Bernard Deweer

Contextual cues and memory retrieval in rats: Alleviation of forgetting by a pretest exposure to background stimuli

With a relatively complex maze, reliable forgetting is clearly seen when the training-test interval is 25 days. This forgetting is evidenced by the longer time taken to run the maze and in an increase in the number of errors from the last training trial to the first test trial. In this case, forgetting is a lapse, not a loss, since performance attains the last training trial level at a subsequent test. Furthermore, a reminder which does not in itself contain sufficient information to facilitate performance of a naive animal, significantly improves maze performance of animals which have “forgotten,” even on the first retention test. With the use of additional control groups, it is shown that there must be a memory lapse before contextual cues can be demonstrated to be effective in facilitating memory retrieval.

Memory retrieval enhanced by amphetamine after a long retention interval

Four experiments investigated the effects of amphetamine on retrieval of a “forgotten” maze task. Rats were trained to run a six-unit spatial discrimination maze for food reward. After a 3-week training to test interval, control animals showed significant forgetting as measured by errors, run time, and retracings. Treatments with amphetamine 20 min before the retention test alleviated this forgetting, in that treated animals had a performance which was not different from the last training trial. The effect was dose dependent, effective doses being 0.25 and 0.50 mg/kg; larger doses were ineffective. There was no state-dependent retrieval effect; animals which showed facilitation under amphetamine treatment, continued to perform well when retested without the drug, 24 hr later. A final experiment showed that there was no effect on acquisition, whether the treatment was given before the first, second, or third acquisition trial. The results are discussed in terms of the possible role of catecholamines, central or peripheral, in memory retrieval processes.

Reticular stimulation facilitates retrieval of a ‘forgotten’ maze habit

Rats tested 25 days after training in a complex maze showed significant forgetting. Stimulation of the mesencephalic reticular formation immediately prior to retention testing facilitated performance in that stimulated rats made fewer errors (but did not run faster) than non-stimulated controls. Rats exposed to a contextual cue as a reminder before testing ran faster and made fewer errors than controls. Results are discussed in terms of forgetting being due to retrieval failure, and the reticular stimulation facilitating retrieval of information concerning the spatial configuration of the maze.

Suppression de l'amnésie rétrograde et consolidation d'un apprentissage à essai unique par stimulation réticulaire

Abstract In order to avoid possible cumulative effects of repeated treatments, a one-trial learning task based on a discrimination reversal technique with water as reinforcement is used. Four groups of rats with chronically implanted intracranial and cortical electrodes are studied in a one-trial discriminative learning based on water reinforcement. Group I, 90 sec after the trial, receives fluothane anesthesia in order to interrupt the consolidation period. Group II receives very low intensity stimulation of the mesencephalic reticular formation during the 90 sec interval between the end of the trial and anesthesia. Group III receives reticular stimulation not followed by anesthesia. Group IV has a pseudotreatment (control). All groups are tested for retention of learning. Results show no retention for group I; normal retention for II, with a performance identical with that of control group; III shows better retention than control group. This study indicates that a mild reticular stimulation during consolidation period even when delivered once, enhances memory storage. These results are discussed in relation to theory of consolidation and mechanisms of arousal.

Background stimuli as a reminder after spontaneous forgetting: role of duration of cuing and cuing-test interval

With a relatively complex maze, reliable forgetting is seen clearly when the training-to-test interval is 25 days. This forgetting is demonstrated by longer time to run the maze and by an increase in the number of errors and retracings from the last training trials to the first test trial. In this case, forgetting is a lapse, not a loss, since performance attains the last training trial level at a subsequent test. Furthermore, a reminder—a 90-sec exposure to background stimuli in the experimental room just prior to the test trial—that does not in itself contain sufficient information to facilitate performance in naive animals, significantly improves maze performance in rats that have “forgotten,” even on the first test trial. Two additional experiments were aimed at assessing the role of time and duration of pretest cuing. In the first experiment, the animals were presented the reminder (90 sec in duration) at different times before the test trial. The results show that this reminder significantly alleviates forgetting only when presented just prior to the test, and is less effective when given 1 or 24 h before the test. In the second experiment, contextual cues, which were presented just prior to testing in all experimental groups, varied in duration. The results showed that (1) animals given the reminder treatment for only 10 sec perform at the same level as controls; (2) cuing for 30 sec and especially for 90 sec alleviates forgetting; and (3) a longer exposure to background stimuli (300 sec) leads to intermediate levels of performance, probably due to a partial extinction of the cue value of these stimuli.

Background stimuli as a reminder after spontaneous forgetting: Potentiation by stimulation of the mesencephalic reticular formation

Reliable forgetting of a relatively complex maze is seen in rats when the training-to-test interval is 24 days. Forgetting is measured by longer time to run the maze and by an increase in the number of errors from the last training trials to the test trial. A contextual cue reminder alleviates forgetting when presented just prior to the test trial, but is only marginally effective when given 1 h before testing. The present experiment, using the same forgetting paradigm, was aimed at testing the hypothesis that such a marginal cuing effect can be potentiated by a stimulation of the mesencephalic reticular formation. Half of the animals were exposed for 90 sec to the experimental room (reminder condition), whereas the other half were exposed to the colony room (pseudoreminder condition), 1 h before testing. These two groups were divided into two subgroups, stimulated and pseudostimulated. The results showed that there was no effect of reminder alone and no effect of the stimulation given with the pseudoreminder; the stimulation given concurrently with the reminder alleviated forgetting as measured by errors. The results suggest that the stimulation acts to potentiate retrieval processes already triggered by the significant contextual cue reminder.

Pretest cuing after forgetting of a food-motivated maze task in rats: Synergistic action of context and reinforcement

It is now well known in animal studies that spontaneous forgetting, as well as performance decrements resulting from other sources, can be alleviated by means of pretest treatments. Several previous experiments have shown that reliable forgetting is observed after a 25-day training-to-test interval on a relatively complex maze. This forgetting can be alleviated by pretest exposure to background stimuli in the experimental room. The effects of this treatment can be modulated by varying either the duration of the treatment or the length of the cuing-test interval. The purpose of the experiment presented here was twofold: (1) to demonstrate the multidimensionality of memories in our paradigm by comparing the effects of different reminder treatments, namely pretest access to the reinforcer and contextual cuing, and (2) to test the general hypothesis, formulated by Spear (1978, p. 418), that “the elicitation and retrieval of the target attribute of a memory depend on the arousal of a sufficientnumber or kind of the remaining attributes of this memory.” The data showed that pretest food reinforcement had no significant effect on retention performance; that pretest contextual cuing did, in some precise conditions, alleviate forgetting; and that some combinations of context and reinforcement had what amounted to a synergistic action on retention performance.

Interaction between conflicting memories in the rat: Contextual pretest cuing reverses control of behavior by testing context

Two experiments were run in order to investigate the influence of external contextual cues on the interaction between two conflicting memories. Rats were trained, in the same apparatus, on a passive-avoidance task and, 10 min later, on an active-avoidance task, then submitted to a 24-h delayed test (without reinforcement). When contextual cues remained unchanged throughout these three phases, the animals exhibited proactive interference, as shown by longer response latencies than those of control animals that had learned only the active-avoidance task (Experiment 1). When training contexts were made slightly different from one task to the other (by the presence of a continuous tone during either the first or the second task), the animals behaved at subsequent testing in accordance with the response contingencies present in the context exactly similar to the test context; this control of behavior by testing context was demonstrated both when the tone was absent (Experiments 1 and 2) and present (Experiment 2) at testing. A cuing procedure—a 90-sec exposure to the tone in the experimental room 5 min prior to testing—led the animals to behave in accordance with the response previously acquired in the presence of the tone (Experiments 1 and 2), exactly in the same way as animals tested in the presence of the tone (Experiment 2). The same cuing treatment was ineffective when administered 1 h before testing (Experiment 2). These results are interpreted in terms of a dual function of contextual cues at the time of retrieval: the general contextual information present during testing or during pretest cuing is assumed to induce concurrent reactivation of both memories. Consequently, the experimentally manipulated contextual cue (the tone) would have a discriminative function, leading the animals to choose between the two equally available representations.

Effects of post-exposure anaesthesia on the retention of imprinting.

Chicks anesthetized immediately, 6 min or 15 min after the end of a 20-min imprinting session show significantly less following behavior than untreated controls, when tested 3 days later. The results are interpreted in terms of consolidation disruption, implying a consolidation period of at least 15-min duration for this type of perceptual learning.

Retroactive and proactive effects of anaesthesia on following in chicks

Abstract Halothane anaesthesia, administered after imprinting session in day old chicks, produces 2 different effects on the following response behavior tested 3 days later. First, it delays the onset of the following response for animals treated immediately after imprinting. Second, it disturbs the general performance of the following response, an effect apparently unrelated to the time of treatment. We suggest that only the first of these effects reflects a disturbance of retention.