Unpacking the cognitive map: the parallel map theory of hippocampal function

Simple gaze analysis and special design of a virtual Morris water maze provides a new method for differentiating egocentric and allocentric navigational strategy choice

We present a novel method of combining eye tracking with specially designed virtual environments to provide objective evidence of navigational strategy selection. A simple, inexpensive video camera with an easily built infrared LED array is used to capture eye movements at 60Hz. Simple algorithms analyze gaze position at the start of each virtual maze trial to identify stimuli used for navigational orientation. To validate the methodology, human participants were tested in two virtual environments which differed with respect to features usable for navigation and which forced participants to use one or another of two well-known navigational strategies. Because the environmental features for the two kinds of navigation were clustered in different regions of the environment (and the video display), a simple analysis of gaze-position during the first (i.e., orienting) second of each trial revealed which features were being attended to, and therefore, which navigational strategy was about to be employed on the upcoming trial.

Gender differences in landmark learning for virtual navigation: the role of distance to a goal

We used a new virtual program in two experiments to prepare subjects to perform the Morris water task (www.nesplora.com). The subjects were Psychology students; they were trained to locate a safe platform amidst the presence of four pinpoint landmarks spaced around the edge of the pool (i.e., two landmarks relatively near the platform and two landmarks relatively distant away from it). At the end of the training phase, we administered one test trial without the platform and recorded the amount of time that the students had spent in the platform quadrant. In Experiment 1, we conducted the test trial in the presence of one or two of the distant landmarks. When only one landmark was present during testing, performance fell to chance. However, the men outperformed the women when the two distant landmarks were both present. Experiment 2 replicated the previous results and extended it by showing that no sex differences exist when the searching process is based on the near landmarks. Both the men and the women had similarly good performances when the landmarks were present both individually and together. When present together, an addition effect was found. Far landmark tests favor configural learning processes, whereas near landmark tests favor elemental learning. Our findings suggest that other factors in addition to the use of directional cues can underlie the sex differences in the spatial learning process. Thus, we expand upon previous research in the field.

Accurate memory for object location by individuals with intellectual disability: absolute spatial tagging instead of configural processing?

Using head-mounted eye tracker material, we assessed spatial recognition abilities (e.g., reaction to object permutation, removal or replacement with a new object) in participants with intellectual disabilities. The "Intellectual Disabilities (ID)" group (n = 40) obtained a score totalling a 93.7% success rate, whereas the "Normal Control" group (n = 40) scored 55.6% and took longer to fix their attention on the displaced object. The participants with an intellectual disability thus had a more accurate perception of spatial changes than controls. Interestingly, the ID participants were more reactive to object displacement than to removal of the object. In the specific test of novelty detection, however, the scores were similar, the two groups approaching 100% detection. Analysis of the strategies expressed by the ID group revealed that they engaged in more systematic object checking and were more sensitive than the control group to changes in the structure of the environment. Indeed, during the familiarisation phase, the "ID" group explored the collection of objects more slowly, and fixed their gaze for a longer time upon a significantly lower number of fixation points during visual sweeping.

A behavioral perspective on the biophysics of the light-dependent magnetic compass: a link between directional and spatial perception?

In terrestrial organisms, sensitivity to the Earth's magnetic field is mediated by at least two different magnetoreception mechanisms, one involving biogenic ferromagnetic crystals (magnetite/maghemite) and the second involving a photo-induced biochemical reaction that forms long-lasting, spin-coordinated, radical pair intermediates. In some vertebrate groups (amphibians and birds), both mechanisms are present; a light-dependent mechanism provides a directional sense or 'compass', and a non-light-dependent mechanism underlies a geographical-position sense or 'map'. Evidence that both magnetite- and radical pair-based mechanisms are present in the same organisms raises a number of interesting questions. Why has natural selection produced magnetic sensors utilizing two distinct biophysical mechanisms? And, in particular, why has natural selection produced a compass mechanism based on a light-dependent radical pair mechanism (RPM) when a magnetite-based receptor is well suited to perform this function? Answers to these questions depend, to a large degree, on how the properties of the RPM, viewed from a neuroethological rather than a biophysical perspective, differ from those of a magnetite-based magnetic compass. The RPM is expected to produce a light-dependent, 3-D pattern of response that is axially symmetrical and, in some groups of animals, may be perceived as a pattern of light intensity and/or color superimposed on the visual surroundings. We suggest that the light-dependent magnetic compass may serve not only as a source of directional information but also provide a spherical coordinate system that helps to interface metrics of distance, direction and spatial position.

Neural structures and mechanisms involved in scene recognition: a review and interpretation

Since the discovery in 1996 that a region within caudal parahippocampal cortex subserves learning and recall of topographical information, numerous studies aimed at elucidating the structures and pathways involved in scene recognition have been published. Neuroimaging studies, in particular, have revealed the locations and identities of some of the principal cortical structures that mediate these faculties. In the present study the detailed organization of the system is examined, based on a meta-analysis of neuroimaging studies of scene processing in human subjects, combined with reviews of the results of lesions on this type of processing, single neuron studies, and available hodological data in non-human primates. A cortical hierarchy of structures that mediate scene recognition is established based on these data, and an attempt is made to determine the function of the individual components of the system.

Does spatial learning ability of common voles (Microtus arvalis) and bank voles (Myodes glareolus) constrain foraging efficiency?

Place learning abilities represent adaptations that contribute also to foraging efficiency under given spatio-temporal conditions. We investigated if this ability in turn constrains decision making in two sympatric vole species: while the herbivorous common vole (Microtus arvalis) feeds on spatio-temporally predictable food resources (e.g. roots, tubers and shoots of plant tubers), the omnivorous bank vole (Myodes glareolus) additionally subsists on temporally unpredictable food resources (e.g. insects and seeds). Here, we compare the spatial reference memory and working memory of the two species. In an automated operant home cage with eight water places, female voles either had to learn the fixed position of non-depletable places (reference memory task) or learn and avoid previously visited water places depleted in a single visit (win-shift task). In the reference memory task, Microtus females required significantly more choices to find all water places, initially performed slightly worse than Myodes females, and displayed slightly lower asymptotic performance. Both species were highly similar in new learning of the same task. In the more complex win-shift task, asymptotic performance was significantly lower in Microtus (72% correct) than in Myodes (79%). Our results suggest that both vole species resemble each other in their efficiency to exploit habitats with low spatio-temporal complexity but may differ in their efficiency at exploiting habitats with temporally changing spatial food distributions. The results imply that spatial ability adjusted to specific food distributions may impair flexible use of habitats that differ in their food distribution and therefore, decrease a species' chances of survival in highly dynamic environments.

Everyday memory deficits in very mild Alzheimer's disease

Memory complaints of patients sometimes are not verified via standard cognitive testing. Acquisition of information in everyday life requires memorization in complex three-dimensional environments. The authors mimicked this with a photorealistic virtual environment (VE). Memory for verbal material and spatial scenery was tested in healthy controls (HC) and patients with mild Alzheimer's disease (AD); mini-mental state evaluation (MMSE) 25.7 ± 1.8 (mean ± standard deviation). The number of memorized items increased to 90% in both classical list learning and for items memorized in VE in HC. In contrast, only 40% of items were recalled in list learning and 20% in VE in AD patients. Unlike the gender difference favoring female HC on list learning, performance was alike for both genders in VE. We conclude that verbal learning abilities in healthy elderly subjects are alike in standard settings and under virtual reality conditions. In AD patients memory deficits that are relevant to everyday life yet not detectable with list learning are unmasked in virtual reality. In future, this may aid objective appraisal of interventions with regard to their everyday relevance.

Agonist-induced restoration of hippocampal neurogenesis and cognitive improvement in a model of cholinergic denervation

Loss of basal forebrain cholinergic innervation of the hippocampus and severe neuronal loss within the hippocampal CA1 region are early hallmarks of Alzheimer's disease, and are strongly correlated with cognitive status. Various therapeutic approaches involve attempts to enhance neurotransmission or to provide some level of neuroprotection for remaining cells. An alternative approach may involve the generation of new cells to replace those lost in AD. Indeed, a simple shift in the balance between cell generation and cell loss may slow disease progression and possibly even reverse existing cognitive deficits. One potential neurogenic regulator might be acetylcholine, itself, which has been shown to play a critical role in hippocampal development. Here, we report the effects of various cholinergic compounds on indices of hippocampal neurogenesis, demonstrating a significant induction following pharmacological activation of muscarinic M1 receptors, located on hippocampal progenitors in the adult brain. This is the first report that a small-molecule agonist may induce neurogenesis in the hippocampal CA1 region. Furthermore, such treatment reversed deficits in markers of neurogenesis and spatial working memory triggered by cholinergic denervation in a rodent model. This study suggests the use of small molecule, receptor agonists may represent a novel means to trigger the restoration of specific neuronal populations lost to a variety of neurodegenerative disorders, such as Parkinson's, Alzheimer's, Huntington's and Amyotrophic Lateral Sclerosis.

Effects of cue types on sex differences in human spatial memory

We examined the effects of cue types on human spatial memory in 3D virtual environments adapted from classical animal and human tasks. Two classes of cues of different functions were investigated: those that provide directional information, and those that provide positional information. Adding a directional cue (geographical slant) to the spatial delayed-match-to-sample task improved performance in males but not in females. When the slant directional cue was removed in a hidden-target location task, male performance was impaired but female performance was unaffected. The removal of positional cues, on the other hand, impaired female performance but not male performance. These results are consistent with results from laboratory rodents and thus support the hypothesis that sex differences in spatial memory arise from the dissociation between a preferential reliance on directional cues in males and on positional cues in females.

The dynamic process of cognitive mapping in the absence of visual cues: human data compared with animal studies

The present study aimed to investigate the behavior involved in constructing spatial representation in humans. For this, blindfolded adult human subjects were introduced into an unfamiliar environment, where they were requested to move incessantly for 10 min. Analysis of the locomotor activity of the participants revealed the following exploratory behaviors: (1) ;looping'; (2) ;wall-following'; (3) ;step-counting'; (4) ;cross-cutting'; and (5) ;free traveling'. Looping is a typical exploratory mode of sightless explorers, based on returning to a recently traveled place. Wall-following is common in enclosed spaces, whereby explorers follow the perimeter of the environment. Both looping and wall-following are based on an egocentric frame of reference by which explorers obtain information about the shape, size and landmarks in the environment. Blindfolded explorers displayed step-counting in order to scale the environment and the relationships in it. Altogether, exploration by looping, wall-following and step-counting resulted in an allocentric spatial representation. The acquisition of spatial representation was manifested by cross-cutting and free travel, with subjects walking in a relatively fast and decisive manner. In light of the above modes of activity, we suggest that exploration of an unfamiliar environment is a synergetic self-organized process (synergetic inter-representation networks, SIRN model); an interplay between external and internal representations. According to this model, the interplay gives rise to an order parameter, such as the environment's dimensions or geometry, enabling progression to a subsequent exploratory behavior. This dynamic and sequential interplay reaches a steady state when a spatial representation (i.e. ;cognitive map') is established.

Exploration and navigation in the blind mole rat (Spalax ehrenbergi): global calibration as a primer of spatial representation

The aim of this study was to uncover the process of initial spatial mapping of the environment. For this, blind mole rats (Spalax ehrenbergi), were tested in an unfamiliar square arena, in order to reveal how they construct a spatial representation. The mole rats first displayed a build-up phase, in which they gradually formed a path along the perimeter while travelling slowly, frequently pausing and repeating previously travelled segments of the path. This behaviour was followed by a free-travel phase, in which the mole rats appeared to locomote smoothly along the perimeter and through the centre of the arena while travelling faster with fewer stops or repetitions of path segments. Familiarity with the environment was reflected in local shortcuts at the arena corners and global shortcuts (crosscuts) through the arena centre. We suggest that scanning the perimeter throughout the build-up phase constitute a process of calibration, i.e. forming an initial representation of the size and perhaps the shape of the environment--a sort of basic global map. We further suggest that this calibration is later used for navigation, as indicated by the emergence of global crosscuts in the subsequent phase. Further investigation of the build-up phase, e.g. by manipulating environment size, might provide additional insight into the course of establishment of global environment representation (mapping).

Flexibility of cue use in the fox squirrel (Sciurus niger)

Recent work on captive flying squirrels has demonstrated a novel degree of flexibility in the use of different orientation cues. In the present study, we examine to what extent this flexibility is present in a free-ranging population of another tree squirrel species, the fox squirrel. We trained squirrels to a rewarded location within a square array of four feeders and then tested them on transformations of the array that either pitted two cue types against one cue type, the majority tests, or all cue types against each other, the forced-hierarchy test. In Experiment 1, squirrels reoriented to the two-cue-type location in all majority tests and to the location indicated by the visual features of the feeders in the forced-hierarchy test. This preference for visual features runs contrary to previous studies that report the use of spatial cues over visual features in food-storing species. In Experiments 2-5 we tested squirrels with different trial orders (Experiments 2 and 3), a different apparatus (Experiment 4) and at different times of the year (Experiment 5) to determine why these squirrels had chosen to orient using visual features in the first experiment. Like captive flying squirrels, free-ranging fox squirrels showed a large degree of flexibility in their use of cues. Furthermore, their cue use appeared to be sensitive both to changes in the test apparatus and the season in which we tested. Altogether our results suggest that the study of free-ranging animals over a variety of conditions is necessary for understanding spatial cognition.

Qualitatively different hippocampal subfield engagement emerges with mastery of a spatial memory task by rats

The parallel, entorhinal cortex projections to different hippocampal regions potentially support separate mnemonic functions. To examine this possibility, rats were trained in a radial-arm maze task so that hippocampal activity could be compared after "early" (two sessions) or "late" (five sessions) learning. Induction of the immediate-early gene Zif268 was then measured, so revealing possible activity differences across hippocampal subfields and the parahippocampal cortices. Each rat in the two experimental groups (early, late) was also yoked to a control rat that obtained the same number of rewards, visited the same number of maze arms, and spent a comparable amount of time in the maze. Although overall Zif268 levels did not distinguish the four groups, significant correlations were found between spatial memory performance and levels of dentate gyrus Zif268 expression in the early but not the late training group. Conversely, hippocampal fields CA3 and CA1 Zif268 expression correlated with performance in the late but not the early training group. This reversal in the correlation pattern was echoed by structural equation modeling, which revealed dynamic changes in effective network connectivity. With early training, the dentate gyrus appeared to help determine CA1 activity, but by late training the dentate gyrus reduced its neural influence. Furthermore, CA1 was distinguished from CA3, each subfield developing opposite relations with task mastery. Thus, functional entorhinal cortex coupling with CA1 activity became more direct with additional training, so producing a trisynaptic circuit bypass. The present study reveals qualitatively different patterns of hippocampal subfield engagement dependent on task demands and mastery.

Toward an integrative perspective on hippocampal function: from the rapid encoding of experience to adaptive behavior

The mammalian hippocampus has been associated with learning and memory, as well as with many other behavioral processes. In this article, these different perspectives are brought together, and it is pointed out that integration of diverse functional domains may be a key feature enabling the hippocampus to support not only the encoding and retrieval of certain memory representations, but also their translation into adaptive behavior. The hippocampus appears to combine: (i) sensory afferents and synaptic mechanisms underlying certain types of rapid learning; and (ii) links to motivational, emotional, executive, and sensorimotor functions. Recent experiments are highlighted, indicating that the induction of hippocampal synaptic plasticity is required to encode rapidly aspects of experience, such as places, into memory representations; subsequent retrieval of these representations requires transmission through the previously modified hippocampal synapses, but no further plasticity. In contrast, slow incremental place learning may not absolutely require hippocampal contributions. The neocortical sensory inputs, especially visuo-spatial information, necessary for hippocampus-dependent rapid learning, are preferentially associated with the septal to intermediate hippocampus. In contrast, connectivity with the prefrontal cortex and subcortical sites, which link the hippocampus to motivational, emotional, executive, and sensorimotor functions, is primarily associated with the intermediate to temporal hippocampus. A model of functional differentiation and integration along the septo-temporal axis of the hippocampus is proposed, describing key hippocampal contributions to adaptive behavior based on information encoded during a single or a few past experiences.

Virtual environment navigation tasks and the assessment of cognitive deficits in individuals with brain injury

Navigation in real environments is often impaired by traumatic brain injury (TBI). These deficits in wayfinding appear to be due to disruption of cognitive processes underlying navigation and may in turn be due to damage to the hippocampus and frontal lobes. These wayfinding problems after TBI were investigated using a virtual simulation of a Morris Water Maze (MWM), a standard test of hippocampal function in laboratory animals. The virtual environment consisted of a large virtual arena in a very large virtual room whose walls provided views of a naturalistic landscape. Eleven community-dwelling TBI survivors and 12 comparison participants, matched for gender, age and education were tested to see if they could find a location in the arena marked by one of the following: (a) a visible platform, (b) a single proximal object, (c) a single proximal object among seven other distracter objects, or (d) distal features inside and outside the room. The proximal objects allowed participants to use egocentric (body-centered) navigational strategies that rely on relatively simple stimulus-response associations. The absence of proximal cues forced the participants to rely on distal features of the environment (room walls, landscape elements) and tested their ability to use allocentric (world-based) navigational strategies requiring cognitive mapping. Results indicated that the navigation of TBI survivors was not impaired when the proximal cues were present but was impaired when proximal cues were absent. These results provide more evidence that the navigational deficit after TBI is due to an inability to form, remember or use cognitive maps.

The Hippocampal CA1 Region and Dentate Gyrus Differentiate between Environmental and Spatial Feature Encoding through Long-Term Depression

Novel spatial information is encoded in the hippocampus by plastic changes of synaptic properties. Novel space consists of several types of information that may evoke differential synaptic responses in individual hippocampal subregions. To examine this possibility, we recorded field potentials from the dentate gyrus (DG) and CA1 region in freely moving adult rats. Stimulation protocols that were marginally subthreshold for the induction of persistent long-term potentiation (LTP) or long-term depression (LTD) were implemented, concurrent with exposure to novel spatial information. We found that in both hippocampal subregions, exploration of a novel empty hole board facilitated LTP. However, LTD facilitation was subregion specific and dependent on the nature of the cues. In the CA1 region, partially concealed cues had a facilitatory effect on LTD. LTD in the DG was facilitated by large directional cues. Thus, although LTP was facilitated uniformly in both areas by the same novel environment, LTD was facilitated in a region-specific manner, based on the nature of the cue. This implies that spatial changes within an environment elicit local changes of synaptic weights dependent on the type of information and, hence, generate a complete cognitive map as a consequence of cooperation of synaptic plasticity in all participating subregions.

Modulation of quinpirole-induced compulsive-like behavior in rats by environmental changes: implications for OCD rituals and for exploration and navigation

Background

Rats treated chronically with the D_2–3 dopamine agonist quinpirole were previously proposed as an animal model of obsessive-compulsive disorder (OCD) since their behavior is based on repeated, compulsive-like persistent traveling between a few places in the open field. The aim of the present study was to determine properties of the physical environment that shape such behavior. For this, quinpirole-treated rats were first exposed to an arena with an array of objects (landmarks) and after the development of compulsive-like behavior, the arena was manipulated by multiplying the number of objects, changing their spacing, relocating object array, or removing the objects.

Results

When the number of objects was retained but they were spaced further apart, rat routes converged at 1–2 of the objects and at the corner at which the rats had been introduced into the arena (start corner). When object spacing was retained but their number was increased, the rats traveled between the objects with the routes converging only at the start corner. Finally, when object array was relocated to different places within the arena, the rats extended their routes from the start corner to the object array, regardless of array location.

Conclusion

Quinpirole-treated rats organized and updated their progression primarily according to the proximal layout of landmarks, but did so with excessive repetitions compared with saline-treated rats. The behavior of quinpirole-treated rats paralleled human OCD rituals that are linked to the immediate physical environment, featuring an excessive rate of performance. Finally, when only a few objects were present, they were perceived by the rats as positional cues (beacons) that routes converged at them. In contrast, in the presence of many objects, the routes passed between the objects as if using them as directional cues.

Hippocampal long-term depression: master or minion in declarative memory processes?

The neural mechanisms for the formation of declarative memory (memory for facts and events) are believed to be integrated from processes mediated by hippocampal long-term potentiation (LTP) and long-term depression (LTD). Traditionally, LTP has been designated as the main mediator of spatial memory storage in the hippocampus, whereas LTD has been assigned an auxiliary role in signal-to-noise regulation or in forgetting. It has recently become apparent, however, that LTD contributes directly to hippocampal information storage. In fact, LTD could dominate in the processing of precise spatial characteristics. Accumulating evidence supports the idea that LTP and LTD enable distinct and separate forms of information storage, which together facilitate the generation of a spatial cognitive map.

The impact of landmark properties in shaping exploration and navigation

This study was aimed at uncovering physical and geometric properties that make a particular landmark a target of exploration and navigation. Rats were tested in a square open-field arena with additional portable corners featuring the same properties as the arena corners. It was found that the routes of progression converged upon the added corners, whether located at the arena wall or the arena center. Route convergence upon the added corners involved numerous visits to these corners. However, time spent at the added corners was relatively short compared with the arena corners, including that from which rats were introduced into the arena. There was no differential effect of testing rats in light or dark, or with a low versus a high portable corner. It is suggested that the added corners were distinct against the background of the arena enclosure, whereas the four arena corners and walls were encoded by the rats as one geometric module. This distinctness, together with the greater accessibility of the added corners, made them salient landmarks and a target of exploration. Thus, the impact of a landmark extended beyond its specific self-geometry to include accessibility and distinctness, which are contextual properties. In addition to the contextual impact on locomotor behavior there was also a temporal effect, with security initially dominating the rats' behavior but then declining along with an increased attraction to salient landmarks. These spatiotemporal patterns characterized behavior in both lit and dark arenas, indicating that distal cues were secondary to local proximal cues in shaping routes.

Flexible use of spatial cues in the southern flying squirrel (Glaucomys volans)

Insects, birds, and mammals have been shown capable of encoding spatial information in memory using multiple strategies or frames of reference simultaneously. These strategies include orientation to a goal-specific cue or beacon, to the position of the goal in an array of local landmarks, or to its position in the array of distant landmarks, also known as the global frame of reference. From previous experiments, it appears that birds and mammals that scatter hoard rely primarily on a global frame of reference, but this generalization depends on evidence from only a few species. Here we examined spatial memory in a previously unstudied scatter hoarder, the southern flying squirrel. We dissociated the relative weighting of three potential spatial strategies (beacon, global, or relative array strategy) with three probe tests: transposition of beacon and the rotation or the expansion of the array. The squirrels' choices were consistent with a spatial averaging strategy, where they chose the location dictated by at least two of the three strategies, rather than using a single preferred frame of reference. This adaptive and flexible heuristic has not been previously described in animal orientation studies, yet it may be a common solution to the universal problem of encoding and recalling spatial locations in an ephemeral physical landscape.

Impairment of spatial learning by estradiol treatment in female mice is attenuated by estradiol exposure during development

High doses of estradiol (E(2)) can impair spatial learning in the Morris water maze, in ovariectomized mice, but the same dose has no effect on adult castrated males. Here, we test the hypothesis that this sex difference is caused by neonatal actions of E(2). In Experiment 1, C57BL/6J pups were given daily estradiol benzoate (EB) or oil injections from the day of birth until postnatal Day 3. Adults were gonadectomized and received EB (s.c.) or oil 28 h before the first day of training, and 4 h before each of four daily training sessions on the Morris water maze. Females given oil as neonates, and EB prior to training displayed the poorest performance. Females that received EB as neonates and EB prior to training were insensitive to the deleterious effects of adult EB and performed better than males given the same hormone treatments. We conducted a second experiment using aromatase enzyme knockout (ArKO) mice. Adult male and female ArKO and wild-type (WT) littermates were gonadectomized and received either injections of oil or EB prior to and during water maze training (as described above). Hormone treatment failed to affect performance, yet, female but not male ArKO mice showed impaired learning compared to WT littermates. Thus, exposure to estradiol during neonatal development can counteract the deleterious effects of EB on adult spatial learning.

The neural mechanisms of long distance animal navigation

Animal navigation is a complex process involving the integration of many sources of specialized sensory information for navigation in near and far space. Our understanding of the neurobiological underpinnings of near-space navigation is well-developed, whereas the neural mechanisms of long-distance navigation are just beginning to be unraveled. One crucial question for future research is whether the near space concepts of place cells, head direction cells, and maps in the entorhinal cortex scale up to animals navigating over very long distances and whether they are related to the map and compass concepts of long-distance navigation.

Deficits in acquisition of spatial learning after dorsomedial telencephalon lesions in goldfish

Acquisition of spatial learning is an important function of mammalian hippocampus. In order to identify the brain areas in teleost fish that are homologous to mammalian hippocampus, the present study examined the effects of lesions in the dorsal area of the caudal telencephalon of goldfish (Carassius auratus) on the acquisition of spatial learning. An open-field maze that was similar to the dry version of the Morris water maze was used. The task consisted of habituation and postoperative training to reach the position of the bait. Extramaze cues were visible in the habituation sessions in experiment 1, while they were blocked and not visible in the habituation sessions in experiment 2. Only in experiment 2, there was a significant deficit in the performance in the training sessions in the goldfish with damage to the dorsomedial area of the caudal telencephalon (DM). These data showed that blocking of the extramaze cues in the habituation sessions caused deficits in postoperative acquisition of spatial learning in the training sessions in the goldfish with DM lesions. Latent learning in the habituation sessions, however, eliminated the effects of the DM lesions on spatial learning. The present study suggests that the DM plays a critical role in acquisition of spatial learning.

The relative importance of global and local landmarks in navigation by Columbian ground squirrels (Spermophilus columbianus)

In order to survive, small burrowing mammals need to remember the locations of escape burrows. Therefore, it is important to know what types of landmarks are used to aid navigation in the wild. The author tested the ability of free-ranging Columbian ground squirrels (Spermophilus columbianus) to locate escape burrows when local (e.g., vegetation pattern, local relief), global (e.g., forest edge, mountain outline), or both types of landmarks were obstructed. Results suggest that squirrels need both local and global landmarks of the environment for successful navigation, and that the upper portion of the horizon is especially important for orientation. Moreover, the lack of information from one type of landmark (local or global) cannot be completely compensated by the other type.

Metabolic activation pattern of distinct hippocampal subregions during spatial learning and memory retrieval

Activation dynamics of hippocampal subregions during spatial learning and their interplay with neocortical regions is an important dimension in the understanding of hippocampal function. Using the (14C)-2-deoxyglucose autoradiographic method, we have characterized the metabolic changes occurring in hippocampal subregions in mice while learning an eight-arm radial maze task. Autoradiogram densitometry revealed a heterogeneous and evolving pattern of enhanced metabolic activity throughout the hippocampus during the training period and on recall. In the early stages of training, activity was enhanced in the CA1 area from the intermediate portion to the posterior end as well as in the CA3 area within the intermediate portion of the hippocampus. At later stages, CA1 and CA3 activations spread over the entire longitudinal axis, while dentate gyrus (DG) activation occurred from the anterior to the intermediate zone. Activation of the retrosplenial cortex but not the amygdala was also observed during the learning process. On recall, only DG activation was observed in the same anterior part of the hippocampus. These results suggest the existence of a functional segmentation of the hippocampus, each subregion being dynamically but also differentially recruited along the acquisition, consolidation, and retrieval process in parallel with some neocortical sites.

The effects of chronic nicotine on spatial learning and bromodeoxyuridine incorporation into the dentate gyrus of the rat

RATIONALE

Nicotine is reported to improve learning and memory in experimental animals. Improved learning and memory has also been related to increased neurogenesis in the dentate gyrus (DG) of the hippocampal formation. Surprisingly, recent studies suggest that self-administered nicotine depresses cell proliferation in the DG.

OBJECTIVE

To test the hypothesis that the effects of nicotine on cell proliferation in the DG and learning and memory depend upon the nicotine dose administered.

METHODS

Rats were chronically infused from subcutaneous osmotic mini pumps with nicotine (0.25 or 4 mg kg(-1) day(-1)) or the saline vehicle for 10 days. Half the rats in each treatment group were trained to locate a hidden platform in a water maze task on days 4-7; a probe trial was performed on day 8. The remaining rats remained in their home cages. The effects of nicotine and of training in the water maze task on cell genesis in the DG were determined by measuring 5-bromo-2'-deoxyuridine (BrDU) uptake using fluorescence immunohistochemistry.

RESULTS

Training in the water maze task increased cell proliferation in the DG. Infusions of nicotine at 4 mg kg(-1) day(-1), but not 0.25 mg kg(-1) day(-1), decreased cell proliferation in both untrained animals and animals trained in the maze and impaired spatial learning.

CONCLUSIONS

The data suggest that learning in the water maze task is impaired by higher doses of nicotine tested, and that this response may be related to reduced cell genesis in the DG.

Distinct contributions of hippocampal NMDA and AMPA receptors to encoding and retrieval of one-trial place memory

Allocentric place memory may serve to specify the context of events stored in human episodic memory. Recently, our laboratory demonstrated that, analogous to event-place associations in episodic memory, rats could associate, within one trial, a specific food flavor with an allocentrically defined place in an open arena. Encoding, but not retrieval, of such flavor-place associations required hippocampal NMDA receptors; retrieval depended on hippocampal AMPA receptors. This might have partly reflected the contributions of these receptors to encoding and retrieval of one-trial place, rather than flavor-place, memory. Therefore, the present study developed a food-reinforced arena paradigm to study encoding and retrieval of one-trial allocentric place memory in rats; memory relied on visuospatial information and declined with increasing retention delay, still being significant after 6 h, the longest delay tested (experiments 1 and 2). Hippocampal infusion of the NMDA receptor antagonist d-AP-5 blocked encoding without affecting retrieval; hippocampal infusion of the AMPA receptor antagonist CNQX impaired retrieval (experiment 3). Finally, we confirmed that the d-AP-5 infusions selectively blocked induction of long-term potentiation, a form of synaptic plasticity, whereas CNQX impaired fast excitatory transmission, at perforant-path dentate gyrus synapses in the dorsal hippocampus in vivo (experiment 4). Our results support that encoding, but not retrieval, of one-trial allocentric place memory requires the NMDA receptor-dependent induction of hippocampal synaptic plasticity, whereas retrieval depends on AMPA receptor-mediated fast excitatory hippocampal transmission. The contributions of hippocampal NMDA and AMPA receptors to one-trial allocentric place memory may be central to episodic memory and related episodic-like forms of memory in rats.

Timing and Hippocampal Theta in Animals

All animals have at least two different internal clocks, one governing cognition of time of day, and the other concerning awareness of seconds and minutes. In the latter case, organisms show scalar properties. The timing mechanisms in the brain may function similarly throughout the animal kingdom, but this is not yet clear. Previous studies have shown that the hippocampus is intricately involved with the process of interval timing. Data concerning electrophysiological field potentials in the hippocampus show obviously rhythmic activity, known as hippocampal theta activity. An information-processing model of interval timing postulates three distinct stages: a clock, a memory, and a decision stage /11/. The timing process includes memory processing, which means that the hippocampus works together with working memory to estimate current time passing.

Global and local spatial landmarks: their role during foraging by Columbian ground squirrels (Spermophilus columbianus)

Locating food and refuge is essential for an animal's survival. However, little is known how mammals navigate under natural conditions and cope with given environmental constraints. In a series of six experiments, I investigated landmark-based navigation in free-ranging Columbian ground squirrels (Spermophilus columbianus). Squirrels were trained individually to find a baited platform within an array of nine identical platforms and artificial landmarks set up on their territories. After animals learned the location of the food platform in the array, the position of the latter with respect to local artificial, local natural, and global landmarks was manipulated, and the animal's ability to find the food platform was tested. When only positions of local artificial landmarks were changed, squirrels located food with high accuracy. When the location of the array relative to global landmarks was altered, food-finding accuracy decreased but remained significant. In the absence of known global landmarks, the presence of a familiar route and natural local landmarks resulted in significant but not highly accurate performance. Squirrels likely relied on multiple types of cues when orienting towards a food platform. Local landmarks were used only as a secondary mechanism of navigation, and were not attended to when a familiar route and known global landmarks were present. This study provided insights into landmark use by a wild mammal in a natural situation, and it demonstrated that an array of platforms can be employed to investigate landmark-based navigation under such conditions.

Methylazoxymethanol acetate does not fully block cell genesis in the young and aged dentate gyrus

During adulthood, new neurons are continuously added to the mammalian dentate gyrus (DG). An increasing number of studies have correlated changes in rates of dentate neurogenesis with memory abilities. One study based on subchronic treatment with the toxin methylazoxymethanol acetate (MAM) has provided causal evidence that neurogenesis is involved in hippocampal-dependent trace conditioning. In contrast, spatial learning is not impaired following MAM treatment. We hypothesized that this was due to the small residual number of new cells produced following MAM treatment. In the present experiment, we attempted to achieve a higher level of reduction of adult-generated cells following MAM treatment in young and aged rats. We found only a partial reduction of adult-generated cells in the DG. More importantly, independently of the age of the animals, MAM treatment at a dose necessary to reduce neurogenesis altered the overall health of the animals. In conclusion, the behavioural results obtained following subchronic treatment with high doses of MAM in adulthood must be interpreted with extreme caution.

Is there a geometric module for spatial orientation? Squaring theory and evidence

There is evidence, beginning with Cheng (1986), that mobile animals may use the geometry of surrounding areas to reorient following disorientation. Gallistel (1990) proposed that geometry is used to compute the major or minor axes of space and suggested that such information might form an encapsulated cognitive module. Research reviewed here, conducted on a wide variety of species since the initial discovery of the use of geometry and the formulation of the modularity claim, has supported some aspects of the approach, while casting doubt on others. Three possible processing models are presented that vary in the way in which (and the extent to which) they instantiate the modularity claim. The extant data do not permit us to discriminate among them. We propose a modified concept of modularity for which an empirical program of research is more tractable.

Generalization in place learning across geometrically different environments is impaired by hippocampal lesions in rats

A recent study (Tommasi & Thinus-Blanc, 2004) found that when rats trained to search for food hidden in the center of a square-shaped enclosure were tested in the absence of food, they searched precisely in the center of the enclosure. Transfer tests carried out in enclosures differing in shape or size (rectangular-, large square-, and equilateral triangle-shaped) showed that rats searched in the geometric center of the novel environments. In the present work, half of those rats were lesioned bilaterally to the hippocampus and were then re-trained and re-tested in the same task, to be compared to sham rats that received the same retraining/re-test protocol. Results show that lesioned rats, although unimpaired in relearning to localize the center of the square-shaped enclosure, performed considerably worse than sham rats in the transfer tests in the enclosures differing in shape, suggesting a relevant role of the hippocampus in geometry-driven place generalization.