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

The temporal context model in spatial navigation and relational learning: toward a common explanation of medial temporal lobe function across domains

The medial temporal lobe (MTL) has been studied extensively at all levels of analysis, yet its function remains unclear. Theory regarding the cognitive function of the MTL has centered along 3 themes. Different authors have emphasized the role of the MTL in episodic recall, spatial navigation, or relational memory. Starting with the temporal context model (M. W. Howard & M. J. Kahana, 2002a), a distributed memory model that has been applied to benchmark data from episodic recall tasks, the authors propose that the entorhinal cortex supports a gradually changing representation of temporal context and the hippocampus proper enables retrieval of these contextual states. Simulation studies show this hypothesis explains the firing of place cells in the entorhinal cortex and the behavioral effects of hippocampal lesion in relational memory tasks. These results constitute a first step toward a unified computational theory of MTL function that integrates neurophysiological, neuropsychological, and cognitive findings.

Parallel processing across neural systems: Implications for a multiple memory system hypothesis

A common conceptualization of the organization of memory systems in brain is that different types of memory are mediated by distinct neural systems. Strong support for this view comes from studies that show double (or triple) dissociations between spatial, response, and emotional memories following selective lesions of hippocampus, striatum, and the amygdala. Here, we examine the extent to which hippocampal and striatal neural activity patterns support the multiple memory systems view. A comparison is made between hippocampal and striatal neural correlates with behavior during asymptotic performance of spatial and response maze tasks. Location- (or place), movement, and reward-specific firing patterns were found in both structures regardless of the task demands. Many, but not all, place fields of hippocampal and striatal neurons were similarly affected by changes in the visual and reward context regardless of the cognitive demands. Also, many, but not all, hippocampal and striatal movement-sensitive neurons showed significant changes in their behavioral correlates after a change in visual context, irrespective of cognitive strategy. Similar partial reorganization was observed following manipulations of the reward condition for cells recorded from both structures, again regardless of task. Assuming that representations that persist across context changes reflect learned information, we make the following conclusions. First, the consistent pattern of partial reorganization supports a view that the analysis of spatial, response, and reinforcement information is accomplished via an error-driven, or match-mismatch, algorithm across neural systems. Second, task-relevant processing occurs continuously within hippocampus and striatum regardless of the cognitive demands of the task. Third, given the high degree of parallel processing across allegedly different memory systems, we propose that different neural systems may effectively compete for control of a behavioral expression system. The strength of the influence of any one neural system on behavioral output is likely modulated by factors such as motivation, experience, or hormone status.

Spatial response properties of homing pigeon hippocampal neurons: correlations with goal locations, movement between goals, and environmental context in a radial-arm arena

The amniote hippocampal formation plays an evolutionarily-conserved role in the neural representation of environmental space. However, species differences in spatial ecology nurture the expectation of species differences in how hippocampal neurons represent space. To determine the spatial response properties of homing pigeon ( Columba livia) HFneurons, we recorded from isolated units in birds freely navigating a radial arena in search of food present at four goal locations. Fifty of 76 neurons displayed firing rate variations that could be placed into three response categories. Location cells ( n=25) displayed higher firing rates at restricted locations in the arena space, often in proximity to goal locations. Path cells ( n=13) displayed higher firing rates as a pigeon moved between a subset of goal locations. Arena-off cells ( n=12) were more active when a pigeon was in a baseline holding space compared to inside the arena. Overall, reliability and coherence scores of the recorded neurons were lower compared to rat place cells. The differences in the spatial response profiles of pigeon hippocampal formation neurons, when compared to rats, provide a departure point for better understanding the relationship between spatial behavior and how hippocampal formation neurons participate in the representation of space.

Hippocampal Place Cells Show Increased Sensitivity to Changes in the Local Environment Following Prefrontal Cortex Lesions

It has been proposed that the prefrontal cortex modulates neural activity in posterior cortex via inhibitory mechanisms. As a result, damage to the former area may produce disinhibition in posterior regions and increase sensitivity to extraneous information. This hypothesis was investigated by examining how prefrontal cortex lesions affected the firing of hippocampal place cells in freely moving rats. In experiment 1, the positional firing of lesion-group cells was altered to a greater extent than that of control-group cells when objects were introduced into the recording environment. This suggested that place cell firing was overly influenced by local cues in the prefrontal-lesioned animals. In experiment 2 place cells were recorded while rats foraged on a circular track with access to both local and distal multimodal cues. Although the position of place fields in lesion-group cells was not excessively tied to local cues, a greater proportion of the fields lost their spatial selectivity following a rotation of these cues. The cue-related effects were associated with larger extracellular action-potential amplitudes and a greater incidence of burst-firing in lesion-group cells. This finding is consistent with the hypothesis that lesions of the prefrontal cortex result in a disinhibition of posterior cortex.

Coupling between place cells and head direction cells during relative translations and rotations of distal landmarks

Hippocampal place cells are selectively active when a rat occupies restricted locations in an environment, and head direction cells fire selectively when the rat's head is pointed in a particular direction in allocentric space. Both place cells and head direction cells are usually coupled, and they are controlled by a complex interaction between external landmarks and idiothetic cues. Most studies have investigated this interaction by rotating the landmarks in the environment. In contrast, a recent study translated the apparatus relative to the landmarks in an environment and found that most place cells maintained the same preferred location on the apparatus regardless of the location of the apparatus in the room. Because head direction cells are insensitive to the rat's location in an environment, the distal landmarks may influence the place field firing locations primarily by controlling the bearing of the head direction cell system. To address this question, ensembles of CA1 place cells and head direction cells of the anterior thalamus were recorded simultaneously, as a rectangular or circular track was moved to different locations in a room with distinct visual landmarks. Most place cells maintained their firing fields relative to the track when the track was translated, and head direction cells maintained the same preferred firing direction. When the distal landmarks were rotated around the track, the firing fields of place cells and the preferred directions of head direction cells rotated with the cues. These results suggest that the precise firing locations of place cells are controlled by an interaction between local and idiothetic cues, and the orientation of the CA1 ensemble representation relative to the distal landmarks may be controlled indirectly by the distal landmarks' influence over the bearing of the head direction cell system.

Local sensory cues and place cell directionality: additional evidence of prospective coding in the hippocampus

In tasks involving goal-directed, stereotyped trajectories on uniform tracks, the spatially selective activity of hippocampal principal cells depends on the animal's direction of motion. Principal cell ensemble activity while the rat moves in opposite directions through a given location is typically uncorrelated. It is shown here, with data from three experiments, that multimodal, local sensory cues can change the directional properties of CA1 pyramidal cells, inducing bidirectionality in a significant proportion of place cells. For a majority of these bidirectional place cells, place field centers in the two directions of motion were displaced relative to one another, as would be the case if the cells were representing a position in space approximately 5-10 cm ahead of the rat or if place cells were subject to strong accommodation or inhibition in the latter half of their input fields. However, place field density was not affected by the presence of local cues, but in the experimental condition with the most salient sensory cues, the CA1 population vectors in the "cue-rich" condition were sparser and changed more quickly in space than in the "cue-poor" condition. These results suggest that "view-invariant" object representations are projected to the hippocampus from lower cortical areas and can have the effect of increasing the correlation of the hippocampal input vectors in the two directions, hence decreasing the orthogonality of hippocampal output.

Distinct ensemble codes in hippocampal areas CA3 and CA1

The hippocampus has differentiated into an extensively connected recurrent stage (CA3) followed by a feed-forward stage (CA1). We examined the function of this structural differentiation by determining how cell ensembles in rat CA3 and CA1 generate representations of rooms with common spatial elements. In CA3, distinct subsets of pyramidal cells were activated in each room, regardless of the similarity of the testing enclosure. In CA1, the activated populations overlapped, and the overlap increased in similar enclosures. After exposure to a novel room, ensemble activity developed slower in CA3 than CA1, suggesting that the representations emerged independently.

Representation of objects in space by two classes of hippocampal pyramidal cells

Humans can recognize and navigate in a room when its contents have been rearranged. Rats also adapt rapidly to movements of objects in a familiar environment. We therefore set out to investigate the neural machinery that underlies this capacity by further investigating the place cell-based map of the surroundings found in the rat hippocampus. We recorded from single CA1 pyramidal cells as rats foraged for food in a cylindrical arena (the room) containing a tall barrier (the furniture). Our main finding is a new class of cells that signal proximity to the barrier. If the barrier is fixed in position, these cells appear to be ordinary place cells. When, however, the barrier is moved, their activity moves equally and thereby conveys information about the barrier's position relative to the arena. When the barrier is removed, such cells stop firing, further suggesting they represent the barrier. Finally, if the barrier is put into a different arena where place cell activity is changed beyond recognition ("remapping"), these cells continue to discharge at the barrier. We also saw, in addition to barrier cells and place cells, a small number of cells whose activity seemed to require the barrier to be in a specific place in the environment. We conclude that barrier cells represent the location of the barrier in an environment-specific, place cell framework. The combined place + barrier cell activity thus mimics the current arrangement of the environment in an unexpectedly realistic fashion.

Comparison of population coherence of place cells in hippocampal subfields CA1 and CA3

The hippocampus, a critical brain structure for navigation, context-dependent learning and episodic memory, is composed of anatomically heterogeneous subregions. These regions differ in their anatomical inputs as well as in their internal circuitry. A major feature of the CA3 region is its recurrent collateral circuitry, by which the CA3 pyramidal cells make excitatory synaptic contacts on each other. In contrast, pyramidal cells in the CA1 region are not extensively interconnected. Although these differences have inspired numerous theoretical models of differential processing capacities of these two regions, there have been few reports of robust differences in the firing properties of CA1 and CA3 neurons in behaving animals. The most extensively studied of these properties is the spatially selective firing of hippocampal 'place cells'. Here we report that in a dynamically changing environment, in which familiar landmarks on the behavioural track and along the wall are rotated relative to each other, the population representation of the environment is more coherent between the original and cue-altered environments in CA3 than in CA1. These results demonstrate a functional heterogeneity between the place cells of CA3 and CA1 at the level of neural population representations.

A proposed architecture for the neural representation of spatial context

The role of context in guiding animal behavior has attracted increasing attention in recent years, but little is known about what constitutes a context, nor how and where in the brain it is represented. Contextual stimuli can take many forms, but of particular importance are those that collectively define a particular place or situation. The representation of place has been linked to the hippocampus, because its principal neurons ('place cells') are spatially responsive; behavioral experiments also implicate this structure in the processing of contextual stimuli. Together, these findings suggest a hippocampal role in representing 'spatial context'. The present article outlines a proposed architecture for the encoding of spatial context in which spatial inputs to place cells are modulated (or 'gated') by non-spatial stimuli. We discuss recent experimental evidence that spatial context is population-coded, a property which could allow both discrimination between overlapping contexts and generalization across them, and thus provide a foundation for animals' capacity for flexible context-linked place learning.

Context-Dependent Reorganization of Spatial and Movement Representations by Simultaneously Recorded Hippocampal and Striatal Neurons During Performance of Allocentric and Egocentric Tasks

Hippocampal and striatal place- and movement-correlated cell firing was recorded as rats performed place or response tasks in a familiar environment, and then after cue manipulation. In a familiar environment, place field properties did not differ across brain structures or task conditions. Movement correlates were stronger during place task performance only in hippocampal neurons. After cue manipulations, place- and movement-sensitive hippocampal and striatal neurons changed their correlate strength, regardless of behavioral strategy. Thus, for both structures, place-correlated cells may encode spatial context information, whereas movement-correlated cells may represent both egocentric movement and learned behavioral responses. The striking overall similarity between hippocampal and striatal neural responses to context manipulation (regardless of strategy) suggests that these structures operate continuously, and in parallel, during multiple forms of learning.

Modulation of the Impairment of Hippocampectomized Rats on the Radial-Arm Maze Cue Task by Visual Characteristics and Subicular Damage

Rats with N-methyl-D-aspartate lesions of the hippocampus that partially damaged the subiculum and controls were trained on 2 versions of the radial-arm maze cue task, with either proximal or distal visual stimuli. In Experiment 1, the relative positions of the stimuli varied across trials. Lesioned rats were impaired when trained on the distal version, as opposed to transiently slowed down when trained on the proximal version. In Experiment 2, the relative positions of the stimuli were fixed throughout training. Lesioned rats were impaired when trained on the distal or the proximal version. Further analyses showed that combined damage to the hippocampus and the subiculum was required to impair performance in the proximal, but not the distal, version.

Heterogeneous modulation of place cell firing by changes in context

Hippocampal place cells show spatially localized activity that can be modulated by both geometric information (e.g., the distances and directions of features in the environment) and nongeometric information (e.g., colors, odors, and possibly behaviors). Nongeometric information may allow the discrimination of different spatial contexts. Understanding how nongeometric (or contextual) information affects hippocampal activity is important in light of proposals that the hippocampus may play a role in constructing a representation of spatial context. We investigated the contextual modulation of place cell activity by recording hippocampal place cells while rats foraged in compound contexts comprising black or white color paired with lemon or vanilla odor. Some cells responded to the color or odor changes alone, but most responded to varying combinations of both. Thus, we demonstrate, for the first time, that there is a heterogeneous input by contextual inputs into the hippocampus. We propose a model of contextual remapping of place cells in which the geometric inputs are selectively activated by subsets of contextual stimuli. Because it appears that different place cells are affected by different subsets of contextual stimuli, the representation of the entire context would require activity at the population level, supporting a role for the hippocampus in constructing a representation of spatial context.

Distal landmarks and hippocampal place cells: effects of relative translation versus rotation

Hippocampal neurons are selectively active when a rat occupies restricted locations in an environment. These place cells derive their specificity from a multitude of sources, including idiothetic cues and sensory input derived from both distal and local landmarks. Most experiments have attempted to dissociate the relative strengths and roles played by these sources by rotating one set against the other. Few studies have addressed the effects of relative translation of the local cue set versus salient distal landmarks. To address this question, ensembles of place cells were recorded as a rectangular or circular track was moved to different locations in a room with controlled visual landmarks. Place cells primarily maintained their firing fields relative to the track (i.e., occupying new locations relative to the distal landmarks), even though the track could occupy completely nonoverlapping regions of the room. When the distal landmarks were rotated around the circular track, however, the place fields rotated with the landmarks, demonstrating that the cues were perceptible to the rat. These results suggest that, under these conditions, the spatial tuning of place cells may derive from an interaction between local and idiothetic cues, which define the precise firing locations of the cells and the relationships between them, and distal landmarks, which set the orientation of the ensemble representation relative to the external environment.

Long-term effects of permanent vestibular lesions on hippocampal spatial firing

The hippocampus is thought to be important for spatial representation processes that depend on the integration of both self-movement and allocentric cues. The vestibular system is a particularly important source of self-movement information that may contribute to this spatial representation. To test the hypothesis that the vestibular system provides self-movement information to the hippocampus, rats were given either a bilateral labyrinthectomy (n = 6) or a sham surgery (n = 6), and at least 60 d after surgery hippocampal CA1 neurons were recorded extracellularly while the animals foraged freely in an open arena. Recorded cells were classified as complex spiking (n = 80) or noncomplex spiking (n = 33) neurons, and their spatial firing fields (place fields) were examined. The most striking effect of the lesion was that it appeared to completely abolish location-related firing. The results of this and previous studies provide converging evidence demonstrating that vestibular information is processed by the hippocampus. The disruption of the vestibular input to the hippocampus may interfere with the reconciliation of internal self-movement signals with the changes to the external sensory inputs that occur as a result of that movement. This would disrupt the ability of the animal to integrate allocentric and egocentric information into a coherent representation of space.

Positional firing properties of postrhinal cortex neurons

Hippocampal cell firing in awake, behaving rats is often spatially selective, and such cells have been called place cells. Similar spatial correlates have also been described for neurons in the medial entorhinal and perirhinal cortices. All three regions receive sensory associational input from postrhinal cortex, which, in turn, is heavily interconnected with visuospatial neocortical regions. The spatial selectivity of postrhinal cells, however, has never been examined. Here, we report the activity of neurons in postrhinal cortex of freely moving rats performing a spatial task on a four-arm radial maze. Data are also reported for visual association cortex neurons. The four-arm radial maze was defined by multisensory cues on the surfaces of the maze arms (proximal) and complex visual cues at the surround (distal). On each recording day, rats were run in three conditions: baseline, double cue rotation (proximal +90 degrees; distal -90 degrees ), and baseline. In this task, hippocampal place field activity is robust and can be controlled by proximal or distal cues. The majority of postrhinal neurons (64%) exhibited positional correlates during performance on the task; however, characteristics of these postrhinal cells were substantially different from those previously described for hippocampal place cells. Most postrhinal cells with firing fields exhibited split or multiple subfields (93%). Unlike hippocampal place fields, the large majority of postrhinal firing fields (84%) adopted new spatial correlates when experimental cues were rotated, but did so neither predictably nor concordantly. This is the first report of positional firing correlates in the postrhinal cortex. The data are consistent with the idea that postrhinal cortex participates in visuospatial functions by monitoring changes in environmental stimuli rather than encoding stable spatial cues. Thus, postrhinal neurons appear to participate in higher-level perceptual functions rather than mnemonic functions. We propose that the response properties of postrhinal neurons represent an early step in a spatial pathway that culminates in the specific and stable place fields of the hippocampus.

Olfactory and/or visual cues for spatial navigation through ontogeny: olfactory cues enable the use of visual cues

This study analyzed the spatial memory capacities of rats in darkness with visual and/or olfactory cues through ontogeny. Tests were conducted with the homing board, where rats had to find the correct escape hole. Four age groups (24 days, 48 days, 3-6 months, and 12 months) were trained in 3 conditions: (a) 3 identical light cues; (b) 5 different olfactory cues; and (c) both types of cues, followed by removal of the olfactory cues. Results indicate that immature rats first take into account olfactory information but are unable to orient with only the help of discrete visual cues. Olfaction enables the use of visual information by 48-day-old rats. Visual information predominantly supports spatial cognition in adult and 12-month-old rats. Results point out cooperation between vision and olfaction for place navigation during ontogeny in rats.

Impaired Performance of Fornix-Transected Rats on a Distal, but Not on a Proximal, Version of the Radial-Arm Maze Cue Task

Fornix-transected and sham-operated rats were trained on radial maze cue tasks in which the relative positions of the cues were either fixed (F condition) or varied (V condition) across trials. Proximal and distal visual stimuli were used in 2 different experiments. With proximal stimuli, fornix-transected rats were transiently impaired in the V condition and performed as well as controls in the F condition. However, using extramaze stimuli, fornix-transected rats were severely impaired in the V condition but performed normally in the F condition. According to histological analyses, performance on these cue tasks varied along with the extent of cholinergic depletion in the hippocampus. At the behavioral level, the location and stability of stimuli's relative positions seemed to have influenced rats' performance.

Concordant and discordant coding of spatial location in populations of hippocampal CA1 pyramidal cells

Pyramidal cells in the rat hippocampus commonly show place-related activity, but it has been difficult to understand the factors that govern them. A particularly important question is whether individual cells have identifiable correlates that can be manipulated independently of the correlates of other cells. Recently Tanila et al. examined the activity of small ensembles of hippocampal cells in rats running on a plus-maze with distinct intra- and extramaze cues. When the two sets of cues were rotated 90 degrees in opposite directions, some cells followed the intramaze cues, others followed the extramaze cues, and others "remapped" unpredictably; moreover, all possible combinations were seen within simultaneously recorded ensembles. In the current study, CA1 pyramidal cell population activity was recorded from four rats in a similar paradigm, using a recording system that permitted the analysis of ensembles of 4-70 simultaneously recorded units. The results were consistent with the data from the earlier study in showing an increase in remapping over time and in showing some place fields following one of the defined sets of cues while others remapped. When the possibility of random remapping was controlled for, however, the analysis did not show significant numbers of place fields following both sets of cues simultaneously. Furthermore, all rats initially showed fully concordant responses with all place fields following the local cues. For two rats, this pattern continued until a new configuration was introduced at which time all fields switched to follow the distal cues. Taken together, the results are difficult to reconcile with the hypothesis that individual hippocampal cells encode information about different subsets of cues in the environment.

Properties of the extra-positional signal in hippocampal place cell discharge derived from the overdispersion in location-specific firing

There is a good deal of evidence that in the rodent, an internal model of the external world is encoded by hippocampal pyramidal cells, called 'place cells'. During free exploration, the activity of place cells is higher within a small part of the space, called the firing field, and virtually silent elsewhere. We have previously shown that the spiking activity during passes through the firing field is characterized not only by the high firing rate, but also by its very high variability ('overdispersion'). This overdispersion indicates that place cells carry information in addition to position. Here we demonstrate by simulations of an integrate-and-fire neuronal model that while a rat is foraging in an open space this additional information may arise from a process that alternatingly modulates the inputs to place cells by about 10% with a mean period of about 1 s. We propose that the overdispersion reflects switches of the rats attention between different spatial reference frames of the environment. This predicts that the overdispersion will not be observed in rats that use only room-based cues for navigation. We show that while place cell firing is overdispersed in rats during foraging in an open arena, the firing is less overdispersed during the same behavior in the same environment, when the rats have been trained to use only room-based and not arena-based cues to navigate.

Dynamic interactions between local surface cues, distal landmarks, and intrinsic circuitry in hippocampal place cells

A number of computational models of hippocampal place cells incorporate attractor neural network architecture to simulate key findings in the place cell literature, including the properties of pattern completion, firing in the absence of visual input, and nonlinear responses to environmental manipulations. To test for evidence of attractor dynamics, ensembles of place cells were recorded using multiple-tetrode techniques. After many days of experience in an environment with salient local surface cues on a circular track and salient distal landmarks on the wall, the local surface cues were rotated as a set in opposition to the distal landmarks. The amount of mismatch between the local and distal sets of cues varied from 45 to 180 degrees. If place cells were parts of strong attractors, then their place fields should follow either the local cues or the distal cues as an integrated ensemble. Instead, in single recording sessions, some place cells were controlled by the distal landmarks, other cells were controlled by the local cues, and other cells became silent or gained new fields. In some cases, individual place fields split in half, following both the local and distal cues. If place cells are indeed parts of attractor networks in the hippocampus, then the attractors may be weak relative to the inputs from external sources, such as representations of the sensory environment and representations of heading direction, in a familiar, well explored environment.

Hippocampal spatial representations require vestibular input

The hippocampal formation is essential for forming declarative representations of the relationships among multiple stimuli. The rodent hippocampal formation, including the entorhinal cortex and subicular complex, is critical for spatial memory. Two classes of hippocampal neurons fire in relation to spatial features. Place cells collectively map spatial locations, with each cell firing only when the animal occupies that cell's "place field," a particular subregion of the larger environment. Head direction (HD) cells encode directional heading, with each HD cell firing when the rat's head is oriented in that cell's particular "preferred firing direction." Both landmarks and internal cues (e.g., vestibular, motor efference copy) influence place and HD cell activity. However, as is the case for navigation, landmarks are believed to exert greater influence over place and HD cell activity. Here we show that temporary inactivation of the vestibular system led to the disruption of location-specific firing in hippocampal place cells and direction-specific discharge of postsubicular HD cells, without altering motor function. Place and HD cell activity recovered over a time course similar to that of the restoration of vestibular function. These results indicate that vestibular signals provide an important influence over the expression of hippocampal spatial representations, and may explain the navigational deficits of humans with vestibular dysfunction.

Neurogenesis may relate to some but not all types of hippocampal-dependent learning

The hippocampal formation generates new neurons throughout adulthood. Recent studies indicate that these cells possess the morphology and physiological properties of more established neurons. However, the function of adult generated neurons is still a matter of debate. We previously demonstrated that certain forms of associative learning can enhance the survival of new neurons and a reduction in neurogenesis coincides with impaired learning of the hippocampal-dependent task of trace eyeblink conditioning. Using the toxin methylazoxymethanol acetate (MAM) for proliferating cells, we tested whether reduction of neurogenesis affected learning and performance associated with different hippocampal dependent tasks: spatial navigation learning in a Morris water maze, fear responses to context and an explicit cue after training with a trace fear paradigm. We also examined exploratory behavior in an elevated plus maze. Rats were injected with MAM (7 mg/kg) or saline for 14 days, concurrent with BrdU, to label new neurons on days 10, 12, and 14. After treatment, groups of rats were tested in the various tasks. A significant reduction in new neurons in the adult hippocampus was associated with impaired performance in some tasks, but not with others. Specifically, treatment with the antimitotic agent reduced the amount of fear acquired after exposure to a trace fear conditioning paradigm but did not affect contextual fear conditioning or spatial navigation learning in the Morris water maze. Nor did MAM treatment affect exploration in the elevated plus maze. These results combined with previous ones suggest that neurogenesis may be associated with the formation of some but not all types of hippocampal-dependent memories.

Finding your way in the dark: the retrosplenial cortex contributes to spatial memory and navigation without visual cues

Path integration is presumed to rely on self-motion cues to identify locations in space and is subject to cumulative error. The authors tested the hypothesis that rats use memory to reduce such errors and that the retrosplenial cortex contributes to this process. Rats were trained for 1 week to hoard food in an arena after beginning a trial from a fixed starting location; probe trials were then conducted in which they began a trial from a novel place in light or darkness. After control injections, rats searched around the training location, showing normal spatial memory. Inactivation of the retrosplenial cortex disrupted this search preference. To assess accuracy during navigation, rats were then trained to perform multiple trials daily, with a fixed or a different starting location in light or darkness. Retrosplenial cortex inactivation impaired accuracy in darkness. The retrosplenial cortex may provide mnemonic information, which decreases errors when navigating in the dark.

Inactivating one hippocampus impairs avoidance of a stable room-defined place during dissociation of arena cues from room cues by rotation of the arena

Unilateral intrahippocampal injections of tetrodotoxin were used to temporarily inactivate one hippocampus during specific phases of training in an active allothetic place avoidance task. The rat was required to use landmarks in the room to avoid a room-defined sector of a slowly rotating circular arena. The continuous rotation dissociated room cues from arena cues and moved the arena surface through a part of the room in which foot-shock was delivered. The rat had to move away from the shock zone to prevent being transported there by the rotation. Unilateral hippocampal inactivations profoundly impaired acquisition and retrieval of the allothetic place avoidance. Posttraining unilateral hippocampal inactivation also impaired performance in subsequent sessions. This allothetic place avoidance task seems more sensitive to hippocampal disruption than the standard water maze task because the same unilateral hippocampal inactivation does not impair performance of the variable-start, fixed hidden goal task after procedural training. The results suggest that the hippocampus not only encodes allothetic relationships amongst landmarks, it also organizes perceived allothetic stimuli into systems of mutually stable coordinates. The latter function apparently requires greater hippocampal integrity.

Age-related deficits in the ability to encode contextual change: a place cell analysis

Aging is known to impair the formation of episodic memory, a process dependent upon the integrity of the hippocampal region. To investigate this issue, hippocampal place cells were recorded from middle-aged and old F-344 male rats while running on a "figure-8" track. The top and bottom arcs of the track were removed, converting it into a plus maze, and the animals were required to conduct a working memory task. Following this change in task, the arcs were replaced and the animals again ran the figure-8 task. Analysis of place fields across the recording session demonstrated that both middle-aged and old rats had reliable representations of the figure-8 task. A comparison of place fields between different behavioral tasks (figure-8 and plus maze) demonstrated a change in the hippocampal representation of the environment in both age groups, despite the fact that the animals remained on the maze throughout the recording session. Notably, place cells in old animals were less affected by the change in task than those in middle-aged animals. The results suggest that hippocampal neurons reflect significant behavioral events within a given environment. Furthermore, the data indicate that age-related episodic memory deficits may result from decreased sensitivity of the hippocampal network to respond to meaningful changes in the environment.

Disparate effects of long-term potentiation on evoked potentials and single CA1 neurons in the hippocampus of anesthetized rats

To examine the effects of long-term potentiation (LTP) on individual neurons in the intact brain, anesthetized rats were implanted with a recording stereotrode in the right CA1 layer of the hippocampus and a stimulating electrode in the right and left CA3 layers. The evoked and spontaneous firing of single CA1 neurons was characterized before and after LTP of the contralateral (commissural) Schaffer collaterals and again after LTP of the ipsilateral (associational) Schaffer collaterals. Individual CA1 neurons displayed either increases or decreases in evoked and spontaneous firing after LTP. As many as five discriminated cells were recorded simultaneously, and they typically responded discordantly, so that after LTP, firing in some neurons increased while in others it decreased. The response of individual neurons to in vivo LTP may be modulated by heterogeneous synaptic changes on individual and local groups of cells, and by changes in feed-forward excitation and inhibition provided by local hippocampal circuitry.

A two-platform task reveals a deficit in the ability of rats to return to the start location in the water maze

The ability of rats to return to the start location was examined with a 4-arm radial water maze. The task required rats to find 2 hidden platforms in sequence. Rats were released from 1 of 3 arms and there was a platform located in the fourth arm. Once a rat found this platform, a 2nd platform was raised in another location, which was either the start location, for 1 group, or another fixed location, for a control group. Across 3 experiments, all rats learned the location of the 1st fixed platform in 80 to 120 trials. However, rats had difficulty finding a 2nd platform if it was at the start location. Control groups revealed that rats could learn 2 platform locations and that the difficulty in learning to return to the start location did not seem to be attributable to its aversive nature. In separate groups, exposure to the start location was increased by starting the rats from an initially stable platform. Rats still did not readily learn to return to the start location. The authors suggest that start location, when varied, cannot readily be used to define the location of a hidden platform.

Hippocampal place-cell firing during movement in three-dimensional space

"Place" cells of the rat hippocampus are coupled to "head direction" cells of the thalamus and limbic cortex. Head direction cells are sensitive to head direction in the horizontal plane only, which leads to the question of whether place cells similarly encode locations in the horizontal plane only, ignoring the z axis, or whether they encode locations in three dimensions. This question was addressed by recording from ensembles of CA1 pyramidal cells while rats traversed a rectangular track that could be tilted and rotated to different three-dimensional orientations. Cells were analyzed to determine whether their firing was bound to the external, three-dimensional cues of the environment, to the two-dimensional rectangular surface, or to some combination of these cues. Tilting the track 45 degrees generally provoked a partial remapping of the rectangular surface in that some cells maintained their place fields, whereas other cells either gained new place fields, lost existing fields, or changed their firing locations arbitrarily. When the tilted track was rotated relative to the distal landmarks, most place fields remapped, but a number of cells maintained the same place field relative to the x-y coordinate frame of the laboratory, ignoring the z axis. No more cells were bound to the local reference frame of the recording apparatus than would be predicted by chance. The partial remapping demonstrated that the place cell system was sensitive to the three-dimensional manipulations of the recording apparatus. Nonetheless the results were not consistent with an explicit three-dimensional tuning of individual hippocampal neurons nor were they consistent with a model in which different sets of cells are tightly coupled to different sets of environmental cues. The results are most consistent with the statement that hippocampal neurons can change their "tuning functions" in arbitrary ways when features of the sensory input or behavioral context are altered. Understanding the rules that govern the remapping phenomenon holds promise for deciphering the neural circuitry underlying hippocampal function.

The hippocampal lamella hypothesis revisited

We have re-examined the hippocampal lamellar organization of the CA3-to-CA1 connection. Based on a new technique with electrophysiological quantification of Schaffer collateral density, and a review of recent literature, we conclude that the lamellar organization remains a useful concept for understanding hippocampal connectivity. Using a sheet-like hippocampal preparation, containing the whole CA1 region, we mapped the distribution of Schaffer collaterals by two procedures. First, we recorded the amplitude of the Schaffer compound action potential in various parts of CA1 after stimulation of a point in CA3. Second, we charted the CA1 positions from which we could antidromically excite individual CA3 neurones. Although the Schaffer collaterals radiated from their CA3 cells of origin within a wide, fan-shaped area, covering a large part of the septo-temporal extent of CA1, the amplitude of the compound action potential was largest in a slightly oblique, transverse band across the CA1 towards the subicular region.

Neurophysiology of Spatial Cognition

Understanding of the neurophysiology of cognition is advancing through the study of how animals navigate and understand space. Manipulating various classes of spatial information and recording from hippocampal neurons provides a robust model for understanding how the brain stores and constructs the spatial memories that are critical for organizing daily experience.

Hippocampectomized rats can use a constellation of landmarks to recognize a place

The hippocampus, one of the most studied regions of the mammalian forebrain, plays some well-established roles in topographic navigation. For two decades, one widely accepted explanation for the observed impairment of hippocampectomized rats in spatial navigation has been an inability to form place representations. In this report, we present a direct experimental evidence that animals with hippocampal lesions can learn to recognize places using the constellation of distinct landmarks. The extrahippocampal implementation of all three basic constituents of topographic orientation - guidance, vector navigation, and place recognition - shows that the hippocampus, and its place cells, serve a much more specialized cognitive function than previously thought. We propose that this function includes multi-place and multi-vector topographic integration.

Conjoint control of hippocampal place cell firing by two visual stimuli. Ii. A vector-field theory that predicts modifications of the representation of the environment

Changing the angular separation between two visual stimuli attached to the wall of a recording cylinder causes the firing fields of place cells to move relative to each other, as though the representation of the floor undergoes a topological distortion. The displacement of the firing field center of each cell is a vector whose length is equal to the linear displacement and whose angle indicates the direction that the field center moves in the environment. Based on the observation that neighboring fields move in similar ways, whereas widely separated fields tend to move relative to each other, we develop an empirical vector-field model that accounts for the stated effects of changing the card separation. We then go on to show that the same vector-field equation predicts additional aspects of the experimental results. In one example, we demonstrate that place cell firing fields undergo distortions of shape after the card separation is changed, as though different parts of the same field are affected by the stimulus constellation in the same fashion as fields at different locations. We conclude that the vector-field formalism reflects the organization of the place-cell representation of the environment for the current case, and through suitable modification may be very useful for describing motions of firing patterns induced by a wide variety of stimulus manipulations.

Conjoint control of hippocampal place cell firing by two visual stimuli. I. The effects of moving the stimuli on firing field positions

To better understand how hippocampal place cell activity is controlled by sensory stimuli, and to further elucidate the nature of the environmental representation provided by place cells, we have made recordings in the presence of two distinct visual stimuli under standard conditions and after several manipulations of these stimuli. In line with a great deal of earlier work, we find that place cell activity is constant when repeated recordings are made in the standard conditions in which the centers of the two stimuli, a black card and a white card, are separated by 135 degrees on the wall of a cylindrical recording chamber. Rotating the two stimuli by 45 degrees causes equal rotations of place cell firing fields. Removing either card and rotating the other card also causes fields to rotate equally, showing that the two stimuli are individually salient. Increasing or decreasing the card separation (card reconfiguration) causes a topological distortion of the representation of the cylinder floor such that field centers move relative to each other. We also found that either kind of reconfiguration induces a position-independent decrease in the intensity of place cell firing. We argue that these results are not compatible with either of two previously stated views of the place cell representation; namely, a nonspatial theory in which each place cell is tuned to an arbitrarily selected subset of available stimuli or a rigid map theory. We propose that our results imply that the representation is map-like but not rigid; it is capable of undergoing stretches without altering the local arrangement of firing fields.

Latent attractors: a model for context-dependent place representations in the hippocampus

Cells throughout the rodent hippocampal system show place-specific patterns of firing called place fields, creating a coarse-coded representation of location. The dependencies of this place code--or cognitive map--on sensory cues have been investigated extensively, and several computational models have been developed to explain them. However, place representations also exhibit strong dependence on spatial and behavioral context, and identical sensory environments can produce very different place codes in different situations. Several recent studies have proposed models for the computational basis of this phenomenon, but it is still not completely understood. In this article, we present a very simple connectionist model for producing context-dependent place representations in the hippocampus. We propose that context dependence arises in the dentate gyrus-hilus (DGH) system, which functions as a dynamic selector, disposing a small group of granule and pyramidal cells to fire in response to afferent stimulus while depressing the rest. It is hypothesized that the DGH system dynamics has "latent attractors," which are unmasked by the afferent input and channel system activity into subpopulations of cells in the DG, CA3, and other hippocampal regions as observed experimentally. The proposed model shows that a minimally structured hippocampus-like system can robustly produce context-dependent place codes with realistic attributes.

Non-spatial water radial-arm maze learning in mice

Recently, we published a method for examining working and reference memory in mice using a spatial version of the water radial-arm maze. Here we describe a non-spatial version of the same maze. BXSB mice were able to learn the maze as shown by the decrease in the number of working and reference memory errors over sessions. This maze was used to examine learning differences between males and females and between mice with misplaced clusters of neurons in layer I of cortex (ectopias) and those without. In a prior study using the spatial version of the water radial-arm maze, male BXSB mice had poorer working memory than females during the acquisition phase. Similarly, in this study male BXSB mice demonstrated impaired working memory during the asymptotic phase of non-spatial radial-arm maze learning. Two prior studies showed that mice with neocortical ectopias demonstrated working memory impairments compared to non-ectopic littermates in the spatial version of the water radial-arm maze. Contrary to this, in the non-spatial radial-arm maze used here, ectopic mice were not impaired in working memory and showed better memory when the working memory 'load' was the highest. Overall, both versions of the maze can be useful tools to assess spatial and non-spatial working and reference memory in mice.

Contribution of multiple sensory information to place field stability in hippocampal place cells

Hippocampal place cells in rats display spatially selective firing in relation to both external and internal cues. In the present study, we assessed the effects of removing visual and/or olfactory cues on place field stability. Place cell activity was recorded as rats searched for randomly scattered food in a cylinder. During an initial recording session, the lights were on and the only available cue was a single white cue card. Following this session, three sessions were run in a row with the cue card removed. In addition, the lights were either turned off or left on and the floor was either cleaned or left unchanged, thus creating four conditions: dark/cleaning, dark/no cleaning, light/cleaning, and light/no cleaning. A fifth session was run with the cue card back on the cylinder wall and the lights turned on. The rat remained in the cylinder during all sessions without being removed at any time. In the dark/cleaning and light/cleaning conditions, most place fields were not stable (i.e., abruptly shifted position). In addition, half of the cells stopped firing in the dark/cleaning condition. In contrast, in the dark/no cleaning and light/no cleaning conditions, most place fields remained stable across sessions. These results suggest that 1) rats are not able to rely on only movement-related information to maintain a stable place representation, 2) visual input is necessary for the firing of a large number of cells, and 3) olfactory information can be used to compensate for the lack of visuospatial information.

Understanding hippocampal activity by using purposeful behavior: place navigation induces place cell discharge in both task-relevant and task-irrelevant spatial reference frames

Continuous rotation of an arena in a cue-rich room dissociates the stationary room-bound information from the rotating arena-bound information. This disrupted spatial discharge in the majority of place cells from rats trained to collect randomly scattered food. In contrast, most place cell firing patterns recorded from rats trained to solve a navigation task on the rotating arena were preserved during the rotation. Spatial discharge was preserved in both the task-relevant stationary and the task-irrelevant rotating reference frames, but firing was more organized in the task-relevant frame. It is concluded that, (i) the effects of environmental manipulations can be understood with confidence only when the rat's purposeful behavior is used to formulate interpretations of the data, and (ii) hippocampal place cell activity is organized in multiple overlapping spatial reference frames.

Understanding hippocampal activity by using purposeful behavior: Place navigation induces place cell discharge in both task-relevant and task-irrelevant spatial reference frames

Continuous rotation of an arena in a cue-rich room dissociates the stationary room-bound information from the rotating arena-bound information. This disrupted spatial discharge in the majority of place cells from rats trained to collect randomly scattered food. In contrast, most place cell firing patterns recorded from rats trained to solve a navigation task on the rotating arena were preserved during the rotation. Spatial discharge was preserved in both the task-relevant stationary and the task-irrelevant rotating reference frames, but firing was more organized in the task-relevant frame. It is concluded that, (i) the effects of environmental manipulations can be understood with confidence only when the rat's purposeful behavior is used to formulate interpretations of the data, and (ii) hippocampal place cell activity is organized in multiple overlapping spatial reference frames.

Synaptogenesis of mossy fibers induced by spatial water maze overtraining

Synaptic plasticity has been proposed as a mechanism underlying learning and memory. Synaptic reorganization of hippocampal mossy fibers has been observed after experimentally induced epilepsy, and after brief high-frequency activation inducing long-term potentiation. Furthermore, it has been suggested that synaptic changes in the hippocampus may occur after spatial learning. In this study, by using a zinc-detecting histologic technique (Timm), we demonstrate a significant increase of mossy fiber terminals in the CA3 stratum oriens region induced by training rats during 3 days in a spatial Morris water maze. In contrast, animals trained for only 1 day and animals that were just allowed to swim or were overtrained in a stress-motivated inhibitory avoidance task did not show increments of mossy fiber terminals in the stratum oriens. Electron microscopy confirmed that synaptic density of mossy fiber terminals in the stratum oriens increases significantly in water maze overtrained animals compared with the swimming control animals. Taken together, these results suggest that overtraining in a spatial learning task induces mossy fiber synaptogenesis that could be involved in the mechanisms underlying long-term memory storage. Hippocampus 1999;9:631-636.

Distribution of spatial and nonspatial information in dorsal hippocampus

The hippocampus in the mammalian brain is required for the encoding of current and the retention of past experience. Previous studies have shown that the hippocampus contains neurons that encode information required to perform spatial and nonspatial short-term memory tasks. A more detailed understanding of the functional anatomy of the hippocampus would provide important insight into how such encoding occurs. Here we show that hippocampal neurons in the rat are distributed anatomically in distinct segments along the length of the hippocampus. Each longitudinal segment contains clusters of neurons that become active when the animal performs a task with spatial attributes. Within these same segments are ordered arrangements of neurons that encode the nonspatial aspects of the task appropriate to those spatial features. Thus, anatomical segregation of spatial information, together with the interleaved representation of nonspatial information, represents a structural framework that may help to resolve conflicting views of hippocampal function.

Hippocampal encoding of non-spatial trace conditioning

Trace eyeblink classical conditioning is a non-spatial learning paradigm that requires an intact hippocampus. This task is hippocampus-dependent because the auditory tone conditioned stimulus (CS) is temporally separated from the corneal airpuff unconditioned stimulus (US) by a 500-ms trace interval. Our laboratory has performed a series of neurophysiological experiments that have examined the activity of pyramidal cells in the CA1 area of the hippocampus during trace eyeblink conditioning. We have found that the non-spatial stimuli involved in this paradigm are encoded in the hippocampus in a logical order that is necessary for their association and the subsequent expression of behavioral learning. Although there were many profiles of single neurons responding to the CS-US trial during training, the majority of the neurons showed an increase in activity to the airpuff-US. Prior to learning, it appears that hippocampal cells and ensembles of cells were preferentially attending to the stimulus with immediate behavioral importance, the US. Hippocampal cells then began to respond to the associated neutral stimulus, the CS. Shortly thereafter, animals began to show increases in the behavioral expression of CRs. In some experiments, hippocampal neurons from aged animals exhibited impairments in the encoding of CS and US information. These aged animals were not able to associate these stimuli and acquire trace eyeblink CRs. Our findings along with the findings of other spatial learning studies, suggest that the hippocampus is involved in encoding information about discontiguous sets of stimuli, either spatial or nonspatial, especially early in the learning process.

Further study of the control of place cell firing by intra-apparatus objects

The angular positions of hippocampal place cell firing fields are accurately controlled by the position of a single salient cue card attached to the wall of a recording cylinder; when the card is rotated, fields rotate equally. In contrast, the control exerted by 3-dimensional objects placed directly in the recording arena depends on their arrangement. When three objects lie on the vertices of an isosceles triangle near the center of the cylinder they rarely exert any control over the angular positions of firing fields. However, if the isosceles triangle is dilated so that its vertices are against the apparatus wall, the objects exert virtually ideal control over angular field position. Why do the objects gain control when they are against the cylinder wall? One possibility is that the asymmetry in the object set is more easily detected when the objects are far apart so that they provide a better polarizing cue. This hypothesis assumes that the identity of individual landmarks is not recognized by the place cell system whereas their geometric arrangement provides crucial information for controlling place field positions. If this is true, putting the 3 objects against the cylinder wall on the vertices of an equilateral triangle should cause a loss of stimulus control over the angular positions of firing fields. To the contrary, we found that the firing fields of most place cells (23/29) were accurately controlled by the equilateral object arrangement. Moreover, 5/6 of the uncontrolled cells were in a single animal. These results bolster our previous suggestion that the centrally placed objects fail to control place field positions because the computations necessary to form a stable reference frame are very difficult when the animal can go between stimuli.

Complimentary roles for hippocampal versus subicular/entorhinal place cells in coding place, context, and events

At least two important questions are posed by the existence of hippocampal place cells. The first of these has to do with how the complex, abstract properties exhibited by these cells can be explained mechanistically. The second has to do with the implications of place cells for our conception of the broader role of the hippocampus in spatial and other behaviors. Here, evidence is reviewed that: (1) Hippocampal cells show different "maps" (place cell representations) for each environment the animal visits and, in fact, can show multiple maps even for any one environment. The choice of the current map for any one environment depends on environmental, contextual, and event-related variables. (2) Cells in the subiculum and entorhinal cortex also show location-specific firing patterns (like hippocampal place cells), but show the same pattern for each environment the animal visits. A model is presented that is a variant of hippocampus-based path integration models developed by McNaughton and colleagues. In this version, the subiculum and entorhinal cortex work together to form a single, universal map that is used for each environment, and that can exhibit path integration abilities. The universal subicular/entorhinal representation is postulated to assist the hippocampal layer to rapidly form new environment and context specific "maps" for each new environment/temporal context ("episode") the animal experiences. In this view, hippocampal layer activity is always obligatorily spatial, due to the input from the entorhinal universal "map." However, the fact that the hippocampus generates a new map in response to global, non-spatial, contextual attributes of each situation, means that the hippocampus is always coding non-spatial aspects of a situation using its obligatorily spatial code. This brings the hippocampal place cell activity in to line with the broader role that has been postulated for the hippocampus in learning and memory functions.