The human hippocampus and spatial and episodic memory

Vestibular syndrome: a change in internal spatial representation

The vestibular system contributes to a wide range of functions from reflexes to spatial representation. This paper reviews behavioral, perceptive, and cognitive data that highlight the role of changes in internal spatial representation on the vestibular syndrome. Firstly, we review how visual vertical perception and postural orientation depend on multiple reference frames and multisensory integration and how reference frames are selected according to the status of the peripheral vestibular system (i.e., unilateral or bilateral hyporeflexia), the environmental constraints (i.e., sensory cues), and the postural constraints (i.e., balance control). We show how changes in reference frames are able to modify vestibular lesion-induced postural and locomotor deficits and propose that fast changes in reference frame may be considered as fast-adaptive processes after vestibular loss. Secondly, we review data dealing with the influence of vestibular loss on higher levels of internal representation sustaining spatial orientation and navigation. Particular emphasis is placed on spatial performance according to task complexity (i.e., the required level of spatial knowledge) and to the sensory cues available to define the position and orientation within the environment (i.e., real navigation in darkness or visual virtual navigation without any actual self-motion). We suggest that vestibular signals are necessary for other sensory cues to be properly integrated and that vestibular cues are involved in extrapersonal space representation. In this respect, vestibular-induced changes would be based on a dynamic mental representation of space that is continuously updated and that supports fast-adaptive processes.

The short and long of it: neural correlates of temporal-order memory for autobiographical events

Previous functional neuroimaging studies of temporal-order memory have investigated memory for laboratory stimuli that are causally unrelated and poor in sensory detail. In contrast, the present functional magnetic resonance imaging (fMRI) study investigated temporal-order memory for autobiographical events that were causally interconnected and rich in sensory detail. Participants took photographs at many campus locations over a period of several hours, and the following day they were scanned while making temporal-order judgments to pairs of photographs from different locations. By manipulating the temporal lag between the two locations in each trial, we compared the neural correlates associated with reconstruction processes, which we hypothesized depended on recollection and contribute mainly to short lags, and distance processes, which we hypothesized to depend on familiarity and contribute mainly to longer lags. Consistent with our hypotheses, parametric fMRI analyses linked shorter lags to activations in regions previously associated with recollection (left prefrontal, parahippocampal, precuneus, and visual cortices), and longer lags with regions previously associated with familiarity (right prefrontal cortex). The hemispheric asymmetry in prefrontal cortex activity fits very well with evidence and theories regarding the contributions of the left versus right prefrontal cortex to memory (recollection vs. familiarity processes) and cognition (systematic vs. heuristic processes). In sum, using a novel photo-paradigm, this study provided the first evidence regarding the neural correlates of temporal-order for autobiographical events.

Scenes unseen: the parahippocampal cortex intrinsically subserves contextual associations, not scenes or places per se

The parahippocampal cortex (PHC) has been implicated in both place/scene processing and episodic memory. We proposed that this region should instead be seen as intrinsically mediating contextual associations and not place/scene processing or episodic memory exclusively. Given that place/scene processing and episodic memory both rely on associations, this modified framework provides a platform for reconciling what seemed like different roles assigned to the same region. Comparing scenes with scenes, we show here that the PHC responds significantly more strongly to scenes with rich contextual associations compared with scenes of equal visual qualities but less associations. This result adds unequivocal support to the view that the PHC mediates contextual associations in general rather than places or scenes proper, and necessitates a revision of the current view that the PHC contains a dedicated place/scenes "module."

Modulation of brain response to emotional images by alcohol cues in alcohol-dependent patients

Alcohol is often used to modulate mood states. Alcohol drinkers report that they use alcohol both to enhance positive affect and to reduce dysphoria, and alcohol-dependent patients specifically state reduction of negative affect as a primary reason for drinking. The current study proposes that alcohol cues may reduce negative affect in alcoholics. We used functional magnetic resonance imaging to examine brain activation in response to combination images that juxtaposed negative or positive International Affective Picture System (IAPS) images with an alcohol or non-alcohol-containing beverage. We found that in the absence of the alcohol cue, alcoholics showed more activation to negative than to positive images and greater activation than controls to negative images. When the IAPS images were presented with the alcohol cue, there was a decreased difference in activation between the positive and negative images among the alcoholics, and a decreased difference in response to the negative images between controls and alcoholics. Additionally, in the neutral-beverage conditions, anxiety ratings significantly predicted activation in the right parahippocampal gyrus but did not predict activation when the alcohol cues were presented. In conclusion, the alcohol cues may have modulated cortical networks involved in the processing of emotional stimuli by eliciting a conditioned response in the alcoholics, but not in the controls, which may have decreased responsiveness to the negative images.

Modulation of hippocampus-dependent learning and synaptic plasticity by nicotine

A long-standing relationship between nicotinic acetylcholine receptors (nAChRs) and cognition exists. Drugs that act at nAChRs can have cognitive-enhancing effects and diseases that disrupt cognition such as Alzheimer's disease and schizophrenia are associated with altered nAChR function. Specifically, hippocampus-dependent learning is particularly sensitive to the effects of nicotine. However, the effects of nicotine on hippocampus-dependent learning vary not only with the doses of nicotine used and whether nicotine is administered acutely, chronically, or withdrawn after chronic nicotine treatment but also vary across different hippocampus-dependent tasks such as the Morris water maze, the radial arm maze, and contextual fear conditioning. In addition, nicotine has variable effects across different types of hippocampal long-term potentiation (LTP). Because different types of hippocampus-dependent learning and LTP involve different neural and molecular substrates, comparing the effects of nicotine across these paradigms can yield insights into the mechanisms that may underlie the effects of nicotine on learning and memory and aid in understanding the variable effects of nicotine on cognitive processes. This review compares and contrasts the effects of nicotine on hippocampus-dependent learning and LTP and briefly discusses how the effects of nicotine on learning could contribute to nicotine addiction.

Crinkling and crumpling: an auditory fMRI study of material properties

Knowledge of an object's material composition (i.e., what it is made of) alters how we interact with that object. Seeing the bright glint or hearing the metallic crinkle of a foil plate for example, confers information about that object before we have even touched it. Recent research indicates that the medial aspect of the ventral visual pathway is sensitive to the surface properties of objects. In the present functional magnetic resonance imaging (fMRI) study, we investigated whether the ventral pathway is also sensitive to material properties derived from sound alone. Relative to scrambled material sounds and non-verbal human vocalizations, audio recordings of materials being manipulated (i.e., crumpled) in someone's hands elicited greater BOLD activity in the right parahippocampal cortex of neurologically intact listeners, as well as a cortically blind participant. Additional left inferior parietal lobe activity was also observed in the neurologically intact group. Taken together, these results support a ventro-medial pathway that is specialized for processing the material properties of objects, and suggest that there are sub-regions within this pathway that subserve the processing of acoustically-derived information about material composition.

Gaze behavior when reaching to remembered targets

People naturally direct their gaze to visible hand movement goals. Doing so improves reach accuracy through use of signals related to gaze position and visual feedback of the hand. Here, we studied where people naturally look when acting on remembered target locations. Four targets were presented on a screen, in peripheral vision, while participants fixed a central cross (encoding phase). Four seconds later, participants used a pen to mark the remembered locations while free to look wherever they wished (recall phase). Visual references, including the screen and the cross, were present throughout. During recall, participants neither looked at the marked locations nor prevented eye movements. Instead, gaze behavior was erratic and was comprised of gaze shifts loosely coupled in time and space with hand movements. To examine whether eye and hand movements during encoding affected gaze behavior during recall, in additional encoding conditions, participants marked the visible targets with either free gaze or with central cross fixation or just looked at the targets. All encoding conditions yielded similar erratic gaze behavior during recall. Furthermore, encoding mode did not influence recall performance, suggesting that participants, during recall, did not exploit sensorimotor memories related to hand and gaze movements during encoding. Finally, we recorded a similar lose coupling between hand and eye movements during an object manipulation task performed in darkness after participants had viewed the task environment. We conclude that acting on remembered versus visible targets can engage fundamentally different control strategies, with gaze largely decoupled from movement goals during memory-guided actions.

Psychosis and autism as diametrical disorders of the social brain

Autistic-spectrum conditions and psychotic-spectrum conditions (mainly schizophrenia, bipolar disorder, and major depression) represent two major suites of disorders of human cognition, affect, and behavior that involve altered development and function of the social brain. We describe evidence that a large set of phenotypic traits exhibit diametrically opposite phenotypes in autistic-spectrum versus psychotic-spectrum conditions, with a focus on schizophrenia. This suite of traits is inter-correlated, in that autism involves a general pattern of constrained overgrowth, whereas schizophrenia involves undergrowth. These disorders also exhibit diametric patterns for traits related to social brain development, including aspects of gaze, agency, social cognition, local versus global processing, language, and behavior. Social cognition is thus underdeveloped in autistic-spectrum conditions and hyper-developed on the psychotic spectrum.;>We propose and evaluate a novel hypothesis that may help to explain these diametric phenotypes: that the development of these two sets of conditions is mediated in part by alterations of genomic imprinting. Evidence regarding the genetic, physiological, neurological, and psychological underpinnings of psychotic-spectrum conditions supports the hypothesis that the etiologies of these conditions involve biases towards increased relative effects from imprinted genes with maternal expression, which engender a general pattern of undergrowth. By contrast, autistic-spectrum conditions appear to involve increased relative bias towards effects of paternally expressed genes, which mediate overgrowth. This hypothesis provides a simple yet comprehensive theory, grounded in evolutionary biology and genetics, for understanding the causes and phenotypes of autistic-spectrum and psychotic-spectrum conditions.

Spatial cognition and the brain

Recent advances in the understanding of spatial cognition are reviewed, focusing on memory for locations in large-scale space and on those advances inspired by single-unit recording and lesion studies in animals. Spatial memory appears to be supported by multiple parallel representations, including egocentric and allocentric representations, and those updated to accommodate self-motion. The effects of these representations can be dissociated behaviorally, developmentally, and in terms of their neural bases. It is now becoming possible to construct a mechanistic neural-level model of at least some aspects of spatial memory and imagery, with the hippocampus and medial temporal lobe providing allocentric environmental representations, the parietal lobe egocentric representations, and the retrosplenial cortex and parieto-occipital sulcus allowing both types of representation to interact. Insights from this model include a common mechanism for the construction of spatial scenes in the service of both imagery and episodic retrieval and a role for the remainder of Papez's circuit in orienting the viewpoint used. In addition, it appears that hippocampal and striatal systems process different aspects of environmental layout (boundaries and local landmarks, respectively) and do so using different learning rules (incidental learning and associative reinforcement, respectively).

Does clinical memory fMRI provide a comprehensive map of medial temporal lobe structures?

Successful clinical application of fMRI tasks requires reliable knowledge about the brain structures mapped by the task. With memory fMRI, diverging evidence exists concerning the location of major signal sources as well as hippocampal contributions. To clarify these issues, we investigated a frequently applied memory test (home town walking) in 33 patients with unilateral medial temporal lobe pathology, comparing healthy and diseased hemispheres. We focused on a detailed investigation of individual fMRI maps on non-transformed high-resolution functional images. Results show a clear dominance of activations around the collateral sulcus, corresponding to parahippocampal and entorhinal cortex activities. Hippocampus activity was absent in the vast majority of patients. The diseased hemispheres showed lower activation than the healthy hemispheres. We conclude that (1) the investigated memory test may be successfully applied for evaluation of the parahippocampal cortex, (2) the hippocampus is not reliably mapped by the task, and (3) the methods described for investigation of individual high-resolution functional images allow generation of application profiles for clinical fMRI tasks.

Firing rate dynamics in the hippocampus induced by trajectory learning

The hippocampus is essential for spatial navigation, which may involve sequential learning. However, how the hippocampus encodes new sequences in familiar environments is unknown. To study the impact of novel spatial sequences on the activity of hippocampal neurons, we monitored hippocampal ensembles while rats learned to switch from two familiar trajectories to a new one in a familiar environment. Here, we show that this novel spatial experience induces two types of changes in firing rates, but not locations of hippocampal place cells. First, place-cell firing rates on the two familiar trajectories start to change before the actual behavioral switch to the new trajectory. Second, repeated exposure on the new trajectory is associated with an increased dependence of place-cell firing rates on immediate past locations. The result suggests that sequence encoding in the hippocampus may involve integration of information about the recent past into current state.

Human hippocampal and parahippocampal theta during goal-directed spatial navigation predicts performance on a virtual Morris water maze

The hippocampus and parahippocampal cortices exhibit theta oscillations during spatial navigation in animals and humans, and in the former are thought to mediate spatial memory formation. Functional specificity of human hippocampal theta, however, is unclear. Neuromagnetic activity was recorded with a whole-head 275-channel magnetoencephalographic (MEG) system as healthy participants navigated to a hidden platform in a virtual reality Morris water maze. MEG data were analyzed for underlying oscillatory sources in the 4-8 Hz band using a spatial filtering technique (i.e., synthetic aperture magnetometry). Source analyses revealed greater theta activity in the left anterior hippocampus and parahippocampal cortices during goal-directed navigation relative to aimless movements in a sensorimotor control condition. Additional analyses showed that left anterior hippocampal activity was predominantly observed during the first one-half of training, pointing to a role for this region in early learning. Moreover, posterior hippocampal theta was highly correlated with navigation performance, with the former accounting for 76% of the variance of the latter. Our findings suggest human spatial learning is dependent on hippocampal and parahippocampal theta oscillations, extending to humans a significant body of research demonstrating such a pivotal role for hippocampal theta in animal navigation.

Protein kinase CK2 modulates synaptic plasticity by modification of synaptic NMDA receptors in the hippocampus

Synaptic plasticity is the foundation of learning and memory. The protein kinase CK2 phosphorylates many proteins related to synaptic plasticity, but whether it is directly involved in it has not been clarified. Here, we examined the role of CK2 in synaptic plasticity in hippocampal slices using the CK2 selective inhibitors 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and 4,5,6,7-tetrabromobenzotriazole (TBB). These significantly inhibited N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). DRB also inhibited NMDA receptor-mediated synaptic transmission, while leaving NMDA receptor-independent LTP unaffected. NMDA receptors thus appear to be the primary targets of CK2. Although both long-term depression (LTD) and LTP are induced by the influx of Ca(2+) through NMDA receptors, surprisingly, LTD was not affected by CK2 inhibitors. We postulated that the LTP-selective modulation by CK2 is due to selective modulation of NMDA receptors, and tested two hypotheses concerning the modulation of NMDA receptors: (i) CK2 selectively modulates NR2A subunits possibly related to LTP, but not NR2B subunits possibly related to LTD; and (ii) CK2 selectively affects synaptic but not extrasynaptic NMDA receptors whose activation is sufficient to induce LTD. DRB decreased NMDA receptor-mediated synaptic transmission in the presence of selective NR2A subunit antagonist. The former hypothesis thus appears unlikely to be correct. However, DRB decreased synaptic NMDA receptor responses in cultured hippocampal neurons without affecting extrasynaptic NMDA receptor current. These findings support the latter hypothesis, that CK2 selectively affects LTP by selective modification of synaptic NMDA receptors in a receptor-location-specific manner.

A principle for learning egocentric-allocentric transformation

Numerous single-unit recording studies have found mammalian hippocampal neurons that fire selectively for the animal's location in space, independent of its orientation. The population of such neurons, commonly known as place cells, is thought to maintain an allocentric, or orientation-independent, internal representation of the animal's location in space, as well as mediating long-term storage of spatial memories. The fact that spatial information from the environment must reach the brain via sensory receptors in an inherently egocentric, or viewpoint-dependent, fashion leads to the question of how the brain learns to transform egocentric sensory representations into allocentric ones for long-term memory storage. Additionally, if these long-term memory representations of space are to be useful in guiding motor behavior, then the reverse transformation, from allocentric to egocentric coordinates, must also be learned. We propose that orientation-invariant representations can be learned by neural circuits that follow two learning principles: minimization of reconstruction error and maximization of representational temporal inertia. Two different neural network models are presented that adhere to these learning principles, the first by direct optimization through gradient descent and the second using a more biologically realistic circuit based on the restricted Boltzmann machine (Hinton, 2002; Smolensky, 1986). Both models lead to orientation-invariant representations, with the latter demonstrating place-cell-like responses when trained on a linear track environment.

Hippocampal lesions in rats impair learning and memory for locations on a touch-sensitive computer screen: the "ASAT" task

It has been repeatedly demonstrated across species that the hippocampus is critical for spatial learning and memory. Consequently, numerous paradigms have been created to study spatial learning in the rodent. Most of these tasks, such as the Morris water maze, 8-arm radial maze, and T-maze, are non-automated procedures. It was our goal to create an automated task in the rodent that is quickly learned, hippocampal-dependent, and minimizes the confounding variables present in most tests measuring hippocampal-dependent learning and memory. To accomplish this, we created a novel search task using a standard operant box fitted with a touch-sensitive computer monitor. Subjects were required to locate an S+ "hidden" amongst other identical stimuli on the monitor. In two versions of the task the S+ stayed in the same location within a session but shifted location between sessions. In a third version of the task the S+ was moved to a new location after every 10 trials. It was found that the location of the S+ was quickly acquired each day (within 10 trials), and that the hippocampal-lesion group was impaired when compared to their control cohort. With the benefits inherent in automation, these tasks confer significant advantages over traditional tasks used to study spatial learning and memory in the rodent. When combined with previously developed non-spatial cognitive tests that can also be run in the touch-screen apparatus, the result is a powerful cognitive test battery for the rodent.

Parallel striatal and hippocampal systems for landmarks and boundaries in spatial memory

How the memory systems centered on the hippocampus and dorsal striatum interact to support behavior remains controversial. We used functional MRI while people learned the locations of objects by collecting and replacing them over multiple trials within a virtual environment comprising a landmark, a circular boundary, and distant cues for orientation. The relative location of landmark and boundary was occasionally changed, with specific objects paired with one or other cue, allowing dissociation of learning and performance relative to either cue. Right posterior hippocampal activation reflected learning and remembering of boundary-related locations, whereas right dorsal striatal activation reflected learning and remembering of landmark-related locations. Within the right hippocampus, anterior processing of environmental change (spatial novelty) was dissociated from posterior processing of location. Behavioral studies show that landmark-related learning obeys associative reinforcement, whereas boundary-related learning is incidental [Doeller CF, Burgess N (2008) Proc Natl Acad Sci USA 105:5909-5914]. The distinct incidental hippocampal processing of boundaries is suggestive of a "geometric module" or "cognitive map" and may explain the hippocampal support of incidental/observational learning in "declarative" or "episodic" memory versus the striatal support of trial-and-error learning in "procedural" memory. Finally, the hippocampal and striatal systems appear to combine "bottom-up," simply influencing behavior proportional to their activations, without direct interaction, with "top-down" ventromedial prefrontal involvement when both are similarly active.

Long-term consequences of soman poisoning in mice: part 2. Emotional behavior

The organophosphorus compound soman produces long-lasting epileptic seizure activity which is associated to brain damage, more particularly in the hippocampus and the amygdala. The companion paper (see part 1 in the same journal issue) describes the neuropathology in the amygdala of soman-poisoned mice. The present paper examines the long-term effects of soman poisoning on emotional reactivity in mice, 30 or 90 days after intoxication using behavioral tasks involving amygdala function. The emotional behavior was estimated in animal tests of unconditioned fear (light/dark boxes, elevated plus-maze) and conditioned fear (auditory and contextual response). In the light/dark boxes and elevated plus-maze, mice intoxicated with soman (110 microg/kg, 1.2 LD(50)) showed an anxiety-like behavior profile at post-poisoning days 30 and 90. In conditioned fear, results showed that both auditory and contextual conditioned responses are increased on post-soman day 30 but no longer on post-soman day 90, evidencing behavioral recovery overtime. This latter behavioral result is in accordance with the delayed neuronal regeneration patterns described in the companion paper (part 1).

Using imagination to understand the neural basis of episodic memory

Functional MRI (fMRI) studies investigating the neural basis of episodic memory recall, and the related task of thinking about plausible personal future events, have revealed a consistent network of associated brain regions. Surprisingly little, however, is understood about the contributions individual brain areas make to the overall recollective experience. To examine this, we used a novel fMRI paradigm in which subjects had to imagine fictitious experiences. In contrast to future thinking, this results in experiences that are not explicitly temporal in nature or as reliant on self-processing. By using previously imagined fictitious experiences as a comparison for episodic memories, we identified the neural basis of a key process engaged in common, namely scene construction, involving the generation, maintenance and visualization of complex spatial contexts. This was associated with activations in a distributed network, including hippocampus, parahippocampal gyrus, and retrosplenial cortex. Importantly, we disambiguated these common effects from episodic memory-specific responses in anterior medial prefrontal cortex, posterior cingulate cortex and precuneus. These latter regions may support self-schema and familiarity processes, and contribute to the brain's ability to distinguish real from imaginary memories. We conclude that scene construction constitutes a common process underlying episodic memory and imagination of fictitious experiences, and suggest it may partially account for the similar brain networks implicated in navigation, episodic future thinking, and the default mode. We suggest that additional brain regions are co-opted into this core network in a task-specific manner to support functions such as episodic memory that may have additional requirements.

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

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

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.

Constitutive activation of the G-protein subunit Galphas within forebrain neurons causes PKA-dependent alterations in fear conditioning and cortical Arc mRNA expression

Memory formation requires cAMP signaling; thus, this cascade has been of great interest in the search for cognitive enhancers. Given that medications are administered long-term, we determined the effects of chronically increasing cAMP synthesis in the brain by expressing a constitutively active isoform of the G-protein subunit Galphas (Galphas*) in postnatal forebrain neurons of mice. Previously, we showed that Galphas* mice exhibit increased adenylyl cyclase activity but decreased cAMP levels in cortex and hippocampus due to a PKA-dependent increase in total cAMP phosphodiesterase (PDE) activity. Here, we extend previous findings by determining if Galphas* mice show increased activity of specific PDE families that are regulated by PKA, if Galphas* mice show PKA-dependent deficits in fear memory, and if these memory deficits are associated with PKA-dependent alterations in neuronal activity as mapped by Arc mRNA expression. Consistent with previous findings, we show here that Galphas* mice exhibit a significant compensatory increase in cAMP PDE1 activity and a trend toward increased cAMP PDE4 activity. Further, inhibiting the presumably elevated PKA activity in Galphas* mice fully rescues short- and long-term memory deficits in a fear-conditioning task, while extending the training session from one to four CS-US pairings partially rescues these deficits. Mapping of Arc mRNA levels suggests these PKA-dependent memory deficits may be related to decreased neuronal activity specifically within the cortex. Galphas* mice show decreased Arc mRNA expression in CA1, orbital cortex, and cortical regions surrounding the hippocampus; however, only the deficits in cortical regions surrounding the hippocampus are PKA dependent. Our results imply that chronically stimulating targets upstream of cAMP may detrimentally affect cognition.

Hippocampal involvement in contextual modulation of fear extinction

Extinction of fear conditioning in animals is an excellent model for the study of fear inhibition in humans. Substantial evidence has shown that extinction is a new learning process that is highly context-dependent. Several recovery effects (renewal, spontaneous recovery, and reinstatement) after extinction suggest that the contextual modulation of extinction is a critical behavioral mechanism underlying fear extinction. In addition, recent studies demonstrate a critical role for hippocampus in the context control of extinction. A growing body of evidence suggests that the hippocampus not only plays a role in contextual encoding and retrieval of fear extinction memories, but also interacts with other brain structures to regulate context-specificity of fear extinction. In this article, the authors will first discuss the fundamental behavioral features of the context effects of extinction and its underlying behavioral mechanisms. In the second part, the review will focus on the brain mechanisms for the contextual control of extinction.

The effect of scene context on episodic object recognition: parahippocampal cortex mediates memory encoding and retrieval success

Previous research has investigated intentional retrieval of contextual information and contextual influences on object identification and word recognition, yet few studies have investigated context effects in episodic memory for objects. To address this issue, unique objects embedded in a visually rich scene or on a white background were presented to participants. At test, objects were presented either in the original scene or on a white background. A series of behavioral studies with young adults demonstrated a context shift decrement (CSD)-decreased recognition performance when context is changed between encoding and retrieval. The CSD was not attenuated by encoding or retrieval manipulations, suggesting that binding of object and context may be automatic. A final experiment explored the neural correlates of the CSD, using functional Magnetic Resonance Imaging. Parahippocampal cortex (PHC) activation (right greater than left) during incidental encoding was associated with subsequent memory of objects in the context shift condition. Greater activity in right PHC was also observed during successful recognition of objects previously presented in a scene. Finally, a subset of regions activated during scene encoding, such as bilateral PHC, was reactivated when the object was presented on a white background at retrieval. Although participants were not required to intentionally retrieve contextual information, the results suggest that PHC may reinstate visual context to mediate successful episodic memory retrieval. The CSD is attributed to automatic and obligatory binding of object and context. The results suggest that PHC is important not only for processing of scene information, but also plays a role in successful episodic memory encoding and retrieval. These findings are consistent with the view that spatial information is stored in the hippocampal complex, one of the central tenets of Multiple Trace Theory.

Course of relational and non-relational recognition memory across the adult lifespan

Human recognition memory shows a decline during normal ageing, which is thought to be related to age-associated dysfunctions of mediotemporal lobe structures. Whether the hippocampus is critical for human general relational memory or for spatial relational memory only is still disputed. The human perirhinal cortex is thought to be critically involved in non-relational memory, but another view postulates hippocampal involvement in both relational and non-relational memory. Investigating whether there is a differential impact of ageing on these memory processes may shed further light into these issues. Thus, in the present study, 106 healthy adults performed three recognition memory tasks in a consecutive age groups design involving a range from age 20 to 76. This allowed the separate assessment of spatial and nonspatial relational memory as well as non-relational memory. Both spatial and nonspatial relational memory declined in the 66-76 yr group. This pattern is consistent with the presumed course of hippocampal changes across normal ageing and points to the hippocampal role in relational memory in general. An impairment of non-relational memory commenced earlier in the 51-65 yr group. This finding is discussed in relation to perceptual/attentional mediation of memory and its potential brain correlates in ageing.

A new account of the neurocognitive foundations of impairments in space, time and number processing in children with chromosome 22q11.2 deletion syndrome

In this article, I present an updated account that attempts to explain, in cognitive processing and neural terms, the nonverbal intellectual impairments experienced by most children with deletions of chromosome 22q11.2. Specifically, I propose that this genetic syndrome leads to early developmental changes in the structure and function of clearly delineated neural circuits for basic spatiotemporal cognition. This dysfunction then cascades into impairments in basic magnitude and then numerical processes, because of the central role that representations of space and time play in their construction. I propose that this takes the form of "spatiotemporal hypergranularity"; the increase in grain size and thus reduced resolution of mental representations of spatial and temporal information. The result is that spatiotemporal processes develop atypically and thereby produce the characteristic impairments in nonverbal cognitive domains that are a hallmark feature of chromosome 22q11.2 deletion syndrome. If this hypothesis driven account is supported by future research, the results will create a neurocognitive explanation of spatiotemporal and numerical impairments in the syndrome that is specific enough to be directly translated into the development of targeted therapeutic interventions.

Detecting concealed information using brain-imaging technology

Many conventional techniques for revealing concealed information have focused on detecting whether a person is responding truthfully to specific questions, typically using some form of lie detector. However, lie detection has faced a number of criticisms and it is still unclear to what degree conventional lie detectors can be used to reveal concealed knowledge in applied real-world settings. Here, we review the key problems with conventional lie-detection technology and critically discuss the potential of novel techniques that aim to directly read concealed mental states out of patterns of brain activity.

Toward an integrative perspective on hippocampal function: from the rapid encoding of experience to adaptive behavior

The mammalian hippocampus has been associated with learning and memory, as well as with many other behavioral processes. In this article, these different perspectives are brought together, and it is pointed out that integration of diverse functional domains may be a key feature enabling the hippocampus to support not only the encoding and retrieval of certain memory representations, but also their translation into adaptive behavior. The hippocampus appears to combine: (i) sensory afferents and synaptic mechanisms underlying certain types of rapid learning; and (ii) links to motivational, emotional, executive, and sensorimotor functions. Recent experiments are highlighted, indicating that the induction of hippocampal synaptic plasticity is required to encode rapidly aspects of experience, such as places, into memory representations; subsequent retrieval of these representations requires transmission through the previously modified hippocampal synapses, but no further plasticity. In contrast, slow incremental place learning may not absolutely require hippocampal contributions. The neocortical sensory inputs, especially visuo-spatial information, necessary for hippocampus-dependent rapid learning, are preferentially associated with the septal to intermediate hippocampus. In contrast, connectivity with the prefrontal cortex and subcortical sites, which link the hippocampus to motivational, emotional, executive, and sensorimotor functions, is primarily associated with the intermediate to temporal hippocampus. A model of functional differentiation and integration along the septo-temporal axis of the hippocampus is proposed, describing key hippocampal contributions to adaptive behavior based on information encoded during a single or a few past experiences.

The neural correlates of implicit sequence learning in schizophrenia

Twenty-seven schizophrenia spectrum patients and 25 healthy controls performed a probabilistic version of the serial reaction time task (SRT) that included sequence trials embedded within random trials. Patients showed diminished, yet measurable, sequence learning. Postexperimental analyses revealed that a group of patients performed above chance when generating short spans of the sequence. This high-generation group showed SRT learning that was similar in magnitude to that of controls. Their learning was evident from the very 1st block; however, unlike controls, learning did not develop further with continued testing. A subset of 12 patients and 11 controls performed the SRT in conjunction with positron emission tomography. High-generation performance, which corresponded to SRT learning in patients, correlated to activity in the premotor cortex and parahippocampus. These areas have been associated with stimulus-driven visuospatial processing. Taken together, these results suggest that a subset of patients who showed moderate success on the SRT used an explicit stimulus-driven strategy to process the sequential stimuli. This adaptive strategy facilitated sequence learning but may have interfered with conventional implicit learning of the overall stimulus pattern.

Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease

Neural network dysfunction may play an important role in Alzheimer's disease (AD). Neuronal circuits vulnerable to AD are also affected in human amyloid precursor protein (hAPP) transgenic mice. hAPP mice with high levels of amyloid-beta peptides in the brain develop AD-like abnormalities, including cognitive deficits and depletions of calcium-related proteins in the dentate gyrus, a region critically involved in learning and memory. Here, we report that hAPP mice have spontaneous nonconvulsive seizure activity in cortical and hippocampal networks, which is associated with GABAergic sprouting, enhanced synaptic inhibition, and synaptic plasticity deficits in the dentate gyrus. Many Abeta-induced neuronal alterations could be simulated in nontransgenic mice by excitotoxin challenge and prevented in hAPP mice by blocking overexcitation. Aberrant increases in network excitability and compensatory inhibitory mechanisms in the hippocampus may contribute to Abeta-induced neurological deficits in hAPP mice and, possibly, also in humans with AD.

The role of hippocampus in the pathophysiology of bipolar disorder

Bipolar disorder (BD) is thought to be associated with abnormalities within discrete brain regions associated with emotional regulation, particularly in fronto-limbic-subcortical circuits. Several reviews have addressed the involvement of the prefrontal cortex in the pathophysiology of BD, whereas little attention has been given to the role of the hippocampus. This study critically reviews data from brain imaging, postmortem, neuropsychological, and preclinical studies, which suggested hippocampal abnormalities in BD. Most of the structural brain imaging studies did not find changes in hippocampal volume in BD, although a few studies suggested that anatomical changes might be restricted to the psychotic, pediatric, or unmedicated BD subgroups. Functional imaging studies showed abnormal brain activation in the hippocampus and its closely related regions during emotional, attentional, and memory tasks. This is consistent with neuropsychological findings that revealed a wide range of cognitive disturbances during acute mood episodes and a significant impairment in declarative memory during remission. Postmortem studies indicate abnormal glutamate and GABA transmission in the hippocampus of BD patients, whereas data from preclinical studies suggest that the regulation of hippocampal plasticity and survival might be associated with the therapeutic effects of mood stabilizers. In conclusion, the available evidence suggests that the hippocampus plays an important role in the pathophysiology of BD.

Age-dependent effects of neonatal methamphetamine exposure on spatial learning

Neonatal rats exposed to (+)-methamphetamine (MA) display spatial learning and reference memory deficits in the Morris water maze. In separate experiments the emergence and permanence of these effects were determined. Twenty litters were used in each experiment, and two male/female pairs/litter received saline or MA (5 mg/kg four times a day) on postnatal days (P) 11-20. In experiment 1, one MA and one saline pair from each litter began testing on either P30 or P40, whereas in experiment 2, testing began on P180 or P360. Animals received trials in a straight swimming channel and then in the Morris maze (acquisition, reversal, and reduced platform phases). In both experiments, MA-treated groups showed impaired learning in the platform trials and impaired reference memory in the probe trials, which were largely independent of age. The P30 and P40 MA impairments were seen on acquisition and reduced platform trials but not on reversal. In the probe trials, MA effects were seen during all phases. The P180 and P360 MA-induced deficits were seen in all phases of the platform trials. In probe trials, deficits were only seen during the reversal and reduced platform phases. The results demonstrate that neonatal MA treatment induces spatial learning and reference memory deficits that emerge early and persist until at least 1 year of age, suggesting permanence.

Dendritic spikes induce single-burst long-term potentiation

The hippocampus is essential for episodic memory, which requires single-trial learning. Although long-term potentiation (LTP) of synaptic strength is a candidate mechanism for learning, it is typically induced by using repeated synaptic activation to produce precisely timed, high-frequency, or rhythmic firing. Here we show that hippocampal synapses potentiate robustly in response to strong activation by a single burst. The induction mechanism of this single-burst LTP requires activation of NMDA receptors, L-type voltage-gated calcium channels, and dendritic spikes. Thus, dendritic spikes are a critical trigger for a form of LTP that is consistent with the function of the hippocampus in episodic memory.

The neuroscience of remote spatial memory: A tale of two cities

Most of our everyday activities take place in familiar environments learned in the past which we need to constantly navigate. Despite our obvious reliance on these remote spatial memories, until quite recently relatively little was known about how they are instantiated in the human brain. Here we will consider developments in the neuropsychological and neuroimaging domains where innovative methodologies and novel analysis techniques are providing new opportunities for exploring the brain dynamics underpinning the retrieval and use of remotely learned spatial information. These advances allow three key questions to be considered anew: What brain areas in humans support the retrieval and use of remotely learned spatial information? Where in the brain are spatial memories stored? Do findings relating to remote spatial memory inform theoretical debates about memory consolidation? In particular, the hippocampus, parahippocampus, retrosplenial and parietal cortices are scrutinized, revealing new insights into their specific contributions to representing spaces and places from the past.

Spatial and non-spatial contextual working memory in patients with diencephalic or hippocampal dysfunction

Damage to the medial temporal lobe (MTL) and diencephalon results in impaired long-term memory, which relies on the binding of multiple, mostly contextual, features. Recent neuroimaging and patient studies have suggested that impairments may also be present in working memory after MTL or diencephalic damage. To examine whether patients with damage to these brain structures have impairments in working memory for contextual information, 15 patients with damage to the diencephalon due to Korsakoff's syndrome and 12 patients with unilateral MTL lesions, and 30 age-matched healthy controls performed a delayed match-to-sample (DMS) task in which they had to maintain either object-location associations, color-number associations, single colors or single locations. Compared to their age-matched controls, performance on the DMS task was generally impaired in both patient groups, whereas no deficits were found on standard neuropsychological span tasks that do not rely on maintenance aspects of working memory. The patients did not show disproportionate impairments on the binding condition. In all, the results clearly show that impairments in working memory maintenance are present in patients with MTL or diencephalic lesions. However, we did not find a disproportionate inability in maintaining spatial or non-spatial associations within working memory as previously demonstrated in long-term memory.

Quantitative and qualitative sex differences in spatial navigation

We examined sex differences in spatial navigation performance using an ecologically relevant experimental paradigm in which virtual maze-like museums are projected in front of a treadmill. Thirty-two 20-30-year-old adults (16 women/16 men) performed a way-finding task in city-block (straight corridors) or variable (irregular corridors) topographies while walking on the treadmill. Sex differences in spatial navigation performance were reduced in variable topographies, suggesting less reliance on spatial relational learning among women. Also, spatial geometric knowledge of the mazes continued to be higher in men after all participants had attained perfect place-finding performance. Results indicate that sex differences in spatial navigation performance are modulated by interactions between environmental demands and sex differences in spatial processing.