Hippocampal system dysfunction and odor discrimination learning in rats: Impairment of facilitation depending on representational demands

Effect of chronic inhibition of calpains in the hippocampus on spatial discrimination learning and protein kinase C

Several behavioral and electrophysiological studies have suggested that a sustained activation of protein kinase C would be required to underlie persistent changes associated with memory formation. Limited proteolysis of PKCs by calpains, calcium-activated proteases, cleaves the catalytic and the regulatory domains, generating a free catalytic fragment termed PKM, constitutively active. In order to investigate the potential physiological importance of this limited proteolysis as a mechanism of PKC activation, we have studied the effect of the calpastatin peptide, a specific calpain inhibitor, on the learning of a spatial discrimination task in a radial maze. Thus, using osmotic micro-pumps, the calpastatin peptide was infused bilaterally into the dorsal hippocampus during the six sessions of training and the probe test. The treatment was shown to facilitate the performance of the mice on the two last training sessions and on the probe test. This behavioral effect was shown to correspond to the reduced calpain activity observed in the hippocampus at the very end of the 7-day infusion of the calpastatin peptide, suggesting a relation between both events. In addition, PKC activity measured immediately after the probe test was notably decreased in the membrane fraction of the hippocampus. Although protein levels of PKCs and calpains quantified by western blot were not affected by calpastatin infusion, we found a noticeable correlation between mu-calpain and PKCgamma levels confirming the particular relationship between both proteins. These results suggest that calpains influence on PKCs activity may affect cellular mechanisms during memory processes.

Learning and memory in two different reward tasks in a radial arm maze in rats

In an eight-arm radial maze, working and reference memory can be assessed simultaneously in the fixed position of reward task (FPRT) in which half of the arms are baited and their positions are fixed throughout the training trails. We characterized performance of rats in the variable position of reward task (VPRT), in which four out of eight arms were baited, but the positions were varied in every training trial. In the VPRT, the rats learned to choose all arms without any discrimination between baited and non-baited arms and the memory retention was time-dependent. The performance of rats in the FPRT was impaired by altering the spatial organization of the extramaze cues while it was not affected in the VPRT. The number of Fos-positive cells transiently increased in the cerebral cortex and hippocampus of both groups of animals during the training. Finally, bilateral lesions of the dorsal hippocampus resulted in an impairment of working memory in the FPRT and the performance of the rats in the VPRT. These results suggest that different strategies are used between the FPRT and VPRT but the hippocampus plays an important role in performance of rats trained for the VPRT as well as FPRT.

Social and individual recognition in rodents: Methodological aspects and neurobiological bases

What animals know about each other, and how they construct and use knowledge of their social world involves at least an ability to recognise different social categories. Although much evidence has accumulated that animals are able to identify and classify other individuals into different categories, few studies have definitively demonstrated true individual recognition, i.e. discrimination between individuals on the basis of their idiosyncratic characteristics. Furthermore, the neural structures and pathways involved in social and, a fortiori, individual recognition have as yet been poorly investigated. This paper discusses various methods and measures currently used to assess different forms of social categorisations in animals, with special reference to rodents. Recent progress concerning the neurobiological bases involved in social recognition is also discussed. Finally, integrative perspectives for studying individual recognition in the context of social cognition is underlined in relation to different approaches investigating rodents' ability to use learned olfactory information.

Dissociating basal forebrain and medial temporal amnesic syndromes: insights from classical conditioning

In humans, anterograde amnesia can result from damage to the medial temporal (MT) lobes (including hippocampus), as well as to other brain areas such as basal forebrain. Results from animal classical conditioning studies suggest that there may be qualitative differences in the memory impairment following MT vs. basal forebrain damage. Specifically, delay eyeblink conditioning is spared after MT damage in animals and humans, but impaired in animals with basal forebrain damage. Recently, we have likewise shown delay eyeblink conditioning impairment in humans with amnesia following anterior communicating artery (ACoA) aneurysm rupture, which damages the basal forebrain. Another associative learning task, a computer-based concurrent visual discrimination, also appears to be spared in MT amnesia while ACoA amnesics are slower to learn the discriminations. Conversely, animal and computational models suggest that, even though MT amnesics may learn quickly, they may learn qualitatively differently from controls, and these differences may result in impaired transfer when familiar information is presented in novel combinations. Our initial data suggests such a two-phase learning and transfer task may provide a double dissociation between MT amnesics (spared initial learning but impaired transfer) and ACoA amnesics (slow initial learning but spared transfer). Together, these emerging data suggest that there are subtle but dissociable differences in the amnesic syndrome following damage to the MT lobes vs. basal forebrain, and that these differences may be most visible in non-declarative tasks such as eyeblink classical conditioning and simple associative learning.

Multiple parallel memory systems in the brain of the rat

A theory of multiple parallel memory systems in the brain of the rat is described. Each system consists of a series of interconnected neural structures. The "central structures" of the three systems described are the hippocampus, the matrix compartment of the dorsal striatum (caudate-putamen), and the amygdala. Information, coded as neural signals, flows independently through each system. All systems have access to the same information from situations in which learning occurs, but each system is specialized to represent a different kind of relationship among the elements (stimulus events, responses, reinforcers) of the information that flows through it. The speed and accuracy with which a system forms a coherent representation of a learning situation depend on the correspondence between the specialization of the system and the relationship among the elements of the situation. The coherence of these stored representations determines the degree of control exerted by each system on behavior in the situation. Although they process information independently the systems interact in at least two ways: by simultaneous parallel influence on behavioral output and by directly influencing each other. These interactions can be cooperative (leading to similar behaviors) or competitive (leading to different behaviors). Experimental findings consistent with these ideas, mostly from experiments with rats, are reviewed.

Olfactory-mediated fear-potentiated startle

Recently, R. Richardson, A. Vishney, and J. Lee (1999) reported that ambient odor cues that were previously paired with footshock potentiate the acoustic startle response in rats. The authors of the present study extend those findings by using a discrete 4-s amyl acetate odor paired with footshock to address several parametric issues that might be important for using odorants as conditioned stimuli (CSs) in this paradigm. Amyl acetate (5%) had no significant effect on startle in untrained rats but did potentiate startle in rats that received 1, 2, 5, or 10 odor-shock pairings. Fear-potentiated startle decreased but was still significant up to 40 days after conditioning and could be measured in test trials separated by as little as 30 s. The magnitude of potentiated startle decreased with decreasing concentrations of amyl acetate (5%-5 x 10-9%). The anxiolytic compound buspirone (10 mg/kg) significantly attenuated olfactory-mediated fear-potentiated startle.

The hippocampus as an olfacto-motor mechanism: were the classical anatomists right after all?

The relations between behavior, olfactory input (monitored by recording the activity of the olfactory mucosa), and the spontaneous field potentials of the dentate gyrus were studied in freely moving rats. Bursts of 30-80 Hz (gamma) waves were elicited in the dentate gyrus when a rat sniffed at a variety of objects but were not elicited by auditory, somesthetic, or visual inputs and were not related to the occurrence of locomotion. The presence of gamma wave activity was associated with an enhancement of the population spike elicited in the dentate gyrus by stimulation of the perforant path. Odorized air blown into a nostril via a cannula, inserted under light urethane anesthesia, elicited a gamma wave response bilaterally in the dentate gyrus. These and other data were reviewed to support the general hypothesis that the hippocampus is primarily an olfacto-motor mechanism and does not play any unique role in learning and memory, cognitive mapping, or emotion. The role of the hippocampus in the control of some forms of motor activity is supported by numerous anatomical and electrophysiological studies, studies of the effect of hippocampal lesions on behavior, and studies of the effects of electrical or chemical stimulation of the hippocampus on behavior.

Interactive memory systems in the human brain

Learning and memory in humans rely upon several memory systems, which appear to have dissociable brain substrates. A fundamental question concerns whether, and how, these memory systems interact. Here we show using functional magnetic resonance imaging (FMRI) that these memory systems may compete with each other during classification learning in humans. The medial temporal lobe and basal ganglia were differently engaged across subjects during classification learning depending upon whether the task emphasized declarative or nondeclarative memory, even when the to-be-learned material and the level of performance did not differ. Consistent with competition between memory systems suggested by animal studies and neuroimaging, activity in these regions was negatively correlated across individuals. Further examination of classification learning using event-related FMRI showed rapid modulation of activity in these regions at the beginning of learning, suggesting that subjects relied upon the medial temporal lobe early in learning. However, this dependence rapidly declined with training, as predicted by previous computational models of associative learning.

Are memories for stimulus-stimulus associations or stimulus-response associations responsible for serial-pattern learning in rats?

Previous research has provided convincing evidence that rats can learn to anticipate the individual elements of a stimulus series consisting of differing amounts of food reinforcement. Rats prepared with lesions of the dorsal striatum or hippocampus were initially trained to acquire a three-element series consisting of 21 sucrose pellets, followed by 0- and 7-pellets (Noyes standard), respectively. During the initial 30 days of training, the animals were run in two adjacent runways; the runways included either of a white, rough runway or a black, smooth runway as additional series cues. Thus, training included both floor (S-R) cues and the series (inter-item memory) cues. Anticipation was defined as faster running on the 21- than on the 7-pellet element and 7- than on the 0-pellet element. While anticipation developed more slowly in the lesion groups than in the control group, all animals eventually demonstrated the ability to track the elements of the series. Reversal of the floor cues disrupted tracking in the hippocampus-lesioned and control animals; dorsal striatum-lesioned rats were also affected but did continue tracking. As a final test, shifting the order of the series produced a marked disruption in performance in the dorsal striatum-lesioned rats but not in the hippocampus-lesioned or control rats. The results are consistent with the proposal that integrated neural mediation is required for anticipation, with a system that includes the dorsal striatum necessary for the promotion of a reinforced approach response and a system that includes the hippocampus necessary for associating and temporarily maintaining an internal record of the different elements of the stimulus series.

Characterizing the neural mechanisms of skill learning and repetition priming: evidence from mirror reading

The changes in brain activity related to skill learning and repetition priming in a mirror-reading task were examined using functional MRI. Subjects exhibited significant learning across five training sessions and this learning generalized significantly to different spatial transformations (inverted-mirror reversed text and normal letters spelled backwards). Mirror reading, compared with reading normal text, was associated with extensive activation in occipital, temporal, parietal and frontal regions. Learning to read mirror-reversed (MR) text was associated with increased activation in left inferior temporal, striatal, left inferior prefrontal and right cerebellar regions and with decreased activity in the left hippocampus and left cerebellum. Short-term repetition priming was associated with reduced activity in many of the regions active during mirror reading and extensive item-specific practice (long-term repetition priming) resulted in a virtual elimination of activity in those regions. Short- and long-term repetition priming thus appeared to rely upon common neural mechanisms. Nearly all of the regions exhibiting significant learning-related changes also exhibited increased repetition priming effects, suggesting common neural substrates for priming and skill learning in this task. Comparison of MR items with other spatially transformed typographies showed that the learning-related changes were general to all of the spatial transformations. The results confirm the importance of striatofrontal neural networks for the acquisition of skills, and suggest that skill learning and repetition priming may have common substrates within a particular task.

Differential modulation of the 5-HT(4) receptor agonists and antagonist on rat learning and memory

Recent data suggest that activation of 5-HT(4) receptors may modulate cognitive processes such as learning and memory. In the present study, the effects of two potent and selective 5-HT(4) agonists, RS 17017 [1-(4-amino-5-chloro-2-methoxyphenyl)-5- (piperidin-1-yl)-1-pentanone hydrochloride] and RS 67333 [1(4-amino-5-chloro-2-methoxyphenyl)-3- (1-n-butyl-4-piperidinyl)-1-propanone], were studied in an olfactory associative discrimination task. The implication of 5-HT(4) receptors in the associative discriminative task was suggested by the following observation. Injection of a selective 5-HT(4) receptor antagonist RS 67532 [1-(4-amino-5-chloro-2-(3, 5-dimethoxybenzyloxyphenyl)-5-(1-piperidinyl)-1-pentanone; 1 mg/kg: i.p.] before the third training session induced a consistent deficit in associative memory during the following training sessions. This deficit was absent when the antagonist was injected together with either a specific hydrophilic 5-HT(4) (RS 17017, 1 mg/kg) or a specific hydrophobic (RS 67333, 1 mg/kg) 5-HT(4) receptor agonist. RS 67333 was more potent than RS 17017. This difference in potency certainly reflects a difference in their capacity to enter into the brain. This is also likely to be the reason why, injected alone, the hydrophobic 5-HT(4) agonist (RS 67333) but not the hydrophilic 5-HT(4) agonist (RS 17017) improved learning and memory performance.

Dissociation of place and cue learning by telencephalic ablation in goldfish

This study examined the spatial strategies used by goldfish (Carassius auratus) to find a goal in a 4-arm maze and the involvement of the telencephalon in this spatial learning. Intact and telencephalon-ablated goldfish were trained to find food in an arm placed in a constant room location and signaled by a local visual cue (mixed place-cue procedure). Both groups learned the task, but they used different learning strategies. Telencephalon-ablated goldfish learned the task more quickly and made fewer errors to criterion than controls. Probe trials revealed that intact goldfish could use either a place or a cue strategy, whereas telencephalon-ablated goldfish learned only a cue strategy. The results offer additional evidence that place and cue learning in fish are subserved by different neural substrates and that the telencephalon of the teleost fish, or some unspecified structure within it, is important for spatial learning and memory in a manner similar to the hippocampus of mammals and birds.

Neurotoxic hippocampal lesions have no effect on odor span and little effect on odor recognition memory but produce significant impairments on spatial span, recognition, and alternation

Recent work has shown that lesions of the hippocampus in monkeys cause deficits in the capacity to remember increasing numbers of objects, colors, and spatial locations (). However, others have observed that hippocampectomized monkeys can show intact memory for a list of objects or locations (). We wished to explore the effects of hippocampal damage on the capacity of memory in the rodent and, to do so, developed novel "span" tasks in which a variable number of odors or locations had to be remembered. In the odor span task (experiment 1), rats were trained on a nonmatching to sample task in which increasing numbers of odors had to be remembered. Half of the trained rats received ibotenic acid lesions of the hippocampus. Postoperatively, hippocampectomized animals did not differ from control animals even when required to remember up to 24 odors. However, when tested on delayed retention of a list of 12 odors, rats with hippocampal lesions were impaired at a long delay. Also, these rats were impaired on a subsequent test of delayed spatial alternation. In a spatial span task (experiment 2), naive rats were trained on a nonmatching to sample task in which a variable number of locations had to be remembered. After this, half of the animals received ibotenic acid lesions. Postoperatively, hippocampectomized animals performed above chance levels when required to remember a single cup location, but were unable to remember more. Subsequent testing on another spatial delayed alternation task suggested that hippocampectomized rats could recognize, but could not inhibit their approach to previously visited locations.

The effects of combined lesions of the subicular complex and the entorhinal cortex on two forms of spatial navigation in the water maze

The role of the subicular complex and entorhinal cortex (SUB-EC) in spatial learning was examined in 2 water maze experiments. In Experiment 1, rats had to locate a hidden platform that was always a fixed distance and direction from an intramaze landmark. Each day, the landmark and platform were moved to a new location. Both control and SUB-EC-lesioned rats learned to locate the platform equally readily during training. However, the control group was impaired in locating the platform when the visual extramaze cues were concealed, whereas the lesioned group was unaffected by this manipulation. In Experiment 2, the lesioned rats were impaired in finding a hidden platform that was in a fixed place in the water maze and showed no evidence of having learned its location in a probe test. These results suggest that damage to the SUB-EC impairs the integration of geometric information but spares a more general navigational-directional strategy.

Odor regulates the expression of the mitogen-activated protein kinase phosphatase gene hVH-5 in bilateral entorhinal cortex-lesioned rats

Since it is known that several immediate early genes are induced by olfactory stimuli, we determined whether an olfactory stimulus also induces the expression of the mitogen-activated protein kinase (MAPK) phosphatase gene hVH-5 (homologue of vaccinia virus H1 phosphatase gene, clone 5), a member of a novel class of immediate early genes encoding dual-specificity protein phosphatases. The expression was studied by in situ hybridization in different brain structures involved in odor processing, in control and bilateral entorhinal cortex (EC) lesioned rats. EC-lesion did not significantly affect hVH-5 gene expression in the glomerular cell layer of the olfactory bulb (OB), while odor stimulation induced it in both control and EC-lesioned groups. In contrast, odor-induced expression of hVH-5 gene in mitral/granular cell layers was only evident after lesion of the EC. Similar results were obtained in the piriform cortex (PCx), a structure intimately connected to the mitral cell layer. In the CA1 hippocampal subfield, odor stimulation induced hVH-5 gene expression in both control and EC-lesioned animals, the increase being potentiated in lesioned rats. CA3 and dentate gyrus exhibited a similar pattern of gene expression, the odor stimulating gene expression in both control and lesioned groups. The amygdala (Am) displayed no significant change. It appears that through the induction of a MAPK phosphatase, the EC controls MAPK activities differently after odor stimulation in OB, PCx and hippocampus (Hip). The results illustrate the notion that odor representation in the brain requires plastic modifications at both anatomical and functional levels.

Reference memory, anxiety and estrous cyclicity in C57BL/6NIA mice are affected by age and sex

Age-related changes in learning and memory are common in rodents. However, direct comparisons of the effects of aging on learning and memory in both males and females are lacking. The present study examined whether memory deteriorates with increasing age in C57BL/6NIA mice, and whether age-related changes in learning and memory are similar in both sexes. Male and female mice (five, 17 and 25 months of age) were tested in a battery of behavioral tasks including the Morris water maze (spatial and non-spatial reference memory), simple odor discrimination (olfactory reference memory), plus maze (anxiety/exploration), locomotor activity, and basic reflexes. Five-month-old mice learned the water maze and odor discrimination tasks rapidly. Relative to five-month-old mice, 25-month-old mice exhibited impaired spatial and olfactory reference memory, but intact non-spatial reference memory. The spatial reference memory of 17-month-old mice was also impaired, but less so than 25-month mice. Seventeen-month-old mice exhibited intact non-spatial (visual and olfactory) reference memory. Five and 25-month-old mice had similar levels of plus maze exploration and locomotor activity, whereas 17-month-old mice were more active than both groups and were slightly less exploratory than five-month-old mice. Although sex differences were not observed in the five- and 25-month groups, 17-month-old females exhibited more impaired spatial reference memory and increased anxiety relative to 17-month-old males. Estrous cycling in females deteriorated significantly with increased age; all 25-month-old females had ceased cycling and 80% of 17-month-old females displayed either irregular or absent estrous cycling. This study is the first to directly compare age-related mnemonic decline in male and female mice. The results suggest that: (i) aged mice exhibit significant deficits in spatial and olfactory reference memory relative to young mice, whereas middle-aged mice exhibit only a moderate spatial memory deficit and; (ii) spatial reference memory decline begins at an earlier age in females than in males, a finding that may be related to the cessation of estrous cycling.

Do hippocampal pyramidal cells signal non-spatial as well as spatial information?

It is generally agreed that the rat hippocampus is involved in spatial memory. Whether this is its sole or primary function, or merely one component of a broader function, is still debated. It has been suggested, for example, that the hippocampus stores information about flexible relations between stimuli, both spatial and non-spatial. In this paper, I reiterate the basic tenet of the cognitive map theory that the processing and storage of spatial information is the primary and perhaps the exclusive role of the hippocampus in the rat, and that data that appear to contradict this have been misinterpreted. These data are found in reports of non-spatial correlates of unit activity recorded in the awake animals and reports of deficits on non-spatial tasks following hippocampal lesions. In this paper, I examine both claims and suggest alternative explanations of the data. The first part of the paper contains a review of some of the properties of hippocampal place cells, which might be misinterpreted as non-spatial in "non-spatial" tasks. For example, if an animal is trained to carry out a sequence of stereotyped actions in different parts of an environment, there will be a strong correlation between the performance of each behaviour and the animal's location, and it is necessary to rule out the locational correlate as the cause of the firing pattern. The second part of the paper looks at the results of experiments on conditioning and non-spatial discrimination tasks and concludes that the results are less supportive of a more general relational theory of hippocampal function than has been suggested. Furthermore, there is often a discrepancy between the correlates of unit firing in non-spatial tasks and the absence of an effect of hippocampal damage on these same or similar tasks. It is concluded that, contrary to the claims of its detractors, the cognitive map theory is still the theory of hippocampal function that is most clearly specified, makes the most testable predictions, and for which there is the strongest experimental support.

Knowing which and knowing what: a potential mouse model for age-related human declarative memory decline

The present study was built on the original report of Eichenbaum et al. [Eichenbaum, H., Fagan, A., Mathews, P. & Cohen, N.J. (1988), Behav. Neurosci., 102, 3531-3542] on the contrasting effects of fornix lesion in different versions of an odour-guided discrimination task in rats, and attempted to extend this into a mouse model for the preferential loss of declarative memory seen in human senescence. Each of the two experiments reported here consisted of a two-stage paradigm, with an initial learning phase followed by a test phase. The information acquired in the first stage was identical in both experiments, i.e. the valence or reward contingency associated with six (three positive and three negative) arms of a radial maze. The only parameter which was varied between Experiment A and B, and also between the two successive stages within each experiment, was the way of presenting the arms to the mice, i.e. either in pairs (simultaneous discriminations) or one at a time (successive go : no-go discrimination). Performance in the first stage demonstrated that our aged mice were impaired in learning concurrent simultaneous discriminations but not successive go/no-go discrimination, thereby resembling that reported in rats with hippocampal damage. Most importantly, our present set of data supports the conclusion that two forms of memory expression for the same piece of acquired experience can be assessed in the same subjects by manipulating the way of presenting two arms that were previously experienced separately. These two forms of memory expressions are differentially affected in aged mice, thereby demonstrating the highly selective and specific deleterious effect of ageing.

Effects of vomeronasal organ removal on individual odor discrimination, sex-odor preference, and scent marking by female hamsters

Removal of the vomeronasal organ (VNX) did not eliminate the ability of female hamsters to discriminate between individual male's flank gland or urine odors in a habituation/discrimination task nor did it impair preference for male odors over female odors from a distance. Vomeronasal organ removal did reduce overall levels of investigation of flank gland odor in the habituation/discrimination task. Although VNX females did not show severe impairments in the frequency of either flank or vaginal marking in response to odors, they did show an abnormal pattern of marking. VNX females, unlike shams, did not flank mark more to female odors than to male odors, nor did they vaginal mark more to male odors than to female odors. Thus, the vomeronasal organ in female hamsters appears to be important for differences in scent marking toward male and female odors, but is not essential for discrimination of individual odors or for preferences for male over female odors.

Analysing hippocampal function in transgenic mice: an ethological perspective

Advances in molecular genetics and technology have led to the dawn of a new era for neuroscience: manipulation of single genes now makes it possible to dissect the complexities of neurobiological phenotypes and to understand many of the intricacies of brain and behaviour, even in mammals. The phenotypical analysis of these mutant animals is complicated because the potential outcome of gene manipulation is difficult to predict. While behavioural analysis should form an integral part of any multidisciplinary research programme investigating the phenotypical effects of single genes on hippocampal function, it is crucial that the behavioural tests are designed and conducted appropriately. Approaches that take species-specific behavioural characteristics into account and use ethological methods could be the most useful for interpreting these behavioural findings and understanding the biological mechanisms of brain function.

Fimbria-fornix cut affects spontaneous activity, two-way avoidance and delayed non matching to sample, but not latent inhibition

Latent inhibition (LI) consists of a decrement in conditioning to a stimulus as a result of its prior nonreinforced preexposure. Based on evidence pointing to the involvement of the hippocampus and the nucleus accumbens (NAC) in LI disruption, it has been proposed that LI depends on the integrity of the subicular input to the NAC. Since fibers originating in the subiculum and destined for the NAC run through the fimbria-fornix, we assessed the effects of fimbria-fornix lesion, made using a knife cut, on LI. In addition, we assessed the effects of the fimbria-fornix cut in three tests known to be sensitive to lesions to the hippocampal region, namely, spontaneous activity, two-way active avoidance and delayed-non-matching-to-sample. In accord with previously documented effects of lesions to the hippocampus and related structures, the fimbria-fornix cut increased spontaneous activity (Experiment 1), facilitated the acquisition of two-way active avoidance (Experiment 3), and produced a delay-dependent deficit in the delayed-non-match-to-sample task (Experiment 4), demonstrating that it disrupted hippocampal functioning. In contrast, LI remained unaffected by the fimbria-fornix cut (Experiment 2), indicating that disruption of subicular input to the NAC is not responsible for the attenuation of LI following non-selective hippocampal lesions. The implications of these results for the neural circuitry of LI are discussed.

Selective spatial memory impairment after right unilateral temporal lobectomy

Deficits in performance of both spatial and visual tasks are common following tissue loss in the right temporal lobe. Since spatial and visual attributes are frequently confounded in experimental tasks, we have studied patients following unilateral temporal lobectomy, in an attempt to determine which aspect mediates the observed deficits. Spatial and visual memory performance was compared in normal controls (n = 16), left temporal (LTL; n = 19) and right temporal (RTL; n = 19) lobectomy patients, by presentation of eight abstract designs in a spatial array for subsequent recall and recognition of the designs (visual memory) and recall of their spatial position (spatial memory). By varying the retention intervals for each group, all three groups were matched on both recall and recognition of the designs at sub-ceiling levels. In contrast, recall of the position of the designs (spatial memory), tested at equivalent delays to those of the visual memory tests, revealed a deficit in the RTL patients compared to both controls and LTL patients (p < 0.05). Magnetic resonance imaging (MRI) was used to quantify the extent of resection of the hippocampus and parahippocampal regions in the two patient groups and showed a significant correlation between hippocampal and parahippocampal removal and spatial memory in the RTL group only. These data support the notion of a disproportionately large involvement of the right hippocampus and adjacent regions in spatial memory.

Hippocampal lesions enhance configural learning by reducing proactive interference

Rats were trained on an operant conditional discrimination in which an ambiguous stimulus (X) indicated both the occasions on which responding in the presence of a second cue (A) would be reinforced and the occasions on which responding in the presence of a third cue (B) would not be reinforced (X --> A+, A-, X --> B-, B+). Both rats with neurotoxic lesions of the hippocampus and control rats learned this discrimination more rapidly when the training trials were widely spaced (mean intertrial interval of 8 min) than when they were massed (mean intertrial interval of 1 min). With spaced practice, lesioned and control rats learned this discrimination equally well. But when the training trials were massed, lesioned rats learned more rapidly than controls. At the end of training, performance of all rats was tested at three different intertrial intervals, 0.5 min, 1 min, and 8 min. The control rats trained with 8-min intertrial intervals showed deficits in discrimination performance when the test intertrial interval was 0.5 min or 1 min. An analysis of sequential effects indicated that a major source of this performance deficit was the control rats' failure to withhold responding on nonreinforced trials when those trials were immediately preceded by reinforced trials within 0.5 min or 1 min. In contrast, performance of lesioned rats was not affected by either the test intertrial interval or by the nature of preceding trials. Thus, with short intertrial intervals, hippocampal lesions may have improved learning or performance on a given trial by reducing proactive interference from the previous trial.

Cues that hippocampal place cells encode: dynamic and hierarchical representation of local and distal stimuli

Hippocampal place fields were recorded as rats explored a four-arm radial maze surrounded by curtains holding distal stimuli and with distinct local tactile, olfactory, and visual cues covering each arm. Systematic manipulations of the individual cues and their interrelationships showed that different hippocampal neurons encoded individual local and distal cues, relationships among cues within a stimulus set, and the relationship between the local and distal cues. Double rotation trials, which maintained stimulus relationships within distal and local cue sets, but altered the relationship between them, often changed the responses of the sampled neural population and produced new representations. After repeated double rotation trials, the incidence of new representations increased, and the likelihood of a simple rotation with one of the cue sets diminished. Cue scrambling trials, which altered the topological relationship within the local or distal stimulus set, showed that the cells that followed one set of controlled stimuli responded as often to a single cue as to the constellation. These cells followed the single cue when the stimulus constellation was scrambled, but often continued firing in the same place when the stimulus was removed or switched to respond to other cues. When the maze was surrounded by a new stimulus configuration, all of the cells either developed new place fields or stopped firing, showing that the controlled stimuli had persistent and profound influence over hippocampal neurons. Together, the results show that hippocampal neurons encode a hierarchical representation of environmental information.

Hippocampal lesions alter conditioning to conditional and contextual stimuli

The control of conditioned fear behaviour by a conditional stimulus (CS) and contextual stimuli (CXT) was compared in rats with lesions to the hippocampus (HPC) or neocortex (CO), and operated controls (OC). After classical fear conditioning in a distinctive context, rats were subsequently tested in the presence of the CS and CXT (CS + CXT), the CS alone (CS-only), or context alone (CXT-only). Two experiments were conducted in which conditioned fear was measured by an active avoidance response (experiment 1) or by response suppression (experiment 2). Groups did not differ in acquiring the conditioned fear response, as measured in the CS + CON test but, in both experiments, hippocampal (HPC) groups exhibited more conditioned fear behaviour than controls in the CXT-Only and CS-Only conditions. It was suggested that control rats conditioned the fear response to a stimulus complex that incorporated the CS and CTX. Rats with HPC lesions did not form this association between the stimulus elements; instead they segregated the CS and CXT and formed independent associations between the conditioned response (CR) and each component. In showing that HPC damage disrupts the process of forming associations between environmental stimuli and that the effect is not restricted to contextual cues, the results help to resolve apparently contradictory findings regarding the role of HPC in contextual information processing.

The flexible use of multiple cue relationships in spatial navigation: a comparison of water maze performance following hippocampal, medial septal, prefrontal cortex, or posterior parietal cortex lesions

Rats prepared with lesions of the prefrontal cortex, posterior parietal cortex, hippocampus, or medial septal area were tested for acquisition of a number of variations of the open-field water maze using a version of place learning assessment described by Eichenbaum, Stewart, and Morris (1991). Specifically, the individual role of the aforementioned cortical and subcortical structures in tasks with differing representational demands on navigation were assessed. The results suggest that the sham-operated control, posterior parietal cortex-lesioned rats, and medial septal area-lesioned rats were able to navigate effectively under changing task conditions. Conversely, the navigational performances of the prefrontal cortex- and hippocampal formation-lesioned rats were impaired when task demands changed. The results are discussed in terms of the flexible use of multiple distal cues to guide navigation and the resulting loss of this flexibility after lesions to either the prefrontal cortex or the hippocampus.

Neonatal gamma-ray irradiation impairs learning and memory of an olfactory associative task in adult rats

Adult neonatally gamma-irradiated rats were compared with control animals in a non-spatial olfactory associative task using two different procedures. Irradiation induced a clear reduction in the total mean area of the olfactory bulbs and hippocampus but not of the orbital prefrontal cortex, diagonal band and cell layers of the entorhinal and piriform cortex. The gamma-irradiation affected the granule cells of the olfactory bulbs and differentially altered the cell layers of the subfields of the ammonic fields and the dorsal and ventral blades of the dentate gyrus. In the CA1 ammonic field, dorsal and ventral blades of the dentate gyrus, the cellular loss was significant in comparison with control adult rats. The behavioural data indicated that irradiated rats were deeply disturbed in learning the odour-reward association, and substantially impaired in a reversal experiment, but not in the discrimination of the odours per se. The cellular loss in the olfactory bulbs, in the CA1 and in the ventral blade of the gyrus dentatus was positively correlated with the deficit in behavioural performance. The data support the findings that the hippocampal system participates in the odour-reward associations and facilitates the long-term storage of associations after learning is achieved in this olfactory associative task.

Declarative memory: insights from cognitive neurobiology

The discovery of declarative memory as distinct from other forms of memory is a major recent achievement in cognitive science. Basic issues about the nature of declarative memory are considered in this review from the perspective of studies on its underlying brain mechanisms. These studies have shown that declarative memory is mediated by a specific brain system including areas of the cerebral cortex and hippocampal region that make distinct functional contributions to memory processing. These processing mechanisms mediate the organization of memories in ways that can support the special properties of declarative or explicit memory expression. Furthermore, the basic properties of declarative memory in human beings can be viewed as evolving from a capacity for organized memory representation and flexible memory expression in animals.

Extending models of hippocampal function in animal conditioning to human amnesia

Although most analyses of amnesia have focused on the loss of explicit declarative and episodic memories following hippocampal-region damage, considerable insights into amnesia can also be realised by studying hippocampal function in simple procedural, or habit-based, associative learning tasks. Although many simple forms of associative learning are unimpaired by hippocampal damage, more complex tasks which require sensitivity to unreinforced stimuli, configurations of multiple stimuli, or contextual information are impaired by hippocampal damage. In several recent papers we have developed a computational theory of hippocampal function which argues that this brain region plays a critical role in the formation of new stimulus representations during learning (Gluck & Myers, 1993, 1995; Myers & Gluck, 1996; Myers, Gluck, & Granger, 1995). We have applied this theory to a broad range of empirical data from studies of classical conditioning in both intact and hippocampal-lesioned animals, and the model correctly accounts for these data. The classical conditioning paradigm can be adapted for use in humans, and similar results for acquisition are obtained in both normal and hippocampal-damaged humans. More recently, we have begun to address an important set of category learning studies in both normals and hippocampal-damaged amnesics. This work integrates experimental studies of amnesic category learning (Knowlton, Squire, & Gluck, 1994) with theoretical accounts of associative learning, and builds on previously established behavioural correspondences between animal conditioning and human category learning (Gluck & Bower, 1988a). Our work to date illustrates some initial progress towards a more integrative understanding of hippocampal function in both animal and human learning, which may be useful in guiding further empirical and theoretical research in human memory and amnesia.

Spatial memory deficits in patients with unilateral damage to the right hippocampal formation

Patients with unilateral temporal lobe damage resulting from intractable temporal lobe epilepsy (TLE, n = 30) or from temporal lobe resection (temporal lobectomy, TLR, n = 47) were investigated on the Nine-box Maze. The task, analogous to the radial arm maze, was designed to compare spatial mapping and working memory theories of the functions of the hippocampus. The task provides measures of spatial, object, working and reference memory, incorporated into a within subjects design. The spatial component was designed to encourage the formation of allocentric rather than egocentric spatial representations. Spatial memory deficits were found (across working and reference memory components) in both TLE and TLR patients with right temporal lobe damage, with intact spatial memory in patients with corresponding left temporal lobe damage. Performance on the matched non-spatial (object) working memory component was equal to healthy controls for all groups. However all patient groups showed a deficit on object reference memory. These findings are discussed in relation to the underlying temporal lobe pathology and particularly atrophy of the hippocampal formation. Overall, the results support the cognitive mapping theory of hippocampal function, with the demonstration of a selective (and probably allocentric) spatial memory deficit in patients with right hippocampal damage.

Systematic comparison of the effects of hippocampal and fornix-fimbria lesions on acquisition of three configural discriminations

The effects of lesions to the hippocampal system on acquisition of three different configural tasks by rats were tested. Lesions of either the hippocampus (kainic acid/colchicine) or fornix-fimbria (radiofrequency current) were made before training. After recovery from surgery, rats were trained to discriminate between simple and compound-configural cues that signaled the availability or nonavailability of food when a bar was pressed. When positive cues were present, one food pellet could be earned by pressing a lever after a variable time had elapsed. The trial terminated on food delivery (variable interval 15 s). This procedure eliminates some possible alternative explanations of the results of previous experiments on configural learning. Hippocampal lesions increased rates of responding and retarded acquisition of a negative patterning task (A+, B+, AB-); using a ratio measure of discrimination performance these lesions had a milder retarding effect on a biconditional discrimination (AX+, AY-, BY+, BX-), and they had no effect on a conditional context discrimination (X: A+, B-; Y: A-, B+). Fornix-fimbria lesions did not affect acquisition of any of these tasks but increased rates of responding. The results suggest that several task parameters determine the involvement of the hippocampus in configural learning; however, all tasks tested can also be learned to some extent in the absence of an intact hippocampal system, presumably by other learning/memory systems that remain intact following surgery. The lack of effect of fornix-fimbria lesions on any of these tasks suggests that retrohippocampal connections with other brain areas may mediate hippocampal contributions to the learning of some configural tasks. An analysis of these results and of experiments on spatial learning situations suggests that involvement of the hippocampus is a function of the degree to which correct performance depends on a knowledge of relationships among cues in a situation.

Smells are no surer: rapid improvement in olfactory discrimination is not due to the acquisition of a learning set

The claim that rats can demonstrate the 'primate-like' learning capacity of learning set formation when trained with olfactory cues, rather than visual or auditory cues, has generated considerable interest in recent years. In this study, the claim is evaluated in detail by using a series of experimental and control procedures to determine whether rats do indeed develop the abstract 'win-stay, lose-shift' strategy which underlies learning set formation in monkeys. We report here that although exposure to a series of novel olfactory discrimination problems gives rise to progressive improvement in the rate of learning, it is not a necessary condition for the development of that rapid learning. Furthermore, even rats which fail to display progressive improvement in olfactory reversal learning show rapid learning on novel olfactory discrimination problems. Each of these findings suggests that although olfactory learning may be rapid, learning set formation does not occur over a short series of problems.

Memory for items and memory for relations in the procedural/declarative memory framework

A major area of research in memory and amnesia concerns the item specificity of implicit memory. In this paper we address several issues about the nature of implicit memory phenomena and about what constitutes an "item", using the procedural/declarative memory theory to guide us. We consider the nature of memory for items and of memory for relations among items, within the context of the procedural/declarative framework, providing us with the foundation necessary to analyse the basis for item-specific implicit memory phenomena. We review recent work from our laboratories demonstrating the fundamentally relational and flexible nature of declarative memory representation, in both humans and animals, and the essential role of the hippocampal system in relational memory processing. We show, further, that the memory representations supporting implicit memory phenomena are inflexible and nonrelational, and are tied to specific processing modules. Finally, we introduce empirical approaches that blur the distinction between skill learning and repetition priming, and show computational modelling results that demonstrate how these two implicit memory phenomena can be mediated by a single incremental learning mechanism, in accord with the claims of the procedural-declarative theory. Taken together, these various analyses of memory for items and memory for relations help to illuminate the nature of the functional deficit in amnesia and the memory systems of the brain.

New approaches to the study of amnesic patients: what can a neurofunctional philosophy and neural network methods offer?

In this paper I first consider a neurofunctional approach to the study of amnesic patients. This approach stresses the need for theorising about the processing operations of brain regions and circuits rather than for theorising about neuropsychological syndromes. A syndrome such as amnesia-may not exist, in any meaningful sense, if there is marked heterogeneity within the patients grouped together in this way. Powerful neuroimaging techniques may now allow a more useful basis for grouping patients in terms of lesion location rather than aetiology. In turn this will allow an evaluation of the information processing functions subserved by the lesioned structures. The second strand to the present paper stresses the weakness in the specification of current theories. This has made it difficult to select experimental tasks that decisively measure the key components of those theories. The paper makes the case that explicit neural network models are a useful way to try to overcome this problem. In line with these ideas, the paper begins to build a model of how the brain may achieve useful kinds of stimulus representations. Considerations of human behaviour in category learning tasks have emphasised parallel and interacting roles for both exemplar- and element-based stimulus representations. It is suggested that the hippocampus itself may encode exemplar representations, and these may provide a basis for episodic memory as well as some types of category learning. It is further suggested that the ventral striatum may encode the element-based representations. The model allows some new and detailed predictions for the performance of amnesic subjects related to lesion location.

Neonatal hippocampal damage in rats: long-term spatial memory deficits and associations with magnitude of hippocampal damage

This study investigated the effects of neonatal hippocampal ablation on the development of spatial learning and memory abilities in rats. Newborn rats sustained bilateral electrolytic lesions of the hippocampus or were sham-operated on postnatal day 1 (PN1). At PN20-25, PN50-55, or PN90-95, separate groups of rats were tested in a Morris water maze on a visible "cue" condition (visible platform in a fixed location of the maze), a spatial "place" condition (submerged platform in a fixed location), or a no-contingency "random" condition (submerged platform in a random location). Rats were tested for 6 consecutive days, with 12 acquisition trials and 1 retention (probe) trial per day. During acquisition trials, the rat's latency to escape the maze was recorded. During retention trials (last trial for each day, no escape platform available), the total time the rat spent in the probe quadrant was recorded. Data from rats with hippocampal lesions tested as infants (PN20-25) or as adults (PN50-55 and PN90-95) converged across measures to reveal that 1) spatial (place) memory deficits were evident throughout developmental testing, suggesting that the deficits in spatial memory were long-lasting, if not permanent, and 2) behavioral performance measures under the spatial (place) condition were significantly correlated with total volume of hippocampal tissue damage, and with volume of damage to the right and anterior hippocampal regions. These results support the hypothesis that hippocampal integrity is important for the normal development of spatial learning and memory functions, and show that other brain structures do not assume hippocampal-spatial memory functions when the hippocampus is damaged during the neonatal period (even when testing is not begun until adulthood). Thus, neonatal hippocampal damage in rats may serve as a rodent model for assessing treatment strategies (e.g., pharmacological) relevant to human perinatal brain injury and developmental disabilities within the learning and memory realm.

Opposite effects depending on learning and memory demands in dorsomedial prefrontal cortex lesioned rats performing an olfactory task

In this study, the functional properties of the dorsomedial prefrontal cortex (dmPFC) of the rat were examined in two olfactory tasks. In a successive cue olfactory discrimination task, dmPFC lesioned animals improved performance across sessions more rapidly than operated control animals. In an olfactory task using fixed interval training, animals with similar lesions were impaired. Both effects, although opposite, can be explained by a temporal processing deficit. The present results seem to indicate that the dmPFC is required for timing, classified as part of non-declarative memory. As reference memory improved in the lesioned animals, the finding is that the dmPFC supports non-declarative memory and thus interacts with declarative memory in the long-term formation of the associations between a particular stimulus (olfactory cue) and particular responses.

Psychobiological models of hippocampal function in learning and memory

We review current computational models of hippocampal function in learning and memory, concentrating on those that make strongest contact with psychological issues and behavioral data. Some models build upon Marr's early theories for modeling hippocampal field CA3's putative role in the fast, temporary storage of episodic memories. Other models focus on hippocampal involvement in incrementally learned associations, such as classical conditioning. More recent efforts have attempted to bring functional interpretations of the hippocampal region in closer contact with underlying anatomy and physiology. In reviewing these psychobiological models, three major themes emerge. First, computational models provide the conceptual glue to bind together data from multiple levels of analysis. Second, models serve as important tools to integrate data from both animal and human studies. Third, previous psychological models that capture important behavioral principles of memory provide an important top-down constraint for developing computational models of the neural bases of these behaviors.

Impaired hippocampal representation of space in CA1-specific NMDAR1 knockout mice

To investigate the role of synaptic plasticity in the place-specific firing of the hippocampus, we have applied multiple electrode recording techniques to freely behaving mice with a CA1 pyramidal cell-specific knockout of the NMDAR1 gene. We have discovered that although the CA1 pyramidal cells of these mice retain place-related activity, there is a significant decrease in the spatial specificity of individual place fields. We have also found a striking deficit in the coordinated firing of pairs of neurons tuned to similar spatial locations. Pairs have uncorrelated firing even if their fields overlap. These results demonstrate that NMDA receptor-mediated synaptic plasticity is necessary for the proper representation of space in the CA1 region of the hippocampus.

Late-training amnesic deficits in probabilistic category learning: a neurocomputational analysis

Building upon earlier behavioral models of animal and human learning, we explore how a psychobiological model of animal conditioning can be applied to amnesic category learning. In particular, we show that the late-training deficit found in Knowlton, Squire, and Gluck's 1994 study of amnesic category learning can be understood as a natural consequence of Gluck and Myers's (1993) theory of hippocampal-region function, a theory that has heretofore been applied only to studies of animal learning. When applied to Knowlton et al.'s category learning task, Gluck and Myers's model assumes that the hippocampal region induces new stimulus representations over multiple training trials that reflect stimulus-stimulus regularities in the training set. As such, the model expects an advantage for control subjects over hippocampal-damaged amnesic patients only later in training when control subjects have developed new hippocampal-dependent stimulus representations; in contrast, both groups are expected to show equivalent performance early in training. A potentially analogous early/late distinction is described for animal studies of stimulus generalization. Our analyses suggest that careful comparisons between early and late-training differences in learning may be an important factor in understanding amnesia and the neural bases of both animal and human learning.

Effects of pharmacologically facilitating glutamatergic transmission in the trisynaptic intrahippocampal circuit

The effects of a recently synthesized benzoyl-piperidine drug that enhances currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors were tested on monosynaptic and polysynaptic responses in hippocampal slices of the rat. Stimulation of perforant path inputs to the dentate gyrus evoked extracellular responses in field CA1 that had latencies and laminar profiles indicating that they were relayed through the trisynaptic intrahippocampal circuit. Under control conditions, trisynaptic field excitatory postsynaptic potentials did not show larger paired-pulse facilitation than monosynaptic responses and failed to exhibit frequency facilitation. Low concentrations of picrotoxin greatly enhanced trisynaptic responses and, under these conditions, frequency facilitation was obtained. Benzoyl-piperidine-12 (250 microM) had a three-fold greater effect on the amplitude of trisynaptic responses than on monosynaptic field excitatory postsynaptic potentials, indicating that the drug's effect is amplified across the successive stages of a polysynaptic circuit. The AMPA receptor modulator did not change the frequency characteristics of monosynaptic potentials and had only a modest influence on those of the trisynaptic response. The effect of benzoyl-piperidine-12 on trisynaptic responses was significantly greater when GABAergic inhibition was partially blocked with picrotoxin; the GABA blocker did not alter the effects of benzoyl-piperidine-12 on monosynaptic responses. These results indicate that centrally active AMPA receptor modulators are likely to have a greater influence on brain operations involving long chains of connections than on those mediated by simple reflex-like circuits, and will vary markedly in their effects depending upon the excitability of local interneurons.

Serial pattern learning in senescent rats: a comparison of acquisition performance with young and hippocampal-lesioned rats

The acquisition of a serial pattern was assessed in young (3-month), senescent (24-month), and hippocampal-lesioned rats. All animals were trained for 25 days on a four-element, nonmonotonic pattern consisting of 14, 0, 3, and 7 food pellets, respectively. Young rats were capable of distinguishing among the elements of the series, with the exception of the 3- and 7-pellet elements, as indexed by running times. Conversely, the senescent rats were largely incapable of patterned responding, and the tracking performance of the hippocampal-lesioned rats was intermediate between those of the young and old rats. Results are discussed in terms of the possible changes that accompany aging in rodents, including age-related changes in the hippocampal system.

Integrating behavioral and physiological models of hippocampal function

In recent modeling of hippocampal function, we have attempted to integrate formal behavioral analyses of classical conditioning with psychobiological data on brain lesions (Gluck and Myers [1993] Hippocampus 3:491-516; Myers and Gluck [1994] Behav Neurosci 108(5):835-847). Based on comparative behavioral analyses, we have argued that animals with hippocampal region damage are unable to alter stimulus similarity based on experience. While hippocampal-damaged animals can still learn whether to respond to an individual stimulus, they are notably impaired at many tasks involving learning relationships between stimuli-especially in the absence of explicit reinforcement. These analyses lead to a computational theory which identifies two representational recoding processes-predictive differentiation and redundancy compression-which alter stimulus similarity relationships in intact animals but are dependent on intact hippocampal region processing. More recent, and ongoing, modeling aims to broaden this model of hippocampal region function in classical conditioning, with an emphasis on physiological and anatomical constraints, including the role of the fornix and subcortical modulation, preprocessing in sensory cortices, and localization of the proposed representational functions within more precisely identified hippocampal region substrates (Myers et al. [1995] Psychology 23(2):116-138; Myers and Gluck [1996] Behav Neurosci; Myers et al. [1996] Neurobiol Learning Memory). Working to bridge between behavioral and physiological levels of analysis, we ultimately hope to develop a more complete understanding of hippocampal region function in memory across a wider range of behavioral paradigms, elucidating how this functionality emerges from underlying physiological and anatomical substrates.

Spatial recognition memory deficits without notable CNS pathology in rats following herpes simplex encephalitis

Survivors of herpes simplex encephalitis (HSE) experience intellectual impairment and an inability to store and recall information. Because the temporal lobes and associated limbic structures are central to storage and retrieval of memories, and are predominantly affected in adult HSE, injury to these areas is postulated to cause behavioral and learning disabilities. A previous study (Beers et al., 1993) demonstrated that intranasal inoculation of Lewis rats with herpes simplex virus type-1 (HSV-1) induced acute partial complex seizures, and hemorrhagic and inflammatory lesions of the hippocampus and entorhinal cortex. Consequently, it was of interest to determine whether rats that had recovered from HSE had limbic system-associated memory impairments. Therefore, rats were evaluated when signs and symptoms of encephalitis were no longer apparent using an eight arm radial maze to assess the acquisition and retention of learned information. An allocentric-spatial location paradigm revealed HSV-1 infected rats performed at chance levels on both acquisition and retention which were statistically different from sham-inoculated controls. However, using an egocentric-spatial left/right discrimination task, infected rats performed statistically similar to sham-inoculated controls. Furthermore, HSV-1 nucleic acids were detected in the nuclei of neurons within the hippocampus and entorhinal cortex using in situ hybridization techniques. Of interest was the observation that rats with learning and memory deficits had no apparent histopathological or immunocytochemical evidence of antecedent CNS infection. This is the first experimental demonstration that HSV-1 can cause behavioral impairments in the absence of obvious inflammatory injury to the temporal lobe memory system.

Cognitive deficits induced by global cerebral ischaemia: relationship to brain damage and reversal by transplants

The CA1 and hilar fields of the hippocampus are highly vulnerable to lack of oxygen after interruption of blood flow to the brain. Severe anterograde memory loss, seen in a significant proportion of heart attack survivors, has been attributed to selective bilateral ischaemic damage to the hippocampus. Animal models of global ischaemia, induced by extracranial occlusion of the major ascending arteries, enable assessment of the neuropathological and functional consequences of transient interruption of cerebral blood flow, and can inform strategies to reduce or alleviate ischaemic brain damage. This review focuses firstly on the nature of cognitive deficits induced by global ischaemia, how far they are consistent with lesion-based accounts of hippocampal function, and the extent to which these deficits can be correlated with CA1 cell loss. The second focus of the review is to examine the limited evidence for graft-induced recovery of cognitive function in animals subjected to global ischaemia. Recent findings that grafted foetal cells from discrete hippocampal fields follow appropriate laminar routes to form functional connections with host neurons, and that growth factors protect cells from ischaemic damage, have suggested that CA1 or trophic grafts placed in the region of ischaemic CA1 cell loss might restore or protect this vulnerable sector, and reduce cognitive deficits.