Hippocampal place cells: parallel input streams, subregional processing, and implications for episodic memory

Neural systems analysis of decision making during goal-directed navigation

The ability to make adaptive decisions during goal-directed navigation is a fundamental and highly evolved behavior that requires continual coordination of perceptions, learning and memory processes, and the planning of behaviors. Here, a neurobiological account for such coordination is provided by integrating current literatures on spatial context analysis and decision-making. This integration includes discussions of our current understanding of the role of the hippocampal system in experience-dependent navigation, how hippocampal information comes to impact midbrain and striatal decision making systems, and finally the role of the striatum in the implementation of behaviors based on recent decisions. These discussions extend across cellular to neural systems levels of analysis. Not only are key findings described, but also fundamental organizing principles within and across neural systems, as well as between neural systems functions and behavior, are emphasized. It is suggested that studying decision making during goal-directed navigation is a powerful model for studying interactive brain systems and their mediation of complex behaviors.

Modular realignment of entorhinal grid cell activity as a basis for hippocampal remapping

Hippocampal place fields, the local regions of activity recorded from place cells in exploring rodents, can undergo large changes in relative location during remapping. This process would appear to require some form of modulated global input. Grid-cell responses recorded from layer II of medial entorhinal cortex in rats have been observed to realign concurrently with hippocampal remapping, making them a candidate input source. However, this realignment occurs coherently across colocalized ensembles of grid cells (Fyhn et al., 2007). The hypothesized entorhinal contribution to remapping depends on whether this coherence extends to all grid cells, which is currently unknown. We study whether dividing grid cells into small numbers of independently realigning modules can both account for this localized coherence and allow for hippocampal remapping. To do this, we construct a model in which place-cell responses arise from network competition mediated by global inhibition. We show that these simulated responses approximate the sparsity and spatial specificity of hippocampal activity while fully representing a virtual environment without learning. Place-field locations and the set of active place cells in one environment can be independently rearranged by changes to the underlying grid-cell inputs. We introduce new measures of remapping to assess the effectiveness of grid-cell modularity and to compare shift realignments with other geometric transformations of grid-cell responses. Complete hippocampal remapping is possible with a small number of shifting grid modules, indicating that entorhinal realignment may be able to generate place-field randomization despite substantial coherence.

Theta and gamma coherence across the septotemporal axis during distinct behavioral states

Theta (4-12 Hz) and gamma (40-100 Hz) field potentials represent the interaction of synchronized synaptic input onto distinct neuronal populations within the hippocampal formation. Theta is quite prominent during exploratory activity, locomotion, and REM sleep. Although it is generally acknowledged that theta is coherent throughout most of the hippocampus, there is significant variability in theta, as well as gamma, coherence across lamina at any particular septotemporal level of the hippocampus. Larger differences in theta coherence are observed across the septotemporal (long) axis. We have reported that during REM sleep there is a decrease in theta coherence across the long axis that varies with the topography of CA3/mossy cell input rather than the topography of the prominent entorhinal input. On the basis of differences in the rat's behavior as well as the activity of neuromodulatory inputs (e.g., noradrenergic and serotonergic), we hypothesized that theta coherence across the long axis would be greater during locomotion than REM sleep and exhibit a pattern more consistent with the topography of entorhinal inputs. We examined theta and gamma coherence indices at different septotemporal and laminar sites during distinct theta states: locomotion during maze running, REM sleep, following acute treatment with a θ-inducing cholinomimetic (physostigmine) and for comparison during slow-wave sleep. The results demonstrate a generally consistent pattern of theta and gamma coherence across the septotemporal axis of the hippocampus that is quite indifferent to sensory input and overt behavior. These results are discussed with regards to the neurobiological mechanisms that generate theta and gamma and the growing body of evidence linking theta and gamma indices to memory and other cognitive functions.

Postnatal development of the hippocampal formation: a stereological study in macaque monkeys

We performed a stereological analysis of neuron number, neuronal soma size, and volume of individual regions and layers of the macaque monkey hippocampal formation during early postnatal development. We found a protracted period of neuron addition in the dentate gyrus throughout the first postnatal year and a concomitant late maturation of the granule cell population and individual dentate gyrus layers that extended beyond the first year of life. Although the development of CA3 generally paralleled that of the dentate gyrus, the distal portion of CA3, which receives direct entorhinal cortex projections, matured earlier than the proximal portion of CA3. CA1 matured earlier than the dentate gyrus and CA3. Interestingly, CA1 stratum lacunosum-moleculare, in which direct entorhinal cortex projections terminate, matured earlier than CA1 strata oriens, pyramidale, and radiatum, in which the CA3 projections terminate. The subiculum developed earlier than the dentate gyrus, CA3, and CA1, but not CA2. However, similarly to CA1, the molecular layer of the subiculum, in which the entorhinal cortex projections terminate, was overall more mature in the first postnatal year compared with the stratum pyramidale in which most of the CA1 projections terminate. Unlike other hippocampal fields, volumetric measurements suggested regressive events in the structural maturation of presubicular neurons and circuits. Finally, areal and neuron soma size measurements revealed an early maturation of the parasubiculum. We discuss the functional implications of the differential development of distinct hippocampal circuits for the emergence and maturation of different types of "hippocampus-dependent" memory processes, including spatial and episodic memories.

Gateways of ventral and dorsal streams in mouse visual cortex

It is widely held that the spatial processing functions underlying rodent navigation are similar to those encoding human episodic memory (Doeller et al., 2010). Spatial and nonspatial information are provided by all senses including vision. It has been suggested that visual inputs are fed to the navigational network in cortex and hippocampus through dorsal and ventral intracortical streams (Whitlock et al., 2008), but this has not been shown directly in rodents. We have used cytoarchitectonic and chemoarchitectonic markers, topographic mapping of receptive fields, and pathway tracing to determine in mouse visual cortex whether the lateromedial field (LM) and the anterolateral field (AL), which are the principal targets of primary visual cortex (V1) (Wang and Burkhalter, 2007) specialized for processing nonspatial and spatial visual information (Gao et al., 2006), are distinct areas with diverse connections. We have found that the LM/AL border coincides with a change in type 2 muscarinic acetylcholine receptor expression in layer 4 and with the representation of the lower visual field periphery. Our quantitative analyses also show that LM strongly projects to temporal cortex as well as the lateral entorhinal cortex, which has weak spatial selectivity (Hargreaves et al., 2005). In contrast, AL has stronger connections with posterior parietal cortex, motor cortex, and the spatially selective medial entorhinal cortex (Haftig et al., 2005). These results support the notion that LM and AL are architecturally, topographically, and connectionally distinct areas of extrastriate visual cortex and that they are gateways for ventral and dorsal streams.

Functional division of hippocampal area CA1 via modulatory gating of entorhinal cortical inputs

The hippocampus receives two streams of information, spatial and nonspatial, via major afferent inputs from the medial (MEC) and lateral entorhinal cortexes (LEC). The MEC and LEC projections in the temporoammonic pathway are topographically organized along the transverse-axis of area CA1. The potential for functional segregation of area CA1, however, remains relatively unexplored. Here, we demonstrated differential novelty-induced c-Fos expression along the transverse-axis of area CA1 corresponding to topographic projections of MEC and LEC inputs. We found that, while novel place exposure induced a uniform c-Fos expression along the transverse-axis of area CA1, novel object exposure primarily activated the distal half of CA1 neurons. In hippocampal slices, we observed distinct presynaptic properties between LEC and MEC terminals, and application of either DA or NE produced a largely selective influence on one set of inputs (LEC). Finally, we demonstrated that differential c-Fos expression along the transverse axis of area CA1 was largely abolished by an antagonist of neuromodulatory receptors, clozapine. Our results suggest that neuromodulators can control topographic TA projections allowing the hippocampus to differentially encode new information along the transverse axis of area CA1.

Intact landmark control and angular path integration by head direction cells in the anterodorsal thalamus after lesions of the medial entorhinal cortex

The medial entorhinal cortex (MEC) occupies a central position within neural circuits devoted to the representation of spatial location and orientation. The MEC contains cells that fire as a function of the animal's head direction (HD), as well as grid cells that fire in multiple locations in an environment, forming a repeating hexagonal pattern. The MEC receives inputs from widespread areas of the cortical mantle including the ventral visual stream, which processes object recognition information, as well as information about visual landmarks. The role of the MEC in processing the HD signal or landmark information is unclear. We addressed this issue by neurotoxically damaging the MEC and recording HD cells within the anterodorsal thalamus (ADN). Direction-specific activity was present in the ADN of all animals with MEC lesions. Moreover, the discharge characteristics of ADN HD cells were only mildly affected by MEC lesions, with HD cells exhibiting greater anticipation of future HDs. Tests of landmark control revealed that HD cells in lesioned rats were capable of accurately updating their preferred firing directions in relation to a salient visual cue. Furthermore, cells from lesioned animals maintained stable preferred firing directions when locomoting in darkness and demonstrated stable HD cell tuning when locomoting into a novel enclosure, suggesting that MEC lesions did not disrupt the integration of idiothetic cues, or angular path integration, by HD cells. Collectively, these findings suggest that the MEC plays a limited role in the formation and spatial updating of the HD cell signal.

Spatial representation along the proximodistal axis of CA1

CA1 cells receive direct input from space-responsive cells in medial entorhinal cortex (MEC), such as grid cells, as well as more nonspatial cells in lateral entorhinal cortex (LEC). Because MEC projects preferentially to the proximal part of the CA1, bordering CA2, whereas LEC innervates only the distal part, bordering subiculum, we asked if spatial tuning is graded along the transverse axis of CA1. Tetrodes were implanted along the entire proximodistal axis of dorsal CA1 in rats. Data were recorded in cylinders large enough to elicit firing at more than one location in many neurons. Distal CA1 cells showed more dispersed firing and had a larger number of firing fields than proximal cells. Phase-locking of spikes to MEC theta oscillations was weaker in distal CA1 than in proximal CA1. The findings suggest that spatial firing in CA1 is organized transversally, with the strongest spatial modulation occurring in the MEC-associated proximal part.

Disconnection of the hippocampal-perirhinal cortical circuits severely disrupts object-place paired associative memory

The hippocampus and the perirhinal cortex (PR) are reciprocally connected both directly and indirectly via the entorhinal cortex. Although it has been hypothesized that the two regions should have intimate functional interactions with each other on the basis of the anatomical connectivity, many lesion studies have demonstrated functional dissociations instead between the hippocampus and PR. To show a tight functional relationship between the two regions, we used reversible inactivation techniques targeting both the hippocampus and PR within subjects, combined with a biconditional memory task in which the rat must consider information about objects and their locations. Specifically, rats were implanted with two sets of bilateral cannulas into the hippocampus and PR, and were tested in an object-place paired-associate task in a radial maze. While alternating between two arms, the rats were required to choose one of the objects exclusively associated with a given arm for food. Bilateral muscimol (MUS) injections into either the hippocampus or PR equally produced chance level performance. When a functional disconnection procedure was used to disrupt the interaction between the hippocampus and PR, contralateral MUS injections into the hippocampus and PR resulted in severe impairment in performance. However, inactivating the hippocampus and PR ipsilaterally did not affect the performance. In a simple object discrimination task, the same functional disconnection protocol with MUS did not affect the performance. The results powerfully demonstrate that the hippocampus, the PR, and their functional interactions are all indispensable when objects and their spatial locations must be processed at the same time.

Maternal immune activation alters nonspatial information processing in the hippocampus of the adult offspring

The observation that maternal infection increases the risk for schizophrenia in the offspring suggests that the maternal immune system plays a key role in the etiology of schizophrenia. In a mouse model, maternal immune activation (MIA) by injection of poly(I:C) yields adult offspring that display abnormalities in a variety of behaviors relevant to schizophrenia. As abnormalities in the hippocampus are a consistent observation in schizophrenia patients, we examined synaptic properties in hippocampal slices prepared from the offspring of poly(I:C)- and saline-treated mothers. Compared to controls, CA1 pyramidal neurons from adult offspring of MIA mothers display reduced frequency and increased amplitude of miniature excitatory postsynaptic currents. In addition, the specific component of the temporoammonic pathway that mediates object-related information displays increased sensitivity to dopamine. To assess hippocampal network function in vivo, we used expression of the immediate-early gene, c-Fos, as a surrogate measure of neuronal activity. Compared to controls, the offspring of poly(I:C)-treated mothers display a distinct c-Fos expression pattern in area CA1 following novel object, but not novel location, exposure. Thus, the offspring of MIA mothers may have an abnormality in modality-specific information processing. Indeed, the MIA offspring display enhanced discrimination in a novel object recognition, but not in an object location, task. Thus, analysis of object and spatial information processing at both synaptic and behavioral levels reveals a largely selective abnormality in object information processing in this mouse model. Our results suggest that altered processing of object-related information may be part of the pathogenesis of schizophrenia-like cognitive behaviors.

The shift from a response strategy to object-in-place strategy during learning is accompanied by a matching shift in neural firing correlates in the hippocampus

Hippocampal-dependent tasks often involve specific associations among stimuli (including egocentric information), and such tasks are therefore prone to interference from irrelevant task strategies before a correct strategy is found. Using an object-place paired-associate task, we investigated changes in neural firing patterns in the hippocampus in association with a shift in strategy during learning. We used an object-place paired-associate task in which a pair of objects was presented in two different arms of a radial maze. Each object was associated with reward only in one of the arms, thus requiring the rats to consider both object identity and its location in the maze. Hippocampal neurons recorded in CA1 displayed a dynamic transition in their firing patterns during the acquisition of the task across days, and this corresponded to a shift in strategy manifested in behavioral data. Specifically, before the rats learned the task, they chose an object that maintained a particular egocentric relationship with their body (response strategy) irrespective of the object identity. However, as the animal acquired the task, it chose an object according to both its identity and the associated location in the maze (object-in-place strategy). We report that CA1 neurons in the hippocampus changed their prospective firing correlates according to the dominant strategy (i.e., response versus object-in-place strategy) employed at a given stage of learning. The results suggest that neural firing pattern in the hippocampus is heavily influenced by the task demand hypothesized by the animal and the firing pattern changes flexibly as the perceived task demand changes.

Attractor-map versus autoassociation based attractor dynamics in the hippocampal network

The autoassociative memory model of hippocampal field CA3 postulates that Hebbian associations among external input features produce attractor states embedded in a recurrent synaptic matrix. In contrast, the attractor-map model postulates that a two-dimensional continuum of attractor states is preconfigured in the network during development and that transitions among these states are governed primarily by self-motion information ("path-integration"), giving rise to the strong spatial characteristic of hippocampal activity. In this model, learned associations between "coordinates" on the attractor map and external cues can result in abrupt jumps between states, in the case of mismatches between the current input and previous associations between internal coordinates and external landmarks. Both models predict attractor dynamics, but for fundamentally different reasons; however, the two models are not a priori mutually exclusive. We contrasted these two models by comparing the dynamics of state transitions when two previously learned environmental shapes were morphed between their endpoints, in animals that had first experienced the environments either at the same location, or at two different locations, connected by a passageway through which they walked. As predicted from attractor-map theory, the latter animals expressed abrupt transitions between representations at the midpoint of the morph series. Contrary to the predictions of autoassociation theory, the former group expressed no evidence of attractor dynamics during the morph series; there was only a gradual transition between endpoints. The results of this critical test thus cast the autoassociator theory for CA3 into doubt and indicate the need for a new theory for this structure.

Hebbian analysis of the transformation of medial entorhinal grid-cell inputs to hippocampal place fields

The discovery of grid cells in the medial entorhinal cortex (MEC) permits the characterization of hippocampal computation in much greater detail than previously possible. The present study addresses how an integrate-and-fire unit driven by grid-cell spike trains may transform the multipeaked, spatial firing pattern of grid cells into the single-peaked activity that is typical of hippocampal place cells. Previous studies have shown that in the absence of network interactions, this transformation can succeed only if the place cell receives inputs from grids with overlapping vertices at the location of the place cell's firing field. In our simulations, the selection of these inputs was accomplished by fast Hebbian plasticity alone. The resulting nonlinear process was acutely sensitive to small input variations. Simulations differing only in the exact spike timing of grid cells produced different field locations for the same place cells. Place fields became concentrated in areas that correlated with the initial trajectory of the animal; the introduction of feedback inhibitory cells reduced this bias. These results suggest distinct roles for plasticity of the perforant path synapses and for competition via feedback inhibition in the formation of place fields in a novel environment. Furthermore, they imply that variability in MEC spiking patterns or in the rat's trajectory is sufficient for generating a distinct population code in a novel environment and suggest that recalling this code in a familiar environment involves additional inputs and/or a different mode of operation of the network.

Changes in hippocampal excitatory synaptic transmission during cholinergically induced theta and slow oscillation states

Neural processing in the hippocampus (HPC) during sleep is important for declarative memory storage. Previously, we have shown that alternations of sleep-like REM and non-REM brain states that involve changing patterns of synchronized oscillatory network activity in the HPC [i.e., theta and the slow oscillation (SO), respectively] robustly and differentially influence excitatory synaptic transmission in a variety of hippocampal pathways. Given that state in the HPC is dependent on variations in cholinergic tone in both sleep and under urethane anesthesia, in the present study we induced theta and SO states via systemic cholinergic manipulations in urethane-anesthetized rats to confirm similar changes in synaptic responsiveness. This was conducted using linear multiprobe recordings and current source density analysis of electrically evoked potentials in commissural and temporal ammonic inputs to CA1 and medial and lateral perforant path inputs to dentate gyrus (DG). Cholinergic agonism and antagonism induced theta and the SO, respectively, and similarly to the case with spontaneous states, also diminished and promoted, respectively, excitatory synaptic currents in all pathways (except for the medial perforant path input to DG which showed the opposite modulation). These results suggest that both state and cholinergic tone bias the hippocampal network during natural sleep across REM and non-REM episodes and that this modulation may play an important role in the consolidation of declarative memories.

Imaging the human medial temporal lobe with high-resolution fMRI

High-resolution functional MRI (hr-fMRI) affords unique leverage on the functional properties of human medial temporal lobe (MTL) substructures. We review initial hr-fMRI efforts to delineate (1) encoding and retrieval processes within the hippocampal circuit, (2) hippocampal subfield contributions to pattern separation and pattern completion, and (3) the representational capabilities of distinct MTL subregions. Extant data reveal functional heterogeneity within human MTL and highlight the promise of hr-fMRI for bridging human, animal, and computational approaches to understanding MTL function.

Role of the hippocampus in goal-oriented tasks requiring retrieval of spatial versus non-spatial information

The role of the hippocampus in non-spatial memory has been issue of some controversy. To investigate the nature of dorsal hippocampus engagement in spatial and non-spatial memory we performed discrete excitotoxic lesions of this region before mice (C57/BL6) were trained in one of two tasks that required the animals to retrieve a hidden food reward. In the visuospatial task animals had to remember a particular spatial location, independent of odor cues. In contrast, in a non-spatial olfactory task animals had to remember a particular odor, independent of spatial location. The mice were trained in one of these tasks over a period of three days. We found that lesions restricted to the dorsal hippocampus affected performance only in the spatial task. In contrast, lesions that also encompassed a larger portion of the ventral hippocampus caused a moderate deficit in the olfactory task. These results are consistent with the role of the dorsal hippocampus in long-term spatial episodic memory, and support the involvement of larger portions of the hippocampus on the encoding of non-spatial olfactory representations.

Perirhinal cortex is necessary for acquiring, but not for retrieving object-place paired association

Remembering events frequently involves associating objects and their associated locations in space, and it has been implicated that the areas associated with the hippocampus are important in this function. The current study examined the role of the perirhinal cortex in retrieving familiar object-place paired associates, as well as in acquiring novel ones. Rats were required to visit one of two locations of a radial-arm maze and choose one of the objects (from a pair of different toy objects) exclusively associated with a given arm. Excitotoxic lesions of the perirhinal cortex initially impaired the normal retrieval of object-place paired-associative memories that had been learned presurgically, but the animals relearned gradually to the level of controls. In contrast, when required to associate a novel pair of objects with the same locations of the maze, the same lesioned rats were severely impaired with minimal learning, if any, taking place throughout an extensive testing period. However, the lesioned rats were normal in discriminating two different objects presented in a fixed arm in the maze. The results suggest that the perirhinal cortex is indispensable to forming discrete representations for object-place paired associates. Its role, however, may be compensated for by other structures when familiar object-place paired associative memories need to be retrieved.

Paradoxical sleep as a tool for understanding the hippocampal mechanisms of contextual memory

Existing data on the involvement of the hippocampus in contextual memory and the fact that contextual memory is impaired in dreams occurring during paradoxical sleep allowed us to suggest that one of the causes of this impairment consists of changes in the efficiency of synaptic transmission in the hippocampus due to increases (as compared with waking) in the concentrations of acetylcholine, dopamine, and cortisol, as well as the absence of serotonin and noradrenaline. Our previous analysis showed that in paradoxical sleep, long-term depression can be induced all components of the polysynaptic pathway through the hippocampal formation, while potentiation can occur at the inputs from the entorhinal cortex to hippocampal fields CA1 and CA3 and in the associative connections in field CA3. It is hypothesized that the correct functioning of episodic memory requires efficient transmission of signals in each component of the polysynaptic pathway through the hippocampus, allowing a neuronal representation of the context to be created within it. In the state of waking, reproduction of the context of an episode simultaneously activates the neuronal representation of the context remembered in the hippocampus and neuronal representations of the details of the episode remembered in those areas of the cortex in which they were processed. It follows from the proposed mechanism that any neurotransmitter or neuropeptide able to promote longterm potentiation in all components of the polysynaptic pathway through the hippocampus can improve episodic memory. As the consequences of the mechanism are consistent with experimental data, it can be used to seek agents improving episodic memory.

A cognitive map for object memory in the hippocampus

The hippocampus has been proposed to support a cognitive map, a mental representation of the spatial layout of an environment as well as the nonspatial items encountered in that environment. In the present study, we recorded simultaneously from 43 to 61 hippocampal pyramidal cells as rats performed an object recognition memory task in which novel and repeated objects were encountered in different locations on a circular track. Multivariate analyses of the neural data indicated that information about object identity was represented secondarily to the primary information dimension of object location. In addition, the neural data related to performance on the recognition memory task. The results suggested that objects were represented as points of interest on the hippocampal cognitive map and that this map was useful in remembering encounters with particular objects in specific locations.

Role of dentate gyrus in aligning internal spatial map to external landmark

Humans and animals form internal representations of external space based on their own body movement (dead reckoning) as well as external landmarks. It is poorly understood, however, how different types of information are integrated to form a unified representation of external space. To examine the role of dentate gyrus (DG) in this process, we conducted physiological and behavioral experiments using Bax knockout (Bax-KO) mice in which newly generated granule cells continue to accumulate disrupting neural circuitry specifically in the DG. Unlike in wild-type (WT) littermates, spatial firing of hippocampal neurons was completely dissociated from a distinct visual cue and instead, tended to stay constant relative to the recording room in Bax-KO mice. Behaviorally, whereas spatial learning was intact in conventional spatial reference memory tasks, Bax-KO mice were impaired in finding a target location based on visual landmarks when target locations predicted by dead reckoning and visual landmarks were made incongruent. These results provide converging evidence for the role of DG in binding animal's internal spatial map with the sensory information on external landmarks in building a distinct spatial representation for each environment.

Cycloheximide-induced inhibition of protein synthesis in hippocampal pyramidal neurons is time-dependent: differences between CA1 and CA3 areas

Cycloheximide (CHI), an inhibitor of protein synthesis, is widely used for studying the mechanisms of consolidation of long-term memory (LTM). High concentrations of CHI inhibit the protein synthesis in brain homogenates by more than 80% and impair LTM consolidation. For understanding the mechanisms of consolidation, it is important to know how protein synthesis inhibitors affect hippocampal neurons. However, the effect of CHI on protein synthesis in CA1 and CA3 hippocampal pyramidal neurons is still poorly understood. In the present work, the state of ribosomes in CA1 and CA3 pyramidal neurons from the dorsal hippocampus of Wistar rats 1, 2, 4, and 72 h after the introcerebroventricular (i.c.v.) injection of a high concentration of CHI was determined using the fluorescent dye acridine orange. We showed that CHI induces great differences in the dynamics of the intensity of protein synthesis in CA1 and CA3 pyramidal neurons. The suppression of the intensity of protein synthesis in CA1 pyramidal neurons 1h after the injection of CHI was more than threefold stronger than in CA3, and by 4h, it was most pronounced in CA3 neurons. We suggest that the protein synthesis in CA1 pyramidal neurons contributes significantly to the synaptic consolidation of declarative memory in the first critical period.

Bilateral NMDA lesions centered on the postrhinal cortex have minimal effects on hippocampal place cell firing

The postrhinal cortex (POR) receives input from parietal cortex and sends output to the hippocampus. It may, therefore, relay spatial information to the hippocampus and as a result, lesions of POR may disturb the spatial firing patterns of hippocampal place cells. To test this hypothesis, the firing of hippocampal CA1 place cells in rats with bilateral N-methyl-D-aspartic acid lesions centered on the POR (n = 83 cells) and rats with sham lesions (n = 77 cells) was compared, while animals foraged freely. The main effect of postrhinal lesions on the basic firing properties of hippocampal place cells was to decrease the coherence of their firing fields. In contrast to the previously reported effects of lesions of neighboring perirhinal cortex, however, there was no effect of postrhinal lesions on the location stability of the fields over time or in the response of these cells to the animal's movement. These data indicate that information originating from the POR has relatively little influence on hippocampal place cell firing while an animal is engaged in foraging behavior. This also suggests that perirhinal and postrhinal cortices make distinct contributions to hippocampal functioning.

A specific role of the human hippocampus in recall of temporal sequences

There is a growing interest in how temporal order of episodic memories is represented within the medial temporal lobe (MTL). Animal studies suggest that the hippocampal formation (HF) is critical for retrieving the temporal order of past experiences. However, human imaging studies that have tested recency discrimination between pairs of previously encoded items have generally failed to report HF activation. We hypothesized that recalling a naturalistic sequence of past events would be particularly sensitive to HF function, attributable to greater involvement of associative processes. To test this prediction, we let subjects watch a novel movie and later, during functional magnetic resonance imaging, asked them to rearrange and "replay" scenes from the movie in correct order. To identify areas specifically involved in retrieval of temporal order, we used a control condition where subjects logically inferred the order of scenes from the same movie. Extensive MTL activation was observed during sequence recall. Activation within the right HF was specifically related to retrieval of temporal order and correlated positively with accuracy of sequence recall. Also, the bilateral parahippocampal cortex responded to retrieval of temporal order, but the activation here was not related to performance. Our study is the first to unequivocally demonstrate that correct sequence recall depends on HF.

Novelty signals: a window into hippocampal information processing

The function of the human hippocampus is a contentious subject among neuroscientists. Theoreticians have long viewed the hippocampus as a computational device, with researchers in humans increasingly adopting this perspective, buoyed by recent reports that its role is not limited to declarative memory. Here, we set out a new strategy for discovering the nature of information processing within the human hippocampus. We argue that novelty responses, measured by functional magnetic resonance imaging, provide a window into the neural representations and computations sustained by the hippocampus. More generally, we suggest that a renewed emphasis on the information processing qualities of the human hippocampus offers the promise of a long awaited union between theoretical and empirical research across species.

Episodic-like and procedural memory impairments in histamine H1 Receptor knockout mice coincide with changes in acetylcholine esterase activity in the hippocampus and dopamine turnover in the cerebellum

We investigated episodic-like (ELM) and procedural memory (PM) in histamine H1 receptor knockout (H1R-KO) mice. In order to relate possible behavioral deficits to neurobiological changes, we examined H1R-KO and wild-type (WT) mice in terms of acetylcholine esterase (AChE) activity in subregions of the hippocampus and AChE and tyrosine hydroxylase (TH) expression in the striatum. Furthermore, we analyzed acetylcholine (ACh), 5-HT and dopamine (DA) levels, including metabolites, in the cerebellum of H1R-KO and WT mice. The homozygous H1R-KO mice showed impaired ELM as compared with the heterozygous H1R-KO and WT mice. The performance of homozygous H1R-KO mice in the ELM task was primarily driven by familiarity-based memory processes. While the homozygous H1R-KO mice performed similar to the heterozygous H1R-KO and WT mice during the acquisition of a PM, as measured with an accelerating rotarod, after a retention interval of 7 days their performance was impaired relative to the heterozygous H1R-KO and WT mice. These findings suggest that, both, ELM and long-term PM are impaired in the homozygous H1R-KO mice. Neurochemical assays revealed that the H1R-KO mice had significantly lower levels of AChE activity in the dentate gyrus (DG) and CA1 subregions of the hippocampus as compared with the WT mice. The homozygous H1R-KO mice also displayed significantly reduced dihydroxyphenylacetic acid (DOPAC) levels and a reduced DOPAC/DA ratio in the cerebellum, suggesting that the DA turnover in the cerebellum is decelerated in homozygous H1R-KO mice. In conclusion, homozygous H1R-KO mice display severe long-term memory deficits in, both, ELM and PM, which coincide with changes in AChE activity in the hippocampus as well as DA turnover in the cerebellum. The importance of these findings for Alzheimer's (AD) and Parkinson's disease (PD) is discussed.

Potential anatomical basis for attentional modulation of hippocampal neurons

Lesions of the hippocampus and related structures produce profound anterograde amnesia. The amnesia is specific to what has been called "explicit," "declarative," and "episodic" memory. These memories are frequently believed to be central to the human condition, requiring such advanced cognitive functions as attention and even consciousness. However, the hippocampus and associated structures are evolutionarily conserved, which argues that the memories of lower mammals should be qualitatively similar in nature. Just as attention and arousal are critical components of appropriate memory formation in humans, an emerging body of evidence suggests that these processes bear on the firing patterns of hippocampal neurons in rodents. Here the evidence favoring this hypothesis is discussed and then the potential anatomical basis for such modulation is considered.

Unstable CA1 place cell representation in rats with entorhinal cortex lesions

Recent studies emphasize the importance of the entorhinal cortex in spatial representation and navigation. Furthermore, evidence is accumulating to show that spatial processing depends on interactions between the entorhinal cortex and the hippocampus. To investigate these interactions, we examined the effects of entorhinal cortex lesions on the activity of hippocampal CA1 place cells. Rats received bilateral radiofrequency lesions of the entorhinal cortex or sham lesions before place cell recording. Place cells were recorded as the rats performed a pellet-chasing task in a cylinder containing three cue-objects. Entorhinal cortex lesions did not abolish place cell spatial firing but reduced noticeably discharge rate and field size. Most importantly, the lesions affected firing field stability when cells were recorded both in constant conditions and following cue manipulations (object rotation, object removal). These findings indicate that the entorhinal cortex is necessary for the stability of hippocampal representations across exposures to a familiar environment. Consistent with the recent discovery of grid cells in the medial entorhinal cortex, our results suggest that the entorhinal cortex contributes to providing a spatial framework that would enable the hippocampus to maintain stable environment-specific representations.

Electrolytic lesions of dorsal CA3 impair episodic-like memory in rats

Episodic memory is the ability to recollect one's past experiences occurring in an unique spatial and temporal context. In non-human animals, it is expressed in the ability to combine "what", "where" and "when" factors to form an integrated memory system. During the search for its neural substrates, the hippocampus has attracted a lot of attentions. Yet, it is not yet possible to induce a pure episodic-like memory deficit in animal studies without being confounded by impairments in the spatial cognition. Here, we present a lesion study evidencing direct links between the hippocampus CA3 region and the episodic-like memory in rats. In a spontaneous object exploration task, lesioned rats showed no interaction between the temporal and spatial elements in their memory associated with the objects. In separate tests carried out subsequently, the same animals still expressed abilities to process spatial, temporal, and object recognition memory. In conclusions, our results support the idea that the hippocampus CA3 has a particular status in the neural mechanism of the episodic-like memory system. It is responsible for combining information from different modules of cognitive processes.

Gradual changes in hippocampal activity support remembering the order of events

The hippocampus is thought to contribute to episodic memory in part by binding stimuli to their spatiotemporal context. The present study examined how hippocampal neuronal populations encode spatial and temporal context as rats performed a task in which they were required to remember the order of trial-unique sequences of odors. The results suggest that a gradual change in the pattern of hippocampal activity served as a temporal context for odor-sampling events and was important for successful subsequent memory of the order of those odors.

The relationship between the field-shifting phenomenon and representational coherence of place cells in CA1 and CA3 in a cue-altered environment

Subfields of the hippocampus display differential dynamics in processing a spatial environment, especially when changes are introduced to the environment. Specifically, when familiar cues in the environment are spatially rearranged, place cells in the CA3 subfield tend to rotate with a particular set of cues (e.g., proximal cues), maintaining a coherent spatial representation. Place cells in CA1, in contrast, display discordant behaviors (e.g., rotating with different sets of cues or remapping) in the same condition. In addition, on average, CA3 place cells shift their firing locations (measured by the center of mass, or COM) backward over time when the animal encounters the changed environment for the first time, but not after that first experience. However, CA1 displays an opposite pattern, in which place cells exhibit the backward COM-shift only from the second day of experience, but not on the first day. Here, we examined the relationship between the environment-representing behavior (i.e., rotation vs. remapping) and the COM-shift of place fields in CA1 and CA3. Both in CA1 and CA3, the backward (as well as forward) COM-shift phenomena occurred regardless of the rotating versus remapping of the place cell. The differential, daily time course of the onset/offset of backward COM-shift in the cue-altered environment in CA1 and CA3 (on day 1 in CA1 and from day 2 onward in CA3) stems from different population dynamics between the subfields. The results suggest that heterogeneous, complex plasticity mechanisms underlie the environment-representating behavior (i.e., rotate/remap) and the COM-shifting behavior of the place cell.

Frequency-dependent gating of synaptic transmission and plasticity by dopamine

The neurotransmitter dopamine (DA) plays an important role in learning by enhancing the saliency of behaviorally relevant stimuli. How this stimulus selection is achieved on the cellular level, however, is not known. Here, in recordings from hippocampal slices, we show that DA acts specifically at the direct cortical input to hippocampal area CA1 (the temporoammonic (TA) pathway) to filter the excitatory drive onto pyramidal neurons based on the input frequency. During low-frequency patterns of stimulation, DA depressed excitatory TA inputs to both CA1 pyramidal neurons and local inhibitory GABAergic interneurons via presynaptic inhibition. In contrast, during high-frequency patterns of stimulation, DA potently facilitated the TA excitatory drive onto CA1 pyramidal neurons, owing to diminished feedforward inhibition. Analysis of DA's effects over a broad range of stimulus frequencies indicates that it acts as a high-pass filter, augmenting the response to high-frequency inputs while diminishing the impact of low-frequency inputs. These modulatory effects of DA exert a profound influence on activity-dependent forms of synaptic plasticity at both TA-CA1 and Schaffer-collateral (SC)-CA1 synapses. Taken together, our data demonstrate that DA acts as a gate on the direct cortical input to the hippocampus, modulating information flow and synaptic plasticity in a frequency-dependent manner.

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

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

Switching between “On” and “Off” states of persistent activity in lateral entorhinal layer III neurons

Persistent neural spiking maintains information during a working memory task when a stimulus is no longer present. During retention, this activity needs to be stable to distractors. More importantly, when retention is no longer relevant, cessation of the activity is necessary to enable processing and retention of subsequent information. Here, by means of intracellular recording with sharp microelectrode in in vitro rat brain slices, we demonstrate that single principal layer III neurons of the lateral entorhinal cortex (EC) generate persistent spiking activity with a novel ability to reliably toggle between spiking activity and a silent state. Our data indicates that in the presence of muscarinic receptor activation, persistent activity following an excitatory input may be induced and that a subsequent excitatory input can terminate this activity and cause the neuron to return to a silent state. Moreover, application of inhibitory hyperpolarizing stimuli is neither able to decrease the frequency of the persistent activity nor terminate it. The persistent activity can also be initiated and terminated by synchronized synaptic stimuli of layer II/III of the perirhinal cortex. The neuronal ability to switch "On" and "Off" persistent activity may facilitate the concurrent representation of temporally segregated information arriving in the EC and being directed toward the hippocampus.