The behavioral neurobiology of learning and memory: a conceptual reorientation

Hippocampal lesions and path integration

Research on spatial problem-solving over the past two years has linked the hippocampus to path integration, that is, the use of movement-related cues to guide spatial behavior. Path integration may underlie the forms of place learning that are impaired by hippocampal damage. It remains a challenge to determine whether path integration is the central function of the hippocampus or but one of many.

Testing hypotheses of spatial learning: the role of NMDA receptors and NMDA-mediated long-term potentiation

The role of NMDA receptors and NMDA-mediated hippocampal long-term potentiation (LTP) in spatial learning was studied in rats using the competitive, systemically administered NMDA receptor antagonists CGS19755 ((+/-)-cis-4-phosphonomethyl-2-piperidine carboxylic acid) and NPC17742 (2R,4R,5S-2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid). CGS19755 caused sensorimotor disturbances and disrupted acquisition of the water maze in naive rats. The sensorimotor disturbances were greatly reduced and maze learning was normal in spite of the blockade of dentate gyrus LTP by CGS19755 in rats that had first been familiarized with the general task requirements by non-spatial pretraining. In a second experiment, antagonism of NMDA receptors caused small, but reliable, impairments in Y-maze and visible platform visual discrimination tasks. The results indicate that NMDA receptors are not crucial for water maze acquisition using a spatial learning strategy, and that NMDA antagonists cause visual and other sensorimotor disturbances in naive rats that could help account for their poor performance in this task.

Associative and perceptual learning and the concept of memory systems

An introductory review is followed by some new experimental data and a final discussion. The primate temporal lobe contains multiple qualitatively distinct memory systems. The functional properties of these memory systems can be explained by reference to the nature of the afferent information which they process, rather than by reference to any putative specialization in memory processing. In this way, the plasticity of 'memory systems' in associative memory is probably similar in principle to the plasticity of 'perceptual systems' in perceptual learning. Therefore, it is important to consider the relationship between perceptual and associative learning. Two experiments investigated perceptual learning in the Rhesus monkey (Macaca mulatta). Substantial perceptual learning was observed both with complex scenes and with simple colours. Two hypotheses as to the basis of perceptual learning are discussed. A physiological hypothesis is that training with a particular set of stimuli expands the cortical representation of those stimuli. This can explain the effects in both experiments. A psychological hypothesis is that perceptual learning is produced by learned associations among multiple features of complex stimuli. This can explain the effects in Expt. 1 but not in Expt. 2. The psychological associative hypothesis is therefore redundant. Furthermore, associative learning can itself be viewed as an expansion of the cortical representation of a complex event. Thus, the distinction between perceptual and memory systems will need to be abandoned as deeper understanding of cortical plasticity is achieved.

CR 2249: a new putative memory enhancer. Behavioural studies on learning and memory in rats and mice

The effects of S-4-amino-5-[4,4-dimethylcyclohexyl)amino]-5-oxopentanoic acid (CR 2249), a new entity selected from a new series of glutamic acid derivatives, has been investigated in different paradigms for screening nootropics. CR 2249 ameliorated the memory retention deficit produced by scopolamine in step-through-type passive avoidance in rats and by electroconvulsive shock in step-down-type passive avoidance in mice. CR 2249 was also capable of improving performance in behavioural tests of learning and memory in the absence of cholinergic hypofunction or cognitive deficit. The activity was determined using different passive and active avoidance behavioural test procedures on rats. CR 2249 was active only when given 45 min before training and did not show any effect when administered immediately after the learning training or before the retention trial. No changes in the general behaviour or motor activity of the animals were observed, indicating that CR 2249 effects cannot be attributed to sensory-motor deficit. Microdialysis experiments have shown that CR 2249 significantly increased noradrenaline release in the hippocampus of freely moving rats and reduced 3,4-dihydroxyphenylglycol efflux. These effects have led us to hypothesize that CR 2249 memory effect might be mediated by a direct or indirect action on noradrenergic transmission. These behavioural results suggest that this new agent has clinical application in memory disorders.

Of mice and mazes: similarities between mice and rats on dry land but not water mazes

Mice are impaired relative to rats in place and matching-to-place learning when tests are given in a swimming pool. The rat advantage may stem from a superior spatial ability or from adaptation to a niche that has prepared them for competency in the water. In the present study, mice (C57BL/6) were compared with rats (Long-Evans) in a number of dry-land spatial tasks given on a radial arm maze and in a place task given in a swimming pool. The performance of the mice matched that of the rats in all dry-land tasks, but was inferior to that of the rats in the swimming pool. The results provide further evidence for a species difference in swimming-pool performance but do not support the idea that there are necessary differences in spatial abilities between mice and rats. It is suggested that, if optimal place learning is required for neurobehavioral studies of mice, such performance is more likely to be obtained in dry-land tasks than in swimming-pool tasks. Nevertheless, the species differences warrant further study because they could provide important insights into species differences is spatial learning more generally.

The effects of different types of pre-training on the rat's retention performance in a swim-to-platform task following administration of scopolamine

Previous research has found that centrally acting antimuscarinic drugs strongly impair the acquisition of a variety of learned behaviors in rats but have little effect on these same behaviors if training is given prior to drug treatment. We gave groups of rats different types of pre-training followed by treatment with scopolamine hydrobromide and subsequent testing on a simple swim-to-platform test. Factors such as practice in swimming without a platform to escape to, or learning to swim to a platform in a different apparatus or even to the test platform located in a different place did not protect the rats from the behavioral disruption produced by scopolamine. However, five training trials on the specific swim-to-platform task used in the retention test afforded almost complete protection against the effect of scopolamine. It appears that the protective effect of pre-training is highly specific and does not involve acquisition of some type of general rule which might survive antimuscarinic blockade.

The role of brain noradrenaline in cortical activation and behavior: a study of lesions of the locus coeruleus, medial thalamus and hippocampus-neocortex and of muscarinic blockade in the rat

Local injection of 6-hydroxydopamine in the locus coeruleus resulted in a 90% depletion of noradrenaline (NA) in the cerebral cortex as assessed by high-pressure liquid chromatography. This NA depletion had no effect on scopolamine-resistant hippocampal rhythmical slow activity and only an occasional effect on scopolamine-resistant neocortical low voltage fast activity. However, NE depletion resulted in a slight deficit in a behavioral swim-to-platform test and increased the deficit produced on the test by systemic treatment with scopolamine. Large surgical lesions of the medial thalamus or hippocampal formation plus posterior neocortex greatly increased the behavioral deficit produced by scopolamine. It is concluded that ascending noradrenergic projections play only a modest and possibly indirect role in the control of electrocortical activation and that a number of different brain lesions increase the behavioral impairment produced by central muscarinic blockade.

The primate temporal pole: its putative role in object recognition and memory

In this article, we consider both the ventral temporopolar cortex and the perirhinal cortex (areas 35 and 36) as the anterior ventromedial temporal (aVMT) cortex, and discuss its role based on recent data in monkeys and human subjects. In monkeys, the aVMT cortex receives its primary input from area TE, and only minor input from other cortical areas. Laminar patterns of connections suggest that the aVMT cortex is a hierarchically higher-order area than area TE. Lesions of this cortex produce deficits in the learning and performance of visual memory tasks. Neurons in the aVMT cortex respond selectively to complex stimuli and changes in activity related to visual memory tasks. In humans, damage of this cortex induces deficits in the recognition of familiar objects and faces. The aVMT cortex is activated during recognition of familiar faces. In addition, the aVMT cortex is one of the most vulnerable areas in Alzheimer's disease. All these data indicate that the aVMT cortex is a higher-order visual cortical area that is related to object recognition and memory. The anterior area TE has been implicated in both functions. We propose here that these areas and the anterior entorhinal cortex are designated as the temporal pole, a brain region which is specialized for both object recognition and memory.

PSYCHOLOGY IN CANADA

▪ Abstract  This chapter reviews how psychology in Canada evolved over half a century to become the most popular discipline in universities and a respected health-care and helping profession. The organization, journals, and funding of the scientific discipline are described. The importance of DO Hebb's research as the stimulus and foundation for discipline growth and significant research contributions to basic processes is identified. The multicultural mosaic of Canadian society and early research on second-language learning are shown to have influenced cross-cultural and social research. Canadian research contributions to basic processes and to the social and health sciences are reviewed. Although late to begin in Canada, clinical research and the profession of psychology are shown to have substantially developed over the past two decades. With large numbers of quality researchers and practitioners, psychology has a bright future in Canada.

Memory, sleep, and dynamic stabilization of neural circuitry: evolutionary perspectives

Some aspects of the evolution of mechanisms for enhancement and maintenance of synaptic efficacy are treated. After the origin of use-dependent synaptic plasticity, frequent synaptic activation (dynamic stabilization, DS) probably prolonged transient efficacy enhancements induced by single activations. In many "primitive" invertebrates inhabiting essentially unvarying aqueous environments, DS of synapses occurs primarily in the course of frequent functional use. In advanced locomoting ectotherms encountering highly varied environments, DS is thought to occur both through frequent functional use and by spontaneous "non-utilitarian" activations that occur primarily during rest. Non-utilitarian activations are induced by endogenous oscillatory neuronal activity, the need for which might have been one of the sources of selective pressure for the evolution of neurons with oscillatory firing capacities. As non-sleeping animals evolved increasingly complex brains, ever greater amounts of circuitry encoding inherited and experiential information (memories) required maintenance. The selective pressure for the evolution of sleep may have been the need to depress perception and processing of sensory inputs to minimize interference with DS of this circuitry. As the higher body temperatures and metabolic rates of endothermy evolved, mere skeletal muscle hypotonia evidently did not suffice to prevent sleep-disrupting skeletal muscle contractions during DS of motor circuitry. Selection against sleep disruption may have led to the evolution of further decreases in muscle tone, paralleling the increase in metabolic rate, and culminating in the postural atonia of REM (rapid eye movement) sleep. Phasic variations in heart and respiratory rates during REM sleep may result from superposition of activations accomplishing non-utilitarian DS of redundant and modulatory motor circuitry on the rhythmic autonomic control mechanisms. Accompanying non-utilitarian DS of circuitry during sleep, authentic and variously modified information encoded in the circuitry achieves the level of unconscious awareness as dreams and other sleep mentation.

Behavioral consequences of intracerebral vasopressin and oxytocin: focus on learning and memory

Since the pioneering work of David de Wied and his colleagues, the neuropeptides arginine vasopressin and oxytocin have been thought to play a pivotal role in behavioral regulation in general, and in learning and memory in particular. The present review focuses on the behavioral effects of intracerebral arginine vasopressin and oxytocin, with particular emphasis on the role of these neuropeptides as signals in interneuronal communication. We also discuss several methodological approaches that have been used to reveal the importance of these intracerebral neuropeptides as signals within signaling cascades. The literature suggests that arginine vasopressin improves, and oxytocin impairs, learning and memory. However, a critical analysis of the subject indicates the necessity for a revision of this generalized concept. We suggest that, depending on the behavioral test and the brain area under study, these endogenous neuropeptides are differentially involved in behavioral regulation; thus, generalizations derived from a single behavioral task should be avoided. In particular, recent studies on rodents indicate that socially relevant behaviors triggered by olfactory stimuli and paradigms in which the animals have to cope with an intense stressor (e.g., foot-shock motivated active or passive avoidance) are controlled by both arginine vasopressin and oxytocin released intracerebrally.

Acetylcholine: a neurotransmitter for learning and memory?

The cholinergic hypothesis claims that the decline in cognitive functions in dementia is predominantly related to a decrease in cholinergic neurotransmission. This hypothesis has led to great interest in the putative involvement of the cholinergic neurotransmission in learning and memory processes. This review aims to assess the data of studies in which the role of acetylcholine (ACh) in cognitive functions was investigated. For this purpose, studies from three different fields of research, namely: (1) behavioral pharmacology (effects of drugs on behavior); (2) behavioral neuroscience (effects of brain lesions on behavior); and (3) dementia, are discussed separately. The experimental tools that have been used in pharmacological studies may appear to be inadequate to enable conclusions to be drawn about the involvement of ACh in learning and memory processes. Especially, the use of scopolamine as a pharmacological tool is criticized. In the field of behavioral neuroscience a highly specific cholinergic toxin has been developed. It appears that the greater and more specific the cholinergic damage, the fewer effects can be observed at the behavioral level. The correlation between the decrease in cholinergic markers and the cognitive decline in dementia may not be as clearcut as has been assumed. The involvement of other neurotransmitter systems in cognitive functions is briefly discussed. Taking into account the results of the different fields of research, the notion that ACh plays a pivotal role in learning and memory processes seems to be overstated. Even when the role of other neurotransmitter systems in learning and memory is taken into consideration, it is unlikely that ACh has a specific role in these processes. On basis of the available data, ACh seems to be more specifically involved in attentional processes than in learning and memory processes.

The neurotoxins colchicine and kainic acid block odor-induced fast waves and olfactory-evoked potentials in the dentate gyrus of the behaving rat

It has previously been shown that the hilus of the dentate gyrus responds to odors (e.g. toluene) with a burst of fast waves and to electrical stimulation of the olfactory bulb with an evoked potential consisting of an early component immediately after the stimulus artifact, a second component with a 16-18-ms latency and additional late components. Spectral analysis revealed that odor-induced fast-wave bursts in the olfactory bulb and dentate gyrus both had a peak frequency of 15-20 Hz and were highly coherent. Unilateral intrahippocampal injections of colchicine or kainic acid were used in an attempt to destroy granule and pyramidal cells, respectively, while saline was injected on the opposite side as a control. Recordings from chronically implanted electrodes in the olfactory bulb and dentate gyrus demonstrated that saline had no effect while either neurotoxin abolished the odor-induced fast waves. In addition, the late 16-18-ms component of the dentate-evoked potentials after single-pulse stimulation of the olfactory bulb was abolished by either kainic acid or colchicine; the early dentate response, probably a volume-conducted olfactory response, was not abolished. Histological analysis indicated that kainic acid produced widespread non-specific damage in the hippocampal formation. Kainic acid-treated tissue exhibits a thinning of granule cell and molecular layers of the dentate gyrus as well as cell loss in CA3 and part of CA1.

Nootropic drugs have different effects on kindling-induced learning deficits in rats

Kindling represents an accepted model of human epileptogenesis. Furthermore, it has been demonstrated that kindled rats show a diminished learning performance in an active avoidance task. In our study we administered different nootropic drugs to kindled rats to test their effects on learning a two-way active avoidance task in the shuttle-box. Kindling was induced by repeated intraperitoneal injections of 45 mg kg-1 pentylenetetrazol (PTZ) once every 48 h. The substances vinpocetine (0.1 and 1.0 mg kg-1), methylglucamin orotate (225 and 450 mg kg-1), piracetam (100 mg kg-1), and meclofenoxate (100 mg kg-1) were administered during kindling development and after kindling completion prior to each session in the learning experiment. The nootropic drugs had little if any effect on severity of seizures. Concerning their effect on learning the substances each acted in a specific manner. Methylglucamin orotate enhanced the learning deficit induced by kindling. Meclofenoxate injected prior to the kindling stimulation was ineffective, whereas administration prior to the learning test improved the learning performance effectively. A complementary action was shown in experiments with vinpocetine. Only piracetam prevented the occurrence of kindling-induced learning deficits regardless the administration schedule.

Synaptic plasticity: stairway to memory

Since the idea that memory is associated with alterations in synaptic strength was accepted, studies on the cellular and molecular mechanisms responsible for the plastic changes in neurons have attracted wide interest in the scientific community. Recent studies on memory processes have also pointed out some unifying themes emerging from a wide range of nervous systems, suggesting that regardless of the species or brain regions, a common denominator for memory may exist. Thus, the present review attempted to create a hypothetical and universal synaptic model valid for a variety of nervous systems, ranging from molluscs to mammals. The cellular and molecular events leading to short- and long-term modifications of memory have been described in a sequential order, from the triggering signals to the gene expression, synthesis of new proteins and neuronal growth. These events are thought to represent the late phases of memory consolidation leading to persistent modifications in synaptic plasticity, thereby facilitating the permanent storage of acquired information throughout the individual's life.

Reduced spatial learning in mice infected with the nematode, Heligmosomoides polygyrus

Parasite modification of host behaviour influences a number of critical responses, but little is known about the effects on host spatial abilities. This study examined the effects of infection with the intestinal trichostrongylid nematode, Heligmosomoides polygyrus, on spatial water maze learning by male laboratory mice, Mus musculus. In this task individual mice had to learn the spatial location of a submerged hidden platform using extramaze visual cues. Determinations of spatial performance were made on day 19 post-infection with mice that had been administered either 50 or 200 infective larvae of H. polygyrus. The infected mice displayed over 1 day of testing (6 blocks of 4 trials) significantly poorer acquisition and retention of the water maze task than either sham-infected or control mice, with mice that had received 200 infective larvae displaying significantly poorer spatial performance than individuals receiving 50 larvae. The decrease in spatial learning occurred in the absence of either any symptoms of illness and malaise, or any evident motor, visual and motivational impairments. It is suggested that in this single host system the parasitic infection-induced decrease in spatial learning arises as a side-effect of the host's immunological and neuromodulatory responses and represents a fitness cost of response to infection.

Does a history of convulsions increase the amnestic effect of temporal region brain lesions?

Rats with surgical lesions of the hippocampal formation, amygdala, and nearby neocortex and pyriform cortex were impaired on a swim-to-platform test and on a passive avoidance test. These impairments were not increased by presurgical treatment with a series of 21 electroshock convulsions. To the extent that this rat preparation can be compared with human patients, the data suggest that a previous history of epileptic seizures does not increase the amnestic effect of temporal lobe lesions.