Hippocampal neurons responding to first-time dislocation of a target object

Hippocampal and neocortical interactions during context discrimination: electrophysiological evidence from the rat

There is substantial evidence that hippocampus plays an important role in the processing of contextual information. Its specific role, however, remains unclear. One possibility is that single hippocampal neurons represent context information so that local circuits can construct representations of the current context, and the context that is expected based on past experience. Population codes derived from input by multiple local circuits may then engage match-mismatch algorithms that compare current and expected context information to determine the extent to which an expected context has changed. The results of such match-mismatch comparisons can be used to discriminate contexts. When context changes are detected, efferent messages may be passed on to connected neocortical areas so that informed "decisions" regarding future behavioral and cognitive strategies can be made. Here, a brief review describes evidence that a primary consequence of hippocampal processing is the discrimination of meaningful contexts. Then, the functional significance of neocortical circuits that likely receive hippocampal output messages are described in terms of their contribution to the control of ongoing behavioral and cognitive strategy, especially during active navigation. It is clear from this systems view that studies of spatial navigation continue to provide researchers with an excellent model of hippocampal-neocortical interactions during learning.

Environmental novelty is signaled by reduction of the hippocampal theta frequency

The hippocampal formation (HF) plays a key role in novelty detection, but the mechanisms remain unknown. Novelty detection aids the encoding of new information into memory-a process thought to depend on the HF and to be modulated by the theta rhythm of EEG. We examined EEG recorded in the HF of rats foraging for food within a novel environment, as it became familiar over the next five days, and in two more novel environments unexpectedly experienced in trials interspersed with familiar trials over three further days. We found that environmental novelty produces a sharp reduction in the theta frequency of foraging rats, that this reduction is greater for an unexpected environment than for a completely novel one, and that it slowly disappears with increasing familiarity. These results do not reflect changes in running speed and suggest that the septo-hippocampal system signals unexpected environmental change via a reduction in theta frequency. In addition, they provide evidence in support of a cholinergically mediated mechanism for novelty detection, have important implications for our understanding of oscillatory coding within memory and for the interpretation of event-related potentials, and provide indirect support for the oscillatory interference model of grid cell firing in medial entorhinal cortex.

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 CA1 place cells encode intended destination on a maze with multiple choice points

The hippocampus encodes both spatial and nonspatial aspects of a rat's ongoing behavior at the single-cell level. In this study, we examined the encoding of intended destination by hippocampal (CA1) place cells during performance of a serial reversal task on a double Y-maze. On the maze, rats had to make two choices to access one of four possible goal locations, two of which contained reward. Reward locations were kept constant within blocks of 10 trials but changed between blocks, and the session of each day comprised three or more trial blocks. A disproportionate number of place fields were observed in the start box and beginning stem of the maze, relative to other locations on the maze. Forty-six percent of these place fields had different firing rates on journeys to different goal boxes. Another group of cells had place fields before the second choice point, and, of these, 44% differentiated between journeys to specific goal boxes. In a second experiment, we observed that rats with hippocampal damage made significantly more errors than control rats on the Y-maze when reward locations were reversed. Together, these results suggest that, at the start of the maze, the hippocampus encodes both current location and the intended destination of the rat, and this encoding is necessary for the flexible response to changes in reinforcement contingencies.

A method of extracellular recording of neuronal activity in swimming mice

The design of a removable miniature microdrive-headstage waterproof assembly for extracellular recordings of single unit activity with high-impedance electrodes in swimming mice is presented. The assembly provides perfect protection of the critical components and electric contacts from water. Neuronal activity may be recorded even if the animal is diving and swimming under the water surface. The advantages of this construction include simple installation and removal of the electrodes, rapid attachment of the assembly to the animal's skull, and rapid removal after recording. The device provides precise vertical positioning of the electrode without rotation or lateral shift, stable recordings of single units for several hours and the possibility to change the penetration track many times in the same animal. The assembly weight is less than 160mg. This work is the first successful attempt to record neuronal activity in mice performing spatial task in water maze.

Match mismatch processes underlie human hippocampal responses to associative novelty

The hippocampus has long been proposed to play a critical role in novelty detection through its ability to act as a comparator between past and present experience. A recent study provided evidence for this hypothesis by characterizing hippocampal responses to sequence novelty, a type of associative novelty where familiar items appear in a new temporal order. Here, we ask whether a hippocampal match-mismatch (i.e., comparator) mechanism operates selectively to identify the violation of predictions within the temporal domain or instead also underlies the processing of associative novelty in other domains (e.g., spatial). We used functional magnetic resonance imaging and a repetition paradigm in which subjects viewed sequences of objects presented in distinct locations on the screen and performed an incidental target detection task. The left hippocampus exhibited a pattern of activity consistent with that of an associative match-mismatch detector, with novelty signals generated only in conditions where one contextual component was novel and the other repeated. In contrast, right hippocampal activation signaled the presence of objects in familiar locations. Our results suggest that hippocampal match-mismatch computations constitute a general mechanism underpinning the processing of associative novelty. These findings support a model in which hippocampal mismatch signals rely critically on the recall of previous experience, a process that only occurs when novel sensory inputs overlap significantly with stored representations. More generally, the current study also offers insights into how the hippocampus automatically represents the spatiotemporal context of our experiences, a function that may relate to its role in episodic memory.

Adaptation of prefrontal cortical firing patterns and their fidelity to changes in action-reward contingencies

Animals adapt action-selection policies when the relationship between possible actions and associated outcomes changes. Prefrontal cortical neurons vary their discharge patterns depending on action choice and rewards received and undoubtedly play a pivotal role in maintaining and adapting action policies. Here, we recorded neurons from the medial precentral subregion of mouse prefrontal cortex to examine neural substrates of goal-directed behavior. Discharge patterns were recorded after animals developed stable action-selection policies, wherein four possible action sequences were invariably related to different reward magnitudes and during adaptation to changes in the action-reward contingencies. During the adaptation period, when the same action sequence resulted in different reward magnitudes, many neurons (38%) exhibited significantly different discharge patterns for identical action sequences, well before reaching the reward site. In addition, trial-to-trial reliability of ensemble pattern production leading up to reward was found to vary both positively and negatively with increases and decreases in reward magnitude, respectively. Pairwise analyses of simultaneously recorded neurons revealed that decreased reliability in part reflected fluctuations between different ensemble activity patterns as opposed to within-pattern variability. Increases in reliability were related to an increased probability of both selecting highly rewarding actions and completing such actions without pause or reversal, whereas decreases in reliability were associated with the opposite pattern. Thus, we suggest that both the spatiotemporal pattern and fidelity of prefrontal cortical discharge are impacted by action-outcome relationships and that each of these features serve to adapt action choices and maintain behaviors leading to reward.

Spatial representations of hippocampal CA1 neurons are modulated by behavioral context in a hippocampus-dependent memory task

Although it is well known that hippocampal neurons code spatial information, it is less clear how these spatial representations are influenced by memory demands, especially in hippocampus-dependent tasks. Recently, our laboratory has demonstrated that hippocampal spatial representations are influenced by mnemonic factors in a T-maze continuous alternation task. Another unique experimental approach that might reveal the ways in which task-related factors impact hippocampal spatial representations is to compare firing patterns between events that require distinct episodic memory processes. Therefore, we recorded from CA1 single neurons during a discrete trial delayed-nonmatch-to-place task that allowed within-trial comparison between an encoding (sample) phase and a retrieval (choice) phase. A large subset of neurons that fired on the central stem of the maze showed dramatic selectivity for either the sample or choice phase of the trial. However, surprisingly, there were fewer neurons that showed differential firing rates between left- and right-bound trajectories. Our results suggest that trial-phase-selective coding is common in tasks that require rapid alternation between encoding and retrieval processes.

An unexpected sequence of events: mismatch detection in the human hippocampus

The ability to identify and react to novelty within the environment is fundamental to survival. Computational models emphasize the potential role of the hippocampus in novelty detection, its unique anatomical circuitry making it ideally suited to act as a comparator between past and present experience. The hippocampus, therefore, is viewed to detect associative mismatches between what is expected based on retrieval of past experience and current sensory input. However, direct evidence that the human hippocampus performs such operations is lacking. We explored brain responses to novel sequences of objects using functional magnetic resonance imaging (fMRI), while subjects performed an incidental target detection task. Our results demonstrate that hippocampal activation was maximal when prior predictions concerning which object would appear next in a sequence were violated by sensory reality. In so doing, we establish the biological reality of associative match-mismatch computations within the human hippocampus, a process widely held to play a cardinal role in novelty detection. Our results also suggest that the hippocampus may generate predictions about how future events will unfold, and critically detect when these expectancies are violated, even when task demands do not require it. The present study also offers broader insights into the nature of essential computations carried out by the hippocampus, which may also underpin its unique contribution to episodic memory.

Phase precession in hippocampal interneurons showing strong functional coupling to individual pyramidal cells

Although hippocampal interneurons typically do not show discrete regions of elevated firing in an environment, such as seen in pyramidal cell place fields, they do exhibit significant spatial modulation (McNaughton et al., 1983a). Strong monosynaptic coupling between pyramidal neurons and nearby interneurons in the CA1 stratum pyramidale has been strongly implicated on the basis of significant, short-latency peaks in cross-correlogram plots (Csicsvari et al., 1998). Furthermore, interneurons receiving a putative monosynaptic connection from a simultaneously recorded pyramidal cell appear to inherit the spatial modulation of the latter (Marshall et al., 2002). Buzsaki and colleagues hypothesize that interneurons may also adopt the firing phase dynamics of their afferent place cells, which show a phase shift relative to the hippocampal theta rhythm as a rat passes through the place field ("phase precession"). This study confirms and extends the previous reports by showing that interneurons in the dorsal and middle hippocampus with putative monosynaptic connections with place cells recorded on the same tetrode share other properties with their pyramidal cell afferents, including the spatial scale of the place field of pyramidal cell, a characteristic of the septotemporal level of the hippocampus from which the cells are recorded, and the rate of phase precession, which is slower in middle regions. Furthermore, variations in pyramidal cell place field scale within each septotemporal level attributable to task variations are similarly associated with variations in interneuron place field scale. The available data strongly suggest that spatial selectivity of CA1 stratum pyramidale interneurons is inherited from a small cluster of local pyramidal cells and is not a consequence of spatially selective synaptic input from CA3 or other sources.

Goal-related activity in hippocampal place cells

Place cells are hippocampal neurons whose discharge is strongly related to a rat's location in its environment. The existence of place cells has led to the proposal that they are part of an integrated neural system dedicated to spatial navigation, an idea supported by the discovery of strong relationships between place cell activity and spatial problem solving. To further understand such relationships, we examined the discharge of place cells recorded while rats solved a place navigation task. We report that, in addition to having widely distributed firing fields, place cells also discharge selectively while the hungry rat waits in an unmarked goal location to release a food pellet. Such firing is not duplicated in other locations outside the main firing field even when the rat's behavior is constrained to be extremely similar to the behavior at the goal. We therefore propose that place cells provide both a geometric representation of the current environment and a reflection of the rat's expectancy that it is located correctly at the goal. This on-line feedback about a critical aspect of navigational performance is proposed to be signaled by the synchronous activity of the large fraction of place cells active at the goal. In combination with other (prefrontal) cells that provide coarse encoding of goal location, hippocampal place cells may therefore participate in a neural network allowing the rat to plan accurate trajectories in space.

Integrating associative learning signals across the brain

Associative learning is defined as the ability to link arbitrary stimuli or actions together in memory. The neural correlates of this fundamental form of plasticity were first described in the hippocampus during delay eye blink conditioning and have since been examined using a variety of tasks in both rats and monkeys. In monkeys, the neural correlates of associative learning have been studied using conditional motor learning tasks where animals learn to associate particular visual stimuli with particular motor responses (i.e., touch left or touch right). Similar tasks have also been used to examine learning-related plasticity in motor-related areas throughout the frontal lobe and striatum. Here, we review the patterns of learning-related activity seen in these diverse brain areas during conditional motor learning. While each of these areas exhibits strong associative learning signals, the differential patterns and time courses of these signals provides insight into the unique contribution of each area to associative learning.

Which computational mechanisms operate in the hippocampus during novelty detection?

A fundamental property of adaptive behavior is the ability to rapidly distinguish what is novel from what is familiar in our environment. Empirical evidence and computational work have provided biologically plausible models of the neural substrate and mechanisms underlying the coding of stimulus novelty in the perirhinal cortex. In this article, we highlight the importance of a different category of novelty, namely associative novelty, which has received relatively little attention, despite its clear ecological importance. While previous studies in both animals and humans have documented hippocampal responses in relation to associative novelty, a key issue concerning the computations underlying these novelty signals has not been previously addressed. We argue that this question has importance not only for our understanding of novelty processing, but also for advancing our knowledge of the fundamental computational operations performed by the hippocampus. We suggest a different approach to this problem, and discuss recent evidence supporting the hypothesis that the hippocampus operates as a comparator during the processing of associative novelty, generating mismatch/novelty signals when prior predictions are violated by sensory reality. We also draw on conceptual similarities between associative novelty and contextual novelty to suggest that empirical findings from these two seemingly distant research fields accord with the operation of a comparator mechanism during novelty detection more generally. We therefore conclude that a comparator mechanism may underlie the role of the hippocampus not only in detecting occurrences that are unexpected given specific associatively retrieved predictions, but also events that violate more abstract properties of the experimental context.

Regional differences in hippocampal PKA immunoreactivity after training and reversal training in a spatial Y-maze task

It is suggested that the hippocampus functions as a comparator by making a comparison between the internal representation and actual sensory information from the environment (for instance, comparing a previously learned location of a food reward with an actual novel location of a food reward in a Y-maze). However, it remains unclear to what extent the various hippocampal regions contribute to this comparator function. One of the proteins known to be crucially involved in the formation of hippocampus-dependent long-term memory is the adenosine 3',5' cyclic monophosphate dependent protein kinase (PKA). Here, we examined region-specific changes in immunoreactivity (ir) of the regulatory IIalpha,beta subunits of PKA (PKA RIIalpha,beta-ir) in the hippocampus during various stages of spatial learning in a Y-maze reference task. Thereafter, we compared changes in hippocampal PKA RIIalpha,beta-ir induced by training and reversal training in which the food reward was relocated to the previously unrewarded arm. We show that: (1) There was a clear correlation between behavioral performance and elevated PKA RIIalpha,beta-ir during the acquisition phase of both training and reversal training in area CA3 and dentate gyrus (DG), (2) PKA RIIalpha,beta-ir was similarly enhanced in area CA1 during the acquisition phase of reversal training, but did not correlate with behavioral performance, (3) PKA RIIalpha,beta-ir did not change during training or reversal training in the subiculum (SUB), (4) No changes in PKA RIIalpha,beta protein levels were found using Western blotting, and (5) AMPA receptor phosphorylation at serine 845 (S845p; the PKA site on the glutamate receptor 1 subunit (GluR1)), was enhanced selectively during the acquisition phase of reversal training. These findings reveal that training and reversal training induce region-specific changes in hippocampal PKA RIIalpha,beta-ir and suggest a differential involvement of hippocampal subregions in match-mismatch detection in case of Y-maze reference learning. Alterations in AMPA receptor regulation at the S845 site seems specifically related to the novelty detector function of the hippocampus important for match-mismatch detection.

Space and context in the temporal cortex

The hippocampus has a critical role in certain kinds of spatial memory processes. Hippocampal "place" cells, fire selectively when an animal is in a particular location within the environment. It is thought that this activity underlies a representation of the environment that can be used for memory-based spatial navigation. But how is this representation constructed and how is it "read"? A simple mechanism, based on place field density across an environment, is described that could allow hippocampal representations to be "read" by other brain regions for the purpose of navigation. The possible influence of activity in neighboring brain regions such as the perirhinal cortex, and pre- and para-subiculum on the construction of the hippocampal spatial representation is then discussed.

Dopaminergic modulation of the persistence of one-trial hippocampus-dependent memory

The persistence of new memory traces in the hippocampus, encoded following appropriate activation of glutamatergic receptors and the induction of synaptic plasticity, can be influenced by heterosynaptic activation of neuromodulatory brain systems. We therefore investigated the effects of a hippocampus-specific blockade of dopamine D1/D5 receptors on the persistence of spatial memory encoded in one trial using a delayed matching-to-place (DMP) task in a watermaze in which rats learn a new escape location each day. A within-subjects design was used such that both short (20 min) and long (6 h) retention intervals, and both drug (SCH23390, a D1/D5 receptor antagonist) and vehicle (aCSF) infusions were tested on different days in the same animals. Bilateral intrahippocampal infusion of SCH23390 (5 microg in 1 microL per side) prior to trial 1 (encoding) caused a differential impairment as a function of memory delay-with no effect during trial 2 (memory retrieval) after a 20-min interval, but a block of memory at 6 h. Further experiments revealed that infusion of SCH23390 immediately after trial 1 had no effect on retention 6 h later, and the poor memory seen at long retention intervals when the drug was present at encoding was not due to a state-dependent failure of retrieval. These results suggest that activation of D1/D5 receptors during memory encoding is necessary for the formation of a persistent memory trace in the hippocampus. The complementary effects of D1/D5 receptor blockade on the persistence of LTP and the duration of memory are consistent with the idea that changes in synaptic strength underlie memory.

Gradual translocation of spatial correlates of neuronal firing in the hippocampus toward prospective reward locations

In a continuous T-maze alternation task, CA1 complex-spike neurons in the hippocampus differentially fire as the rat traverses overlapping segments of the maze (i.e., the stem) repeatedly via alternate routes. The temporal dynamics of this phenomenon were further investigated in the current study. Rats learned the alternation task from the first day of acquisition and the differential firing pattern in the stem was observed accordingly. More importantly, we report a phenomenon in which spatial correlates of CA1 neuronal ensembles gradually changed from their original firing locations, shifting toward prospective goal locations in the continuous T-maze alternation task. The relative locations of simultaneously recorded firing fields, however, were preserved within the ensemble spatial representation during this shifting. The within-session shifts in preferred firing locations in the absence of any changes in the environment suggest that certain cognitive factors can significantly alter the location-bound coding scheme of hippocampal neurons.

Elements of a neurobiological theory of hippocampal function: the role of synaptic plasticity, synaptic tagging and schemas

The 2004 EJN Lecture was an attempt to lay out further aspects of a developing neurobiological theory of hippocampal function [Morris, R.G.M., Moser, E.I., Riedel, G., Martin, S.J., Sandin, J., Day, M. & O'Carroll, C. (2003) Phil. Trans. R. Soc. Lond. B Biol. Sci., 358, 773-786.] These are that (i) activity-dependent synaptic plasticity plays a key role in the automatic encoding and initial storage of attended experience; (ii) the persistence of hippocampal synaptic potentiation over time can be influenced by other independent neural events happening closely in time, an idea with behavioural implications for memory; and (iii) that systems-level consolidation of memory traces within neocortex is guided both by hippocampal traces that have been subject to cellular consolidation and by the presence of organized schema in neocortex into which relevant newly encoded information might be stored. Hippocampal memory is associative and, to study it more effectively than with previous paradigms, a new learning task is described which is unusual in requiring the incidental encoding of flavour-place paired associates, with the readout of successful storage being successful recall of a place given the flavour with which it was paired. NMDA receptor-dependent synaptic plasticity is shown to be critical for the encoding and intermediate storage of memory traces in this task, while AMPA receptor-mediated fast synaptic transmission is necessary for memory retrieval. Typically, these rapidly encoded traces decay quite rapidly over time. Synaptic potentiation also decays rapidly, but can be rendered more persistent by a process of cellular consolidation in which synaptic tagging and capture play a key part in determining whether or not it will be persistent. Synaptic tags set at the time of an event, even many trivial events, can capture the products of the synthesis of plasticity proteins set in train by events before, during or even after an event to be remembered. Tag-protein interactions stabilize synaptic potentiation and, by implication, memory. The behavioural implications of tagging are explored. Finally, using a different protocol for flavour-place paired associate learning, it is shown that rats can develop a spatial schema which represents the relative locations of several different flavours of food hidden at places within a familiar space. This schema is learned gradually but, once acquired, enables new paired associates to be encoded and stored in one trial. Their incorporation into the schema prevents rapid forgetting and suggests that schema play a key and hitherto unappreciated role in systems-level memory consolidation. The elements of what may eventually mature into a more formal neurobiological theory of hippocampal memory are laid out as specific propositions with detailed conceptual discussion and reference to recent data.

Rearing on hind legs, environmental novelty, and the hippocampal formation

Many mammals spontaneously rear on their hind legs in response to novelty. The current paper is the first review of rearing behaviour, and is intended to collate findings from different perspectives that are not usually brought together. We suggest that rearing is a useful marker of environmental novelty, that the hippocampal formation is a crucial component of the system controlling rearing in novel environments, and that rearing is one of several ethological measures that can profitably be used to assess hippocampal learning and memory. Consideration is given to the following topics: the possible functions of rearing in information-gathering and escape behaviour; the modulation of rearing by various factors such as anxiety/ fear emotionality; comparative perspectives on rearing; neuroanatomical circuits involved in rearing with particular reference to the hippocampal formation and its afferents and efferents; and the role of the hippocampal formation in uncharted and mismatch environmental novelty. The review concludes with testable predictions about rearing, environmental novelty and the hippocampus.

The dynamic nature of spatial encoding in the hippocampus

Many hippocampal neurons (place cells) appear to represent a particular location within an environment (their place field). This property would appear to be central to hippocampal involvement in navigation based on spatial memory. Although a navigationally useful representation might also include information about distal goals, having a place field and being able to represent a distal goal would appear to be mutually exclusive place cell properties. Our simulations demonstrate, however, that information about goal direction can be simply derived from the changes in place field density that occur when place fields shift location in a goal-directed manner. Previous reports that place fields respond dynamically to shifts in goal location may, therefore, represent the operation of such a system.

Behavioral and neurophysiological analyses of dynamic learning processes

In this article, the authors address two topics relevant to the study of the brain basis of associative learning. In Part 1, they compare and contrast the patterns and time course of dynamic learning-related neural activity that have been reported in the medial temporal lobe, premotor cortex, prefrontal cortex, and striatum during various associative learning tasks. In Part 2, they examine the statistical methodologies that have been used to analyze both behavioral learning and learning-related neural activity. They describe a state-space model of behavioral learning that provides accurate estimates of dynamic learning processes and a point-process filter algorithm that tracks the dynamic changes in neural activity on a millisecond time scale. Future challenges for these statistical methodologies and their application to the study of the brain basis of associative learning are discussed.

Place cells, spatial maps and the population code for memory

The study of population dynamics in hippocampal place cells has emerged as one of the most powerful tools for understanding the encoding, storage and retrieval of declarative memory. Recent work has laid out the contours of an attractor-based hippocampal population code for memory in recurrent circuits of the hippocampus. The code is based on inputs from a topographically organized, path-integration-dependent spatial map that lies upstream in the medial entorhinal cortex. The recurrent networks of the hippocampal formation enable these spatial inputs to be synthesized with nonspatial event-related information.

Topography of Arc/Arg3.1 mRNA expression in the dorsal and ventral hippocampus induced by recent and remote spatial memory recall: dissociation of CA3 and CA1 activation

The understanding of the mechanisms of memory retrieval and its deficits, and the detection of memory underlying neuronal plasticity, is greatly impeded by a lack of precise knowledge of the brain circuitry that underlies the functions of memory. The specific roles of anatomically distinct hippocampal subdivisions in recent and long-term memory retention and recall are essentially unknown. To address these questions, we mapped the expression of Arc/Arg 3.1 mRNA, a neuronal activity marker, in memory retention at multiple rostrocaudal levels of the dentate gyrus, CA3, CA1, subiculum, and lateral and medial entorhinal cortices after a platform search in a water-maze spatial task at 24 h and 1 month compared with swim and naive controls. We found that the entorhinohippocampal neuronal activity underlying the recall of recent and remote spatial memory has an anatomically distributed and time-dependent organization throughout both the dorsal and ventral hippocampus that is subdivision specific. We found a dissociation in the activity of the entorhinal cortex, CA3, and CA1 over a period of memory consolidation. Although CA3, the dorsal hippocampus, and the entorhinal cortex demonstrated the most persistent learning-specific signal during both recent and long-term memory recall, CA1 and the ventral hippocampus displayed the most dramatic signal decline. We determined the coordinates of activity clusters in the hippocampal subdivisions during the platform search and their dynamics over time. Our mapping data suggest that although the level of corticohippocampal interaction is similar during the retrieval of recent and remote spatial memories, the mnemonic function of the hippocampus may have changed, and the activity underlying remote spatial memory could be anatomically segregated within hippocampal subdivisions in small segments.

Progressive Transformation of Hippocampal Neuronal Representations in “Morphed” Environments

Hippocampal neural codes for different, familiar environments are thought to reflect distinct attractor states, possibly implemented in the recurrent CA3 network. A defining property of an attractor network is its ability to undergo sharp and coherent transitions between pre-established (learned) representations when the inputs to the network are changed. To determine whether hippocampal neuronal ensembles exhibit such discontinuities, we recorded in CA3 and CA1 when a familiar square recording enclosure was morphed in quantifiable steps into a familiar circular enclosure while leaving other inputs constant. We observed a gradual noncoherent progression from the initial to the final network state. In CA3, the transformation was accompanied by significant hysteresis, resulting in more similar end states than when only square and circle were presented. These observations suggest that hippocampal cell assemblies are capable of incremental plastic deformation, with incongruous information being incorporated into pre-existing representations.

The hippocampal-VTA loop: controlling the entry of information into long-term memory

In this article we develop the concept that the hippocampus and the midbrain dopaminergic neurons of the ventral tegmental area (VTA) form a functional loop. Activation of the loop begins when the hippocampus detects newly arrived information that is not already stored in its long-term memory. The resulting novelty signal is conveyed through the subiculum, accumbens, and ventral pallidum to the VTA where it contributes (along with salience and goal information) to the novelty-dependent firing of these cells. In the upward arm of the loop, dopamine (DA) is released within the hippocampus; this produces an enhancement of LTP and learning. These findings support a model whereby the hippocampal-VTA loop regulates the entry of information into long-term memory.

Microstructure of a spatial map in the entorhinal cortex

The ability to find one's way depends on neural algorithms that integrate information about place, distance and direction, but the implementation of these operations in cortical microcircuits is poorly understood. Here we show that the dorsocaudal medial entorhinal cortex (dMEC) contains a directionally oriented, topographically organized neural map of the spatial environment. Its key unit is the 'grid cell', which is activated whenever the animal's position coincides with any vertex of a regular grid of equilateral triangles spanning the surface of the environment. Grids of neighbouring cells share a common orientation and spacing, but their vertex locations (their phases) differ. The spacing and size of individual fields increase from dorsal to ventral dMEC. The map is anchored to external landmarks, but persists in their absence, suggesting that grid cells may be part of a generalized, path-integration-based map of the spatial environment.

Hippocampal plasticity across multiple days of exposure to novel environments

The hippocampus is essential for learning complex spatial relationships, but little is known about how hippocampal neural activity changes as animals learn about a novel environment. We studied the formation of new place representations in rats by examining the changes in place-specific firing of neurons in the CA1 region of the hippocampus and the relationship between these changes and behavioral change across multiple days of exposure to novel places. We found that many neurons showed very rapid changes on the first day of exposure to the novel place, including many cases in which a previously silent neuron developed a place field over the course of a single pass through the environment. Across the population, the largest changes in neural activity occurred on day 2 of exposure to a novel place, but only if the animal had little experience (<4 min) in that location on day 1. Longer exposures on day 1 were associated with smaller changes on day 2, suggesting that hippocampal neurons required 5-6 min of experience to form a stable spatial representation. Even after the representation stabilized, the animals' behavior remained different in the novel places, suggesting that other brain regions continued to distinguish novel from familiar locations. These results show that the hippocampus can form new spatial representations quickly but that stable hippocampal representations are not sufficient for a place to be treated as familiar.

Putting fear in its place: remapping of hippocampal place cells during fear conditioning

We recorded hippocampal place cells in two spatial environments: a training environment in which rats underwent fear conditioning and a neutral control environment. Fear conditioning caused many place cells to alter (or remap) their preferred firing locations in the training environment, whereas most cells remained stable in the control environment. This finding indicates that aversive reinforcement can induce place cell remapping even when the environment itself remains unchanged. Furthermore, contextual fear conditioning caused significantly more remapping of place cells than auditory fear conditioning, suggesting that place cell remapping was related to the rat's learned fear of the environment. These results suggest that one possible function of place cell remapping may be to generate new spatial representations of a single environment, which could help the animal to discriminate among different motivational contexts within that environment.

Factors regulating the effects of hippocampal inactivation on renewal of conditional fear after extinction

After extinction of fear to a Pavlovian conditional stimulus (CS), contextual stimuli come to regulate the expression of fear to that CS. There is growing evidence that the context dependence of memory retrieval after extinction involves the hippocampus. In the present experiment, we examine whether hippocampal involvement in memory retrieval after extinction is related to the history of CS presentations in the context used for retrieval testing. We used infusions of muscimol to inactivate the dorsal hippocampus (DH) during postextinction retrieval tests that were conducted in contexts that differed in their history of CS presentations in that context. We found that DH inactivation affected the context-dependent retrieval of extinction (i.e., renewal) when testing occurred in a context that had no history of CS exposure, but not in a context that reliably predicted the CS. These results are discussed in terms of theories regarding the role of the hippocampus in contextual memory retrieval.

The Role of Hippocampal Subregions in Detecting Spatial Novelty

Previous literature suggests that the hippocampus subserves processes associated with the encoding of novel information. To investigate the role of different subregions of the hippocampus, the authors made neurotoxic lesions in different subregions of the dorsal hippocampus (i.e., CA1, dentate gyrus [DG], or CA3) of rats, followed by tests using a spontaneous object exploration paradigm. All lesion groups explored normally an object newly introduced in a familiar location. However, when some of the familiar objects were moved to novel locations, both DG and CA3 lesion groups were severely impaired in reexploring the displaced objects, whereas the CA1 lesion group was only mildly impaired in reexploration. The results suggest that the DG-CA3 network is essential in detecting novelty for spatial, but not for individual object, information.

Differential but complementary mnemonic functions of the hippocampus and subiculum

In this study we describe how the hippocampus and subiculum act in concert to encode information in a spatial delayed-nonmatch-to-sample (DNMS) task. This encoding was functionally partitioned between neurons within subiculum and hippocampus to uniquely identify trial-specific information accounting for both spatial and temporal constraints on performance within and between trials. Encoding by subicular neurons in the task was normally accurate and specific, but only if delays were shorter than 15 s, whereas trial-specific information encoded by hippocampal neurons was subject to strong biases from prior trial sequences and was accessible only when delays exceeded 15 s. The two structures operated in a complementary manner to encode information correctly on 75% of all trials using the above strategies. The remaining 25% of trials were at risk due to inherent idiosyncrasies by which hippocampal and subicular neurons encoded information and became errors when the random sequence of trials conflicted with these constraints.

Maturation of long-term potentiation induction rules in rodent hippocampus: role of GABAergic inhibition

The induction rules of synaptic plasticity are important for the functional operation of a neural network. We asked whether such synaptic plasticity rules change during development from juvenile to adult animals. Using perforated patch and whole-cell recordings from CA1 pyramidal cells in hippocampal slices, we demonstrate here that the postsynaptic requirements for induction of associative long-term potentiation (LTP) shift gradually. Presynaptic stimulation paired with single postsynaptic action potentials became progressively less effective at inducing LTP with advancing developmental age until, in adult hippocampus, postsynaptic bursts of action potentials were necessary to induce synaptic potentiation. This developmental change might be accounted for by changes in GABA(A) receptor-mediated inhibition known to occur in the hippocampus during this postnatal period, because blocking GABA(A) receptor-mediated inhibition re-established the effectiveness of single postsynaptic action potentials at inducing LTP in adult hippocampus. These data reveal a gradual shift in the induction rules for LTP, explained by a maturational change in GABAergic inhibition, and could have implications for our understanding of the role of inhibition in information processing in the brain.

Differential modulation of CA1 and dentate gyrus interneurons during exploration of novel environments

Parallel recordings of hippocampal principal cells and interneurons were obtained as rats foraged in familiar and adjacent, novel environments. Firing rates of each cell type were assessed as a function of spatial location. Many CA1 interneurons exhibited large decreases in activity in the novel compared with the familiar environment. Dentate gyrus interneurons, however, were much more likely to exhibit large increases in firing in the novel environment. Neither effect was correlated with basic interneuron discharge properties such as degree of theta modulation, baseline firing rate or degree of spatially modulated discharge. Both CA1 and dentate gyrus interneuron rate changes extended into regions of the familiar environment bordering the novel environment. Principal cells in CA1 and dentate gyrus exhibited similar patterns of place specific activity each being indicative of incorporation of novel spatial information into the spatial representation of the familiar environment. The data indicate that inhibitory networks in the CA1 and dentate gyrus areas are modulated in a divergent fashion during the acquisition of novel spatial information and that interneuron activities can be used to detect those regions of an environment subject to redistribution of principal cell spatial activity patterns.

Reward modulates neuronal activity in the hippocampus of the rat

In the hippocampus of freely moving rats, neurons have been recorded that fire predominantly when the animal travels through a particular area while exploring the environment (so-called 'place cells'). This study investigates if the neuronal firing characteristics of such cells are modulated by attention, expectation of reward or memory load. A total of 16 electrodes were implanted in the CA1 region of the hippocampus of 3-month-old Long-Evans rats. Using a tetrode recording system, single neurons were recorded while a rat explored an 8-arm maze and retrieved pellets at the end of each arm. It was found that 31 out of 67 neurons showed place cell characteristics, while the other cells either fired in more than one place or fired along whole arms of the maze. Interestingly, 11 of the 31 neurons showed enhanced firing activity when the animal entered a baited arm but did not fire when the arm was visited again after the bait had been retrieved. In a second experiment, only four out of eight arms were baited. Firing rates of 46 neurons were analysed, and all cells (spatial or non-spatial) fired more in baited arms than in non-baited ones (P<0.001). In a reversal task in which the previously unbaited four arms were subsequently baited, neuronal activity was increased in the newly baited arms (42 cells analysed, P<0.001). Since no alterations to the maze or cues have been made, we interpret the increased firing probability of neurons in baited arms compared to unbaited arms as a correlate for 'attention' or 'expectation'.

Single neurons in the monkey hippocampus and learning of new associations

The medial temporal lobe is crucial for the ability to learn and retain new declarative memories. This form of memory includes the ability to quickly establish novel associations between unrelated items. To better understand the patterns of neural activity during associative memory formation, we recorded the activity of hippocampal neurons of macaque monkeys as they learned new associations. Hippocampal neurons signaled learning by changing their stimulus-selective response properties. This change in the pattern of selective neural activity occurred before, at the same time as, or after learning, which suggests that these neurons are involved in the initial formation of new associative memories.

Genetic neuroscience of mammalian learning and memory

Our primary research interest is to understand the molecular and cellular mechanisms on neuronal circuitry underlying the acquisition, consolidation and retrieval of hippocampus-dependent memory in rodents. We study these problems by producing genetically engineered (i.e. spatially targeted and/or temporally restricted) mice and analysing these mice by multifaceted methods including molecular and cellular biology, in vitro and in vivo physiology and behavioural studies. We attempt to identify deficits at each of the multiple levels of complexity in specific brain areas or cell types and deduce those deficits that underlie specific learning or memory. We will review our recent studies on the acquisition, consolidation and recall of memories that have been conducted with mouse strains in which genetic manipulations were targeted to specific types of cells in the hippocampus or forebrain of young adult mice.

Elements of a neurobiological theory of the hippocampus: the role of activity-dependent synaptic plasticity in memory

The hypothesis that synaptic plasticity is a critical component of the neural mechanisms underlying learning and memory is now widely accepted. In this article, we begin by outlining four criteria for evaluating the 'synaptic plasticity and memory (SPM)' hypothesis. We then attempt to lay the foundations for a specific neurobiological theory of hippocampal (HPC) function in which activity-dependent synaptic plasticity, such as long-term potentiation (LTP), plays a key part in the forms of memory mediated by this brain structure. HPC memory can, like other forms of memory, be divided into four processes: encoding, storage, consolidation and retrieval. We argue that synaptic plasticity is critical for the encoding and intermediate storage of memory traces that are automatically recorded in the hippocampus. These traces decay, but are sometimes retained by a process of cellular consolidation. However, we also argue that HPC synaptic plasticity is not involved in memory retrieval, and is unlikely to be involved in systems-level consolidation that depends on HPC-neocortical interactions, although neocortical synaptic plasticity does play a part. The information that has emerged from the worldwide focus on the mechanisms of induction and expression of plasticity at individual synapses has been very valuable in functional studies. Progress towards a comprehensive understanding of memory processing will also depend on the analysis of these synaptic changes within the context of a wider range of systems-level and cellular mechanisms of neuronal transmission and plasticity.

Hippocampal CA3 NMDA Receptors Are Crucial for Memory Acquisition of One-Time Experience

Lesion and pharmacological intervention studies have suggested that in both human patients and animals the hippocampus plays a crucial role in the rapid acquisition and storage of information from a novel one-time experience. However, how the hippocampus plays this role is poorly known. Here, we show that mice with NMDA receptor (NR) deletion restricted to CA3 pyramidal cells in adulthood are impaired in rapidly acquiring the memory of novel hidden platform locations in a delayed matching-to-place version of the Morris water maze task but are normal when tested with previously experienced platform locations. CA1 place cells in the mutant animals had place field sizes that were significantly larger in novel environments, but normal in familiar environments relative to those of control mice. These results suggest that CA3 NRs play a crucial role in rapid hippocampal encoding of novel information for fast learning of one-time experience.

Time, Space and Hippocampal Functions

The hippocampus is one of the most researched structures of the brain. Studies of lesions in humans, primates and rodents have suggested to some that the primary role of the hippocampus is to act as a temporary memory buffer which is required for the consolidation of long-term memory. The famous case study of patient H.M., in particular, seemed to suggest that the hippocampus was of crucial importance for memory formation. However, recordings of single neurons in freely moving rodents did not support this notion. In such recordings, neurons were found that were active predominately when the animal passed through a particular area in space. Consequently, these neurons were termed 'place cells' and a theory was developed that suggested that the hippocampus acts as a 'cognitive map' that is required for spatial orientation. It was then found that H.M. had significant damage to his temporal lobes that included the amygdala, rhinal cortices, and other areas. Further case studies and selective hippocampal lesions in primates resulted in much milder amnestic symptoms, and lesions of defined cortical areas in the temporal lobes showed that a number of functions previously attributed to the hippocampus were in fact linked to these areas. Further analysis of neuronal activity in the hippocampus showed that not only is spatial information represented there, but also additional information, such as speed of movement, direction of movement, match or non-match detection, olfactorial identification, and others. In addition, it was found that selective lesions of the hippocampus in rodents impaired spatial navigation and memory formation only mildly. Only simultaneous lesions of several cortical areas in conjunction with the hippocamus could reproduce the impairments and symptoms that were previously thought to be observed after hippocampal lesions alone. In conclusion it is proposed that information processing and memory formation is shared by several brain areas that act as a functional system. This review presents evidence from many different studies that the hippocampus is part of this system and plays a supportive role in associating complex multimodal information and laying down new memory traces. In addition, the concept of allocating specific functions (such as the development of a cognitive map) exclusively to the hippocampus is rejected.