Deciphering the hippocampal polyglot: the hippocampus as a path integration system

Behavioral and Neural Subsystems of Rodent Exploration

Animals occupy territories in which resources such as food and shelter are often distributed unevenly. While studies of exploratory behavior have typically involved the laboratory rodent as an experimental subject, questions regarding what constitutes exploration have dominated. A recent line of research has utilized a descriptive approach to the study of rodent exploration, which has revealed that this behavior is organized into movement subsystems that can be readily quantified. The movements include home base behavior, which serves as a central point of attraction from which rats and mice organize exploratory trips into the remaining environment. In this review, we describe some of the features of this organized behavior pattern as well as its modulation by sensory cues and previous experience. We conclude the review by summarizing research investigating the neurobiological bases of exploration, which we hope will stimulate renewed interest and research on the neural systems mediating rodent exploratory behavior.

Entorhinal fast-spiking speed cells project to the hippocampus

Our location in space is represented by a spectrum of space and direction-responsive cell types in medial entorhinal cortex and hippocampus. Many cells in these areas respond also to running speed. The presence of local speed-tuned cells is considered a requirement for position to be encoded in a self-motion–dependent manner; however, whether and how speed-responsive cells in entorhinal cortex and hippocampus are functionally connected have not been determined. The present study shows that a large proportion of entorhinal speed cells are fast-spiking with properties similar to those of GABAergic interneurons and that outputs from a subset of these cells, particularly the parvalbumin-expressing subset, form a component of the medial entorhinal input to the hippocampus.

The mammalian positioning system contains a variety of functionally specialized cells in the medial entorhinal cortex (MEC) and the hippocampus. In order for cells in these systems to dynamically update representations in a way that reflects ongoing movement in the environment, they must be able to read out the current speed of the animal. Speed is encoded by speed-responsive cells in both MEC and hippocampus, but the relationship between the two populations has not been determined. We show here that many entorhinal speed cells are fast-spiking putative GABAergic neurons. Using retrograde viral labeling from the hippocampus, we find that a subset of these fast-spiking MEC speed cells project directly to hippocampal areas. This projection contains parvalbumin (PV) but not somatostatin (SOM)-immunopositive cells. The data point to PV-expressing GABAergic projection neurons in MEC as a source for widespread speed modulation and temporal synchronization in entorhinal–hippocampal circuits for place representation.

Navigational Strategies and Their Neural Correlates

Animals depend on navigation to find food, water, mate(s), shelter, etc. Different species use diverse strategies that utilise forms of motion- and location-related information derived from the environment to navigate to their goals and back. We start by describing behavioural studies undertaken to unearth different strategies used in navigation. Then we move on to outline what we know about the brain area most associated with spatial navigation, namely the hippocampal formation. While doing so, we first briefly explain the anatomical connections in the area and then proceed to describe the neural correlates that are considered to play a role in navigation. We conclude by looking at how the strategies might interact and complement each other in certain contexts.

Spatial representation in the hippocampal formation: a history

Since the first place cell was recorded and the cognitive-map theory was subsequently formulated, investigation of spatial representation in the hippocampal formation has evolved in stages. Early studies sought to verify the spatial nature of place cell activity and determine its sensory origin. A new epoch started with the discovery of head direction cells and the realization of the importance of angular and linear movement-integration in generating spatial maps. A third epoch began when investigators turned their attention to the entorhinal cortex, which led to the discovery of grid cells and border cells. This review will show how ideas about integration of self-motion cues have shaped our understanding of spatial representation in hippocampal-entorhinal systems from the 1970s until today. It is now possible to investigate how specialized cell types of these systems work together, and spatial mapping may become one of the first cognitive functions to be understood in mechanistic detail.

Theta sequences of grid cell populations can provide a movement-direction signal

It has been proposed that path integration in mammals is performed by the convergence of internally generated speed and directional inputs onto grid cells. Although this hypothesis has been supported by the discovery that head direction, speed, and grid cells are intermixed within entorhinal cortex and by the recent finding that head-direction inputs are necessary for grid firing, many details on how grid cells are generated have remained elusive. For example, analysis of recording data suggests that substituting head direction for movement direction accrues errors that preclude the formation of grid patterns. To address this discrepancy, we propose that the organization of grid networks makes it plausible that movement-direction signals are an output from grid cells and that temporally precise grid cell sequences provide a robust directional signal to other spatial and directional cell types.

The Aging Navigational System

The discovery of neuronal systems dedicated to computing spatial information, composed of functionally distinct cell types such as place and grid cells, combined with an extensive body of human-based behavioral and neuroimaging research has provided us with a detailed understanding of the brain's navigation circuit. In this review, we discuss emerging evidence from rodents, non-human primates, and humans that demonstrates how cognitive aging affects the navigational computations supported by these systems. Critically, we show 1) that navigational deficits cannot solely be explained by general deficits in learning and memory, 2) that there is no uniform decline across different navigational computations, and 3) that navigational deficits might be sensitive markers for impending pathological decline. Following an introduction to the mechanisms underlying spatial navigation and how they relate to general processes of learning and memory, the review discusses how aging affects the perception and integration of spatial information, the creation and storage of memory traces for spatial information, and the use of spatial information during navigational behavior. The closing section highlights the clinical potential of behavioral and neural markers of spatial navigation, with a particular emphasis on neurodegenerative disorders.

Sparse orthogonal population representation of spatial context in the retrosplenial cortex

Sparse orthogonal coding is a key feature of hippocampal neural activity, which is believed to increase episodic memory capacity and to assist in navigation. Some retrosplenial cortex (RSC) neurons convey distributed spatial and navigational signals, but place-field representations such as observed in the hippocampus have not been reported. Combining cellular Ca²⁺ imaging in RSC of mice with a head-fixed locomotion assay, we identified a population of RSC neurons, located predominantly in superficial layers, whose ensemble activity closely resembles that of hippocampal CA1 place cells during the same task. Like CA1 place cells, these RSC neurons fire in sequences during movement, and show narrowly tuned firing fields that form a sparse, orthogonal code correlated with location. RSC ‘place’ cell activity is robust to environmental manipulations, showing partial remapping similar to that observed in CA1. This population code for spatial context may assist the RSC in its role in memory and/or navigation.

Neurons in the retrosplenial cortex (RSC) encode spatial and navigational signals. Here the authors use calcium imaging to show that, similar to the hippocampus, RSC neurons also encode place cell-like activity in a sparse orthogonal representation, partially anchored to the allocentric cues on the linear track.

Theta-paced flickering between place-cell maps in the hippocampus: A model based on short-term synaptic plasticity

Hippocampal place cells represent different environments with distinct neural activity patterns. Following an abrupt switch between two familiar configurations of visual cues defining two environments, the hippocampal neural activity pattern switches almost immediately to the corresponding representation. Surprisingly, during a transient period following the switch to the new environment, occasional fast transitions between the two activity patterns (flickering) were observed (Jezek, Henriksen, Treves, Moser, & Moser, 2011). Here we show that an attractor neural network model of place cells with connections endowed with short‐term synaptic plasticity can account for this phenomenon. A memory trace of the recent history of network activity is maintained in the state of the synapses, allowing the network to temporarily reactivate the representation of the previous environment in the absence of the corresponding sensory cues. The model predicts that the number of flickering events depends on the amplitude of the ongoing theta rhythm and the distance between the current position of the animal and its position at the time of cue switching. We test these predictions with new analysis of experimental data. These results suggest a potential role of short‐term synaptic plasticity in recruiting the activity of different cell assemblies and in shaping hippocampal activity of behaving animals.

Building a cognitive map by assembling multiple path integration systems

Path integration and cognitive mapping are two of the most important mechanisms for navigation. Path integration is a primitive navigation system which computes a homing vector based on an animal's self-motion estimation, while cognitive map is an advanced spatial representation containing richer spatial information about the environment that is persistent and can be used to guide flexible navigation to multiple locations. Most theories of navigation conceptualize them as two distinctive, independent mechanisms, although the path integration system may provide useful information for the integration of cognitive maps. This paper demonstrates a fundamentally different scenario, where a cognitive map is constructed in three simple steps by assembling multiple path integrators and extending their basic features. The fact that a collection of path integration systems can be turned into a cognitive map suggests the possibility that cognitive maps may have evolved directly from the path integration system.

Segregated Cell Populations Enable Distinct Parallel Encoding within the Radial Axis of the CA1 Pyramidal Layer

Numerous studies have implicated the hippocampus in the encoding and storage of declarative and spatial memories. Several models have considered the hippocampus and its distinct subfields to contain homogeneous pyramidal cell populations. Yet, recent studies have led to a consensus that the dorso-ventral and proximo-distal axes have different connectivities and physiologies. The remaining deep-superficial axis of the pyramidal layer, however, remains relatively unexplored due to a lack of techniques that can record from neurons simultaneously at different depths. Recent advances in transgenic mice, two-photon imaging and dense multisite recording have revealed extensive disparities between the pyramidal cells located in the deep and the superficial layers. Here, we summarize differences between the two populations in terms of gene expression and connectivity with other intra-hippocampal subregions and local interneurons that underlie distinct learning processes and spatial representations. A unified picture will emerge to describe how such local segregations can increase the capacity of the hippocampus to compute and process numerous tasks in parallel.

A Computational Model for Spatial Navigation Based on Reference Frames in the Hippocampus, Retrosplenial Cortex, and Posterior Parietal Cortex

Behavioral studies for humans, monkeys, and rats have shown that, while traversing an environment, these mammals tend to use different frames of reference and frequently switch between them. These frames represent allocentric, egocentric, or route-centric views of the environment. However, combinations of either of them are often deployed. Neurophysiological studies on rats have indicated that the hippocampus, the retrosplenial cortex, and the posterior parietal cortex contribute to the formation of these frames and mediate the transformation between those. In this paper, we construct a computational model of the posterior parietal cortex and the retrosplenial cortex for spatial navigation. We demonstrate how the transformation of reference frames could be realized in the brain and suggest how different brain areas might use these reference frames to form navigational strategies and predict under what conditions an animal might use a specific type of reference frame. Our simulated navigation experiments demonstrate that the model’s results closely resemble behavioral findings in humans and rats. These results suggest that navigation strategies may depend on the animal’s reliance in a particular reference frame and shows how low confidence in a reference frame can lead to fluid adaptation and deployment of alternative navigation strategies. Because of its flexibility, our biologically inspired navigation system may be applied to autonomous robots.

Heterogeneous spatial representation by different subpopulations of neurons in the subiculum

The subiculum is a pivotal structure located in the hippocampal formation that receives inputs from grid and place cells and that mediates the output from the hippocampus to cortical and sub-cortical areas. Previous studies have demonstrated the existence of boundary vector cells (BVC) in the subiculum, as well as exceptional stability during recordings conducted in the dark, suggesting that the subiculum is involved in the coding of allocentric cues and also in path integration. In order to better understand the role of the subiculum in spatial processing and the coding of external cues, we recorded subicular units in freely moving rats while performing two experiments: the "size experiment" in which we modified the arena size, and the "barrier experiment" in which we inserted new barriers in a familiar open field thus dividing the enclosure into four comparable sub-chambers. We hypothesized that if physical boundaries were deterministic of the firing of subicular units a strong spatial replication pattern would be found in most spatially modulated units. In contrast, our results demonstrate heterogeneous space coding by different cell types: place cells, barrier-related units and BVC. We also found units characterized by narrow spike waveforms, most likely belonging to axonal recordings, that showed grid-like patterns. Our data indicate that the subiculum codes space in a flexible manner, and that it is involved in the processing of allocentric information, external cues and path integration, thus broadly supporting spatial navigation.

Spatial coding and physiological properties of hippocampal neurons in the Cornu Ammonis subregions

It is well-established that the feed-forward connected main hippocampal areas, CA3, CA2, and CA1 work cooperatively during spatial navigation and memory. These areas are similar in terms of the prevalent types of neurons; however, they display different spatial coding and oscillatory dynamics. Understanding the temporal dynamics of these operations requires simultaneous recordings from these regions. However, simultaneous recordings from multiple regions and subregions in behaving animals have become possible only recently. We performed large-scale silicon probe recordings simultaneously spanning across all layers of CA1, CA2, and CA3 regions in rats during spatial navigation and sleep and compared their behavior-dependent spiking, oscillatory dynamics and functional connectivity. The accuracy of place cell spatial coding increased progressively from distal to proximal CA1, suddenly dropped in CA2, and increased again from CA3a toward CA3c. These variations can be attributed in part to the different entorhinal inputs to each subregions, and the differences in theta modulation of CA1, CA2, and CA3 neurons. We also found that neurons in the subregions showed differences in theta modulation, phase precession, state-dependent changes in firing rates and functional connectivity among neurons of these regions. Our results indicate that a combination of intrinsic properties together with distinct intra- and extra-hippocampal inputs may account for the subregion-specific modulation of spiking dynamics and spatial tuning of neurons during behavior. © 2016 Wiley Periodicals, Inc.

Influence of Proximal, Distal, and Vestibular Frames of Reference in Object-Place Paired Associate Learning in the Rat

Object-place paired associate learning has been used to test hypotheses regarding the neurobiological basis of memory in rodents. Much of this work has focused on the role of limbic and hippocampal-parahippocampal regions, as well as the use of spatial information derived from allothetic visual stimuli to determine location in an environment. It has been suggested that idiothetic self-motion (vestibular) signals and internal representations of directional orientation might play an important role in disambiguating between spatial locations when forming object-place associations, but this hypothesis has not been explicitly tested. In the present study, we investigated the relationship between allothetic (i.e., distal and proximal cues) and vestibular stimuli on performance of an object-place paired-associate task. The paired-associate task was composed of learning to discriminate between an identical pair of objects presented in 180° opposite arms of a radial arm maze. Thus, animals were required to select a particular object on the basis of spatial location (i.e., maze arm). After the animals acquired the object-place rule, a series of probe tests determined that rats utilize self-generated vestibular cues to discriminate between the two maze arms. Further, when available, animals showed a strong preference for local proximal cues associated with the maze. Together, the work presented here supports the establishment of an object-place task that requires both idiothetic and allothetic stimulus sources to guide choice behavior, and which can be used to further investigate the dynamic interactions between neural systems involved in pairing sensory information with spatial locations.

Nonuniform allocation of hippocampal neurons to place fields across all hippocampal subfields

The mechanisms governing how the hippocampus selects neurons to exhibit place fields are not well understood. A default assumption in some previous studies was the uniform random draw with replacement (URDWR) model, which, theoretically, maximizes spatial "pattern separation", and predicts a Poisson distribution of the numbers of place fields expressed by a given cell per unit area. The actual distribution of mean firing rates exhibited by a population of hippocampal neurons, however, is approximately exponential or log-normal in a given environment and these rates are somewhat correlated across multiple places, at least under some conditions. The advantage of neural activity-dependent immediate-early gene (IEG) analysis, as a proxy for electrophysiological recording, is the ability to obtain much larger samples of cells, even those whose activity is so sparse that they are overlooked in recording studies. Thus, a more accurate representation of the activation statistics can potentially be achieved. Some previous IEG studies that examined behavior-driven IEG expression in CA1 appear to support URDWR. There was, however, in some of the same studies, an under-recruitment of dentate gyrus granule cells, indicating a highly skewed excitability distribution, which is inconsistent with URDWR. Although it was suggested that this skewness might be related to increased excitability of recently generated granule cells, we show here that CA1, CA3, and subiculum also exhibit cumulative under-recruitment of neurons. Thus, a highly skewed excitability distribution is a general principle common to all major hippocampal subfields. Finally, a more detailed analysis of the frequency distributions of IEG intranuclear transcription foci suggests that a large fraction of hippocampal neurons is virtually silent, even during sleep. Whether the skewing of the excitability distribution is cell-intrinsic or a network phenomenon, and the degree to which this excitability is fixed or possibly time-varying are open questions for future studies. © 2016 Wiley Periodicals, Inc.

Neuronal Implementation of Hippocampal-Mediated Spatial Behavior: A Comparative Evolutionary Perspective

The hippocampal formation (HF) of mammals and birds plays a strikingly similar role in the representation of space. This evolutionarily conserved property, however, belies the contrasting spatial ecology of animals such as rats and homing pigeons, differing spatial ecologies that should have promoted the evolution of group-specific adaptations to the HF representation of space. However, the spatial response properties of pigeon and rat HF neurons reveal surprising similarity in the contribution of position, direction, and trajectory toward explaining spatial variation in firing rate. By contrast, the asymmetrical distribution of neuronal response properties in the left and right HF of homing pigeons, but not rats, indicates a difference in network organization. The authors propose that hippocampal evolution may be characterized by inertia with respect to changes in the basic spatial elements that determine the response properties of neurons but considerable plasticity in how the neuronal response elements are organized into functional networks.

Anxiolytic-like effects of leptin on fixed interval responding

Leptin has been shown to affect energy homeostasis, learning and memory, and some models of anxiolytic action. However, leptin has produced inconsistent results in previous non-operant behavioural tests of anxiety. Here, we test the anxiolytic potential of leptin in an operant paradigm that has produced positive results across all classes of anxiolytic so far tested. Rats were tested in the Fixed Interval 60 Seconds (FI60) task following administration of 0/0.5/1.0mg/kg (i.p.) leptin or an active anxiolytic control of 5mg/kg (i.p.) chlordiazepoxide (CDP). By the end of the 14days of testing in the FI60 task, 0.5mg/kg leptin released suppressed responding in a manner similar to CDP, and 1.0mg/kg leptin produced a relative depression in responding, a similar outcome pattern to previously tested 5HT-agonist anxiolytics. This suggests that leptin behaves similarly to established serotonergic anxiolytics such as buspirone and fluoxetine; with the delay in development of effect during testing, and the inverted-U dose-response curve explaining the inconsistent behaviour of leptin in behavioural tests of anxiety, as this type of pattern is common to serotonergic anxiolytics.

Topological Schemas of Cognitive Maps and Spatial Learning

Spatial navigation in mammals is based on building a mental representation of their environment-a cognitive map. However, both the nature of this cognitive map and its underpinning in neural structures and activity remains vague. A key difficulty is that these maps are collective, emergent phenomena that cannot be reduced to a simple combination of inputs provided by individual neurons. In this paper we suggest computational frameworks for integrating the spiking signals of individual cells into a spatial map, which we call schemas. We provide examples of four schemas defined by different types of topological relations that may be neurophysiologically encoded in the brain and demonstrate that each schema provides its own large-scale characteristics of the environment-the schema integrals. Moreover, we find that, in all cases, these integrals are learned at a rate which is faster than the rate of complete training of neural networks. Thus, the proposed schema framework differentiates between the cognitive aspect of spatial learning and the physiological aspect at the neural network level.

The Art of Grid Fields: Geometry of Neuronal Time

The discovery of grid cells in the entorhinal cortex has both elucidated our understanding of spatial representations in the brain, and germinated a large number of theoretical models regarding the mechanisms of these cells' striking spatial firing characteristics. These models cross multiple neurobiological levels that include intrinsic membrane resonance, dendritic integration, after hyperpolarization characteristics and attractor dynamics. Despite the breadth of the models, to our knowledge, parallels can be drawn between grid fields and other temporal dynamics observed in nature, much of which was described by Art Winfree and colleagues long before the initial description of grid fields. Using theoretical and mathematical investigations of oscillators, in a wide array of mediums far from the neurobiology of grid cells, Art Winfree has provided a substantial amount of research with significant and profound similarities. These theories provide specific inferences into the biological mechanisms and extraordinary resemblances across phenomenon. Therefore, this manuscript provides a novel interpretation on the phenomenon of grid fields, from the perspective of coupled oscillators, postulating that grid fields are the spatial representation of phase resetting curves in the brain. In contrast to prior models of gird cells, the current manuscript provides a sketch by which a small network of neurons, each with oscillatory components can operate to form grid cells, perhaps providing a unique hybrid between the competing attractor neural network and oscillatory interference models. The intention of this new interpretation of the data is to encourage novel testable hypotheses.

Driving Performance Among Patients with Cirrhosis Who Drove to Their Outpatient Hepatology Clinic Appointments

BACKGROUND

Minimal hepatic encephalopathy (MHE) may adversely affect driving skills.

AIMS

To compare the driving performance of cirrhotic patients with and without prior HE as well as controls using a driving stimulator and to correlate psychometric testing with driving performance.

METHODS

Adult patients with cirrhosis, who drove to the outpatient clinic for their routine appointments underwent a battery of driving and psychometric tests including number connection tests A & B (NCT-A and NCT-B), digit symbol test (DST) and critical flicker and fusion frequency (CFF) testing.

RESULTS

Cirrhotics had significantly higher NCT-A (39.3 s vs. 31.2 s, P = 0.006) and DST scores (317 s vs. 245 s, P = 0.012), and lower CFF scores Fusion (33 vs. 36 Hz, P = 0.05), Flicker (35 vs. 42 Hz, P = 0.007) than controls. There was no difference in NCT-A, DST and CFF scores between patients with and without HE. Ten (22%) patients, 7 (27%) with prior HE and 3 (15%) without prior HE, had abnormal NCT-A scores (i.e. >control mean ± 2SD), and 12% of patients with prior HE had one or more driving test accidents, while controls and patients without prior HE had none. Patients with cirrhosis were more likely to hit pedestrians compared to controls (P = 0.05). There was no correlation between CFF, DST and NCTB scores with driving performance test results.

CONCLUSIONS

Unlike previous reports, no significant differences were noted between the patients with and without prior HE on psychometric testing, and on the driving simulator, but driving accidents were seen in only those with previous history of HE.

Continuous attractor network models of grid cell firing based on excitatory-inhibitory interactions

Neurons in the medial entorhinal cortex encode location through spatial firing fields that have a grid‐like organisation. The challenge of identifying mechanisms for grid firing has been addressed through experimental and theoretical investigations of medial entorhinal circuits. Here, we discuss evidence for continuous attractor network models that account for grid firing by synaptic interactions between excitatory and inhibitory cells. These models assume that grid‐like firing patterns are the result of computation of location from velocity inputs, with additional spatial input required to oppose drift in the attractor state. We focus on properties of continuous attractor networks that are revealed by explicitly considering excitatory and inhibitory neurons, their connectivity and their membrane potential dynamics. Models at this level of detail can account for theta‐nested gamma oscillations as well as grid firing, predict spatial firing of interneurons as well as excitatory cells, show how gamma oscillations can be modulated independently from spatial computations, reveal critical roles for neuronal noise, and demonstrate that only a subset of excitatory cells in a network need have grid‐like firing fields. Evaluating experimental data against predictions from detailed network models will be important for establishing the mechanisms mediating grid firing.

Environmental boundaries as a mechanism for correcting and anchoring spatial maps

Ubiquitous throughout the animal kingdom, path integration-based navigation allows an animal to take a circuitous route out from a home base and using only self-motion cues, calculate a direct vector back. Despite variation in an animal's running speed and direction, medial entorhinal grid cells fire in repeating place-specific locations, pointing to the medial entorhinal circuit as a potential neural substrate for path integration-based spatial navigation. Supporting this idea, grid cells appear to provide an environment-independent metric representation of the animal's location in space and preserve their periodic firing structure even in complete darkness. However, a series of recent experiments indicate that spatially responsive medial entorhinal neurons depend on environmental cues in a more complex manner than previously proposed. While multiple types of landmarks may influence entorhinal spatial codes, environmental boundaries have emerged as salient landmarks that both correct error in entorhinal grid cells and bind internal spatial representations to the geometry of the external spatial world. The influence of boundaries on error correction and grid symmetry points to medial entorhinal border cells, which fire at a high rate only near environmental boundaries, as a potential neural substrate for landmark-driven control of spatial codes. The influence of border cells on other entorhinal cell populations, such as grid cells, could depend on plasticity, raising the possibility that experience plays a critical role in determining how external cues influence internal spatial representations.

Temporoparietal encoding of space and time during vestibular-guided orientation

When we walk in our environment, we readily determine our travelled distance and location using visual cues. In the dark, estimating travelled distance uses a combination of somatosensory and vestibular (i.e. inertial) cues. The observed inability of patients with complete peripheral vestibular failure to update their angular travelled distance during active or passive turns in the dark implies a privileged role for vestibular cues during human angular orientation. As vestibular signals only provide inertial cues of self-motion (e.g. velocity, °/s), the brain must convert motion information to distance information (a process called ‘path integration’) to maintain our spatial orientation during self-motion in the dark. It is unknown, however, what brain areas are involved in converting vestibular-motion signals to those that enable such vestibular-spatial orientation. Hence, using voxel-based lesion–symptom mapping techniques, we explored the effect of acute right hemisphere lesions in 18 patients on perceived angular position, velocity and motion duration during whole-body angular rotations in the dark. First, compared to healthy controls’ spatial orientation performance, we found that of the 18 acute stroke patients tested, only the four patients with damage to the temporoparietal junction showed impaired spatial orientation performance for leftward (contralesional) compared to rightward (ipsilesional) rotations. Second, only patients with temporoparietal junction damage showed a congruent underestimation in both their travelled distance (perceived as shorter) and motion duration (perceived as briefer) for leftward compared to rightward rotations. All 18 lesion patients tested showed normal self-motion perception. These data suggest that the cerebral cortical regions mediating vestibular-motion (‘am I moving?’) and vestibular-spatial perception (‘where am I?’) are distinct. Furthermore, the congruent contralesional deficit in time (motion duration) and position perception, seen only in temporoparietal junction patients, may reflect a common neural substrate in the temporoparietal junction that mediates the encoding of motion duration and travelled distance during vestibular-guided navigation. Alternatively, the deficits in timing and spatial orientation with temporoparietal junction lesions could be functionally linked, implying that the temporoparietal junction may act as a cortical temporal integrator, combining estimates of self-motion velocity over time to derive an estimate of travelled distance. This intriguing possibility predicts that timing abnormalities could lead to spatial disorientation.

A Prefrontal-Hippocampal Comparator for Goal-Directed Behavior: The Intentional Self and Episodic Memory

The hypothesis of this article is that the interactions between the prefrontal cortex and the hippocampus play a critical role in the modulation of goal-directed self-action and the strengthening of episodic memories. We describe various theories that model a comparator function for the hippocampus, and then elaborate the empirical evidence that supports these theories. One theory which describes a prefrontal-hippocampal comparator for voluntary action is emphasized. Action plans are essential for successful goal-directed behavior, and are elaborated by the prefrontal cortex. When an action plan is initiated, the prefrontal cortex transmits an efference copy (or corollary discharge) to the hippocampus where it is stored as a working memory for the action plan (which includes the expected outcomes of the action plan). The hippocampus then serves as a response intention-response outcome working memory comparator. Hippocampal comparator function is enabled by the hippocampal theta rhythm allowing the hippocampus to compare expected action outcomes to actual action outcomes. If the expected and actual outcomes match, the hippocampus transmits a signal to prefrontal cortex which strengthens or consolidates the action plan. If a mismatch occurs, the hippocampus transmits an error signal to the prefrontal cortex which facilitates a reformulation of the action plan, fostering behavioral flexibility and memory updating. The corollary discharge provides the self-referential component to the episodic memory, affording the personal and subjective experience of what behavior was carried out, when it was carried out, and in what context (where) it occurred. Such a perspective can be applied to episodic memory in humans, and episodic-like memory in non-human animal species.

Is there a pilot in the brain? Contribution of the self-positioning system to spatial navigation

Since the discovery of place cells, the hippocampus is thought to be the neural substrate of a cognitive map. The later discovery of head direction cells, grid cells and border cells, as well as of cells with more complex spatial signals, has led to the idea that there is a brain system devoted to providing the animal with the information required to achieve efficient navigation. Current questioning is focused on how these signals are integrated in the brain. In this review, we focus on the issue of how self-localization is performed in the hippocampal place cell map. To do so, we first shortly review the sensory information used by place cells and then explain how this sensory information can lead to two coding modes, respectively based on external landmarks (allothetic information) and self-motion cues (idiothetic information). We hypothesize that these two modes can be used concomitantly with the rat shifting from one mode to the other during its spatial displacements. We then speculate that sequential reactivation of place cells could participate in the resetting of self-localization under specific circumstances and in learning a new environment. Finally, we provide some predictions aimed at testing specific aspects of the proposed ideas.

Neural mechanisms of self-location

The ability to self-localise and to navigate to remembered goals in complex and changeable environments is crucial to the survival of many mobile species. Electrophysiological investigations of the mammalian hippocampus and associated brain structures have identified several classes of neurons which represent information about an organism's position and orientation. These include place cells, grid cells, head direction cells, and boundary vector cells, as well as cells representing aspects of self-motion. Understanding how these neural representations are formed and updated from environmental sensory information and from information relating to self-motion is an important topic attracting considerable current interest. Here we review the computational mechanisms thought to underlie the formation of these different spatial representations, the interactions between them, and their use in guiding behaviour. These include some of the clearest examples of computational mechanisms of general interest to neuroscience, such as attractor dynamics, temporal coding and multi-modal integration. We also discuss the close relationships between computational modelling and experimental research which are driving progress in this area.

Transitions between Spatial Attractors in Place-Cell Models

The spontaneous transitions between D-dimensional spatial maps in an attractor neural network are studied. Two scenarios for the transition from one map to another are found, depending on the level of noise: (i) through a mixed state, partly localized in both maps around positions where the maps are most similar, and (ii) through a weakly localized state in one of the two maps, followed by a condensation in the arrival map. Our predictions are confirmed by numerical simulations and qualitatively compared to recent recordings of hippocampal place cells during quick-environment-changing experiments in rats.

Understanding Emergent Dynamics: Using a Collective Activity Coordinate of a Neural Network to Recognize Time-Varying Patterns

In higher animals, complex and robust behaviors are produced by the microscopic details of large structured ensembles of neurons. I describe how the emergent computational dynamics of a biologically based neural network generates a robust natural solution to the problem of categorizing time-varying stimulus patterns such as spoken words or animal stereotypical behaviors. The recognition of these patterns is made difficult by their substantial variation in cadence and duration. The neural circuit behaviors used are similar to those associated with brain neural integrators. In the larger context described here, this kind of circuit becomes a building block of an entirely different computational algorithm for solving complex problems. While the network behavior is simulated in detail, a collective view is essential to understanding the results. A closed equation of motion for the collective variable describes an algorithm that quantitatively accounts for many aspects of the emergent network computation. The feedback connections and ongoing activity in the network shape the collective dynamics onto a reduced dimensionality manifold of activity space, which defines the algorithm and computation actually performed. The external inputs are weak and are not the dominant drivers of network activity.

Distinct speed dependence of entorhinal island and ocean cells, including respective grid cells

Entorhinal-hippocampal circuits in the mammalian brain are crucial for an animal's spatial and episodic experience, but the neural basis for different spatial computations remain unknown. Medial entorhinal cortex layer II contains pyramidal island and stellate ocean cells. Here, we performed cell type-specific Ca(2+) imaging in freely exploring mice using cellular markers and a miniature head-mounted fluorescence microscope. We found that both oceans and islands contain grid cells in similar proportions, but island cell activity, including activity in a proportion of grid cells, is significantly more speed modulated than ocean cell activity. We speculate that this differential property reflects island cells' and ocean cells' contribution to different downstream functions: island cells may contribute more to spatial path integration, whereas ocean cells may facilitate contextual representation in downstream circuits.

Hippocampal place cells construct reward related sequences through unexplored space

Dominant theories of hippocampal function propose that place cell representations are formed during an animal's first encounter with a novel environment and are subsequently replayed during off-line states to support consolidation and future behaviour. Here we report that viewing the delivery of food to an unvisited portion of an environment leads to off-line pre-activation of place cells sequences corresponding to that space. Such ‘preplay’ was not observed for an unrewarded but otherwise similar portion of the environment. These results suggest that a hippocampal representation of a visible, yet unexplored environment can be formed if the environment is of motivational relevance to the animal. We hypothesise such goal-biased preplay may support preparation for future experiences in novel environments.

DOI:http://dx.doi.org/10.7554/eLife.06063.001

As an animal explores an area, part of the brain called the hippocampus creates a mental map of the space. When the animal is in one location, a few neurons called ‘place cells’ will fire. If the animal moves to a new spot, other place cells fire instead. Each time the animal returns to that spot, the same place cells will fire. Thus, as the animal moves, a place-specific pattern of firing emerges that scientists can view by recording the cells' activity and which can be used to reconstruct the animal's position.

After exploring a space, the hippocampus may replay the new place-specific pattern of activity during sleep. By doing so, the brain consolidates the memory of the space for return visits. Recent evidence now suggests that these mental rehearsals—or internal simulations of the space—may begin even before a new space has been explored.

Now, Ólafsdóttir, Barry et al. report that whether an animal's brain simulates a first visit to a new space depends on whether the animal anticipates a reward. In the experiments, rats were allowed to run up to the junction in a T-shaped track. The animals could see into each of the arms, but not enter them. Food was then placed in one of the inaccessible arms. Ólafsdóttir, Barry et al. recorded the firing of place cells in the brain of the animals when they were on the track and during a rest period afterwards. The rats were then allowed onto the inaccessible arms, and again their brain activity was recorded.

In the rest period after the rats first viewed the inaccessible arms, the place cell pattern that would later form the mental map of a journey to and from the food-containing arm was pre-activated. However, the place cell pattern that would become the mental map of the other inaccessible arm was not activated before the rat explored that area. Therefore, Ólafsdóttir, Barry et al. suggest that the perception of reward influences which place cell pattern is simulated during rest. An implication of these findings is that the brain preferentially simulates past or future experiences that are deemed to be functionally significant, such as those associated with reward. A future challenge will be to determine whether this goal-related simulation of unvisited spaces predicts and is needed for behaviour such as successful navigation to a goal.

DOI:http://dx.doi.org/10.7554/eLife.06063.002

Environmental boundaries as an error correction mechanism for grid cells

Medial entorhinal grid cells fire in periodic, hexagonally patterned locations and are proposed to support path-integration-based navigation. The recursive nature of path integration results in accumulating error and, without a corrective mechanism, a breakdown in the calculation of location. The observed long-term stability of grid patterns necessitates that the system either performs highly precise internal path integration or implements an external landmark-based error correction mechanism. To distinguish these possibilities, we examined grid cells in behaving rodents as they made long trajectories across an open arena. We found that error accumulates relative to time and distance traveled since the animal last encountered a boundary. This error reflects coherent drift in the grid pattern. Further, interactions with boundaries yield direction-dependent error correction, suggesting that border cells serve as a neural substrate for error correction. These observations, combined with simulations of an attractor network grid cell model, demonstrate that landmarks are crucial to grid stability.

Theta sequences are essential for internally generated hippocampal firing fields

Sensory cue inputs and memory-related internal brain activities govern the firing of hippocampal neurons, but which specific firing patterns are induced by either of the two processes remains unclear. We found that sensory cues guided the firing of neurons in rats on a timescale of seconds and supported the formation of spatial firing fields. Independently of the sensory inputs, the memory-related network activity coordinated the firing of neurons not only on a second-long timescale, but also on a millisecond-long timescale, and was dependent on medial septum inputs. We propose a network mechanism that might coordinate this internally generated firing. Overall, we suggest that two independent mechanisms support the formation of spatial firing fields in hippocampus, but only the internally organized system supports short-timescale sequential firing and episodic memory.

Can we reconcile the declarative memory and spatial navigation views on hippocampal function?

Some argue that hippocampus supports declarative memory, our capacity to recall facts and events, whereas others view the hippocampus as part of a system dedicated to calculating routes through space, and these two contrasting views are pursued largely independently in current research. Here we offer a perspective on where these views can and cannot be reconciled and update a bridging framework that will improve our understanding of hippocampal function.

Decoding signal processing in thalamo-hippocampal circuitry: implications for theories of memory and spatial processing

A major tool in understanding how information is processed in the brain is the analysis of neuronal output at each hierarchical level through which neurophysiological signals are propagated. Since the experimental brain operation performed on Henry Gustav Molaison (known as patient H.M.) in 1953, the hippocampal formation has gained special attention, resulting in a very large number of studies investigating signals processed by the hippocampal formation. One of the main information streams to the hippocampal formation, vital for episodic memory formation, arises from thalamo-hippocampal projections, as there is extensive connectivity between these structures. This connectivity is sometimes overlooked by theories of memory formation by the brain, in favour of theories with a strong cortico-hippocampal flavour. In this review, we attempt to address some of the complexity of the signals processed within the thalamo-hippocampal circuitry. To understand the signals encoded by the anterior thalamic nuclei in particular, we review key findings from electrophysiological, anatomical, behavioural and computational studies. We include recent findings elucidating the integration of different signal modalities by single thalamic neurons; we focus in particular on the propagation of two prominent signals: head directionality and theta rhythm. We conclude that thalamo-hippocampal processing provides a centrally important, substantive, and dynamic input modulating and moderating hippocampal spatial and mnemonic processing. This article is part of a Special Issue entitled SI: Brain and Memory.

Place cells in the hippocampus: eleven maps for eleven rooms

The contribution of hippocampal circuits to high-capacity episodic memory is often attributed to the large number of orthogonal activity patterns that may be stored in these networks. Evidence for high-capacity storage in the hippocampus is missing, however. When animals are tested in pairs of environments, different combinations of place cells are recruited, consistent with the notion of independent representations. However, the extent to which representations remain independent across larger numbers of environments has not been determined. To investigate whether spatial firing patterns recur when animals are exposed to multiple environments, we tested rats in 11 recording boxes, each in a different room, allowing for 55 comparisons of place maps in each animal. In each environment, activity was recorded from neuronal ensembles in hippocampal area CA3, with an average of 30 active cells per animal. Representations were highly correlated between repeated tests in the same room but remained orthogonal across all combinations of different rooms, with minimal overlap in the active cell samples from each environment. A low proportion of cells had activity in many rooms but the firing locations of these cells were completely uncorrelated. Taken together, the results suggest that the number of independent spatial representations stored in hippocampal area CA3 is large, with minimal recurrence of spatial firing patterns across environments.

Head direction is coded more strongly than movement direction in a population of entorhinal neurons

The spatial firing pattern of entorhinal grid cells may be important for navigation. Many different computational models of grid cell firing use path integration based on movement direction and the associated movement speed to drive grid cells. However, the response of neurons to movement direction has rarely been tested, in contrast to multiple studies showing responses of neurons to head direction. Here, we analyzed the difference between head direction and movement direction during rat movement and analyzed cells recorded from entorhinal cortex for their tuning to movement direction. During foraging behavior, movement direction differs significantly from head direction. The analysis of neuron responses shows that only 5 out of 758 medial entorhinal cells show significant coding for both movement direction and head direction when evaluating periods of rat behavior with speeds above 10 cm/s and ±30° angular difference between movement and head direction. None of the cells coded movement direction alone. In contrast, 21 cells in this population coded only head direction during behavioral epochs with these constraints, indicating much stronger coding of head direction in this population. This suggests that the movement direction signal required by most grid cell models may arise from other brain structures than the medial entorhinal cortex. This article is part of a Special Issue entitled SI: Brain and Memory.

Estimating location without external cues

The ability to determine one's location is fundamental to spatial navigation. Here, it is shown that localization is theoretically possible without the use of external cues, and without knowledge of initial position or orientation. With only error-prone self-motion estimates as input, a fully disoriented agent can, in principle, determine its location in familiar spaces with 1-fold rotational symmetry. Surprisingly, localization does not require the sensing of any external cue, including the boundary. The combination of self-motion estimates and an internal map of the arena provide enough information for localization. This stands in conflict with the supposition that 2D arenas are analogous to open fields. Using a rodent error model, it is shown that the localization performance which can be achieved is enough to initiate and maintain stable firing patterns like those of grid cells, starting from full disorientation. Successful localization was achieved when the rotational asymmetry was due to the external boundary, an interior barrier or a void space within an arena. Optimal localization performance was found to depend on arena shape, arena size, local and global rotational asymmetry, and the structure of the path taken during localization. Since allothetic cues including visual and boundary contact cues were not present, localization necessarily relied on the fusion of idiothetic self-motion cues and memory of the boundary. Implications for spatial navigation mechanisms are discussed, including possible relationships with place field overdispersion and hippocampal reverse replay. Based on these results, experiments are suggested to identify if and where information fusion occurs in the mammalian spatial memory system.

Spatial navigation is one of the most important functions of animal brains. Multiple regions and cell types encode the current location in mammalian brains, but the underlying interactions between sensory and memory information remain unclear. Recent experimental and theoretical evidence have been found to suggest that the presence of a boundary fundamentally alters the task of navigation. In this paper, evidence is provided that it is possible to determine the location inside any familiar arena with 1-fold rotational symmetry, while completely ignoring sensory cues from the outside world. Surprisingly, the results show that the mere knowledge of the boundary's existence is enough, without requiring direct physical contact. Localization is robust despite the presence of noise modelled from the rodent head direction system, and even inaccuracies in the navigation system's memory of the boundary or internal models of noise. In circular arenas, rotational asymmetry can arise from interior structures such as barriers or voids, also without contact information. This theoretical evidence highlights the need to distinguish arena-based navigation common to most experimental studies, from open field navigation. These findings also point to novel ways to study information fusion in mammalian brains.

Impairment of the anterior thalamic head direction cell network following administration of the NMDA antagonist MK-801

Head direction (HD) cells, found in the rodent Papez circuit, are thought to form the neural circuitry responsible for directional orientation. Because NMDA transmission has been implicated in spatial tasks requiring directional orientation, we sought to determine if the NMDA antagonist dizocilpine (MK-801) would disrupt the directional signal carried by the HD network. Anterior thalamic HD cells were isolated in female Long-Evans rats and initially monitored for baseline directional activity while the animals foraged in a familiar enclosure. The animals were then administered MK-801 at a dose of .05 mg/kg or 0.1 mg/kg, or isotonic saline, and cells were re-examined for changes in directional specificity and landmark control. While the cells showed no changes in directional specificity and landmark control following administration of saline or the lower dose of MK-801, the higher dose of MK-801 caused a dramatic attenuation of the directional signal, characterized by decreases in peak firing rates, signal to noise, and directional information content. While the greatly attenuated directional specificity of cells in the high dose condition usually remained stable relative to the landmarks within the recording enclosure, a few cells in this condition exhibited unstable preferred directions within and between recording sessions. Our results are discussed relative to the possibility that the findings explain the effects of MK-801 on the acquisition and performance of spatial tasks.

Reconceiving the hippocampal map as a topological template

The role of the hippocampus in spatial cognition is incontrovertible yet controversial. Place cells, initially thought to be location-specifiers, turn out to respond promiscuously to a wide range of stimuli. Here we test the idea, which we have recently demonstrated in a computational model, that the hippocampal place cells may ultimately be interested in a space's topological qualities (its connectivity) more than its geometry (distances and angles); such higher-order functioning would be more consistent with other known hippocampal functions. We recorded place cell activity in rats exploring morphing linear tracks that allowed us to dissociate the geometry of the track from its topology. The resulting place fields preserved the relative sequence of places visited along the track but did not vary with the metrical features of the track or the direction of the rat's movement. These results suggest a reinterpretation of previous studies and new directions for future experiments.

Vestibular pathways involved in cognition

Recent discoveries have emphasized the role of the vestibular system in cognitive processes such as memory, spatial navigation and bodily self-consciousness. A precise understanding of the vestibular pathways involved is essential to understand the consequences of vestibular diseases for cognition, as well as develop therapeutic strategies to facilitate recovery. The knowledge of the “vestibular cortical projection areas”, defined as the cortical areas activated by vestibular stimulation, has dramatically increased over the last several years from both anatomical and functional points of view. Four major pathways have been hypothesized to transmit vestibular information to the vestibular cortex: (1) the vestibulo-thalamo-cortical pathway, which probably transmits spatial information about the environment via the parietal, entorhinal and perirhinal cortices to the hippocampus and is associated with spatial representation and self-versus object motion distinctions; (2) the pathway from the dorsal tegmental nucleus via the lateral mammillary nucleus, the anterodorsal nucleus of the thalamus to the entorhinal cortex, which transmits information for estimations of head direction; (3) the pathway via the nucleus reticularis pontis oralis, the supramammillary nucleus and the medial septum to the hippocampus, which transmits information supporting hippocampal theta rhythm and memory; and (4) a possible pathway via the cerebellum, and the ventral lateral nucleus of the thalamus (perhaps to the parietal cortex), which transmits information for spatial learning. Finally a new pathway is hypothesized via the basal ganglia, potentially involved in spatial learning and spatial memory. From these pathways, progressively emerges the anatomical network of vestibular cognition.