Investigation of task performance variations according to task requirements and alertness across the 24-h day in shift workers.

The study investigated time-of-day effects on task performance in shift workers in different tasks (reaction time, discrimination, probe recognition, free recall), by varying task-specific features. On each of six recordings, each programmed on a different day and in a randomised order, operators rated alertness and performed different tasks. Self-rated alertness varied according to a typical diurnal trend. Time of day also affected reaction time (slower responses at 03:00 hours), discrimination performance (lower accuracy at 03:00 hours in the most difficult condition) and recall (superior recall at 07:00 and 11:00 hours following deeper processing at encoding). The data demonstrated time-of-day effects on cognitive processes also involved in many real-job activities, despite the lack of control for a number of exogenous factors known to interfere with performance in work settings. Since in the cognitively more loaded tasks, time-of-day effects depended on task conditions, the findings are of operational concern in shift-work situations involving differential task requirements. In a real-job setting, performance variations were observed according to time of day and task requirements in a set of cognitively more or less demanding tasks. Task-specific research across the 24-h day enables a better understanding of operators' tasks and the development of supporting technology.

Are glutamate receptors specifically implicated in some forms of memory processes?

Convergent data indicate that certain substances that interact with N-methyl-D-aspartate (NMDA) receptors or metabotropic glutamate receptors (mGluRs) do not affect acquisition processes per se, or retrieval, but interfere specifically with the formation of memory traces. This action differs widely in its amplitude and time-course according to the learning task used. We showed that systemic injection of the competitive NMDA receptor antagonists, gamma-L-glutamyl-L-aspartate (gamma-LGLA) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), or intracerebroventricular infusion of D-2-amino-5-phosphonovalerate (D-AP5), immediately following acquisition of a Y-maze avoidance learning task in mice, deeply impaired retention of the temporal component of the task (leaving the start alley within the first 5 s of a trial), which significantly improved in controls during the hours following acquisition. In contrast the same substances had no or only slight effects on retention of the discrimination component (choice of the correct alley), which did not improve over time in control animals. This retention deficit did not appear to be due to an action on acquisition, retrieval and/or forgetting processes, or to state-dependent effects. Moreover, gamma-LGLA, CPP or AP5, when administered immediately after partial acquisition of a food-reinforced bar-press task, suppressed the spontaneous improvement in post-training performance observed in control mice 24 h after the training session. (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of mGluRs, also suppressed the post-training performance increment and its effects were antagonized by the co-administration of trans-ACPD, an agonist of mGluRs. Post-training improvement of performance over time is thought to reflect an active and dynamic process, leading to the organization of memory traces. According to this hypothesis, our results suggest that synaptic plasticity mediated by NMDA receptors and/or mGluRs activation is involved in mechanisms underlying long-term consolidation of memory traces.

Serial learning in the pigeon in a free correction procedure: role of reinforcing values.

Pigeons were shown four chromatic stimuli simultaneously (A, B, C, D) in daily 15-min sessions. A free correction procedure was used: production of the series A-->B-->C resulted in food delivery and ended a trial, while incorrect colour pecks had no consequences. The learning criterion (five consecutive correct series) was not achieved after 50 sessions, although the number of correct responses increased progressively. From around session 10, birds produced primarily transitions between items B and C or A and C and repeats on items B and C (BC 35%, AC 15%, BB 10%, CC 25%). Despite the low rate of the previously acquired A-->B transition (6%), its production was highly correlated with that of the A-->B-->C series. On the other hand, item C was predominantly pecked and a recency effect occurred for this item. These data extend previous findings indicating that acquisition of series in pigeons involves the formation of associative links between stimuli according to direct and indirect reinforcing values.

Blockade of spontaneous posttraining performance improvement in mice by NMDA antagonists.

We investigated the effects of immediate post-training systemic administration of gamma-L-glutamyl-L-aspartate (gamma-LGLA) and 3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP), antagonists at the N-methyl-D-aspartate receptor, in a lever-press task in two inbred strains of mice. When retention performance was tested in control animals 24 h after partial acquisition of the task. BALB/c mice exhibited a spontaneous performance improvement whereas C57BL/6J mice did not gamma-LGLA at doses of 2.5 and 25 mumol/kg and CPP at doses ranging between 0.025 and 2.5 mumol/kg blocked the spontaneous performance improvement found in BALB/c mice but had no apparent effects on the retention performance of C57BL/6J mice. These data suggest that retention impairment induced by CPP and gamma-LGLA in BALB/c mice results from an interference with posttraining memory processes.

Response strategies during acquisition of serial learning in pigeons (Columba livia).

Pigeons were exposed on fixed duration sessions to four chromatic stimuli and trained to perform a 2-item sequence. A detailed analysis of incorrect responses performed on each trial prior to completion of the sequence led to distinguish three learning stages. Already on initial sessions animals responded significantly more frequently to relevant stimuli than to irrelevant ones indicating stimulus control by salience. During an intermediary stage, responding to item 2 increased significantly and animals started an increasing number of trials with item 2, suggesting stimulus control by recency. On the final session the correct sequence outnumbered significantly the reverse sequence, suggesting control by order, while recency and salience effects were present to a lesser extent. Repeated pecks, i.e. consecutive pecks on the same cue, occurred on the four cues on the first session and decreased significantly on the last session for irrelevant colours. In addition, when key pecks were followed by a brief sound signal, repeats on relevant stimuli decreased while efficient colour pecks (i.e. when repeats were ignored) on these stimuli increased. These findings are discussed in the light of those obtained in traditional chaining paradigms.

Strain-dependent effects of gamma-L-glutamyl-L-aspartate, a NMDA antagonist, on retention of a Y-maze avoidance learning task in mice.

The NMDA receptor antagonist, gamma-L-glutamyl-L-aspartate (gamma-LGLA), suppressed spontaneous improvement in posttraining performance in Swiss mice during the hours following acquisition of a Y-maze avoidance learning task. Since variability in posttraining performance is at least partially due to genetic factors, we compared the effects of gamma-LGLA on retention of Y-maze learning in C57BL/6J, DBA/2J and BALB/c mice. Mice had to leave the start alley of the maze within the first 5 s (temporal component) and to choose the left alley (spatial component). C57BL mice significantly improved their performance from 1 h to 24 h posttraining, whereas DBA/2J and BALB/c mice did not. However, only retention of the temporal component improved over time in C57BL. gamma-LGLA administered immediately posttraining (0.025-25 mumol/kg, i.p.) dose-dependently impaired retention of the temporal component in C57BL mice 48 h later, but had no significant effect on retention of the spatial component. gamma-LGLA administered 24 h posttraining induced a similar but weaker deficit. In contrast, gamma-LGLA did not significantly affect retention of DBA/2J and BALB/c mice, regardless of the component analyzed or the time of administration. It had no effect on locomotor activity or emotional reactivity of animals of any strain. These results support the hypothesis of a specific action of gamma-LGLA on mechanisms involved in the treatment of information during the hours following acquisition, and suggest that NMDA receptors are involved in this action.

The NMDA receptor antagonists, CPP and gamma-L-glutamyl-L-aspartate, selectively block post-training improvement of performance in a Y-maze avoidance learning task.

Behavioral effects of CCP and gamma-L-glutamyl-L-aspartate (gamma-LGLA) were studied in a Y-maze avoidance learning task. Male Swiss mice had to leave the start alley of the maze within the first 5 s of a trial (temporal component) and to choose the left alley (spatial component) to avoid footshocks; they were trained to a criterion of 7 correct out of 8 consecutive trials. CPP and gamma-LGLA when administered immediately following the learning session (0.025-200 mumol/kg, i.p.) significantly impaired retention 48 h later at doses of 0.025-0.25 and 0.25-25 mumol/kg, respectively, but had no significant effect at higher doses. CPP, when administered 30 min before the learning session (0.025-25 mumol/kg) did not affect learning acquisition at any dose, whereas it significantly impaired retention 48 h later but only at the doses of 0.025-0.25 mumol/kg. CPP and gamma-LGLA did not erase all memory traces; posttraining performances on the temporal component, which significantly improved in control animals during the hours following acquisition, were much more affected by CPP and gamma-LGLA than posttraining performances on the spatial component which did not improve over time in controls. Moreover, CPP (0.025-25 mumol/kg) had no effect on spatial recognition memory in an alternation task in which no spontaneous improvement of posttraining performance was observed in controls. These results strongly suggest that CPP and gamma-LGLA interfere with mechanisms underlying posttraining organization of memory traces and that NMDA receptors are involved in this action.

Gamma-L-glutamyl-L-aspartate, interacting with NMDA receptors, affects appetitive visual discrimination tasks in mice.

gamma-L-glutamyl-L-aspartate (gamma-LGLA), which interacts with NMDA receptors, has been shown to impair retention of an active avoidance task in mice. Here, we specified the behavioral effects of gamma-LGLA on acquisition and retention of appetitive nondelayed visual discrimination tasks. Three experiments were conducted: the peptide (0.25 and 2.5 microM/kg/25 ml. ip) was administered 3 min after each of the first six sessions of either original learning, reversal 1 or reversal 3. gamma-LGLA affected acquisition of the original task and of the first reversal, as revealed by an absence of improvement on initial sessions and an increased number of sessions to reach criterion fixed at 7 of 10 correct choices on three consecutive sessions. This deficit did not result from an action of the peptide on position habits (repetition of spatial choices) nor on motivational processes, suggesting a specific interference of gamma-LGLA with acquisition and memorization of the visual rule. In contrast, gamma-LGLA had no effect on acquisition of the third reversal, in which the positively reinforced visual stimulus was identical to that used on the first reversal. These results show that the behavioral deficits of gamma-LGLA, which had previously been demonstrated in an aversive task, can be generalized to appetitive tasks based on acquisition of a new rule.

Positional choices and visual discrimination in non-delayed appetitive discrimination tasks in mice.

Mice were submitted to visual discrimination learning using a large choice compartment positioned in front of two adjacent runways, black and white, one of which was baited. Rate of acquisition, expressed by the mean number of sessions to criterion did not differ between the initial task and two reversals. However, the number of correct choices before criterion was significantly lower on both reversals compared to original learning. Position habit rates did not vary significantly between acquisitions, and they were negatively correlated with visual discrimination rates in the three learning phases. This observation, and the fact that on each phase, position habits were largely absent on initial sessions and only developed gradually, suggest that position habits may occur when animals do not grasp the solution to the visual task.

[Role of neuroexcitatory amino acids in memory processes. Study with gamma-L-glutamyl-L-aspartic acid]. / Rôle des acides aminés neuroexcitateurs dans les phénomènes de mémoire. Etude à partir de l'acide gamma-L-glutamyl-L-aspartique.

Convergent data demonstrate that excitatory amino acid systems (glutamate and aspartate) participate in synaptic plasticity of the central nervous system. Their action is mediated by at least three subclasses of receptors which have been characterized on the basis of their selective affinity to N-methyl-D-aspartate (NMDA), quisqualate and kainate. NMDA receptors appear to be directly involved in the induction of long-term potentiation (LTP) at the hippocampal level, and quisqualate/kainate receptors in the expression of LTP. This suggests that excitatory amino acid systems may have an important role in learning and memory. However, how these systems interfere with memory processes remains largely unknown. We have isolated a pseudopeptide, gamma-L-glutamyl-L-aspartate (gamma-LGLA) (Ungerer et al., 1988), which has the pharmacological properties of a competitive antagonist at NMDA receptors as evidenced by biochemical studies and by the fact that gamma-LGLA selectively blocks the clonico-tonic seizures induced by NMDA, while having no significant action against seizures induced by kainate or quisqualate. Elsewhere, gamma-LGLA is devoid of toxicity at the doses used. Behavioral effects of gamma-LGLA were first studied in a Y-maze avoidance learning task. Animals had to leave the start alley within 5 sec. (temporal component) and to choose the left alley of the maze (spatial component) to avoid footshock. They underwent one trial every minute and were trained to a criterion of 7 correct out of 8 consecutive trials. Retention was tested either 1 h, 3 h, 6h, 24 h, 7 days or 21 days after acquisition.(ABSTRACT TRUNCATED AT 250 WORDS)

gamma-L-glutamyl-L-aspartate induces specific deficits in long-term memory and inhibits [3H]glutamate binding on hippocampal membranes.

gamma-L-Glutamyl-L-aspartic acid (gamma-LGLA) has been isolated from Datura stramonium; its structure has been determined and it was then synthesized. In male Swiss mice intraperitoneal administration of the natural peptide (125 nmol/kg) or of the synthetic peptide (25-2500 nmol/kg) 24 h after acquisition of a Y-maze avoidance task induced a dose-dependent deficit in retention performance 48 h later. gamma-LGLA had no effect on locomotor activity or emotional reactivity at the doses used. Separate or simultaneous administration of aspartate or glutamate (each at 250 nmol/kg) had no effect on learning retention, indicating that deficit induced by gamma-LGLA was specific to the peptide. gamma-LGLA impaired learning acquisition in a time-dependent manner when administered from 3 min to 24 h post-training, but had no effect when administered 3 days following acquisition. gamma-LGLA administered just after the training session did not affect retention performance during the first 3 h, but suppressed the retention improvement observed in control animals from 6 to 24 h after acquisition; this deficit was still evident 7 days after the treatment. gamma-LGLA partially inhibited L-[3H]glutamate binding on crude hippocampal or striatal membrane preparations; this inhibition was not observed on cerebellar membrane preparations. These results suggest a specific action of gamma-LGLA on excitatory amino acid systems which may be responsible for its effects on learning retention.

Spontaneous light or dark preference in albino mice?

The present experiment examined spontaneous visual choice behaviour and acquisition of a positively reinforced visual discrimination task in Swiss albino mice. In experiment I animals were given 4 consecutive trials in which they could freely enter either a dimly illuminated or a darkened arm of a Y-maze; the position of the light stimulus was randomized across trials. D groups and L groups were tested during the dark and the light period of the day respectively. Results revealed a significant spontaneous preference for the illuminated arm of the maze, independent of the testing period. It is suggested that the dim light has a reinforcing value because it provides additional information about a novel environment. In a second experiment an appetitive visual discrimination task was carried out in the same Y-maze. After a pretraining period, half the animals were reinforced in the illuminated arm and half were reinforced in the darkened one, on five consecutive days. On the first test session all groups of animals chose the illuminated arm significantly more frequently, whereas light/dark choices reached chance level on the last test session. Discrimination learning was not acquired and a behavioural analysis revealed an increasing tendency to a side preference across testing.