The Hippocampal Role in Spatial Memory and the Familiarity-Recollection Distinction: A Case Study

Drawing promotes memory retention in a patient with sleep-related anterograde amnesia

Drawing is a powerful tool to enhance memory in healthy participants and patients with probable dementia. Here, we investigated whether the drawing effect could extend to patient CT, a young woman with severe anterograde amnesia. Following surgery for a midline tumor involving her septum pellucidium and extending down into her fornices bilaterally, CT experienced a severe case of sleep-related amnesia. She can remember information encountered throughout the day, but when waking up in the morning or following a nap she forgets information learned prior to sleep. Here, we tested CT and 21 age-matched controls in a 3-day within-subjects design, during which participants encoded words by either drawing or writing them down. Memory for encoded words was tested in two conditions that each followed a 12-h delay, once after a night of sleep, and once after 12 h of wake. Despite her severe memory impairment, CT showed a drawing effect that was comparable to controls in both sleep and wake conditions. Whereas CT's memory for written words was consistently impaired relative to controls, her memory for drawn words was at the lower control range following a waking delay and above chance following a sleep delay. We suggest that amnesic patients may benefit from the drawing effect due to the recruitment of brain regions outside of the hippocampal system for encoding and consolidation. Furthermore, in control participants, sleep benefited memory for written words, but not for drawn words, suggesting that sleep preferentially consolidates memories that are more dependent on the hippocampal system.

Episodic memory development: Bridging animal and human research

Human episodic memory is not functionally evident until about 2 years of age and continues to develop into the school years. Behavioral studies have elucidated this developmental timeline and its constituent processes. In tandem, lesion and neurophysiological studies in non-human primates and rodents have identified key neural substrates and circuit mechanisms that may underlie episodic memory development. Despite this progress, collaborative efforts between psychologists and neuroscientists remain limited, hindering progress. Here, we seek to bridge human and non-human episodic memory development research by offering a comparative review of studies using humans, non-human primates, and rodents. We highlight critical theoretical and methodological issues that limit cross-fertilization and propose a common research framework, adaptable to different species, that may facilitate cross-species research endeavors.

Sleep Differentially and Profoundly Impairs Recall Memory in a Patient with Fornix Damage

In March 2020, C.T., a kind, bright, and friendly young woman underwent surgery for a midline tumor involving her septum pellucidum and extending down into her fornices bilaterally. Following tumor diagnosis and surgery, C.T. experienced significant memory deficits: C.T.'s family reported that she could remember things throughout the day, but when she woke up in the morning or following a nap, she would expect to be in the hospital, forgetting all the information that she had learned before sleep. The current study aimed to empirically validate C.T.'s pattern of memory loss and explore its neurological underpinnings. On two successive days, C.T. and age-matched controls watched an episode of a TV show and took a nap or stayed awake before completing a memory test. Although C.T. performed numerically worse than controls in both conditions, sleep profoundly exacerbated her memory impairment, such that she could not recall any details following a nap. This effect was replicated in a second testing session. High-resolution MRI scans showed evidence of the trans-callosal surgical approach's impact on the mid-anterior corpus callosum, indicated that C.T. had perturbed white matter particularly in the right fornix column, and demonstrated that C.T.'s hippocampal volumes did not differ from controls. These findings suggest that the fornix is important for processing episodic memories during sleep. As a key output pathway of the hippocampus, the fornix may ensure that specific memories are replayed during sleep, maintain the balance of sleep stages, or allow for the retrieval of memories following sleep.

Distribution and Morphological Characteristics of Oligodendrocytes in Selected Areas of the Brain of Male and Female Red Kangaroos (Macropus rufus)

This study was carried out on six adult red kangaroos of both sexes. To determine the location of the oligodendrocytes (OLGs) of the hippocampus (Hip) and corpus callosum (CC), the method of impregnation of the neuroglia with silver salts was applied. The iron distribution in the OLGs was determined by the histochemical method. The Nissl method was used to determine the location of the brain structure and to analyze the number of OLGs. In the Hip, these cells are located one beside another, mainly in blood vessels and neurons; in the neocortex (NC), they are located in layers I–VI; and in the CC, they are arranged in characteristic rows and accompany both nerve fibers and blood vessels. The analysis of the results obtained by the chosen methods in the Hip, NC, and CC in males and females did not show statistically significant differences in the distribution and location of the red kangaroo OLGs. The involvement of these cells is a physiological process that proceeds in a similar manner throughout the life of individuals and actively influences the metabolism of neurons and myelin.

Impaired allocentric spatial memory in patients with affective disorders

BACKGROUND

Memory disturbances are frequent in unipolar depression (UD) and bipolar disorder (BD) and may comprise important predisposing and maintaining factors. Previous studies have demonstrated hippocampal abnormalities in UD and BD but there is a lack of studies specifically assessing hippocampus-dependent memory.

METHODS

We used a virtual task to assess hippocampus-dependent (allocentric) vs non-hipppocampal (egocentric) spatial memory in remitted and partially remitted patients with UD or BD (N = 22) and a healthy control group (N = 32). Participants also completed a range of standard neuropsychological and functional assessments.

RESULTS

Participants in the UD/BD group showed selective impairments on high-load hippocampal (allocentric) memory compared to egocentric memory and this effect was independent of residual mood symptoms. Across both samples, both allocentric and egocentric spatial memory correlated with more general measures of memory and other aspects of cognition measured on standard neuropsychological tests but only high-load allocentric memory showed a significant relationship with functional capacity.

CONCLUSION

Results show a selective impairment in high-load allocentric spatial memory compared to egocentric memory in the patient group, suggesting impaired hippocampal functioning in patients with remitted UD/BD.

Perspective taking and systematic biases in object location memory

The aim of the current study was to develop a novel task that allows for the quick assessment of spatial memory precision with minimal technical and training requirements. In this task, participants memorized the position of an object in a virtual room and then judged from a different perspective, whether the object has moved to the left or to the right. Results revealed that participants exhibited a systematic bias in their responses that we termed the reversed congruency effect. Specifically, they performed worse when the camera and the object moved in the same direction than when they moved in opposite directions. Notably, participants responded correctly in almost 100% of the incongruent trials, regardless of the distance by which the object was displaced. In Experiment 2, we showed that this effect cannot be explained by the movement of the object on the screen, but that it relates to the perspective shift and the movement of the object in the virtual world. We also showed that the presence of additional objects in the environment reduces the reversed congruency effect such that it no longer predicts performance. In Experiment 3, we showed that the reversed congruency effect is greater in older adults, suggesting that the quality of spatial memory and perspective-taking abilities are critical. Overall, our results suggest that this effect is driven by difficulties in the precise encoding of object locations in the environment and in understanding how perspective shifts affect the projected positions of the objects in the two-dimensional image.

Allocentric spatial memory performance predicts intrusive memory severity in posttraumatic stress disorder

BACKGROUND

Posttraumatic stress disorder (PTSD) is characterized by distressing trauma-related memories. According to the dual representation theory, intrusive memories arise from strengthened egocentric encoding and a poor contextual encoding, with spatial context requiring allocentric processing. Contextualization of mental imagery is proposed to be formed hierarchically through the ventral visual stream (VVS) to the hippocampal formation. Here, we tested this notion by investigating whether neuronal aberrations in structures of the VVS or in the hippocampus, as well as allocentric memory performance are associated with intrusive memory severity.

METHODS

The sample comprised 33 women with PTSD due to childhood trauma. Allocentric memory performance was measured with the virtual Town Square Task and T1-weighted images acquired on a 3T Siemens Scanner. Intrusive memories were evoked by presenting an audio script describing parts of their trauma (script-driven imagery).

RESULTS

Using hierarchical linear regression analysis, we found a significant association between lower intrusive memory severity and higher allocentric spatial memory, controlling for age, working memory, and general visuospatial ability. No significant association was found between cortical thickness of VVS structures, hippocampal volume and intrusive memory severity. Post hoc exploratory analyses revealed a negative correlation between years since index trauma and left hippocampal volume.

LIMITATIONS

Our results are based on correlational analyses, causality cannot be inferred.

CONCLUSION

This study supports the dual representation theory, which emphasizes the role of allocentric spatial memory for the contextualization of mental imagery in PTSD. Clinical implications are discussed.

Spatial navigation deficits - overlooked cognitive marker for preclinical Alzheimer disease?

Detection of incipient Alzheimer disease (AD) pathophysiology is critical to identify preclinical individuals and target potentially disease-modifying therapies towards them. Current neuroimaging and biomarker research is strongly focused in this direction, with the aim of establishing AD fingerprints to identify individuals at high risk of developing this disease. By contrast, cognitive fingerprints for incipient AD are virtually non-existent as diagnostics and outcomes measures are still focused on episodic memory deficits as the gold standard for AD, despite their low sensitivity and specificity for identifying at-risk individuals. This Review highlights a novel feature of cognitive evaluation for incipient AD by focusing on spatial navigation and orientation deficits, which are increasingly shown to be present in at-risk individuals. Importantly, the navigation system in the brain overlaps substantially with the regions affected by AD in both animal models and humans. Notably, spatial navigation has fewer verbal, cultural and educational biases than current cognitive tests and could enable a more uniform, global approach towards cognitive fingerprints of AD and better cognitive treatment outcome measures in future multicentre trials. The current Review appraises the available evidence for spatial navigation and/or orientation deficits in preclinical, prodromal and confirmed AD and identifies research gaps and future research priorities.

Effect of age on spatial memory performance in real museum vs. computer simulation

BACKGROUND

Healthy older adults frequently complain on difficulty in recalling the locations of objects of everyday use. Cognitive abilities decline with normal aging; inefficiencies of information processing, as well as deterioration of neuronal structures, may impede the performance of complex cognitive skills such as spatial memory. Extraneous, task-irrelevant cognitive load in real environments is usually high and might interfere with spatial memory abilities of older adults. The purpose of this study was to determine (1) the extent to which older adults maintain their cognitive capacity during a spatial memory task as compared to young adults and (2) whether this capacity is affected by performance of the task in a real environment setting where the cognitive demands are similar to a simulation, but the physical demands (navigating via walking versus via a mouse) vary.

METHODS

In the museum, participants physically moved between display stations to locate hidden tokens performing a task in which an ongoing representation of previous searches had to be remembered. A comparable task was implemented via mouse actions on a computer simulation. Seventeen healthy older (60-80 years) and twenty younger (20-45 years) adults performed both tasks in a counterbalanced order.

RESULTS

The younger group was superior to the older group in terms of success rate and completion time for both conditions. All participants performed better during the simulated task. The delta between the total performance score in the two settings of the older group was significantly larger as compared to the younger group, suggesting a differential impact of setting on the groups.

CONCLUSIONS

Our results highlight the importance and feasibility of experimentation in ecologically relevant settings: differences were found in the way the cognitive performance of older and younger adults was affected by setting. Older adults appear to preserve basic cognitive abilities required for successful performance of object-location memory tasks. However, real museum setting appeared to impose higher demands on the older adults.

S100A9 Protein Aggregates Boost Hippocampal Glutamate Modifying Monoaminergic Neurochemistry: A Glutamate Antibody Sensitive Outcome on Alzheimer-like Memory Decline

Alzheimer's disease (AD) involves dementia conceivably arising from integrated inflammatory processes, amyloidogenesis, and neuronal apoptosis. Glutamate can also cause neuronal death via excitotoxicity, and this is similarly implicated in some neurological diseases. The aim was to examine treatment with in vitro generated proinflammatory protein S100A9 aggregate species alone or with glutamate antibodies (Glu-Abs) on Morris water maze (MWM) spatial learning and memory performance in 12 month old mice. Amino acid and monoamine cerebral neurotransmitter metabolic changes were concurrently monitored. Initially, S100A9 fibrils were morphologically verified by atomic force microscopy and Thioflavin T assay. They were then administered intranasally alone or with Glu-Abs for 14 days followed by a 5 day MWM protocol before hippocampal and prefrontal cortical neurochemical analysis. S100A9 aggregates evoked spatial amnesia which correlated with disrupted glutamate and dopaminergic neurochemistry. Hippocampal glutamate release, elevation of DOPAC and HVA, as well as DOPAC/DA and HVA/DA ratios were subsequently reduced by Glu-Abs which simultaneously prevented the spatial memory deficit. The present outcomes emphasized the pathogenic nature of S100A9 fibrillar aggregates in causing spatial memory amnesia associated with enhanced hippocampal glutamate release and DA-ergic disruption in the aging brain. This finding might be exploited during dementia management through a neuroprotective strategy.

The Contribution of the Human Medial Temporal Lobe to Perception: Bridging the Gap between Animal and Human Studies

The medial temporal lobe (MTL) has been considered traditionally to subserve declarative memory processes only. Recent studies in nonhuman primates suggest, however, that the MTL may also be critical to higher order perceptual processes, with the hippocampus and perirhinal cortex being involved in scene and object perception, respectively. The current article reviews the human neuropsychological literature to determine whether there is any evidence to suggest that these same views may apply to the human MTL. Although the majority of existing studies report intact perception following MTL damage in human amnesics, there have been recent studies that suggest that when scene and object perception are assessed systematically, signifi-cant impairments in perception become apparent. These findings have important implications for current mnemonic theories of human MTL function and our understanding of human amnesia as a result of MTL lesions.

The role of CA3 GABAB receptors on anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 with respect to Ca2+ ions

Glutamatergic and GABAergic systems play key roles in the hippocampus and affect the pathogenesis of anxiety- and memory-related processes. Some investigations have assessed the role of balancing the function of these two systems in different areas of the central nervous system (CNS) as an approach to manage the related disorders. We investigated the anxiety and avoidance memory states using the test-retest protocol in the elevated plus maze to understand the role of GABA_B receptors (GABA_BRs) in relation to the NMDA receptor blockade by D-AP5 (an NMDA receptor antagonist). Also, we examined the function of Ca²⁺ ions by blocking its entrance to the cell using SKF96365 (a Ca²⁺ channel blocker). The drugs were injected into the CA3 region before the test. Our data showed that D-AP5 induced anxiolytic-like behaviors and impaired the avoidance memory. Injection of baclofen (a GABA_BR agonist), but not phaclofen (a GABA_BR antagonist) induced anxiolytic-like behaviors. Neither baclofen nor phaclofen altered avoidance memory-related behaviors. When baclofen was injected before D-AP5, it potentiated the anxiolytic-like behaviors induced by D-AP5, but counteracted its effect on avoidance memory. Phaclofen pretreatment attenuated D-AP5-induced anxiolytic-like behaviors, but potentiated its effect on avoidance memory. The effect of baclofen application before D-AP5 on anxiety and phaclofen application before D-AP5 on avoidance memory at the heist doses were accompanied by a decrease in locomotion. The application of SKF96365 did not alter anxiety-like behaviors but induced avoidance memory impairment. SKF96365 application before the combination of baclofen and D-AP5 counteracted the effects produced by the combination of baclofen and D-AP5 on anxiety and memory states. Our findings showed that the CA3 GABA_BRs had a critical role in anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 and confirmed the role of Ca²⁺ ions in the observed results.

Differential effects of negative emotion on memory for items and associations, and their relationship to intrusive imagery

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Negative emotion can affect memory for items and associations differentially.

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Strengthened item memory reflected in increased amygdala activity.

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Weakened contextual/associative memory reflected in reduced hippocampal activity.

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Imbalance between strong negative items and weak contextual associations predicts intrusions.

A crucial aspect of episodic memory formation is the way in which our experiences are stored within a coherent spatio-temporal context. We review research that highlights how the experience of a negative event can alter memory encoding in a complex manner, strengthening negative items but weakening associations with other items and the surrounding context. Recent evidence suggests that these opposing effects can occur through amygdala up-modulation to facilitate item encoding, while the hippocampal provision of contextual binding is down-modulated. We consider how these characteristics of memory for negative events might contribute to the development and maintenance of distressing intrusive imagery in posttraumatic stress disorder, and how they should influence therapeutic interventions.

Medial temporal lobe and topographical memory

There has been interest in the idea that medial temporal lobe (MTL) structures might be especially important for spatial processing and spatial memory. We tested the proposal that the MTL has a specific role in topographical memory as assessed in tasks of scene memory where the viewpoint shifts from study to test. Building on materials used previously for such studies, we administered three different tasks in a total of nine conditions. Participants studied a scene depicting four hills of different shapes and sizes and made a choice among four test images. In the Rotation task, the correct choice depicted the study scene from a shifted perspective. MTL patients succeeded when the study and test images were presented together but failed the moment the study scene was removed (even at a 0-s delay). In the No-Rotation task, the correct choice was a duplicate of the study scene. Patients were impaired to the same extent in the No-Rotation and Rotation tasks after matching for difficulty. Thus, an inability to accommodate changes in viewpoint does not account for patient impairment. In the Nonspatial-Perceptual task, the correct choice depicted the same overall coloring as the study scene. Patients were intact at a 2-s delay but failed at longer, distraction-filled delays. The different results for the spatial and nonspatial tasks are discussed in terms of differences in demand on working memory. We suggest that the difficulty of the spatial tasks rests on the neocortex and on the limitations of working memory, not on the MTL.

The Association between the Mediterranean Dietary Pattern and Cognitive Health: A Systematic Review

The ageing population is accompanied by increased rates of cognitive decline and dementia. Not only does cognitive decline have a profound impact on an individual's health and quality of life, but also on that of their caregivers. The Mediterranean diet (MD) has been known to aid in reducing the risk of cardiovascular diseases, cancer and diabetes. It has been recently linked to better cognitive function in the elderly population. The purpose of this review was to compile evidence based data that examined the effect of adherence to the MD on cognitive function and the risk of developing dementia or Alzheimer's disease. This review followed PRISMA guidelines and was conducted using four databases and resulted in 31 articles of interest. Cross-sectional studies and cohort studies in the non-Mediterranean region showed mixed results. However, cohort studies in the Mediterranean region and randomized controlled trials showed more cohesive outcomes of the beneficial effect of the MD on cognitive function. Although more standardized and in-depth studies are needed to strengthen the existing body of evidence, results from this review indicate that the Mediterranean diet could play a major role in cognitive health and risk of Alzheimer's disease and dementia.

Dopamine receptor activity participates in hippocampal synaptic plasticity associated with novel object recognition

Physiological and behavioral evidence supports that dopamine (DA) receptor signaling influences hippocampal function. While several recent studies examined how DA influences CA1 plasticity and learning, there are fewer studies investigating the influence of DA signaling to the dentate gyrus. The dentate gyrus receives convergent cortical input through the perforant path fiber tracts and has been conceptualized to detect novelty in spatial memory tasks. To test whether DA-receptor activity influences novelty-detection, we used a novel object recognition (NOR) task where mice remember previously presented objects as an indication of learning. Although DA innervation arises from other sources and the main DA signaling may be from those sources, our molecular approaches verified that midbrain dopaminergic fibers also sparsely innervate the dentate gyrus. During the NOR task, wild-type mice spent significantly more time investigating novel objects rather than previously observed objects. Dentate granule cells in slices cut from those mice showed an increased AMPA/NMDA-receptor current ratio indicative of potentiated synaptic transmission. Post-training injection of a D1-like receptor antagonist not only effectively blocked the preference for the novel objects, but also prevented the increased AMPA/NMDA ratio. Consistent with that finding, neither NOR learning nor the increase in the AMPA/NMDA ratio were observed in DA-receptor KO mice under the same experimental conditions. The results indicate that DA-receptor signaling contributes to the successful completion of the NOR task and to the associated synaptic plasticity of the dentate gyrus that likely contributes to the learning.

A comparison of the delayed outward potassium current between the nucleus ambiguus and hippocampus: sensitivity to paeonol

Whole-cell patch-clamp recordings investigated the electrophysiological effects of 2'-hydroxy-4'-methoxyacetophenone (paeonol), one of the major components of Moutan Cortex, in hippocampal CA1 neurons and nucleus ambiguus (NA) neurons from neonatal rats as well as in lung epithelial H1355 cells expressing Kv2.1 or Kv1.2. Extracellular application of paeonol at 100μM did not significantly affect the spontaneous action potential frequency, whereas paeonol at 300μM increased the frequency of spontaneous action potentials in hippocampal CA1 neurons. Paeonol (300μM) significantly decreased the tetraethylammonium-sensitive outward current in hippocampal CA1 neurons, but had no effect upon the fast-inactivating potassium current (IA). Extracellular application of paeonol at 300μM did not affect action potentials or the delayed outward currents in NA neurons. Paeonol (100μM) reduced the Kv2.1 current in H1355 cells, but not the Kv1.2 current. The inhibitor of Kv2, guangxitoxin-1E, reduced the delayed outward potassium currents in hippocampal neurons, but had only minimal effects in NA neurons. We demonstrated that paeonol decreased the delayed outward current and increased excitability in hippocampal CA1 neurons, whereas these effects were not observed in NA neurons. These effects may be associated with the inhibitory effects on Kv2.1 currents.

Hippocampal Volume Reduction in Humans Predicts Impaired Allocentric Spatial Memory in Virtual-Reality Navigation

The extent to which navigational spatial memory depends on hippocampal integrity in humans is not well documented. We investigated allocentric spatial recall using a virtual environment in a group of patients with severe hippocampal damage (SHD), a group of patients with "moderate" hippocampal damage (MHD), and a normal control group. Through four learning blocks with feedback, participants learned the target locations of four different objects in a circular arena. Distal cues were present throughout the experiment to provide orientation. A circular boundary as well as an intra-arena landmark provided spatial reference frames. During a subsequent test phase, recall of all four objects was tested with only the boundary or the landmark being present. Patients with SHD were impaired in both phases of this task. Across groups, performance on both types of spatial recall was highly correlated with memory quotient (MQ), but not with intelligence quotient (IQ), age, or sex. However, both measures of spatial recall separated experimental groups beyond what would be expected based on MQ, a widely used measure of general memory function. Boundary-based and landmark-based spatial recall were both strongly related to bilateral hippocampal volumes, but not to volumes of the thalamus, putamen, pallidum, nucleus accumbens, or caudate nucleus. The results show that boundary-based and landmark-based allocentric spatial recall are similarly impaired in patients with SHD, that both types of recall are impaired beyond that predicted by MQ, and that recall deficits are best explained by a reduction in bilateral hippocampal volumes.

SIGNIFICANCE STATEMENT

In humans, bilateral hippocampal atrophy can lead to profound impairments in episodic memory. Across species, perhaps the most well-established contribution of the hippocampus to memory is not to episodic memory generally but to allocentric spatial memory. However, the extent to which navigational spatial memory depends on hippocampal integrity in humans is not well documented. We investigated spatial recall using a virtual environment in two groups of patients with hippocampal damage (moderate/severe) and a normal control group. The results showed that patients with severe hippocampal damage are impaired in learning and recalling allocentric spatial information. Furthermore, hippocampal volume reduction impaired allocentric navigation beyond what can be predicted by memory quotient as a widely used measure of general memory function.

Extinction learning is slower, weaker and less context specific after alcohol

Alcohol is frequently involved in psychological trauma and often used by individuals to reduce fear and anxiety. We examined the effects of alcohol on fear acquisition and extinction within a virtual environment. Healthy volunteers were administered alcohol (0.4 g/kg) or placebo and underwent acquisition and extinction from different viewpoints of a virtual courtyard, in which the conditioned stimulus, paired with a mild electric shock, was centrally located. Participants returned the following day to test fear recall from both viewpoints of the courtyard. Skin conductance responses were recorded as an index of conditioned fear. Successful fear acquisition under alcohol contrasted with impaired extinction learning evidenced by persistent conditioned responses (Experiment 1). Participants’ impairments in extinction under alcohol correlated with impairments in remembering object-locations in the courtyard seen from one viewpoint when tested from the other viewpoint. Alcohol-induced extinction impairments were overcome by increasing the number of extinction trials (Experiment 2). However, a test of fear recall the next day showed persistent fear in the alcohol group across both viewpoints. Thus, alcohol impaired extinction rather than acquisition of fear, suggesting that extinction is more dependent than acquisition on alcohol-sensitive representations of spatial context. Overall, extinction learning under alcohol was slower, weaker and less context-specific, resulting in persistent fear at test that generalized to the extinction viewpoint. The selective effect on extinction suggests an effect of alcohol on prefrontal involvement, while the reduced context-specificity implicates the hippocampus. These findings have important implications for the use of alcohol by individuals with clinical anxiety disorders.

The development of hippocampal-dependent memory functions: Theoretical comments on Jabès and Nelson review (2015)

Studies investigating the development of memory processes and their neural substrates have flourished over the last two decades. The review by Jabès and Nelson (2015) adds an important piece to our understanding of the maturation of different elements and circuits within the hippocampal system and their association with the progressive development of hippocampal-dependent memory processes in humans. In this accompanying commentary, we explore some additional connections between the nonhuman primate work and the human data, and take the opportunity to highlight some common and additional interpretations of the results. This commentary makes three points: (1) the recognition processes present in the first few days of life may be linked to the early maturation of the medial temporal cortical areas instead of, or in addition to, the early maturation of the subiculum; (2) recent findings on the differential protracted maturation of spatial relational memory processes in monkeys further support the notion proposed by Jabès and Nelson that this protracted development may reflect progressive maturation of the CA1 field of the hippocampus followed by further maturation of CA3/dentate gyrus; (3) finally, further considerations of the differential maturation of the longitudinal hippocampal axis and of the diencephalon are proposed as additional contributors to the refinement of episodic memory functions during development.

Patterns of preserved and impaired spatial memory in a case of developmental amnesia

The hippocampus is believed to have evolved to support allocentric spatial representations of environments as well as the details of personal episodes that occur within them, whereas other brain structures are believed to support complementary egocentric spatial representations. Studies of patients with adult-onset lesions lend support to these distinctions for newly encountered places but suggest that with time and/or experience, schematic aspects of environments can exist independent of the hippocampus. Less clear is the quality of spatial memories acquired in individuals with impaired episodic memory in the context of a hippocampal system that did not develop normally. Here we describe a detailed investigation of the integrity of spatial representations of environments navigated repeatedly over many years in the rare case of H.C., a person with congenital absence of the mammillary bodies and abnormal hippocampal and fornix development. H.C. and controls who had extensive experience navigating the residential and downtown areas known to H.C. were tested on mental navigation tasks that assess the identity, location, and spatial relations among landmarks, and the ability to represent routes. H.C. was able to represent distances and directions between familiar landmarks and provide accurate, though inefficient, route descriptions. However, difficulties producing detailed spatial features on maps and accurately ordering more than two landmarks that are in close proximity to one another along a route suggest a spatial representation that includes only coarse, schematic information that lacks coherence and that cannot be used flexibly. This pattern of performance is considered in the context of other areas of preservation and impairment exhibited by H.C. and suggests that the allocentric-egocentric dichotomy with respect to hippocampal and extended hippocampal system function may need to be reconsidered.

Impaired allocentric spatial processing in posttraumatic stress disorder

A neurobiological dual representation model of PTSD proposes that reduced hippocampus-dependent contextual processing contributes to intrusive imagery due to a loss of control over hippocampus-independent sensory and affective representations. We investigated whether PTSD sufferers show impaired allocentric spatial processing indicative of reduced hippocampal functioning. Trauma-exposed individuals with (N=29) and without (N=30) a diagnosis of PTSD completed two tests of spatial processing: a topographical recognition task comprising perceptual and memory components, and a test of memory for objects' locations within a virtual environment in which the test is from either the same viewpoint as presentation (solvable with egocentric memory) or a different viewpoint (requiring allocentric memory). Participants in the PTSD group performed significantly worse on allocentric spatial processing than trauma-exposed controls. Groups performed comparably on egocentric memory and non-spatial memory for lists of objects. Exposure to repeated incident trauma was also associated with significantly worse spatial processing in the PTSD group. Results show a selective impairment in allocentric spatial processing, implicating weak hippocampal functioning, as predicted by a neurobiological dual representation model of PTSD. These findings have important clinical implications for cognitive therapy.

Deficits in egocentric-updating and spatial context memory in a case of developmental amnesia

Patients with developmental amnesia usually suffer from both episodic and spatial memory deficits. DM, a developmental amnesic, was impaired in her ability to process self-motion (i.e., idiothetic) information while her ability to process external stable landmarks (i.e., allothetic) was preserved when no self-motion processing was required. On a naturalistic and incidental episodic task, DM was severely and predictably impaired on both free and cued recall tasks. Interestingly, when cued, she was more impaired at recalling spatial context than factual or temporal information. Theoretical implications of that co-occurrence of deficits and those dissociations are discussed and testable cerebral hypothesis are proposed.

Preschool children's proto-episodic memory assessed by deferred imitation

In two experiments, both employing deferred imitation, we studied the developmental origins of episodic memory in two- to three-year-old children by adopting a "minimalist" view of episodic memory based on its What-When-Where ("WWW": spatiotemporal plus semantic) content. We argued that the temporal element within spatiotemporal should be the order/simultaneity of the event elements, but that it is not clear whether the spatial content should be egocentric or allocentric. We also argued that episodic recollection should be configural (tending towards all-or-nothing recall of the WWW elements). Our first deferred imitation experiment, using a two-dimensional (2D) display, produced superior-to-chance performance after 2.5 years but no evidence of configural memory. Moreover, performance did not differ from that on a What-What-What control task. Our second deferred imitation study required the children to reproduce actions on an object in a room, thereby affording layout-based spatial cues. In this case, not only was there superior-to-chance performance after 2.5 years but memory was also configural at both ages. We discuss the importance of allocentric spatial cues in episodic recall in early proto-episodic memory and reflect on the possible role of hippocampal development in this process.

Contextualisation in the revised dual representation theory of PTSD: a response to Pearson and colleagues

Three recent studies (Pearson, 2012; Pearson, Ross, & Webster, 2012) purported to test the revised dual representation theory of posttraumatic stress disorder (Brewin, Gregory, Lipton, & Burgess, 2010) by manipulating the amount of additional information accompanying traumatic stimulus materials and assessing the effect on subsequent intrusive memories. Here we point out that these studies involve a misunderstanding of the meaning of "contextual" within the theory, such that the manipulation would be unlikely to have had the intended effect and the results are ambiguous with respect to the theory. Past and future experimental tests of the theory are discussed.

Development of memory for spatial locations and object/place associations in infant rhesus macaques with and without neonatal hippocampal lesions

This study traces the development of spatial memory abilities in monkeys and reports the effects of selective neonatal hippocampal lesions on performance across development. Two different versions of the visual paired-comparison (VPC) task were used. The VPC-Spatial-Location task tested memory for object-locations that could be solved using an egocentric spatial frame of reference and the VPC-Object-In-Place task taxed memory for spatial relations using an allocentric reference frame. Eleven rhesus macaques (6 neonatal sham-operated controls and 5 with neonatal neurotoxic hippocampal lesions) were tested on both tasks as infants (8 months), juveniles (18 months), and adults (5-6 years). Memory for spatial locations was present by 18 months of age, whereas memory for object-place relations was present only in adulthood. Also, neonatal hippocampal lesions delayed the emergence of memory for spatial locations and abolished memory for object-place associations, particularly in animals that had sustained extensive and bilateral hippocampal lesions. The differential developmental time course of spatial memory functions and of the effects of neonatal hippocampal lesions on these functions are discussed in relation to morphological maturation of the medial temporal lobe structures in monkeys. Implications of the findings for the neural basis of spatial memory development in humans are also considered.

Neonatal hippocampal damage impairs specific food/place associations in adult macaques

This study describes a novel spatial memory paradigm for monkeys and reports the effects of neonatal damage to the hippocampus on performance in adulthood. Monkeys were trained to forage in eight boxes hung on the walls of a large enclosure. Each box contained a different food item that varied in its intrinsic reward value, as determined from food preference testing. Monkeys were trained on a spatial and a cued version of the task. In the spatial task, the boxes looked identical and remained fixed in location whereas in the cued task, the boxes were individuated with colored plaques and changed location on each trial. Ten adult Rhesus macaques (5 neonatal sham-operated and 5 with neonatal neurotoxic hippocampal lesions) were allowed to forage once daily until they preferentially visited boxes containing preferred foods. The data suggest that all monkeys learned to discriminate preferred from nonpreferred food locations, but that monkeys with neonatal hippocampal damage committed significantly more working memory errors than controls in both tasks. Furthermore, following selective satiation, controls altered their foraging pattern to avoid the satiated food, whereas lesioned animals did not, suggesting that neonatal hippocampal lesions prohibit learning of specific food-place associations. We conclude that whereas an intact hippocampus is necessary to form specific item-in-place associations, in its absence, cortical areas may support more broad distinctions between food types that allow monkeys to discriminate places containing highly preferred foods.

Serial position functions following selective hippocampal lesions in monkeys: effects of delays and interference

We examined the role of the hippocampus in list-memory processing. Three rhesus monkeys that had extensive experience in this task and had demonstrated full abstract-concept learning and excellent list memory performance (Katz et al., 2002; Wright et al., 2003) received bilateral neurotoxic hippocampal lesions and were re-tested in the serial list memory task. Effects of delays on memory performance were measured in all monkeys, whereas the effects of proactive interference were assessed in only one. Despite a slight change in performance of one of the three animals during re-learning of the same/different task, selective hippocampal damage had little or no effects on list memory accuracy. In addition, the hippocampal damage did not impact serial list position functions (SPFs) but slightly altered the dynamic of the SPF curves. Finally, even more remarkable was that accurate memory performance of one animal remained intact despite the use of small set size of 8 items that created high proactive interference across lists thereby eliminating any use of familiarity judgments to support performance. Together the findings indicate that, with short list items and extensive training in the task (i.e., reference memory), monkeys with selective hippocampal lesions may be able to use alternative memory processes (i.e., working memory) that are mediated by structures other than the hippocampus.

Contextual memory, psychosis-proneness, and the experience of intrusive imagery

This study tested the hypothesis that the presence and characteristics of naturally occurring involuntary imagery would be related to poorer context-dependent spatial memory and higher levels of proneness to psychotic experiences. Poorer contextual memory was also predicted to be associated with a greater sense of "nowness". Participants completed a virtual environment task that assessed contextual memory through responses that required allocentric and egocentric processing of virtual stimuli. Two questionnaires assessing predisposition to psychotic experiences were employed. Finally, participants completed an interview that required details of recent, naturally occurring involuntary images. Reports of involuntary imagery were associated with greater proneness to psychotic experiences but not with memory. In those participants who reported imagery, however, poorer memory performance was associated with more vivid and detailed intrusive imagery. Poorer contextual memory was specifically associated with a greater sense of "nowness". Possible links between contextual memory and proneness to psychosis are discussed.

The hippocampus and spatial constraints on mental imagery

We review a model of imagery and memory retrieval based on allocentric spatial representation by place cells and boundary vector cells (BVCs) in the medial temporal lobe, and their translation into egocentric images in retrosplenial and parietal areas. In this model, the activity of place cells constrain the contents of imagery and retrieval to be coherent and consistent with the subject occupying a single location, while the activity of head-direction cells along Papez's circuit determine the viewpoint direction for which the egocentric image is generated. An extension of this model is discussed in which a role for grid cells in dynamic updating of representations (mental navigation) is included. We also discuss the extension of this model to implement a version of the dual representation theory of post-traumatic stress disorder (PTSD) in which PTSD arises from an imbalance between weak allocentric hippocampal-mediated contextual representations and strong affective/sensory representations. The implications of these models for behavioral, neuropsychological, and neuroimaging data in humans are explored.

When do objects become landmarks? A VR study of the effect of task relevance on spatial memory

We investigated how objects come to serve as landmarks in spatial memory, and more specifically how they form part of an allocentric cognitive map. Participants performing a virtual driving task incidentally learned the layout of a virtual town and locations of objects in that town. They were subsequently tested on their spatial and recognition memory for the objects. To assess whether the objects were encoded allocentrically we examined pointing consistency across tested viewpoints. In three experiments, we found that spatial memory for objects at navigationally relevant locations was more consistent across tested viewpoints, particularly when participants had more limited experience of the environment. When participants' attention was focused on the appearance of objects, the navigational relevance effect was eliminated, whereas when their attention was focused on objects' locations, this effect was enhanced, supporting the hypothesis that when objects are processed in the service of navigation, rather than merely being viewed as objects, they engage qualitatively distinct attentional systems and are incorporated into an allocentric spatial representation. The results are consistent with evidence from the neuroimaging literature that when objects are relevant to navigation, they not only engage the ventral "object processing stream", but also the dorsal stream and medial temporal lobe memory system classically associated with allocentric spatial memory.

The hippocampus and visual perception

In this review, we will discuss the idea that the hippocampus may be involved in both memory and perception, contrary to theories that posit functional and neuroanatomical segregation of these processes. This suggestion is based on a number of recent neuropsychological and functional neuroimaging studies that have demonstrated that the hippocampus is involved in the visual discrimination of complex spatial scene stimuli. We argue that these findings cannot be explained by long-term memory or working memory processing or, in the case of patient findings, dysfunction beyond the medial temporal lobe (MTL). Instead, these studies point toward a role for the hippocampus in higher-order spatial perception. We suggest that the hippocampus processes complex conjunctions of spatial features, and that it may be more appropriate to consider the representations for which this structure is critical, rather than the cognitive processes that it mediates.

Effects of selective neonatal hippocampal lesions on tests of object and spatial recognition memory in monkeys

Earlier studies in monkeys have reported mild impairment in recognition memory after nonselective neonatal hippocampal lesions. To assess whether the memory impairment could have resulted from damage to cortical areas adjacent to the hippocampus, we tested adult monkeys with neonatal focal hippocampal lesions and sham-operated controls in three recognition tasks: delayed nonmatching-to-sample, object memory span, and spatial memory span. Further, to rule out that normal performance on these tasks may relate to functional sparing following neonatal hippocampal lesions, we tested adult monkeys that had received the same focal hippocampal lesions in adulthood and their controls in the same three memory tasks. Both early and late onset focal hippocampal damage did not alter performance on any of the three tasks, suggesting that damage to cortical areas adjacent to the hippocampus was likely responsible for the recognition impairment reported by the earlier studies. In addition, given that animals with early and late onset hippocampal lesions showed object and spatial recognition impairment when tested in a visual paired comparison task, the data suggest that not all object and spatial recognition tasks are solved by hippocampal-dependent memory processes. The current data may not only help explain the neural substrate for the partial recognition memory impairment reported in cases of developmental amnesia, but they are also clinically relevant given that the object and spatial memory tasks used in monkeys are often translated to investigate memory functions in several populations of human infants and children in which dysfunction of the hippocampus is suspected.

Spatial and temporal aspects of navigation in two neurological patients

We present two cases (A.C. and W.J.) with navigation problems resulting from parieto-occipital right hemisphere damage. For both the cases, performance on the neuropsychological tests did not indicate specific impairments in spatial processing, despite severe subjective complaints of spatial disorientation. Various aspects of navigation were tested in a new virtual reality task, the Virtual Tübingen task. A double dissociation between spatial and temporal deficits was found; A.C. was impaired in route ordering, a temporal test, whereas W.J. was impaired in scene recognition and route continuation, which are spatial in nature. These findings offer important insights in the functional and neural architecture of navigation.

Acute effects of alcohol on intrusive memory development and viewpoint dependence in spatial memory support a dual representation model

BACKGROUND

A dual representation model of intrusive memory proposes that personally experienced events give rise to two types of representation: an image-based, egocentric representation based on sensory-perceptual features; and a more abstract, allocentric representation that incorporates spatiotemporal context. The model proposes that intrusions reflect involuntary reactivation of egocentric representations in the absence of a corresponding allocentric representation. We tested the model by investigating the effect of alcohol on intrusive memories and, concurrently, on egocentric and allocentric spatial memory.

METHODS

With a double-blind independent group design participants were administered alcohol (.4 or .8 g/kg) or placebo. A virtual environment was used to present objects and test recognition memory from the same viewpoint as presentation (tapping egocentric memory) or a shifted viewpoint (tapping allocentric memory). Participants were also exposed to a trauma video and required to detail intrusive memories for 7 days, after which explicit memory was assessed.

RESULTS

There was a selective impairment of shifted-view recognition after the low dose of alcohol, whereas the high dose induced a global impairment in same-view and shifted-view conditions. Alcohol showed a dose-dependent inverted "U"-shaped effect on intrusions, with only the low dose increasing the number of intrusions, replicating previous work. When same-view recognition was intact, decrements in shifted-view recognition were associated with increases in intrusions.

CONCLUSIONS

The differential effect of alcohol on intrusive memories and on same/shifted-view recognition support a dual representation model in which intrusions might reflect an imbalance between two types of memory representation. These findings highlight important clinical implications, given alcohol's involvement in real-life trauma.

Spatial deficits in a virtual water maze in amnesic participants with hippocampal damage

The Morris water maze is a standard paradigm for the testing of hippocampal function in laboratory animals. Virtual versions of the Morris water maze are now available and can be used to assess spatial learning and memory ability in both healthy and brain injured participants. To evaluate the importance of the hippocampus in spatial learning and memory, we tested five amnesic participants with selective hippocampal damage using a virtual water maze called the Arena Maze. The amnesic participants with hippocampal damage were impaired on the invisible platform (place) task that required them to use distal cues, but were able to navigate almost as well as comparison participants when the invisible platform was marked by a single proximal cue. These results not only confirm that the hippocampus plays a necessary role in human navigation in large-scale environments but also provides a new link between the mnemonic and navigational roles of the hippocampus.

Everyday memory deficits in very mild Alzheimer's disease

Memory complaints of patients sometimes are not verified via standard cognitive testing. Acquisition of information in everyday life requires memorization in complex three-dimensional environments. The authors mimicked this with a photorealistic virtual environment (VE). Memory for verbal material and spatial scenery was tested in healthy controls (HC) and patients with mild Alzheimer's disease (AD); mini-mental state evaluation (MMSE) 25.7 ± 1.8 (mean ± standard deviation). The number of memorized items increased to 90% in both classical list learning and for items memorized in VE in HC. In contrast, only 40% of items were recalled in list learning and 20% in VE in AD patients. Unlike the gender difference favoring female HC on list learning, performance was alike for both genders in VE. We conclude that verbal learning abilities in healthy elderly subjects are alike in standard settings and under virtual reality conditions. In AD patients memory deficits that are relevant to everyday life yet not detectable with list learning are unmasked in virtual reality. In future, this may aid objective appraisal of interventions with regard to their everyday relevance.

Going beyond LTM in the MTL: a synthesis of neuropsychological and neuroimaging findings on the role of the medial temporal lobe in memory and perception

Studies in rats and non-human primates suggest that medial temporal lobe (MTL) structures play a role in perceptual processing, with the hippocampus necessary for spatial discrimination, and the perirhinal cortex for object discrimination. Until recently, there was little convergent evidence for analogous functional specialisation in humans, or for a role of the MTL in processes beyond long-term memory. A recent series of novel human neuropsychological studies, however, in which paradigms from the animal literature were adapted and extended, have revealed findings remarkably similar to those seen in rats and monkeys. These experiments have demonstrated differential effects of distinct stimulus categories on performance in tasks for which there was no explicit requirement to remember information across trials. There is also accruing complementary evidence from functional neuroimaging that MTL structures show differential patterns of activation for scenes and objects, even on simple visual discrimination tasks. This article reviews some of these key studies and discusses the implications of these new findings for existing accounts of memory. A non-modular view of memory is proposed in which memory and perception depend upon the same anatomically distributed representations (emergent memory account). The limitations and criticisms of this theory are discussed and a number of outstanding questions proposed, including key predictions that can be tested by future studies.

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).