversed more rapidly than Groups W, S, and C. There were no significant differences in reversal performance among Groups W, S, and C. These findings indicate that the effects of overtraining on reversal learning in concurrent discriminations differ from those observed in single discriminations.     

Effects of acute restraint stress on set-shifting and reversal learning in male rats

Exposure to acute stress alters cognition; however, few studies have examined the effects of acute stress on executive functions such as behavioral flexibility. The goal of the present experiments was to determine the effects of acute periods of stress on two distinct forms of behavioral flexibility: set-shifting and reversal learning. Male Sprague-Dawley rats were trained and tested in an operant-chamber-based task. Some of the rats were exposed to acute restraint stress (30 min) immediately before either the set-shifting test day or the reversal learning test day. Acute stress had no effect on set-shifting, but it significantly facilitated reversal learning, as assessed by both trials to criterion and total errors. In a second experiment, the roles of glucocorticoid (GR) and mineralocorticoid receptors (MR) in the acute-stress-induced facilitation of reversal learning were examined. Systemic administration of the GR-selective antagonist RU38486 (10 mg/kg) or the MR-selective antagonist spironolactone (50 mg/kg) 30 min prior to acute stress failed to block the facilitation on reversal learning. The present results demonstrate a dissociable effect of acute stress on set-shifting and reversal learning and suggest that the facilitation of reversal learning by acute stress may be mediated by factors other than corticosterone.     

Withdrawal from cocaine self-administration produces long-lasting deficits in orbitofrontal-dependent reversal learning in rats

Drug addicts make poor decisions. These decision-making deficits have been modeled in addicts and laboratory animals using reversal-learning tasks. However, persistent reversal-learning impairments have been shown in rats and monkeys only after noncontingent cocaine injections. Current thinking holds that to represent the human condition effectively, animal models of addiction must utilize self-administration procedures in which drug is earned contingently; thus, it remains unclear whether reversal-learning deficits caused by noncontingent cocaine exposure are relevant to addiction. To test whether reversal learning deficits are caused by contingent cocaine exposure, we trained rats to self-administer cocaine, assessed cue-induced cocaine seeking in extinction tests after 1 and 30 d of withdrawal, and then tested for reversal learning more than a month later. We found robust time-dependent increases in cue-induced cocaine seeking in the two extinction tests (incubation of craving) and severe reversal-learning impairments.     

Contextual control in discrimination reversal learning

In 3 human predictive learning experiments, the authors examined contextual control of responding in discrimination reversal learning. In Phase 1, a discrimination between 2 stimuli (A+, B-) was trained in Context 1. During Phase 2, participants received discrimination reversal training (A-, B+) in Context 2. Testing occurred in Context 1 and Context 2 (Experiments 1A and 1B) or in Context 1 and Context 3 (Experiment 2). During the test phase, performance in Context 1 and Context 2 reflected the contingencies trained during Phase 1 and Phase 2, respectively. When testing occurred in Context 3, there was no discriminative responding between A and B. In addition, the experiments demonstrated that discriminating stimuli with a consistent reinforcement history were also affected by contextual manipulations. Results indicate that each training context acquires the ability to control performance. Unique-cue and configural approaches account for a major part of the results.     

Successive olfactory reversal learning in honeybees

Honeybees Apis mellifera can associate an originally neutral odor with a reinforcement of sucrose solution. Forward pairings of odor and reinforcement enable the odor to release the proboscis extension reflex in consecutive tests. Bees can also be conditioned differentially: They learn to respond to a reinforced odor and not to a nonreinforced one. They can also learn to reverse their choice. Here we ask whether honeybees can learn successive olfactory differential conditioning tasks involving different overlapping pairs of odors. The conditioning schedules were established in order to train the animals with 3, 2, 1, or 0 reversals previous to a last differential conditioning phase in which two additional reversals were present. We studied whether or not successive reversal learning is possible and whether or not learning olfactory discrimination reversals affects the solving of subsequent discrimination reversals. Therefore we compared the responses of bees that had experienced reversals with those of bees that had not experienced such reversals when both are confronted with a new reversal situation. In experiment 1 we showed that bees that had experienced three previous reversals were better in solving the final reversal task than bees with no previous reversal experience. In experiment 2, we showed that one reversal learning is enough for bees to perform better in the final reversal task. The successive different reversals trained in our experiments resemble the natural foraging situation in which a honeybee forager has to switch successively from an initial floral species to different ones. The fact that experiencing such changes seems to improve a bee's performance in dealing with further new exploited food sources has therefore an adaptive impact for the individual and for the colony as a whole.     

Changes in cytochrome oxidase activity following spatial working memory learning in rats treated with tacrine

We evaluated change in cytochrome oxidase (COx) activity of the hippocampus and related structures of the limbic system following spatial working memory learning in rats after treatment with tacrine (8.0mg/kg). Control groups treated with saline and tacrine and an untreated group were added. Acetylcholinesterase optical density levels were also measured. The tacrine and saline groups showed similar behavioral results, but a decrease in COx activity was found in the tacrine group in the prefrontal cortex, nucleus accumbens, anterior thalamus, hippocampus and nucleus basalis of Meynert. Similarly, acetylcholinesterase levels of the tacrine group were lower in most of the regions. Learning-related increase in COx activity was found in the prefrontal cortex and dentate gyrus in the saline group. The tacrine group presented the same increase in the anterodorsal thalamus, dentate gyrus, CA3 and mammillary nuclei. These results suggest that inhibition of the acetylcholinesterase produces a different pattern of learning-related neuronal activity in the limbic system of the rat.     

Transfer of serial reversal learning in the pigeon

Pigeons were trained on a series of reversals of a simultaneous visual discrimination and were then shifted to a second series of reversals with different visual discriminanda. Pigeons that were given discrimination reversals with one pair of colours (Group Colour) and then shifted to a second pair of colours made fewer errors with the second pair than the first. In contrast, pigeons that were initially given reversals with a pair of orientations (Group Orientation) and then shifted to colours made as many errors during colour reversals as Group Colour had during initial colour training. When birds in Group Colour were subsequently shifted to orientation discrimination reversals, they performed no better than Group Orientation had during initial orientation training. The present results suggest that positive transfer from one series of discrimination reversals to a second, independent series may be constrained by the nature of the stimulus shift.