One-shot memory in hippocampal CA3 networks

Formation and Retrieval of Cell Assemblies in a Biologically Realistic Spiking Neural Network Model of Area CA3 in the Mouse Hippocampus

The hippocampal formation is critical for episodic memory, with area Cornu Ammonis 3 (CA3) a necessary substrate for auto-associative pattern completion. Recent theoretical and experimental evidence suggests that the formation and retrieval of cell assemblies enable these functions. Yet, how cell assemblies are formed and retrieved in a full-scale spiking neural network (SNN) of CA3 that incorporates the observed diversity of neurons and connections within this circuit is not well understood. Here, we demonstrate that a data-driven SNN model quantitatively reflecting the neuron type-specific population sizes, intrinsic electrophysiology, connectivity statistics, synaptic signaling, and long-term plasticity of the mouse CA3 is capable of robust auto-association and pattern completion via cell assemblies. Our results show that a broad range of assembly sizes could successfully and systematically retrieve patterns from heavily incomplete or corrupted cues after a limited number of presentations. Furthermore, performance was robust with respect to partial overlap of assemblies through shared cells, substantially enhancing memory capacity. These novel findings provide computational evidence that the specific biological properties of the CA3 circuit produce an effective neural substrate for associative learning in the mammalian brain.

Spatial coordinate transforms linking the allocentric hippocampal and egocentric parietal primate brain systems for memory, action in space, and navigation

A theory and model of spatial coordinate transforms in the dorsal visual system through the parietal cortex that enable an interface via posterior cingulate and related retrosplenial cortex to allocentric spatial representations in the primate hippocampus is described. First, a new approach to coordinate transform learning in the brain is proposed, in which the traditional gain modulation is complemented by temporal trace rule competitive network learning. It is shown in a computational model that the new approach works much more precisely than gain modulation alone, by enabling neurons to represent the different combinations of signal and gain modulator more accurately. This understanding may have application to many brain areas where coordinate transforms are learned. Second, a set of coordinate transforms is proposed for the dorsal visual system/parietal areas that enables a representation to be formed in allocentric spatial view coordinates. The input stimulus is merely a stimulus at a given position in retinal space, and the gain modulation signals needed are eye position, head direction, and place, all of which are present in the primate brain. Neurons that encode the bearing to a landmark are involved in the coordinate transforms. Part of the importance here is that the coordinates of the allocentric view produced in this model are the same as those of spatial view cells that respond to allocentric view recorded in the primate hippocampus and parahippocampal cortex. The result is that information from the dorsal visual system can be used to update the spatial input to the hippocampus in the appropriate allocentric coordinate frame, including providing for idiothetic update to allow for self-motion. It is further shown how hippocampal spatial view cells could be useful for the transform from hippocampal allocentric coordinates to egocentric coordinates useful for actions in space and for navigation.

What Learning Systems do Intelligent Agents Need? Complementary Learning Systems Theory Updated

We update complementary learning systems (CLS) theory, which holds that intelligent agents must possess two learning systems, instantiated in mammalians in neocortex and hippocampus. The first gradually acquires structured knowledge representations while the second quickly learns the specifics of individual experiences. We broaden the role of replay of hippocampal memories in the theory, noting that replay allows goal-dependent weighting of experience statistics. We also address recent challenges to the theory and extend it by showing that recurrent activation of hippocampal traces can support some forms of generalization and that neocortical learning can be rapid for information that is consistent with known structure. Finally, we note the relevance of the theory to the design of artificial intelligent agents, highlighting connections between neuroscience and machine learning.

Pattern separation in the hippocampus: distinct circuits under different conditions

Pattern separation is a fundamental hippocampal process thought to be critical for distinguishing similar episodic memories, and has long been recognized as a natural function of the dentate gyrus (DG), supporting autoassociative learning in CA3. Understanding how neural circuits within the DG-CA3 network mediate this process has received much interest, yet the exact mechanisms behind remain elusive. Here, we argue for the case that sparse coding is necessary but not sufficient to ensure efficient separation and, alternatively, propose a possible interaction of distinct circuits which, nevertheless, act in synergy to produce a unitary function of pattern separation. The proposed circuits involve different functional granule-cell populations, a primary population mediates sparsification and provides recurrent excitation to the other populations which are related to additional pattern separation mechanisms with higher degrees of robustness against interference in CA3. A variety of top-down and bottom-up factors, such as motivation, emotion, and pattern similarity, control the selection of circuitry depending on circumstances. According to this framework, a computational model is implemented and tested against model variants in a series of numerical simulations and biological experiments. The results demonstrate that the model combines fast learning, robust pattern separation and high storage capacity. It also accounts for the controversy around the involvement of the DG during memory recall, explains other puzzling findings, and makes predictions that can inform future investigations.

Evidence for functional specialization of hippocampal subfields detected by MR subfield volumetry on high resolution images at 4 T

Animal studies suggest an involvement of CA3 and dentate gyrus (CA3&DG) in memory encoding and early retrieval and an involvement of CA1 in late retrieval, consolidation and recognition. The aim of this study was to test if similar associations could be found between hippocampal subfield volumes measured in vivo using a manual parcellation scheme and selected scores of the California Verbal Learning Test II (CVLTII): total immediate free recall discriminability (IFRD), short free recall discriminability (SFRD), and delayed recall discriminability (DRD). 50 elderly subjects (25 controls and 25 cognitively impaired subjects) had CVLTII and high resolution hippocampal MRI at 4T. Entorhinal cortex, subiculum, CA1, CA1-CA2 transition zone, and CA3&DG were manually marked on five slices in the anterior hippocampal body on the MRI. Pearson correlations followed by stepwise regression analysis were used to test for associations between subfield volumes and CVLTII. IFRD and SFRD, which are measures of encoding/early retrieval, were associated with CA3&DG, and DRD, which measures consolidation/late retrieval, with CA1. These preliminary findings demonstrate that subfield volumetry has the potential to study non invasively subfield specific memory functions.

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.

Reversed and forward buffering of behavioral spike sequences enables retrospective and prospective retrieval in hippocampal regions CA3 and CA1

We propose a mechanism to explain both retrospective and prospective recall activity found in experimental data from hippocampal regions CA3 and CA1. Our model of temporal context dependent episodic memory replicates reverse recall in CA1, as recently recorded and published [Foster, D., & Wilson, M. (2006). Reverse replay of behavioural sequences in hippocampal place cells during the awake state. Nature, 440, 680-683], as well as the prospective and retrospective activity recorded in region CA3 during spatial tasks [Johnson, A., & Redish, A. (2006). Neural ensembles in ca3 transiently encode paths forward of the animal at a decision point: a possible mechanism for the consideration of alternatives. In 2006 neuroscience meeting planner. Atlanta, GA: Society for Neuroscience. (Program no. 574.2)]. We suppose that CA3 encodes episodic memory of both forward and reversed sequences of perforant path spikes representing place input. Using a persistent firing buffer mechanism in layer II of entorhinal cortex, simulated episodic learning involves dentate gyrus, layer III of entorhinal cortex, and hippocampal regions CA3 and CA1. Associations are formed between buffered episodic cues, unique temporal context specific representations in dentate gyrus, and episodic memory in the CA3 recurrent network.

Modulation of CA3 afferent inputs by novelty and theta rhythm

Models of hippocampal function suggest that the modulation of CA3 afferent input during theta rhythm allows for a rapid alternation between encoding and retrieval states, with each phase enhancing either extrinsic or intrinsic CA3 afferents, favoring either encoding or retrieval, respectively. Here, we show that during the initial exploration of a novel environment by rats, intrinsic CA3-CA3 synaptic inputs are attenuated on CA3 theta peaks, favoring extrinsic CA3 inputs, whereas extrinsic perforant path-CA3 synaptic inputs are attenuated on CA3 theta troughs, favoring intrinsic CA3 inputs. This modulation is absent when animals are re-exposed to the same environment 2 or 48 h later and thus habituates with familiarity, suggesting a process involved in learning. Modulation of CA3 synaptic inputs during novelty was blocked by atropine at a dose that blocks type 2 theta rhythm. Re-exposure to the same novel environment 48 h later in the absence of atropine did not result in habituation, but instead modulated CA3 synaptic responses as though the environment were novel and explored for the first time. The NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), administered in a dose that blocks long-term potentiation induction, did not alter CA3 synaptic modulation during initial exploration. However, like atropine, CPP blocked the habituation of synaptic modulation normally observed with re-exposure, as though the environment were novel and explored for the first time. Thus, as predicted theoretically, recurrent and cortical CA3 afferents are differentially modulated during phases of theta rhythm. This modulation is atropine sensitive and habituates in an NMDA receptor-dependent manner, suggesting an NMDA receptor-dependent process that, in conjunction with theta rhythm, contributes to encoding of novel information in the hippocampus.

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.

alphaCaMKII autophosphorylation: a fast track to memory

Alpha Ca(2+)/calmodulin-dependent kinase II (alphaCaMKII), the major synaptic protein in the forebrain, can switch into a state of autonomous activity upon autophosphorylation. It has been proposed that alphaCaMKII autophosphorylation mediates long-term memory (LTM) storage. However, recent evidence shows that synaptic stimulation and behavioural training only transiently increase the autonomous alphaCaMKII activity, implicating alphaCaMKII autophosphorylation in LTM formation rather than storage. Consistent with this, mutant mice deficient in alphaCaMKII autophosphorylation can store LTM after a massed training protocol, but cannot form LTM after a single trial. Here, we review evidence that the role of alphaCaMKII autophosphorylation is in fact to enable LTM formation after a single training trial, possibly by regulating LTM consolidation-specific transcription.

Conserving total synaptic weight ensures one-trial sequence learning of place fields in the hippocampus

The hippocampus plays a critical role in the rapid acquisition of information from a novel experience. Recent theoretical studies on the rat hippocampus have shown the possibility of behavioral sequence learning in a single traversal experience by theta phase coding. Specifically, previous work using computer simulations demonstrated that the extent of overlap among individual events of sequence and rat running velocity should be quantitatively incorporated into the learning rule to ensure one-trial sequence learning. These extents of overlap- and running velocity-dependent properties in the learning rule are called the input-dependent regulation of the learning rule. However, the biological meaning of such learning properties remains poorly understood. In this study, we quantitatively derive these learning properties with mathematical analyses. We further find that the input-dependent regulation of the learning rule allows maintenance of total synaptic weight over a given neuron during one-trial learning. Our results predict that a homeostatic plasticity mechanism should exist for conserving total synaptic weight on a rapid timescale.

Stress suppresses and learning induces plasticity in CA3 of rat hippocampus: a three-dimensional ultrastructural study of thorny excrescences and their postsynaptic densities

Chronic stress and spatial training have been proposed to affect hippocampal structure and function in opposite ways. Previous morphological studies that addressed structural changes after chronic restraint stress and spatial training were based on two-dimensional morphometry which does not allow a complete morphometric characterisation of synaptic features. Here, for the first time in such studies, we examined these issues by using three-dimensional (3-D) reconstructions of electron microscope images taken from thorny excrescences of hippocampal CA3 pyramidal cells. Ultrastructural alterations in postsynaptic densities (PSDs) of thorny excrescences receiving input from mossy fibre boutons were also determined, as were changes in numbers of multivesicular bodies (endosome-like structures) within thorny excrescences and dendrites. Quantitative 3-D data demonstrated retraction of thorny excrescences after chronic restraint stress which was reversed after water maze training, whilst water maze training alone increased thorny excrescence volume and number of thorns per thorny excrescence. PSD surface area was unaffected by restraint stress but water maze training increased both number and area of PSDs per thorny excrescence. In restrained rats that were water maze trained PSD volume and surface area increased significantly. The proportion of perforated PSDs almost doubled after water maze training and restraint stress. Numbers of endosome-like structures in thorny excrescences decreased after restraint stress and increased after water maze training. These findings demonstrate that circuits involving contacts between mossy fibre terminals and CA3 pyramidal cells at stratum lucidum level are affected conversely by water maze training and chronic stress, confirming the remarkable plasticity of CA3 dendrites. They provide a clear illustration of the structural modifications that occur after life experiences noted for their different impact on hippocampal function.