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

The mosaic structure of the mammalian cognitive map

The cognitive map, proposed by Tolman in the 1940s, is a hypothetical internal representation of space constructed by the brain to enable an animal to undertake flexible spatial behaviors such as navigation. The subsequent discovery of place cells in the hippocampus of rats suggested that such a map-like representation does exist, and also provided a tool with which to explore its properties. Single-neuron studies in rodents conducted in small singular spaces have suggested that the map is founded on a metric framework, preserving distances and directions in an abstract representational format. An open question is whether this metric structure pertains over extended, often complexly structured real-world space. The data reviewed here suggest that this is not the case. The emerging picture is that instead of being a single, unified construct, the map is a mosaic of fragments that are heterogeneous, variably metric, multiply scaled, and sometimes laid on top of each other. Important organizing factors within and between fragments include boundaries, context, compass direction, and gravity. The map functions not to provide a comprehensive and precise rendering of the environment but rather to support adaptive behavior, tailored to the species and situation.

Whole-brain tracking of cocaine and sugar rewards processing

Natural rewards, such as food, and sex are appetitive stimuli available for animals in their natural environment. Similarly, addictive rewards such as drugs of abuse possess strong, positive valence, but their action relies on their pharmacological properties. Nevertheless, it is believed that both of these kinds of rewards activate similar brain circuitry. The present study aimed to discover which parts of the brain process the experience of natural and addictive rewards. To holistically address this question, we used a single-cell whole-brain imaging approach to find patterns of activation for acute and prolonged sucrose and cocaine exposure. We analyzed almost 400 brain structures and created a brain-wide map of specific, c-Fos-positive neurons engaged by these rewards. Acute but not prolonged sucrose exposure triggered a massive c-Fos expression throughout the brain. Cocaine exposure on the other hand potentiated c-Fos expression with prolonged use, engaging more structures than sucrose treatment. The functional connectivity analysis unraveled an increase in brain modularity after the initial exposure to both types of rewards. This modularity was increased after repeated cocaine, but not sucrose, intake. To check whether discrepancies between the processing of both types of rewards can be found on a cellular level, we further studied the nucleus accumbens, one of the most strongly activated brain structures by both sucrose and cocaine experience. We found a high overlap between natural and addictive rewards on the level of c-Fos expression. Electrophysiological measurements of cellular correlates of synaptic plasticity revealed that natural and addictive rewards alike induce the accumulation of silent synapses. These results strengthen the hypothesis that in the nucleus accumbens drugs of abuse cause maladaptive neuronal plasticity in the circuitry that typically processes natural rewards.

Navigation task and action space drive the emergence of egocentric and allocentric spatial representations

In general, strategies for spatial navigation could employ one of two spatial reference frames: egocentric or allocentric. Notwithstanding intuitive explanations, it remains unclear however under what circumstances one strategy is chosen over another, and how neural representations should be related to the chosen strategy. Here, we first use a deep reinforcement learning model to investigate whether a particular type of navigation strategy arises spontaneously during spatial learning without imposing a bias onto the model. We then examine the spatial representations that emerge in the network to support navigation. To this end, we study two tasks that are ethologically valid for mammals—guidance, where the agent has to navigate to a goal location fixed in allocentric space, and aiming, where the agent navigates to a visible cue. We find that when both navigation strategies are available to the agent, the solutions it develops for guidance and aiming are heavily biased towards the allocentric or the egocentric strategy, respectively, as one might expect. Nevertheless, the agent can learn both tasks using either type of strategy. Furthermore, we find that place-cell-like allocentric representations emerge preferentially in guidance when using an allocentric strategy, whereas egocentric vector representations emerge when using an egocentric strategy in aiming. We thus find that alongside the type of navigational strategy, the nature of the task plays a pivotal role in the type of spatial representations that emerge.

Most species rely on navigation in space to find water, food, and mates, as well as to return home. When navigating, humans and animals can use one of two reference frames: one based on stable landmarks in the external environment, such as moving due north and then east, or one centered on oneself, such as moving forward and turning left. However, it remains unclear how these reference frames are chosen and interact in navigation tasks, as well as how they are supported by representations in the brain. We therefore modeled two navigation tasks that would each benefit from using one of these reference frames, and trained an artificial agent to learn to solve them through trial and error. Our results show that when given the choice, the agent leveraged the appropriate reference frame to solve the task, but surprisingly could also use the other reference frame when constrained to do so. We also show that the representations that emerge to enable the agent to solve the tasks exist on a spectrum, and are more complex than commonly thought. These representations reflect both the task and reference frame being used, and provide useful insights for the design of experimental tasks to study the use of navigational strategies.

Stress-Induced Enhanced Long-Term Potentiation and Reduced Threshold for N-Methyl-D-Aspartate Receptor- and β-Adrenergic Receptor-Mediated Synaptic Plasticity in Rodent Ventral Subiculum

Stress is a biologically relevant signal and can modulate hippocampal synaptic plasticity. The subiculum is the major output station of the hippocampus and serves as a critical hub in the stress response network. However, stress-associated synaptic plasticity in the ventral subiculum has not been adequately addressed. Therefore, we investigated the impact of a single exposure to an inherently stressful two-way active avoidance conditioning on the induction of long-term potentiation (LTP) at CA1-subiculum synapses in ventral hippocampal slices from young adult rats 1 day after stressor exposure. We found that acute stress enhanced LTP and lowered the induction threshold for a late-onset LTP at excitatory CA1 to subicular burst-spiking neuron synapses. This late-onset LTP was dependent on the activation of β-adrenergic and glutamatergic N-methyl-D-aspartate receptors and independent of D1/D5 dopamine receptor activation. Thereby, we present a cellular mechanism that might contribute to behavioral stress adaptation after acute stressor exposure.

The subiculum and its role in focal epileptic disorders

The subicular complex (hereafter referred as subiculum), which is reciprocally connected with the hippocampus and rhinal cortices, exerts a major control on hippocampal outputs. Over the last three decades, several studies have revealed that the subiculum plays a pivotal role in learning and memory but also in pathological conditions such as mesial temporal lobe epilepsy (MTLE). Indeed, subicular networks actively contribute to seizure generation and this structure is relatively spared from the cell loss encountered in this focal epileptic disorder. In this review, we will address: (i) the functional properties of subicular principal cells under normal and pathological conditions; (ii) the subiculum role in sustaining seizures in in vivo models of MTLE and in in vitro models of epileptiform synchronization; (iii) its presumptive role in human MTLE; and (iv) evidence underscoring the relationship between subiculum and antiepileptic drug effects. The studies reviewed here reinforce the view that the subiculum represents a limbic area with relevant, as yet unexplored, roles in focal epilepsy.

Noncanonical, Dopamine-Dependent Long-Term Potentiation at Hippocampal Output Synapses in a Rodent Model of First-Episode Psychosis

Cognitive deficits and positive symptoms in schizophrenia have both been linked to hippocampal dysfunction. Recently, subregion-specific aberrant and maladaptive hippocampal synaptic plasticity has been suggested as one of the mechanistic underpinnings. The subiculum is the final output hub of the hippocampus and orchestrates hippocampal information transfer to other brain regions. While most CA1 pyramidal neurons show regular-spiking behavior, subicular output neurons comprise bursting and regular-firing pyramidal cells. These two cell types target different brain regions and express unique forms of synaptic plasticity. Here, we used a single systemic application of the noncompetitive glutamatergic N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 to model first-episode psychosis in rats and studied long-term potentiation (LTP) in subicular regular-firing cells in acute hippocampal slices. Previously, we have reported a facilitation of a presynaptic, late-onset LTP in subicular bursting pyramidal cells after systemic NMDAR antagonism. Here, we show that single systemic NMDAR antagonist application also facilitates the induction of a noncanonical, but postsynaptic NMDAR-independent LTP in ventral subicular but not in CA1 regular-firing pyramidal cells. This form of LTP was dependent on D1/D5 dopamine receptor activation. Activation of D1/D5 dopamine receptors by a specific agonist mimicked and occluded LTP induced by electrical high-frequency stimulation (HFS). Furthermore, our results indicate that this form of LTP relies on postsynaptic Ca²⁺ signaling and requires the activation of protein kinase A. Considering the pivotal role of the subiculum as information gatekeeper between the hippocampus and other brain regions, this aberrant LTP in ventral subicular regular-firing neurons is expected to interfere with physiological hippocampal output processing and might thereby contribute to hippocampal dysfunction in psychotic events.

Multiple Maps of the Same Spatial Context Can Stably Coexist in the Mouse Hippocampus

Hippocampal place cells selectively fire when an animal traverses a particular location and are considered a neural substrate of spatial memory. Place cells were shown to change their activity patterns (remap) across different spatial contexts but to maintain their spatial tuning in a fixed familiar context. Here, we show that mouse hippocampal neurons can globally remap, forming multiple distinct representations (maps) of the same familiar environment, without any apparent changes in sensory input or behavior. Alternations between maps occurred only across separate visits to the environment, implying switching between distinct stable attractors in the hippocampal network. Importantly, the different maps were spatially informative and persistent over weeks, demonstrating that they can be reliably stored and retrieved from long-term memory. Taken together, our results suggest that a memory of a given spatial context could be associated with multiple distinct neuronal representations, rather than just one.

Opposing and Complementary Topographic Connectivity Gradients Revealed by Quantitative Analysis of Canonical and Noncanonical Hippocampal CA1 Inputs

Physiological studies suggest spatial representation gradients along the CA1 proximodistal axis. To determine the underlying anatomical basis, we quantitatively mapped canonical and noncanonical inputs to excitatory neurons in dorsal hippocampal CA1 along the proximal-distal axis in mice of both sexes using monosynaptic rabies tracing. Our quantitative analyses show comparable strength of subiculum complex and entorhinal cortex (EC) inputs to CA1, significant inputs from presubiculum and parasubiculum to CA1, and a threefold stronger input to proximal versus distal CA1 from CA3. Noncanonical subicular complex inputs exhibit opposing topographic connectivity gradients whereby the subiculum-CA1 input strength systematically increases but the presubiculum-CA1 input strength decreases along the proximal-distal axis. The subiculum input strength cotracks that of the lateral EC, known to be less spatially selective than the medial EC. The functional significance of this organization is verified physiologically for subiculum-to-CA1 inputs. These results reveal a novel anatomical framework by which to determine the circuit bases for CA1 representations.

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.

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.

Reward-based spatial learning in unmedicated adults with obsessive-compulsive disorder

OBJECTIVE

The authors assessed the functioning of mesolimbic and striatal areas involved in reward-based spatial learning in unmedicated adults with obsessive-compulsive disorder (OCD).

METHOD

Functional MRI blood-oxygen-level-dependent response was compared in 33 unmedicated adults with OCD and 33 healthy, age-matched comparison subjects during a reward-based learning task that required learning to use extramaze cues to navigate a virtual eight-arm radial maze to find hidden rewards. The groups were compared in their patterns of brain activation associated with reward-based spatial learning versus a control condition in which rewards were unexpected because they were allotted pseudorandomly to experimentally prevent learning.

RESULTS

Both groups learned to navigate the maze to find hidden rewards, but group differences in neural activity during navigation and reward processing were detected in mesolimbic and striatal areas. During navigation, the OCD group, unlike the healthy comparison group, exhibited activation in the left posterior hippocampus. Unlike healthy subjects, participants in the OCD group did not show activation in the left ventral putamen and amygdala when anticipating rewards or in the left hippocampus, amygdala, and ventral putamen when receiving unexpected rewards (control condition). Signal in these regions decreased relative to baseline during unexpected reward receipt among those in the OCD group, and the degree of activation was inversely associated with doubt/checking symptoms.

CONCLUSIONS

Participants in the OCD group displayed abnormal recruitment of mesolimbic and ventral striatal circuitry during reward-based spatial learning. Whereas healthy comparison subjects exhibited activation in this circuitry in response to the violation of reward expectations, unmedicated OCD participants did not and instead over-relied on the posterior hippocampus during learning. Thus, dopaminergic innervation of reward circuitry may be altered, and future study of anterior/posterior hippocampal dysfunction in OCD is warranted.

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.

Tactile modulation of hippocampal place fields

Neural correlates of spatial representation can be found in the activity of the hippocampal place cells. These neurons are characterized by firing whenever the animal is located in a particular area of the space, the place field. Place fields are modulated by sensory cues, such as visual, auditory, or olfactory cues, being the influence of visual inputs the most thoroughly studied. Tactile information gathered by the whiskers has a prominent representation in the rat cerebral cortex. However, the influence of whisker-detected tactile cues on place fields remains an open question. Here we studied place fields in an enriched tactile environment where the remaining sensory cues were occluded. First, place cells were recorded before and after blockade of tactile transmission by means of lidocaine applied on the whisker pad. Following tactile deprivation, the majority of place cells decreased their firing rate and their place fields expanded. We next rotated the tactile cues and 90% of place fields rotated with them. Our results demonstrate that tactile information is integrated into place cells at least in a tactile-enriched arena and when other sensory cues are not available.

The anterior thalamus provides a subcortical circuit supporting memory and spatial navigation

The anterior thalamic nuclei (ATN), a central component of Papez' circuit, are generally assumed to be key constituents of the neural circuits responsible for certain categories of learning and memory. Supporting evidence for this contention is that damage to either of two brain regions, the medial temporal lobe and the medial diencephalon, is most consistently associated with anterograde amnesia. Within these respective regions, the hippocampal formation and the ATN (anteromedial, anteroventral, and anterodorsal) are the particular structures of interest. The extensive direct and indirect hippocampal-anterior thalamic interconnections and the presence of theta-modulated cells in both sites further support the hypothesis that these structures constitute a neuronal network crucial for memory and cognition. The major tool in understanding how the brain processes information is the analysis of neuronal output at each hierarchical level along the pathway of signal propagation coupled with neuroanatomical studies. Here, we discuss the electrophysiological properties of cells in the ATN with an emphasis on their role in spatial navigation. In addition, we describe neuroanatomical and functional relationships between the ATN and hippocampal formation.

The adaptive significance of adult neurogenesis: an integrative approach

Adult neurogenesis in mammals is predominantly restricted to two brain regions, the dentate gyrus (DG) of the hippocampus and the olfactory bulb (OB), suggesting that these two brain regions uniquely share functions that mediate its adaptive significance. Benefits of adult neurogenesis across these two regions appear to converge on increased neuronal and structural plasticity that subserves coding of novel, complex, and fine-grained information, usually with contextual components that include spatial positioning. By contrast, costs of adult neurogenesis appear to center on potential for dysregulation resulting in higher risk of brain cancer or psychological dysfunctions, but such costs have yet to be quantified directly. The three main hypotheses for the proximate functions and adaptive significance of adult neurogenesis, pattern separation, memory consolidation, and olfactory spatial, are not mutually exclusive and can be reconciled into a simple general model amenable to targeted experimental and comparative tests. Comparative analysis of brain region sizes across two major social-ecological groups of primates, gregarious (mainly diurnal haplorhines, visually-oriented, and in large social groups) and solitary (mainly noctural, territorial, and highly reliant on olfaction, as in most rodents) suggest that solitary species, but not gregarious species, show positive associations of population densities and home range sizes with sizes of both the hippocampus and OB, implicating their functions in social-territorial systems mediated by olfactory cues. Integrated analyses of the adaptive significance of adult neurogenesis will benefit from experimental studies motivated and structured by ecologically and socially relevant selective contexts.

Spatial information outflow from the hippocampal circuit: distributed spatial coding and phase precession in the subiculum

Hippocampal place cells convey spatial information through a combination of spatially selective firing and theta phase precession. The way in which this information influences regions like the subiculum that receive input from the hippocampus remains unclear. The subiculum receives direct inputs from area CA1 of the hippocampus and sends divergent output projections to many other parts of the brain, so we examined the firing patterns of rat subicular neurons. We found a substantial transformation in the subicular code for space from sparse to dense firing rate representations along a proximal-distal anatomical gradient: neurons in the proximal subiculum are more similar to canonical, sparsely firing hippocampal place cells, whereas neurons in the distal subiculum have higher firing rates and more distributed spatial firing patterns. Using information theory, we found that the more distributed spatial representation in the subiculum carries, on average, more information about spatial location and context than the sparse spatial representation in CA1. Remarkably, despite the disparate firing rate properties of subicular neurons, we found that neurons at all proximal-distal locations exhibit robust theta phase precession, with similar spiking oscillation frequencies as neurons in area CA1. Our findings suggest that the subiculum is specialized to compress sparse hippocampal spatial codes into highly informative distributed codes suitable for efficient communication to other brain regions. Moreover, despite this substantial compression, the subiculum maintains finer scale temporal properties that may allow it to participate in oscillatory phase coding and spike timing-dependent plasticity in coordination with other regions of the hippocampal circuit.

Profound retrograde but absence of anterograde amnesia for cued place learning in rats with hippocampal lesions

Previous studies in our lab have shown that slight modifications in the spatial reference memory procedure can overcome the deficit in spatial learning typically observed in rats with hippocampal damage. However, it is unknown if memory acquired under such training circumstances is spared after hippocampal lesions. With this aim a four-arm plus-shaped maze and a spatial reference memory paradigm were used, in which the goal arm was doubly marked: by an intramaze cue (a piece of sandpaper positioned on the floor of the arm) and by the extramaze constellation of stimuli around the maze. Experiment 1 replicated previous findings showing that hippocampally damaged rats can learn a place response just as well as the controls when the intramaze cue is present during the training, but they are unable to do so in the absence of the intramaze signal. When the learning procedure was doubly signaled, a transfer test performed 24h after the end of acquisition demonstrated that lesioned rats showed perfect memory for the goal arm when the intramaze cue was removed. Experiment 2 investigated the effect of hippocampal damage 1 day after the learning. Results showed that regardless of the training procedure employed (with or without the intramaze cue), hippocampal lesions produced a profound retrograde amnesia. Thus, although the absence of anterograde amnesia suggests that structures other that the hippocampus can take charge of the acquisition, the presence of retrograde amnesia indicates the critical role of the normal hippocampus in the long-term formation of allocentric information.

Computational models of grid cells

Grid cells are space-modulated neurons with periodic firing fields. In moving animals, the multiple firing fields of an individual grid cell form a triangular pattern tiling the entire space available to the animal. Collectively, grid cells are thought to provide a context-independent metric representation of the local environment. Since the discovery of grid cells in 2005, a number of models have been proposed to explain the formation of spatially repetitive firing patterns as well as the conversion of these signals to place signals one synapse downstream in the hippocampus. The present article reviews the most recent developments in our understanding of how grid patterns are generated, maintained, and transformed, with particular emphasis on second-generation computational models that have emerged during the past 2-3 years in response to criticism and new data.

Real-time position reconstruction with hippocampal place cells

Brain–computer interfaces (BCI) are using the electroencephalogram, the electrocorticogram and trains of action potentials as inputs to analyze brain activity for communication purposes and/or the control of external devices. Thus far it is not known whether a BCI system can be developed that utilizes the states of brain structures that are situated well below the cortical surface, such as the hippocampus. In order to address this question we used the activity of hippocampal place cells (PCs) to predict the position of an rodent in real-time. First, spike activity was recorded from the hippocampus during foraging and analyzed off-line to optimize the spike sorting and position reconstruction algorithm of rats. Then the spike activity was recorded and analyzed in real-time. The rat was running in a box of 80 cm × 80 cm and its locomotor movement was captured with a video tracking system. Data were acquired to calculate the rat's trajectories and to identify place fields. Then a Bayesian classifier was trained to predict the position of the rat given its neural activity. This information was used in subsequent trials to predict the rat's position in real-time. The real-time experiments were successfully performed and yielded an error between 12.2 and 17.4% using 5–6 neurons. It must be noted here that the encoding step was done with data recorded before the real-time experiment and comparable accuracies between off-line (mean error of 15.9% for three rats) and real-time experiments (mean error of 14.7%) were achieved. The experiment shows proof of principle that position reconstruction can be done in real-time, that PCs were stable and spike sorting was robust enough to generalize from the training run to the real-time reconstruction phase of the experiment. Real-time reconstruction may be used for a variety of purposes, including creating behavioral–neuronal feedback loops or for implementing neuroprosthetic control.

Hippocampus, amygdala, and stress: interacting systems that affect susceptibility to addiction

Stress is one of the major factors in drug abuse, particularly in relapse and drug-seeking behavior. However, the mechanisms underlying the interactions between stress and drug abuse are unclear. For many years, studies have focused on the role of the dopaminergic reward system in drug abuse. Our results, for example, show that increased dopaminergic activity is induced by drug sensitization and different stressors via potentiation of the ventral subiculum-nucleus accumbens (NAc) pathway. Although the role of the norepinephrine (NE) system in stress is well known, its involvement in drug abuse has received less attention. This review explores the different mechanisms by which stressors can modulate the ventral subiculum-accumbens pathway, and how these modulations can induce alterations in the behavioral response to drug administration. In particular, we will focus on two main afferents to the NAc, the basolateral amygdala and the ventral subiculum of the hippocampus, and their interactions with the locus coeruleus-norepinephrine system.

Dopamine system dysregulation by the hippocampus: implications for the pathophysiology and treatment of schizophrenia

For decades, the predominant hypothesis of schizophrenia centered on dysfunctions of the dopamine system. However, recent evidence now suggests that the dopamine system may be "normal" in its configuration, but instead is regulated abnormally by modulatory processes. Convergent studies in animals and in humans have now focused on the hippocampus as a central component in the generation of psychosis and possibly other symptom states in schizophrenia. Thus, activity in the ventral hippocampus has been shown to regulate dopamine neuron responsivity by controlling the number of dopamine neurons that can be phasically activated by stimuli. In this way, this structure determines the gain of the dopamine signal in response to stimuli. However, in schizophrenia, the hippocampus appears to be hyper-active, possibly due to attenuation of function of inhibitory interneurons. As a result, the dopamine system is driven into an overly responsive state. Current medications have focused on blockade of overstimulated dopamine receptors; however, this now appears to be several synapses downstream from the pathological antecedent. Therapeutic approaches that focus on normalizing hippocampal function may prove to be more effective treatment avenues for the schizophrenia patient.

Aversive stimuli alter ventral tegmental area dopamine neuron activity via a common action in the ventral hippocampus

Stress is a physiological, adaptive response to changes in the environment, but can also lead to pathological alterations, such as relapse in psychiatric disorders and drug abuse. Evidence demonstrates that the dopamine (DA) system plays a role in stress; however, the nature of the effects of sustained stressors on DA neuron physiology has not been adequately addressed. By using a combined electrophysiological, immunohistochemical and behavioral approach, we examined the response of ventral tegmental area DA neurons in rats to acute as well as repeated stressful events using noxious (footshock) and psychological (restraint) stress. We found that aversive stimuli induced a pronounced activation of the DA system both electrophysiologically (population activity; i.e., number of DA neurons firing spontaneously) and behaviorally (response to psychostimulants). Moreover, infusion of TTX into the ventral hippocampus (vHPC) reversed both behavioral and electrophysiological effects of stress, indicating that the hyperdopaminergic condition associated with stress is driven by hyperactivity within the vHPC. Therefore, the stress-induced activation of the DA system may underlie the propensity of stress to exacerbate psychotic disorders or predispose an individual to drug-seeking behavior. Furthermore, the vHPC represents a critical link between context-dependent DA sensitization, stress-induced potentiation of amphetamine responsivity, and the increase in DA associated with stressors.

A virtual reality-based FMRI study of reward-based spatial learning

Although temporo-parietal cortices mediate spatial navigation in animals and humans, the neural correlates of reward-based spatial learning are less well known. Twenty-five healthy adults performed a virtual reality fMRI task that required learning to use extra-maze cues to navigate an 8-arm radial maze and find hidden rewards. Searching the maze in the spatial learning condition compared to the control conditions was associated with activation of temporo-parietal regions, albeit not including the hippocampus. The receipt of rewards was associated with activation of the hippocampus in a control condition when using the extra-maze cues for navigation was rendered impossible by randomizing the spatial location of cues. Our novel experimental design allowed us to assess the differential contributions of the hippocampus and other temporo-parietal areas to searching and reward processing during reward-based spatial learning. This translational research will permit parallel studies in animals and humans to establish the functional similarity of learning systems across species; cellular and molecular studies in animals may then inform the effects of manipulations on these systems in humans, and fMRI studies in humans may inform the interpretation and relevance of findings in animals.

Dopamine system dysregulation by the ventral subiculum as the common pathophysiological basis for schizophrenia psychosis, psychostimulant abuse, and stress

The dopamine system is under multiple forms of regulation, and in turn provides effective modulation of system responses. Dopamine neurons are known to exist in several states of activity. The population activity, or the proportion of dopamine neurons firing spontaneously, is controlled by the ventral subiculum of the hippocampus. In contrast, burst firing, which is proposed to be the behaviorally salient output of the dopamine system, is driven by the brainstem pedunculopontine tegmentum (PPTg). When an animal is exposed to a behaviorally salient stimulus, the PPTg elicits a burst of action potentials in the dopamine neurons. However, this bursting only occurs in the portion of the dopamine neuron population that is firing spontaneously. This proportion is regulated by the ventral subiculum. Therefore, the ventral subiculum provides the gain, or the amplification factor, for the behaviorally salient stimulus. The ventral subiculum itself is proposed to carry information related to the environmental context. Thus, the ventral subiculum will adjust the responsivity of the dopamine system based on the needs of the organism and the characteristics of the environment. However, this finely tuned system can be disrupted in disease states. In schizophrenia, a disruption of interneuronal regulation of the ventral subiculum is proposed to lead to an overdrive of the dopamine system, rendering the system in a constant hypervigilant state. Moreover, amphetamine sensitization and stressors also appear to cause an abnormal dopaminergic drive. Such an interaction could underlie the risk factors of drug abuse and stress in the precipitation of a psychotic event. On the other hand, this could point to the ventral subiculum as an effective site of therapeutic intervention in the treatment or even the prevention of schizophrenia.

Preserved learning about allocentric cues but impaired flexible memory expression in rats with hippocampal lesions

Several studies have shown that slight modifications in the standard reference spatial memory procedure normally used for allocentric learning in the Morris water maze and the radial maze, can overcome the classic deficit in allocentric navigation typically observed in rats with hippocampal damage. In these special paradigms, however, there is only intramaze manipulation of a salient stimulus. The present study was designed to investigate whether extramaze manipulations produce a similar outcome. With this aim a four-arm plus-shaped maze and a reference spatial memory paradigm were used, in which the goal arm was marked in two ways: by a prominent extramaze cue (intermittent light), which maintained a constant relation with the goal, and by the extramaze constellation of stimuli around the maze. Experiment 1 showed that, unlike the standard version of the task, using this special training procedure hippocampally-damaged rats could learn a place response as quickly as control animals; importantly, one day after reaching criterion, lesioned and control subjects performed the task perfectly during a transfer test in which the salient extramaze stimulus used during the acquisition was removed. However, although acquisition deficit was overcomed in these lesioned animals, a profound deficit in retention was detected 15 days later. Experiment 2 suggests that although under our special paradigm hippocampal rats can learn a place response, spatial memory only can be expressed when the requisites of behavioral flexibility are minimal. These findings suggest that, under certain circumstances, extrahippocampal structures are sufficient for building a coherent allocentric representation of space; however, flexible memory expression is dependent, fundamentally, on hippocampal functioning.

Cortico-Basal Ganglia reward network: microcircuitry

Many of the brain's reward systems converge on the nucleus accumbens, a region richly innervated by excitatory, inhibitory, and modulatory afferents representing the circuitry necessary for selecting adaptive motivated behaviors. The ventral subiculum of the hippocampus provides contextual and spatial information, the basolateral amygdala conveys affective influence, and the prefrontal cortex provides an integrative impact on goal-directed behavior. The balance of these afferents is under the modulatory influence of dopamine neurons in the ventral tegmental area. This midbrain region receives its own complex mix of excitatory and inhibitory inputs, some of which have only recently been identified. Such afferent regulation positions the dopamine system to bias goal-directed behavior based on internal drives and environmental contingencies. Conditions that result in reward promote phasic dopamine release, which serves to maintain ongoing behavior by selectively potentiating ventral subicular drive to the accumbens. Behaviors that fail to produce an expected reward decrease dopamine transmission, which favors prefrontal cortical-driven switching to new behavioral strategies. As such, the limbic reward system is designed to optimize action plans for maximizing reward outcomes. This system can be commandeered by drugs of abuse or psychiatric disorders, resulting in inappropriate behaviors that sustain failed reward strategies. A fuller appreciation of the circuitry interconnecting the nucleus accumbens and ventral tegmental area should serve to advance discovery of new treatment options for these conditions.

Roles for the subiculum in spatial information processing, memory, motivation and the temporal control of behaviour

The subiculum is in a pivotal position governing the output of the hippocampal formation. Despite this, it is a rather under-explored and sometimes ignored structure. Here, we discuss recent data indicating that the subiculum participates in a wide range of neurocognitive functions and processes. Some of the functions of subiculum are relatively well-known-these include providing a relatively coarse representation of space and participating in, and supporting certain aspects of, memory (particularly in the dynamic bridging of temporal intervals). The subiculum also participates in a wide variety of other neurocognitive functions too, however. Much less well-known are roles for the subiculum, and particularly the ventral subiculum, in the response to fear, stress and anxiety, and in the generation of motivated behaviour (particularly the behaviour that underlies drug addiction and the response to reward). There is an emerging suggestion that the subiculum participates in the temporal control of behaviour. It is notable that these latter findings have emerged from a consideration of instrumental behaviour using operant techniques; it may well be the case that the use of the watermaze or similar spatial tasks to assess subicular function (on the presumption that its functions are very similar to the hippocampus proper) has obscured rather than revealed neurocognitive functions of subiculum. The anatomy of subiculum suggests it participates in a rather subtle fashion in a very broad range of functions, rather than in a relatively more isolated fashion in a narrower range of functions, as might be the case for "earlier" components of hippocampal circuitry, such as the CA1 and CA3 subfields. Overall, there appears to a strong dorso-ventral segregation of function within subiculum, with the dorsal subiculum relatively more concerned with space and memory, and the ventral hippocampus concerned with stress, anxiety and reward. Finally, it may be the case that the whole subiculum participates in the temporal control of reinforced behaviour, although further experimentation is required to clarify this hypothesis.

Role of deep brain stimulation in modulating memory formation and recall

Deep brain stimulation (DBS) has become an increasingly popular tool for treating a variety of medically refractory neurological and psychiatric disorders such as Parkinson disease, essential tremor, depression, and obsessive-compulsive disorder. Several targets have been identified for ablation or stimulation based on their anatomical location and presumed function. Areas such as the subthalamic nucleus, globus pallidus, and thalamus, for example, are believed to play a key role in motor control and execution, and they are commonly used in the treatment of motor disorders. Limbic structures such as the cingulate cortex and ventral striatum, believed to be important in motivation, emotion, and higher cognition, have also been targeted for treatment of a number of psychiatric disorders. In all of these settings, DBS is largely aimed at addressing the deleterious aspects of these diseases. In Parkinson disease, for example, DBS has been used to reduce rigidity and tremor, whereas in obsessive-compulsive disorder it has been used to limit compulsive behavior. More recently, however, attention has also turned to the potential use of DBS for enhancing or improving otherwise nonpathological aspects of cognitive function. This review explores the potential role of DBS in augmenting memory formation and recall, and the authors discuss recent studies and future trends in this emerging field.

The anatomy of memory: an interactive overview of the parahippocampal-hippocampal network

Converging evidence suggests that each parahippocampal and hippocampal subregion contributes uniquely to the encoding, consolidation and retrieval of declarative memories, but their precise roles remain elusive. Current functional thinking does not fully incorporate the intricately connected networks that link these subregions, owing to their organizational complexity; however, such detailed anatomical knowledge is of pivotal importance for comprehending the unique functional contribution of each subregion. We have therefore developed an interactive diagram with the aim to display all of the currently known anatomical connections of the rat parahippocampal-hippocampal network. In this Review, we integrate the existing anatomical knowledge into a concise description of this network and discuss the functional implications of some relatively underexposed connections.

Grid cells: the position code, neural network models of activity, and the problem of learning

We review progress on the modeling and theoretical fronts in the quest to unravel the computational properties of the grid cell code and to explain the mechanisms underlying grid cell dynamics. The goals of the review are to outline a coherent framework for understanding the dynamics of grid cells and their representation of space; to critically present and draw contrasts between recurrent network models of grid cells based on continuous attractor dynamics and independent-neuron models based on temporal interference; and to suggest open questions for experiment and theory.

A metric for space

Not all areas of neuronal systems investigation have matured to the stage where computation can be understood at the microcircuit level. In mammals, insights into cortical circuit functions have been obtained for the early stages of sensory systems, where signals can be followed through networks of increasing complexity from the receptors to the primary sensory cortices. These studies have suggested how neurons and neuronal networks extract features from the external world, but how the brain generates its own codes, in the higher-order nonsensory parts of the cortex, has remained deeply mysterious. In this terra incognita, a path was opened by the discovery of grid cells, place-modulated entorhinal neurons whose firing locations define a periodic triangular or hexagonal array covering the entirety of the animal's available environment. This array of firing is maintained in spite of ongoing changes in the animal's speed and direction, suggesting that grid cells are part of the brain's metric for representation of space. Because the crystal-like structure of the firing fields is created within the nervous system itself, grid cells may provide scientists with direct access to some of the most basic operational principles of cortical circuits.

Navigating from hippocampus to parietal cortex

The navigational system of the mammalian cortex comprises a number of interacting brain regions. Grid cells in the medial entorhinal cortex and place cells in the hippocampus are thought to participate in the formation of a dynamic representation of the animal's current location, and these cells are presumably critical for storing the representation in memory. To traverse the environment, animals must be able to translate coordinate information from spatial maps in the entorhinal cortex and hippocampus into body-centered representations that can be used to direct locomotion. How this is done remains an enigma. We propose that the posterior parietal cortex is critical for this transformation.

Landmark control and updating of self-movement cues are largely maintained in head direction cells after lesions of the posterior parietal cortex

Head direction (HD) cells discharge as a function of the rat's directional orientation with respect to its environment. Because animals with posterior parietal cortex (PPC) lesions exhibit spatial and navigational deficits, and the PPC is indirectly connected to areas containing HD cells, we determined the effects of bilateral PPC lesions on HD cells recorded in the anterodorsal thalamus. HD cells from lesioned animals had similar firing properties compared to controls and their preferred firing directions shifted a corresponding amount following rotation of the major visual landmark. Because animals were not exposed to the visual landmark until after surgical recovery, these results provide evidence that the PPC is not necessary for visual landmark control or the establishment of landmark stability. Further, cells from lesioned animals maintained a stable preferred firing direction when they foraged in the dark and were only slightly less stable than controls when they self-locomoted into a novel enclosure. These findings suggest that PPC does not play a major role in the use of landmark and self-movement cues in updating the HD cell signal, or in its generation.

Multimodal sensory integration and concurrent navigation strategies for spatial cognition in real and artificial organisms

Flexible spatial behavior requires the ability to orchestrate the interaction of multiple parallel processes. At the sensory level, multimodal inputs must be combined to produce a robust description of the spatiotemporal properties of the environment. At the action-selection level, multiple concurrent navigation policies must be dynamically weighted in order to adopt the strategy that is the most adapted to the complexity of the task. Different neural substrates mediate the processing of spatial information. Elucidating their anatomo-functional interrelations is fundamental to unravel the overall spatial memory function. Here we first address the multisensory integration issue and we review a series of experimental findings (both behavioral and electrophysiological) concerning the neural bases of spatial learning and the way the brain builds unambiguous spatial representations from incoming multisensory streams. Second, we move at the navigation strategy level and present an overview of experimental data that begin to explain the cooperation-competition between the brain areas involved in spatial navigation. Third, we introduce the spatial cognition function from a computational neuroscience and neuro-robotics viewpoint. We provide an example of neuro-computational model that focuses on the importance of combining multisensory percepts to enable a robot to acquire coherent (spatial) memories of its interaction with the environment.

Neurobiologically inspired mobile robot navigation and planning

After a short review of biologically inspired navigation architectures, mainly relying on modeling the hippocampal anatomy, or at least some of its functions, we present a navigation and planning model for mobile robots. This architecture is based on a model of the hippocampal and prefrontal interactions. In particular, the system relies on the definition of a new cell type "transition cells" that encompasses traditional "place cells".

A navigational guidance system in the human brain

Finding your way in large-scale space requires knowing where you currently are and how to get to your goal destination. While much is understood about the neural basis of one's current position during navigation, surprisingly little is known about how the human brain guides navigation to goals. Computational accounts argue that specific brain regions support navigational guidance by coding the proximity and direction to the goal, but empirical evidence for such mechanisms is lacking. Here, we scanned subjects with functional magnetic resonance imaging as they navigated to goal destinations in a highly accurate virtual simulation of a real city. Brain activity was then analyzed in combination with metric measures of proximity and direction to goal destinations that were derived from each individual subject's coordinates at every second of navigation. We found that activity in the medial prefrontal cortex was positively correlated, and activity in a right subicular/entorhinal region was negatively correlated with goal proximity. By contrast, activity in bilateral posterior parietal cortex was correlated with egocentric direction to goals. Our results provide empirical evidence for a navigational guidance system in the human brain, and define more precisely the contribution of these three brain regions to human navigation. In addition, these findings may also have wider implications for how the brain monitors and integrates different types of information in the service of goal-directed behavior in general.

Contributions of the dorsal hippocampus and the dorsal subiculum to processing of idiothetic information and spatial memory

It is well established that the dorsal hippocampal formation is crucial for spatial memory in rats. However, little is known about the distinct functions of the dorsal hippocampus and the dorsal subiculum. To examine the role of the dorsal hippocampus and the dorsal subiculum, Long-Evans rats with excitotoxic lesions (NMDA) of the dorsal hippocampus (DH), the dorsal subiculum (DS), or both (DHDS), and controls were trained on a nonmatching-to-place task. Then, to identify the strategy used by each group, they were tested on the same task in the dark with the T-maze being rotated between the sample and the test runs. In the light, rats with combined lesions were impaired. In the dark, groups DH, DS, and controls performed near chance level except in trials without rotation, suggesting the use of a sense of direction. The same rats were trained on a radial-arm maze task. In the light, where proximal visual cues were accessible, combined lesions affected performance whereas in the dark, it was impaired by all lesions. This experiment demonstrated that the dorsal subiculum and the dorsal hippocampus play a crucial role in processing idiothetic information and/or in maintenance of this information in memory.

Hippocampal remapping and grid realignment in entorhinal cortex

A fundamental property of many associative memory networks is the ability to decorrelate overlapping input patterns before information is stored. In the hippocampus, this neuronal pattern separation is expressed as the tendency of ensembles of place cells to undergo extensive 'remapping' in response to changes in the sensory or motivational inputs to the hippocampus. Remapping is expressed under some conditions as a change of firing rates in the presence of a stable place code ('rate remapping'), and under other conditions as a complete reorganization of the hippocampal place code in which both place and rate of firing take statistically independent values ('global remapping'). Here we show that the nature of hippocampal remapping can be predicted by ensemble dynamics in place-selective grid cells in the medial entorhinal cortex, one synapse upstream of the hippocampus. Whereas rate remapping is associated with stable grid fields, global remapping is always accompanied by a coordinate shift in the firing vertices of the grid cells. Grid fields of co-localized medial entorhinal cortex cells move and rotate in concert during this realignment. In contrast to the multiple environment-specific representations coded by place cells in the hippocampus, local ensembles of grid cells thus maintain a constant spatial phase structure, allowing position to be represented and updated by the same translation mechanism in all environments encountered by the animal.

Path integration and the neural basis of the 'cognitive map'

The hippocampal formation can encode relative spatial location, without reference to external cues, by the integration of linear and angular self-motion (path integration). Theoretical studies, in conjunction with recent empirical discoveries, suggest that the medial entorhinal cortex (MEC) might perform some of the essential underlying computations by means of a unique, periodic synaptic matrix that could be self-organized in early development through a simple, symmetry-breaking operation. The scale at which space is represented increases systematically along the dorsoventral axis in both the hippocampus and the MEC, apparently because of systematic variation in the gain of a movement-speed signal. Convergence of spatially periodic input at multiple scales, from so-called grid cells in the entorhinal cortex, might result in non-periodic spatial firing patterns (place fields) in the hippocampus.

Representation of Spatial Goals in Rat Orbitofrontal Cortex

The orbitofrontal cortex (OFC) is thought to participate in making and evaluating goal-directed decisions. In rodents, spatial navigation is a major mode of goal-directed behavior, and anatomical and lesion studies implicate the OFC in spatial processing, but there is little direct evidence for coding of spatial or motor variables. Here, we recorded from ventrolateral and lateral OFC in an odor-cued two-alternative choice task requiring orientation and approach to spatial goal ports. In this context, over half of OFC neurons encoded choice direction or goal port location. A subset of neurons was jointly selective for the trial outcome and port location, information useful for the selection or evaluation of spatial goals. These observations show that the rodent OFC not only encodes information relating to general motivational significance, as shown previously, but also encodes spatiomotor variables needed to define specific behavioral goals and the locomotor actions required to attain them.