The human hippocampus and spatial and episodic memory

A similar impairment in CA3 mossy fibre LTP in the R6/2 mouse model of Huntington's disease and in the complexin II knockout mouse

Complexin II is reduced in Huntington's disease (HD) patients and in the R6/2 mouse model of HD. Mice lacking complexin II (Cplx2-/- mice) show selective cognitive deficits that reflect those seen in R6/2 mice. To determine whether or not there is a common mechanism that might underlie the cognitive deficits, long-term potentiation (LTP) was examined in the CA3 region of hippocampal slices from R6/2 mice and Cplx2-/- mice. While associational/commissural (A/C) LTP was not significantly different, mossy fibre (MF) LTP was significantly reduced in slices from R6/2 mice and Cplx2-/- mice compared with wild-type (WT) and Cplx2+/+ control mice. MF field excitatory postsynaptic potentials (fEPSPs) in response to paired stimuli were not significantly different between control mice and R6/2 or Cplx2-/- mice, suggesting that MF basal glutamate release is unaffected. Forskolin (30 microm) caused an increase in glutamate release at MF synapses in slices from R6/2 mice and from Cplx2-/- mice that was not significantly different from that seen in control mice, indicating that the capacity for increased glutamate release is not diminished. Thus, R6/2 mice and Cplx2-/- mice have a common selective impairment of MF LTP in the CA3 region. Together, these data suggest that complexin II is required for MF LTP, and that depletion of complexin II causes a selective impairment in MF LTP in the CA3 region. This impairment in MF LTP could contribute to spatial learning deficits observed in R6/2 and Cplx2-/- mice.

Deficits in spatial orientation of children with intrauterine growth retardation

The spatial orientation of intrauterine growth retarded (IUGR) children versus age-matched controls was examined using two spatial tests. The first test was the radial arm maze (RAM), a navigational test frequently used in animal models. The second test was a subtest from the Kaufman assessment battery for children (K-ABC). The IUGR group comprised 28 children aged 6 years. The control group comprised 29 appropriate-for-gestational age children. The performance of the IUGR children was significantly inferior to controls in both tests. In the RAM test, the ratio between the correct entrances to the total entrances was significantly lower in the IUGR group than in the control group (P<0.001). In the K-ABC, the IUGR group could not perform as well as control children (P<0.001). These results suggest that spatial orientation in IUGR children is inferior to their age-matched controls, possibly contributing to their potential learning difficulties. The present results also suggest that the RAM can be potentially used to test spatial orientation of children at-risk.

Hippocampal activation during processing of previously seen visual stimulus pairs

Activity in the hippocampus is modulated by novelty detection, and by the processing of conjunctions between two stimuli. We investigated whether the hippocampus is activated by discrimination of stimulus-stimulus relationships in novel versus familiar pairs of visual stimuli in 15 healthy subjects using functional magnetic resonance imaging. Subjects were asked to recognize the previously rewarded stimulus in each case. We found significantly greater activation of the right hippocampus when discriminating previously seen compared with novel pairs of visual stimuli. This activation was evident in individual subjects and was not related to stimulus novelty, reward contingency, or task instruction. Right hippocampal activation during discrimination of previously seen pairs of objects was correlated with activity in the anteromedial thalamus, cingulate cortex, and contralateral hippocampus.

Neuroprotective and abstinence-promoting effects of acamprosate: elucidating the mechanism of action

Acamprosate is an abstinence-promoting drug widely used in the treatment of alcohol dependence but which has a mechanism of action that has remained obscure for many years. Recently, evidence has emerged that this drug may interact with excitatory glutamatergic neurotransmission in general and as an antagonist of the metabotropic glutamate receptor subtype 5 (mGluR5) in particular. These findings provide, for the first time, a satisfactory, unifying hypothesis that can bring together and explain the diverse neurochemical effects of acamprosate. Glutamic acid is involved in several aspects of alcohol dependence and withdrawal, many of which can be modified by acamprosate. For example, during chronic exposure to alcohol, the glutamatergic system becomes upregulated, leaving the brain exposed to excessive glutamatergic activity when alcohol is abruptly withdrawn. The surge in glutamic acid release that occurs following alcohol withdrawal can be attenuated by acamprosate. The elevated extracellular levels of glutamic acid observed in withdrawal, together with supersensitivity of NMDA receptors, may expose vulnerable neurons to excitotoxicity, possibly contributing to the neuronal loss sometimes observed in chronic alcohol dependence. In vitro studies suggest that the excitotoxicity produced by ethanol can effectively be blocked by acamprosate. Moreover, glutamatergic neurotransmission plays an important role in the acquisition of cue-elicited drinking behaviours, which again can be modulated by acamprosate. In conclusion, the glutamatergic hypothesis of the mechanism of action of acamprosate helps explain many of its effects in human alcohol dependence and points the way to potential new activities, such as neuroprotection, that merit exploration in the clinic.

Visuospatial Memory Computations During Whole-Body Rotations in Roll

We used a memory-saccade task to test whether the location of a target, briefly presented before a whole-body rotation in roll, is stored in egocentric or in allocentric coordinates. To make this distinction, we exploited the fact that subjects, when tilted sideways in darkness, make systematic errors when indicating the direction of gravity (an allocentric task) even though they have a veridical percept of their self-orientation in space. We hypothesized that if spatial memory is coded allocentrically, these distortions affect the coding of remembered targets and their readout after a body rotation. Alternatively, if coding is egocentric, updating for body rotation becomes essential and errors in performance should be related to the amount of intervening rotation. Subjects ( n = 6) were tested making saccades to remembered world-fixed targets after passive body tilts. Initial and final tilt angle ranged between −120° CCW and 120° CW. The results showed that subjects made large systematic directional errors in their saccades (up to 90°). These errors did not occur in the absence of intervening body rotation, ruling out a memory degradation effect. Regression analysis showed that the errors were closely related to the amount of subjective allocentric distortion at both the initial and final tilt angle, rather than to the amount of intervening rotation. We conclude that the brain uses an allocentric reference frame, possibly gravity-based, to code visuospatial memories during whole-body tilts. This supports the notion that the brain can define information in multiple frames of reference, depending on sensory inputs and task demands.

Theta-modulated place-by-direction cells in the hippocampal formation in the rat

We report the spatial and temporal properties of a class of cells termed theta-modulated place-by-direction (TPD) cells recorded from the presubicular and parasubicular cortices of the rat. The firing characteristics of TPD cells in open-field enclosures were compared with those of the following two other well characterized cell classes in the hippocampal formation: place and head-direction cells. Unlike place cells, which code only for the animal's location, or head-direction cells, which code only for the animal's directional heading, TPD cells code for both the location and the head direction of the animal. Their firing is also strongly theta modulated, firing primarily at the negative-to-positive phase of the locally recorded theta wave. TPD theta modulation is significantly stronger than that of place cells. In contrast, the firing of head-direction cells is not modulated by theta at all. In repeated exposures to the same environment, the locational and directional signals of TPD cells are stable. When recorded in different environments, TPD locational and directional fields can uncouple, with the locational field shifting unpredictably ("remapping"), whereas the directional preference remains similar across environments.

Alzheimer's disease behaviors from past self-identities: an exploration of the memory and cognitive features

Alzheimer's disease (AD) patients have been reported by caregivers to display "behaviors from past self-identities " (BPSI); however, there is little known about these distinct behaviors. This study, the first to explore BPSI, hypothesized that BPSI were associated with self-memory and cognitive impairments. Its purpose was to determine if AD subjects with and without BPSI differed on measures of autobiographical memory, selective attention, and fluency. The cross-sectional design compared 35 moderate-stage AD subjects from an AD research center. Subjects demonstrating BPSI (37 percent) recalled significantly fewer recent autobiographical memories than AD subjects without BPSI. The results establish BPSI as a common behavior among moderate-stage AD patients and suggest that paucity of recent self-memories contributes to BPSI.

Functional MRI of galvanic vestibular stimulation with alternating currents at different frequencies

Functional MRI was performed in 28 healthy volunteers to study the effects of galvanic vestibular stimulation with alternating currents (AC-GVS) of different frequencies on brain activation patterns. The aims of this study were (1) to identify specific areas within the vestibular cortical network that are involved in the processing of frequency-specific aspects by correlation analyses, (2) to determine the optimal frequency for stimulation of the vestibular system with respect to perception, and (3) to analyze whether different frequencies of AC-GVS are mediated in different cortical areas or different sites within the vestibular cortex. AC-GVS was performed using sinusoidal stimulation currents with an amplitude of +/-2.5 mA, and frequencies of 0.1 Hz, 0.3 Hz, 0.8 Hz, 1.0 Hz, 2.0 Hz, and 5.0 Hz were applied. Regardless of the applied stimulation frequency, AC-GVS elicited activations within a network of multisensory areas similar to those described in earlier studies using direct currents. No mapping of different stimulation frequencies to different cortical locations was observed. Additional activations of somatosensory cortex areas were observed during stimulation with 5 Hz only. The strongest vestibular sensations were reported during stimulation with 1 Hz and 2 Hz. Correlation analyses between blood oxygenation level dependent (BOLD) signal changes and stimulation frequency revealed a positive dependency in areas of the supramarginal gyrus, posterolateral thalamus, cerebellar vermis, posterior insula, and in the hippocampal region/uncus. These regions represent areas involved in the processing of vestibular information for head and body orientation in space.

Functional neuroanatomy of remote episodic, semantic and spatial memory: a unified account based on multiple trace theory

We review lesion and neuroimaging evidence on the role of the hippocampus, and other structures, in retention and retrieval of recent and remote memories. We examine episodic, semantic and spatial memory, and show that important distinctions exist among different types of these memories and the structures that mediate them. We argue that retention and retrieval of detailed, vivid autobiographical memories depend on the hippocampal system no matter how long ago they were acquired. Semantic memories, on the other hand, benefit from hippocampal contribution for some time before they can be retrieved independently of the hippocampus. Even semantic memories, however, can have episodic elements associated with them that continue to depend on the hippocampus. Likewise, we distinguish between experientially detailed spatial memories (akin to episodic memory) and more schematic memories (akin to semantic memory) that are sufficient for navigation but not for re-experiencing the environment in which they were acquired. Like their episodic and semantic counterparts, the former type of spatial memory is dependent on the hippocampus no matter how long ago it was acquired, whereas the latter can survive independently of the hippocampus and is represented in extra-hippocampal structures. In short, the evidence reviewed suggests strongly that the function of the hippocampus (and possibly that of related limbic structures) is to help encode, retain, and retrieve experiences, no matter how long ago the events comprising the experience occurred, and no matter whether the memories are episodic or spatial. We conclude that the evidence favours a multiple trace theory (MTT) of memory over two other models: (1) traditional consolidation models which posit that the hippocampus is a time-limited memory structure for all forms of memory; and (2) versions of cognitive map theory which posit that the hippocampus is needed for representing all forms of allocentric space in memory.

Cortical areas involved in object, background, and object-background processing revealed with functional magnetic resonance adaptation

Previous work has suggested that object and place processing are neuroanatomically dissociated in ventral visual areas under conditions of passive viewing. It has also been shown that the hippocampus and parahippocampal gyrus mediate the integration of objects with background scenes in functional imaging studies, but only when encoding or retrieval processes have been directed toward the relevant stimuli. Using functional magnetic resonance adaptation, we demonstrated that object, background scene, and contextual integration of selectively repeated objects and background scenes could be dissociated during the passive viewing of naturalistic pictures involving object-scene pairings. Specifically, bilateral fusiform areas showed adaptation to object repetition, regardless of whether the associated scene was novel or repeated, suggesting sensitivity to object processing. Bilateral parahippocampal regions showed adaptation to background scene repetition, regardless of whether the focal object was novel or repeated, suggesting selectivity for background scene processing. Finally, bilateral parahippocampal regions distinct from those involved in scene processing and the right hippocampus showed adaptation only when the unique pairing of object with background scene was repeated, suggesting that these regions perform binding operations.

Entorhinal cortex lesions disrupt the relational organization of memory in monkeys

Recent accounts suggest that the hippocampal system critically supports two central characteristics of episodic memory: the ability to establish and maintain representations for the salient relationships between experienced events (relational representation) and the capacity to flexibly manipulate memory (flexible memory expression). To test this proposal in monkeys, intact controls and subjects with bilateral aspiration lesions of the entorhinal cortex were trained postoperatively on two standard memory tasks, delayed nonmatchingto-sample (DNMS) and two-choice object discrimination (OD) learning, and three procedures intended to emphasize relational representation and flexible memory expression: a paired associate (PA) task, a transitive inference (TI) test of learning and memory for hierarchical stimulus relationships, and a spatial delayed recognition span (SDRS) procedure. The latter assessments each included critical "probe" tests that asked monkeys to evaluate the relationships among previously learned stimuli presented in novel combinations. Subjects with entorhinal cortex lesions scored as accurately as controls on all phases of DNMS and OD, procedures that can be solved on the basis of memory for individual stimuli. In contrast, experimental monkeys displayed deficits relative to controls on all phases of the PA, TI, and SDRS tasks that emphasized the flexible manipulation of memory for the relationships between familiar items. Together, the findings support the conclusion that the primate hippocampal system critically enables the relational organization of declarative memory.

Functional, structural, and metabolic abnormalities of the hippocampal formation in Williams syndrome

Williams syndrome (WS), caused by microdeletion of some 21 genes on chromosome 7q11.23, is characterized by dysmorphic features, mental retardation or learning difficulties, elastin arteriopathy, and striking neurocognitive and social-behavioral abnormalities. Recent studies of murine knockouts of key genes in the microdeleted region, LIM kinase 1 (LIMK1) and cytoplasmatic linker protein 2 (CYLN2), demonstrated significant functional and metabolic abnormalities, but grossly normal structure, in the hippocampal formation (HF). Furthermore, deficits in spatial navigation and long-term memory, major cognitive domains dependent on hippocampal function, have been described in WS. We used multimodal neuroimaging to characterize hippocampal structure, function, and metabolic integrity in 12 participants with WS and 12 age-, sex-, and IQ-matched healthy controls. PET and functional MRI studies showed profound reduction in resting blood flow and absent differential response to visual stimuli in the anterior HF in WS. Spectroscopic measures of N-acetyl aspartate, considered a marker of synaptic activity, were reduced. Hippocampal size was preserved, but subtle alterations in shape were present. These data demonstrate abnormalities in HF in WS in agreement with murine models, implicate LIMK1 and CYLN2 in human hippocampal function, and suggest that hippocampal dysfunction may contribute to neurocognitive abnormalities in WS.

Effects of apolipoprotein E genotype on spatial attention, working memory, and their interaction in healthy, middle-aged adults: results From the National Institute of Mental Health's BIOCARD study

The cognitive consequences of the apolipoprotein E-epsilon4 (APOE-epsilon4) allele were examined in middle age, before likely onset of symptoms of Alzheimer's disease. The authors identified 3 cognitive processes--visuospatial attention, spatial working memory, and the effect of visuospatial attention on working memory--and devised "behavioral assays" of the integrity of components of these processes. Redirecting visuospatial attention, retention of memory for location, and attentional modulation of memory of target location were affected by APOE genotype. Visuospatial attention showed additive effects of epsilon4 gene dose; each additional epsilon4 allele inherited further slowed disengagement from invalidly cued space. In contrast, working memory performance was affected only in epsilon4 homozygotes. Effect sizes for the APOE gene were moderate to large, ranging from 14% to 24%. Effects of APOE genotype on component processes of cognition in healthy, middle-aged adults is consistent with the emergence in adulthood of an APOE-epsilon4 cognitive phenotype.

Dissociable contributions within the medial temporal lobe to encoding of object-location associations

The crucial role of the medial temporal lobe (MTL) in episodic memory is well established. Although there is little doubt that its anatomical subregions-the hippocampus, peri-, entorhinal and parahippocampal cortex (PHC)-contribute differentially to mnemonic processes, their specific functions in episodic memory are under debate. Data from animal, human lesion, and neuroimaging studies suggest somewhat contradictory perspectives on this functional specialization: a general participation in declarative memory, an exclusive involvement in associative mnemonic processes, and a specific contribution to spatial memory are reported for the hippocampus, adjacent cortices, and the PHC. A functional lateralization in humans dependent on the verbalizability of the material is also discussed herein. To further elucidate the differential contributions of the various MTL subregions to encoding, we employed an object-location association memory paradigm. The memory for each of the studied associations was tested twice: by the object, and by the location serving as retrieval cue. The memory accuracy in response to both cue types was also assessed parametrically. Brain activity during encoding which leads to different degrees of subsequent memory accuracy under the two retrieval conditions was compared. We found the bilateral posterior PHC to participate in encoding of both the object associated with a location and the location associated with an object. In contrast, activity in an area in the left anterior PHC and the right anterior MTL was only correlated with the memory for the location associated with an object.

Growth hormone prevents neuronal loss in the aged rat hippocampus

Decline of growth hormone (GH) with aging is associated to memory and cognitive alterations. In this study, the number of neurons in the hilus of the dentate gyrus has been assessed in male and female Wistar rats at 3, 6, 12, 14, 18, 22 and 24 months of age, using the optical fractionator method. Male rats had more neurons than females at all the ages studied. Significant neuronal loss was observed in both sexes between 22 and 24 months of age. In a second experiment, 22 month-old male and female rats were treated for 10 weeks with 2 mg/kg/day of GH or saline. At 24 months of age, animals treated with GH had more neurons in the hilus than animals treated with saline. These findings indicate that GH is neuroprotective in old animals and that its administration may ameliorate neuronal alterations associated to aging.

Analysis of long-term gene expression in neurons of the hippocampal subfields following traumatic brain injury in rats

After experimental traumatic brain injury (TBI), widespread neuronal loss is progressive and continues in selectively vulnerable brain regions, such as the hippocampus, for months to years after the initial insult. To clarify the molecular mechanisms underlying secondary or delayed cell death in hippocampal neurons after TBI, we compared long-term changes in gene expression in the CA1, CA3 and dentate gyrus (DG) subfields of the rat hippocampus at 24 h and 3, 6, and 12 months after TBI with changes in gene expression in sham-operated rats. We used laser capture microdissection to collect several hundred hippocampal neurons from the CA1, CA3, and DG subfields and linearly amplified the nanogram samples of neuronal RNA with T7 RNA polymerase. Subsequent quantitative analysis of gene expression using ribonuclease protection assay revealed that mRNA expression of the anti-apoptotic gene, Bcl-2, and the chaperone heat shock protein 70 was significantly downregulated at 3, 6 (Bcl-2 only), and 12 months after TBI. Interestingly, the expression of the pro-apoptotic genes caspase-3 and caspase-9 was also significantly decreased at 3, 6 (caspase-9 only), and 12 months after TBI, suggesting that long-term neuronal loss after TBI is not mediated by increased expression of pro-apoptotic genes. The expression of two aging-related genes, p21 and integrin beta3 (ITbeta3), transiently increased 24 h after TBI, returned to baseline levels at 3 months and significantly decreased below sham levels at 12 months (ITbeta3 only). Expression of the gene for the antioxidant glutathione peroxidase-1 also significantly increased 6 months after TBI. These results suggest that decreased levels of neuroprotective genes may contribute to long-term neurodegeneration in animals and human patients after TBI. Conversely, long-term increases in antioxidant gene expression after TBI may be an endogenous neuroprotective response that compensates for the decrease in expression of other neuroprotective genes.

Complementary memory systems: competition, cooperation and compensation

Spatial navigation depends on dissociable memory systems that have distinct neural bases and employ different forms of representation. One system gradually acquires reliable sequences of responses to given situations (e.g. repeatedly following a fixed route), and depends on the striatum. The other develops flexible representations permitting novel responses (e.g. finding new shortcuts), and depends on the hippocampus. Voermans and colleagues explore the interaction between these two systems using functional neuroimaging and behavioural measures in a clinical population.

“Where to?” Remote Memory for Spatial Relations and Landmark Identity in Former Taxi Drivers with Alzheimer's Disease and Encephalitis

Recent research suggests that the hippocampus is not needed for the maintenance and recovery of extensively used environments learned long ago. Instead, a network of neo-cortical regions differentially supports memory for location-navigation knowledge and visual appearance of well-known places. In this study, we present a patient, S. B., who was diagnosed with probable Alzheimer's disease long after retiring from his 40 years as a taxi driver in downtown Toronto, a place that he has visited rarely, if ever, in the last decade. His performance was compared to that of two other retired taxi drivers, L. R., who developed encephalitis after retirement, and I. L., who is without neurological illness, and a group of eight healthy control participants who were never taxi drivers but all of whom worked or lived in downtown Toronto until at least 10 years ago. Despite S. B.'s widespread atrophy, which has affected mainly his hippocampus and part of his occipitotemporal cortex, he performed at least as well as all other participants on remote memory tests of spatial location and mental navigation between well-known Toronto landmarks. Unlike the comparison populations, however, he was unable to discriminate between the appearances of landmarks that he had visited frequently in his many years as a taxi driver from unknown buildings. This profound deficit extended to famous world landmarks but not to famous faces and does not appear to be semantic in nature. These findings add further support to the claim that the hippocampus is not necessary for mental navigation of old environments and suggest that expertise is not sufficient to protect against landmark agnosia.

Preserved spatial memory after hippocampal lesions: effects of extensive experience in a complex environment

Damage to the hippocampus typically impairs spatial learning and memory in animals, but humans with hippocampal lesions retain spatial memories of premorbidly familiar environments. We showed that, like humans, normal rats reared in a complex environment and then given hippocampal lesions retained allocentric spatial memory for that environment. These results, which ruled out dependency on single cues, landmarks or specific routes, suggest that extensive premorbid experience leads to spatial representations that are independent of the hippocampus.

MR-based in vivo hippocampal volumetrics: 2. Findings in neuropsychiatric disorders

Magnetic resonance imaging (MRI) has opened a new window to the brain. Measuring hippocampal volume with MRI has provided important information about several neuropsychiatric disorders. We reviewed the literature and selected all English-language, human subject, data-driven papers on hippocampal volumetry, yielding a database of 423 records. Smaller hippocampal volumes have been reported in epilepsy, Alzheimer's disease, dementia, mild cognitive impairment, the aged, traumatic brain injury, cardiac arrest, Parkinson's disease, Huntington's disease, Cushing's disease, herpes simplex encephalitis, Turner's syndrome, Down's syndrome, survivors of low birth weight, schizophrenia, major depression, posttraumatic stress disorder, chronic alcoholism, borderline personality disorder, obsessive-compulsive disorder, and antisocial personality disorder. Significantly larger hippocampal volumes have been correlated with autism and children with fragile X syndrome. Preservation of hippocampal volume has been reported in congenital hyperplasia, children with fetal alcohol syndrome, anorexia nervosa, attention-deficit and hyperactivity disorder, bipolar disorder, and panic disorder. Possible mechanisms of hippocampal volume loss in neuropsychiatric disorders are discussed.

Are spatial memories strengthened in the human hippocampus during slow wave sleep?

In rats, the firing sequences observed in hippocampal ensembles during spatial learning are replayed during subsequent sleep, suggesting a role for posttraining sleep periods in the offline processing of spatial memories. Here, using regional cerebral blood flow measurements, we show that, in humans, hippocampal areas that are activated during route learning in a virtual town are likewise activated during subsequent slow wave sleep. Most importantly, we found that the amount of hippocampal activity expressed during slow wave sleep positively correlates with the improvement of performance in route retrieval on the next day. These findings suggest that learning-dependent modulation in hippocampal activity during human sleep reflects the offline processing of recent episodic and spatial memory traces, which eventually leads to the plastic changes underlying the subsequent improvement in performance.

Nonphosphorylated high-molecular-weight neurofilament expression suggests early maturation of the monkey subiculum

We analyzed the distribution of nonphosphorylated high-molecular-weight neurofilaments (NF-H) in the hippocampal formation of infant (3-week-old and 3-month-old) and adult (9-17-year-old) macaque monkeys in order to obtain neuroanatomical evidence of the maturity of these structures shortly after birth. We employed the monoclonal antibody SMI-32, a well-characterized antibody raised against nonphosphorylated NF-H, the expression of which is believed to reflect the maturation of certain neuronal populations. Patterns of SMI-32 immunoreactivity differed dramatically between infant and adult monkeys. In adults, nonphosphorylated NF-H expression was prominent in the CA3 and CA2 fields of the hippocampus, in the subiculum and in the entorhinal cortex. In infants, only the subiculum stained heavily for nonphosphorylated NF-H. These findings suggest that different subregions of the primate hippocampal formation mature at different times during development. The subiculum, the major source of efferent projections from the hippocampal formation toward subcortical structures, matures early during development. In contrast, the entorhinal cortex, the main interface of the hippocampal formation with the neocortex, matures relatively later. These findings have direct implications for the type of information processing that might be subserved by the primate hippocampal formation shortly after birth, as well as for the emergence of particular behavioral and memory processes during postnatal development.

An fMRI study of episodic memory: retrieval of object, spatial, and temporal information

Sixteen participants viewed a videotaped tour of 4 houses that highlighted a series of objects and their spatial locations. Participants were tested for memory of object, spatial, and temporal-order information while undergoing functional magnetic resonance imaging. Preferential activation was observed in the right parahippocampal gyrus during the retrieval of spatial-location information. Retrieval of contextual information (spatial location and temporal order) was associated with activation in the right dorsolateral prefrontal cortex. In bilateral posterior parietal regions, greater activation was associated with processing of visual scenes regardless of the memory judgment. These findings support current theories positing roles for frontal and medial temporal regions during episodic retrieval and suggest a specific role for the hippocampal complex in the retrieval of spatial-location information.

GSA: behavioral, histological, electrophysiological and neurochemical effects

Renal insufficient patients suffer from a variety of complications as direct and indirect consequence of accumulation of retention solutes. Guanidinosuccinic acid (GSA) is an important probable uremic toxin, increased in plasma, urine, cerebrospinal fluid and brain of patients with uremia and supposed to play a role in the pathogenesis of some neurological symptoms. GSA, an NMDA-receptor agonist and GABA-receptor antagonist, is suggested to act as an excitotoxin and shown to be convulsive. The effect of hippocampal (i.h.) GSA injection on behavior and hippocampal volume in mice is presented here. In addition, hippocampal cGMP concentration after systemic injection of GSA was measured. The effect of co-application of NMDA-receptor antagonist CGP37849 with GSA was tested, in vivo, after hippocampal GSA injection and, in vitro, on GSA evoked currents in spinal cord neurons. A significant dose-dependent effect of i.h. injection of GSA on cognitive performance, activity and social exploratory behavior was observed. There was a protective effect of CGP37849 on GSA induced behavioral alterations. Volume of hippocampal cornu ammonis region decreased significantly and dose-dependently after GSA injection. Systemic GSA injection increased cGMP concentration in hippocampal formation. It can be concluded that GSA is an important neurotoxin. As GSA is increased in patients with uremia, it probably contributes to their neurological symptoms. Knowledge of neurotoxic effects and mechanisms of action of GSA and other uremic retention solutes could help in the development of more efficient treatment of uremic patients. Animal models like the 'GSA mouse model' are useful tools for research in this context.

Hippocampal contributions to neurocognitive mapping in humans: A new model

The ability of an organism to develop, maintain, and act upon an abstracted internal representation of spatially extensive environments can provide an increased chance in ensuring that organism's survival. Here, we propose a neurocognitive model of spatial representation describing how several different processes interact and segregate the differing types of information used to produce a unified cognitive map. This model proposes that view-based egocentric and vestibulomotor translational information are functionally and anatomically separate, and that these parallel systems result in independent, but interacting, models within a neurocognitive map of space. In this context, we selectively review relevant portions of the large literature, addressing the establishment and operation of such spatial constructs in humans and the brain systems that underpin them, with particular reference to the hippocampal formation (HF). We present a reinterpretation of the types of knowledge used in the formation of this spatial construct, the processes that act upon this information, the nature of the final spatial representation, and describe how these universal concepts relate to the proposed model of spatial processing. The relevant experimental paradigms used to examine the neural basis of spatial representation and the main findings from previous research are also briefly presented. Finally, we detail a series of testable theoretical, behavioral, and anatomical predictions made by the model.

Pathophysiology and pharmacology of GABA(A) receptors

By controlling spike timing and sculpting neuronal rhythms, inhibitory interneurons play a key role in brain function. GABAergic interneurons are highly diverse. The respective GABA(A) receptor subtypes, therefore, provide new opportunities not only for understanding GABA-dependent pathophysiologies but also for targeting of selective neuronal circuits by drugs. The pharmacological relevance of GABA(A) receptor subtypes is increasingly being recognized. A new central nervous system pharmacology is on the horizon. The development of anxiolytic drugs devoid of sedation and of agents that enhance hippocampus-dependent learning and memory has become a novel and highly selective therapeutic opportunity.

Theta rhythm of navigation: Link between path integration and landmark navigation, episodic and semantic memory

Five key topics have been reverberating in hippocampal-entorhinal cortex (EC) research over the past five decades: episodic and semantic memory, path integration ("dead reckoning") and landmark ("map") navigation, and theta oscillation. We suggest that the systematic relations between single cell discharge and the activity of neuronal ensembles reflected in local field theta oscillations provide a useful insight into the relationship among these terms. In rats trained to run in direction-guided (1-dimensional) tasks, hippocampal cell assemblies discharge sequentially, with different assemblies active on opposite runs, i.e., place cells are unidirectional. Such tasks do not require map representation and are formally identical with learning sequentially occurring items in an episode. Hebbian plasticity, acting within the temporal window of the theta cycle, converts the travel distances into synaptic strengths between the sequentially activated and unidirectionally connected assemblies. In contrast, place representations by hippocampal neurons in 2-dimensional environments are typically omnidirectional, characteristic of a map. Generation of a map requires exploration, essentially a dead reckoning behavior. We suggest that omnidirectional navigation through the same places (junctions) during exploration gives rise to omnidirectional place cells and, consequently, maps free of temporal context. Analogously, multiple crossings of common junction(s) of episodes convert the common junction(s) into context-free or semantic memory. Theta oscillation can hence be conceived as the navigation rhythm through both physical and mnemonic space, facilitating the formation of maps and episodic/semantic memories.

Allocentric spatial memory in humans with hippocampal lesions

An immersive virtual reality (IVR) system was used to investigate allocentric spatial memory in a patient (PR) who had selective hippocampal damage, and also in patients who had undergone unilateral temporal lobectomies (17 right TL and 19 left TL), their performance compared against normal control groups. A human analogue of the Olton [Olton (1979). Hippocampus, space, and memory. Behavioural Brain Science, 2, 315] spatial maze was developed, consisting of a virtual room, a central virtual circular table and an array of radially arranged up-turned 'shells.' The participant had to search these shells in turn in order to find a blue 'cube' that would then 'move' to another location and so on, until all the shells had been target locations. Within-search errors could be made when the participants returned to a previously visited location during a search, and between-search errors when they revisited previously successful, but now incorrect locations. PR made significantly more between-search errors than his control group, but showed no increase in within-search errors. The right TL group showed a similar pattern of impairment, but the left TL group showed no impairment. This finding implicates the right hippocampal formation in spatial memory functioning in a scenario in which the visual environment was controlled so as to eliminate extraneous visual cues.

Specialization in the medial temporal lobe for processing of objects and scenes

There has been considerable debate as to whether the hippocampus and perirhinal cortex may subserve both memory and perception. We administered a series of oddity tasks, in which subjects selected the odd stimulus from a visual array, to amnesic patients with either selective hippocampal damage (HC group) or more extensive medial temporal damage, including the perirhinal cortex (MTL group). All patients performed normally when the stimuli could be discriminated using simple visual features, even if faces or complex virtual reality scenes were presented. Both patient groups were, however, severely impaired at scene discrimination when a significant demand was placed on processing spatial information across viewpoint independent representations, while only the MTL group showed a significant deficit in oddity judgments of faces and objects when object viewpoint independent perception was emphasized. These observations provide compelling evidence that the human hippocampus and perirhinal cortex are critical to processes beyond long-term declarative memory and may subserve spatial and object perception, respectively.

The temporal context model in spatial navigation and relational learning: toward a common explanation of medial temporal lobe function across domains

The medial temporal lobe (MTL) has been studied extensively at all levels of analysis, yet its function remains unclear. Theory regarding the cognitive function of the MTL has centered along 3 themes. Different authors have emphasized the role of the MTL in episodic recall, spatial navigation, or relational memory. Starting with the temporal context model (M. W. Howard & M. J. Kahana, 2002a), a distributed memory model that has been applied to benchmark data from episodic recall tasks, the authors propose that the entorhinal cortex supports a gradually changing representation of temporal context and the hippocampus proper enables retrieval of these contextual states. Simulation studies show this hypothesis explains the firing of place cells in the entorhinal cortex and the behavioral effects of hippocampal lesion in relational memory tasks. These results constitute a first step toward a unified computational theory of MTL function that integrates neurophysiological, neuropsychological, and cognitive findings.

Spatial orientation strategies in Morris-type virtual water task for humans

The present study characterized frequent motion patterns (search strategies) that occurred during spatial navigation in a virtual maze. The research focused on identifying and characterizing some search strategies, the temporal progression of strategy-use, and their role in spatial performance. Participants were 112 undergraduate students (42 males and 70 females). We identified three search strategies that predicted spatial performance. Enfilading refers to an approach-withdrawal pattern of active exploration near a target location. Thigmotaxis refers to a search strategy that involves continuous contact with the circular wall of the maze. Visual scan involves active visual exploration while the subject remains in a fixed spatial location and turns round. In addition to identifying these motion patterns, some significant points of the spatial learning process were also detailed where strategies appeared to shift systematically. The applied search strategies in these transitional points have determined overall spatial performance.

Prefrontal-hippocampal dynamics involved in learning regularities across episodes

Using functional magnetic resonance imaging, the neural correlates of context-specific memories and invariant memories about regularities across episodes were investigated. Volunteers had to learn conjunctions between objects and positions. In an invariant learning condition, positions were held constant, enabling subjects to learn regularities across trials. By contrast, in a context-specific condition object-position conjunctions were trial unique. Performance increase in the invariant learning condition was paralleled by a learning-related increase of inferior frontal gyrus activation and ventral striatal activation and a decrease of hippocampus activation. Conversely, in the context-specific condition hippocampal activation was constant across trials. We argue that the learning-related hippocampal activation pattern might be due to reduced relational binding requirements once regularities are extracted. Furthermore, we propose that the learning-related prefrontal modulation reflects the requirement to extract and maintain regularities across trials and the adjustment of object-position conjunctions on the basis of the extracted knowledge. Finally, our data suggest that the ventral striatum encodes the increased predictability of spatial features as a function of learning. Taken together, these results indicate a transition of the relative roles of distinct brain regions during learning regularities across multiple episodes: regularity learning is characterized by a shift from a hippocampal to a prefrontal-striatal brain system.

How emotion enhances the feeling of remembering

Studies examining memories of arousing 'real-life' events show that emotion heightens the feeling of remembering, without necessarily enhancing the objective accuracy of the memories. We measured brain activity associated with the feeling of remembering emotional and neutral photos. Subjects indicated whether recognition was accompanied by a recollection of details about the study episode ('remember') or not ('know'). 'Remember' judgments were boosted for emotional photos, but accuracy did not differ. For neutral photos, 'remember' judgments were related to enhanced activity in the parahippocampal cortex, previously related to recognition of visual details, which one might expect to supply the retrieval clues for a 'remember' judgment. In contrast, 'remember' judgments for emotional photos were associated with enhanced activity in the amygdala, suggesting that subjects rely on arousal and perceptual fluency to evaluate these memories. For the first time, we identify the neural mechanisms underlying the enhanced feeling of remembering for emotional events.

Associative recognition in a patient with selective hippocampal lesions and relatively normal item recognition

Previous work (Mayes et al., Hippocampus 12:325-340, 2002) found that patient YR, who suffered a selective bilateral lesion to the hippocampus in 1986, showed relatively preserved verbal and visual item recognition memory in the face of clearly impaired verbal and visual recall. In this study, we found that YR's Yes/No as well as forced-choice recognition of both intra-item associations and associations between items of the same kind was as well preserved as her item recognition memory. In contrast, YR was clearly impaired, and more so than she was on the above kinds of recognition, at recognition of associations between different kinds of information. Thus, her recognition memory for associations between objects and their locations, words and their temporal positions, abstract visual items or words and their temporal order, animal pictures and names of professions, faces and voices, faces and spoken names, words and definitions, and pictures and sounds, was clearly impaired. Several of the different information associative recognition tests at which YR was impaired could be compared with related item or inter-item association recognition tests of similar difficulty that she performed relatively normally around the same time. It is suggested that YR's familiarity memory for items, intra-item associations, and associations between items of the same kind was mediated by her intact medial temporal lobe cortices and was preserved, whereas her hippocampally mediated recall/recollection of these kinds of information was impaired. It is also suggested that the components of associations between different kinds of information are represented in distinct neocortical regions and that initially they only converge for memory processing within the hippocampus. No familiarity memory may exist in normal subjects for such associations, and, if so, YR's often chance recognition occurred because of her severe recall/recollection deficit. Conflicting data and views are discussed, and the way in which recall as well as item and associative recognition need to be systematically explored in patients with apparently selective hippocampal lesions, in order to resolve existing conflicts, is outlined.

Impairment of visuospatial memory is associated with decreased slow wave sleep in schizophrenia

Cognitive impairments such as memory deficits and sleep disturbances are common clinical features of schizophrenia. Since sleep plays an important role in consolidation of memory, we hypothesize, that there is an interrelationship between distinct alterations in sleep and memory performance in schizophrenia. We studied 17 patients with schizophrenia on stable antipsychotic medication with amisulpride (age range 22-44 years; 7 women) and 17 healthy controls (matched for age, gender and educational level). Sleep was recorded and scored according to the standard criteria by Rechtschaffen and Kales. Immediately before polysomnography and the morning after we performed neuropsychological tasks including Rey-Osterrieth Complex Figure Test and a test for recall of spatial location for testing aspects of declarative memory and a mirror tracing skill for procedural memory. In comparison to healthy controls, the patients showed a significant increase in sleep onset latency and a significant decrease in sleep efficiency and amount of slow wave sleep (SWS). Furthermore, the patients' performance in recall of the Rey-figure and of spatial location the next morning was significantly impaired. These impairments in the tests for visuospatial memory were positively correlated with reduction in the amount of SWS and in sleep efficiency. These results point to a functional interrelationship between regulation of SWS and performance in visuospatial memory in schizophrenia. If these results of our pilot study hold true, they will allow the development of innovative treatment strategies for neuropsychological deficits in patients with schizophrenia.

Reference Frames for Spatial Cognition: Different Brain Areas are Involved in Viewer-, Object-, and Landmark-Centered Judgments About Object Location

Functional magnetic resonance imaging was used to compare the neural correlates of three different types of spatial coding, which are implicated in crucial cognitive functions of our everyday life, such as visuomotor coordination and orientation in topographical space. By manipulating the requested spatial reference during a task of relative distance estimation, we directly compared viewer-centered, object-centered, and landmark-centered spatial coding of the same realistic 3-D information. Common activation was found in bilateral parietal, occipital, and right frontal premotor regions.

The retrosplenial and ventromedial occipital–temporal cortex (and parts of the parietal and occipital cortex) were significantly more activated during the landmark-centered condition. The ventrolateral occipital–temporal cortex was particularly involved in object-centered coding. Results strongly demonstrate that viewer-centered (egocentric) coding is restricted to the dorsal stream and connected frontal regions, whereas a coding centered on external references requires both dorsal and ventral regions, depending on the reference being a movable object or a landmark.

Cognitive Memory: Cellular and Network Machineries and Their Top-Down Control

A brain-wide distributed network orchestrates cognitive memorizing and remembering of explicit memory (i.e., memory of facts and events). The network was initially identified in humans and is being systematically investigated in molecular/genetic, single-unit, lesion, and imaging studies in animals. The types of memory identified in humans are extended into animals as episodic-like (event) memory or semantic-like (fact) memory. The unique configurational association between environmental stimuli and behavioral context, which is likely the basis of episodic-like memory, depends on neural circuits in the medial temporal lobe, whereas memory traces representing repeated associations, which is likely the basis of semantic-like memory, are consolidated in the domain-specific regions in the temporal cortex. These regions are reactivated during remembering and contribute to the contents of a memory. Two types of retrieval signal reach the cortical representations. One runs from the frontal cortex for active (or effortful) retrieval (top-down signal), and the other spreads backward from the medial temporal lobe for automatic retrieval. By sending the top-down signal to the temporal cortex, frontal regions manipulate and organize to-be-remembered information, devise strategies for retrieval, and also monitor the outcome, with dissociated frontal regions making functionally separate contributions. The challenge is to understand the hierarchical interactions between these multiple cortical areas, not only with a correlational analysis but also with an interventional study demonstrating the causal necessity and the direction of the causality.

Retrosplenial cortex lesions of area Rgb (but not of area Rga) impair spatial learning and memory in the rat

The retrosplenial cortex, which is situated in a critical position in the flow of information between the hippocampal formation and the neocortex, contributes to spatial memory, but no studies have examined the distinct contribution of each area of the retrosplenial cortex to this behavior. This study tests the hypothesis that the two areas of the retrosplenial granular cortex play distinct roles in spatial learning and memory. Adult, male Sprague-Dawley rats with small, bilateral lesions (ibotenic acid) of the retrosplenial granular cortex were tested for 2 weeks in a repeated acquisition water maze task. Compared to controls, rats with complete lesions of the retrosplenial granular b cortex (Rgb) were slightly, but significantly impaired, whereas rats with lesions of the retrosplenial granular a cortex (Rga) displayed no impairment. Further, the Rgb-lesioned (but not the Rga-lesioned) group was impaired in the probe trials at the end of the first week of training. All animals were tested in the same paradigm for a second week to determine if the learning and memory impairment in the Rgb-lesioned rats simply reflected "delayed learning." All animals improved their maze performance during the second week of testing, but the Rgb-lesioned group still had no preference for the correct quadrant in the probe trial. Together, these data indicate that Rgb plays a small, independent role in spatial learning and memory. Further, although selective lesions of Rga or Rgb do not cause a large deficit in learning, concomitant destruction of both areas causes a much greater impairment in learning than would be predicted from their independent contributions. The data highlight the unique and complex contribution of each area of the retrosplenial cortex to behavior.

Enhancement of Cognitive and Electrophysiological Measures of Hippocampal Functioning in Rats by a Low, But Not High, Dose of Dehydroepiandrosterone Sulfate (DHEAS)

Dehydroepiandrosterone sulfate (DHEAS) is a steroid hornone that is synthesized, de novo, in the brain. Endogenous DHEAS levels correlate with the quality of mental and physical health, where the highest levels of DHEAS occur in healthy young adults and reduced levels of DHEAS are found with advanced age, disease, or extreme stress. DHEAS supplementation, therefore, may serve as a therapeutic agent against a broad range of maladies. This paper summarizes laboratory findings on dose-response relationships between DHEAS and cognitive and electrophysiological measures of hippocampal functioning. It was found that a low, but not a high, dose of DHEAS enhanced hippocampal primed burst potentiation (a physiological model of memory) as well as spatial (hippocampal-dependent) memory in rats. This complex dose-response function of DHEAS effects on the brain and memory may contribute toward the inconsistent findings that have been obtained by other investigators in studies on DHEAS administration in people.

Contribution of Ih and GABAB to Synaptically Induced Afterhyperpolarizations in CA1: A Brake on the NMDA Response

CA1 pyramidal cells receive two major excitatory inputs: the perforant path (PP) terminates in the most distal dendrites, whereas the Schaffer collaterals (SC) terminate more proximally. We have examined the mechanism of the afterhyperpolarization (AHP) that follows single subthreshold excitatory postsynaptic potentials (EPSPs) in these inputs. The AHPs were not reduced by a GABAA antagonist or by agents that block Ca2+ entry. Application of the Ih blocker, ZD7288, partially blocked the AHP in the PP; the substantial remaining component was blocked by 2-hydroxysaclofen, a GABAB antagonist. In contrast, the AHP in the SC depends nearly completely on Ih, with almost no GABAB component. Thus postsynaptic GABAB receptors appear to be preferentially involved at distal synapses, consistent with the spatial distribution of GABAB receptors and g protein-coupled inward rectifying potassium (GIRK) channels. GABAB does, however, play a role at proximal synapses through presynaptic suppression of glutamate release, a mechanism that is much weaker at distal synapses. Experiments were conducted to explore the functional role of the AHP in the PP, which has a higher N-methyl-d-aspartate (NMDA)/AMPA ratio than the SC. Blockade of the AHP converted a response that had a small NMDA component to one that had a large component. These results indicate that the Ih and postsynaptic GABAB conductances act as a brake on distally generated NMDA responses.

Influence of the hippocampus on interneurons of the rat prefrontal cortex

The hippocampus and prefrontal cortex (PFC), two structures implicated in learning and memory processes, are linked by a direct hippocampo-prefrontal pathway. It has been shown that PFC pyramidal cells receive monosynaptic excitatory inputs from the hippocampus and, in this study, we sought to determine the influence of the hippocampus on PFC interneurons in anesthetized rats. Extracellular recordings were coupled to juxtacellular injections of neurobiotin or biotinylated dextran amine to morphologically differentiate interneurons from pyramidal cells. In all cases, the action potentials of labeled interneurons were of shorter duration (< 0.70 ms) than those of identified pyramidal cells (> 0.70 ms). Single pulse stimulation of the hippocampal CA1/subiculum region induced an excitatory response in 70% of recorded interneurons in the prelimbic and medial-orbital areas of the PFC. In contrast to the one to two action potentials generated by pyramidal cells, an important group of interneurons fired a burst of action potentials in response to hippocampal stimulation. A large proportion of these excitatory responses was probably monosynaptic as their latency is consistent with the conduction time of the hippocampo-prefrontal pathway. In addition, when both a pyramidal cell and an interneuron were simultaneously recorded and both responded to stimulation, the interneuron consistently fired before the pyramidal cell. In conclusion, the hippocampus exerts a direct excitatory influence on PFC interneurons and is thus capable of feedforward inhibition of pyramidal cells. Hippocampal output is spatially and temporally focalized via this inhibitory process and consequently could facilitate the synchronization of a specific subset of PFC neurons with hippocampal activity.

Autobiographical and episodic memory--one and the same? Evidence from prefrontal activation in neuroimaging studies

Laboratory investigations of episodic memory often require participants to encode and later retrieve lists of items (words, pictures, or faces). The underlying assumption is that recollection of items from the list is analogous to recollection of events from one's past, i.e. autobiographical re-experiencing. Functional neuroimaging studies of episodic memory have provided extensive evidence suggesting that regions of the prefrontal cortex (PFC) play a role in episodic memory retrieval. A review of PFC activations reported in imaging studies of autobiographical memory and matched sub-sets of list-learning episodic memory studies reveals patterns of similarity but also substantial differences. Episodic memory studies often report activations in the right mid-dorsolateral PFC, but such activations are absent in autobiographical memory studies. Additionally, activations in the ventromedial PFC, primarily on the left, are almost invariably found in autobiographical memory studies, but rarely occur in studies of episodic memory. It is suggested that these two regions mediate different modes of post-retrieval monitoring and verification. Autobiographical memory relies on quick intuitive 'feeling of rightness' to monitor the veracity and cohesiveness of retrieved memories in relation to an activated self-schema. Episodic memory for lists requires more conscious elaborate monitoring to avoid omissions, commissions and repetitions. The present analysis suggests that care and caution should be exercised in extrapolating from the way we recollect 'events' from a list learned in the laboratory to the way we recollect events from our lives.

Allocentric spatial memory activation of the hippocampal formation measured with fMRI

Hippocampal activation was investigated, comparing allocentric and egocentric spatial memory. Healthy participants were immersed in a virtual reality circular arena, with pattern-rendered walls. In a viewpoint-independent task, they moved toward a pole, which was then removed. They were relocated to another position and had to move to the prior location of the pole. For viewpoint-dependent memory, the participants were not moved to a new starting point, but the patterns were rotated to prevent them from indicating the final position. Hippocampal and parahippocampal activation were found in the viewpoint-independent memory encoding phase. Viewpoint-dependent memory did not result in such activation. These results suggest differential activation of the hippocampal formation during allocentric encoding, in partial support of the spatial mapping hypothesis as applied to humans.

Recalling spatial information as a component of recently and remotely acquired episodic or semantic memories: an fMRI study

Activations produced by the recall of episodic and semantic memories differing in spatial content and age were examined. Recall of recent episodic memories with differing spatial content activated the medial temporal lobes and the retrosplenial-posterior cingulate cortex-precuneus complex more than recall of recent semantic memories with similarly differing spatial content. Some of these differences related to the amount of spatial information recalled because spatially richer recent memories, regardless of whether they were episodic or semantic, activated the right posterior parahippocampal cortex, precuneus, and posterior parietal cortex more. This spatial effect was found to be independent of memory age for semantic memories, although some episodic-semantic memory differences, including one in the left hippocampus, were not age independent. Episodic-semantic memory recall activation differences are therefore probably a function of the amount recalled, memory age, and what is recalled, particularly with respect to spatial information.

Smaller hippocampal volume in Dutch police officers with posttraumatic stress disorder

BACKGROUND

Previous magnetic resonance imaging studies of posttraumatic stress disorder (PTSD) have reported smaller hippocampal volume, especially in war and sexual abuse victims. Our aim was to assess hippocampal volume in traumatized police officers with and without PTSD in the absence of alcohol abuse and moderate to severe major depression.

METHODS

In a case-matched control study, 14 police officers with current PTSD and 14 traumatized police officers without lifetime PTSD were examined using magnetic resonance imaging. Three temporal lobe areas were manually segmented: hippocampus, amygdala, and parahippocampal gyrus. Volumetric analysis was used to measure gray matter, white matter, and cerebrospinal fluid.

RESULTS

After controlling for total brain volume, the hippocampal volume in the PTSD group was significantly smaller in comparison with the traumatized control group (total 10.6%; left 12.6%). Volumes of amygdala, parahippocampal gyrus, gray matter, white matter, and cerebrospinal fluid were not significantly altered. A significant negative correlation was found between reexperiencing symptoms and hippocampal volume in the PTSD group.

CONCLUSIONS

We confirmed previous findings of smaller hippocampal volume in PTSD in a new population made up of police officers, excluding comorbidity as a confounder. The finding of smaller hippocampal volume was specific to PTSD.

Hippocampus

The hippocampus serves a critical role in declarative memory--our capacity to recall everyday facts and events. Recent studies using functional brain imaging in humans and neuropsychological analyses of humans and animals with hippocampal damage have revealed some of the elemental cognitive processes mediated by the hippocampus. In addition, recent characterizations of neuronal firing patterns in behaving animals and humans have suggested how neural representations in the hippocampus underlie those elemental cognitive processes in the service of declarative memory.

Altered hippocampal transcript profile accompanies an age-related spatial memory deficit in mice

We have carried out a global survey of age-related changes in mRNA levels in the C57BL/6NIA mouse hippocampus and found a difference in the hippocampal gene expression profile between 2-month-old young mice and 15-month-old middle-aged mice correlated with an age-related cognitive deficit in hippocampal-based explicit memory formation. Middle-aged mice displayed a mild but specific deficit in spatial memory in the Morris water maze. By using Affymetrix GeneChip microarrays, we found a distinct pattern of age-related change, consisting mostly of gene overexpression in the middle-aged mice, suggesting that the induction of negative regulators in the middle-aged hippocampus could be involved in impairment of learning. Interestingly, we report changes in transcript levels for genes that could affect synaptic plasticity. Those changes could be involved in the memory deficits we observed in the 15-month-old mice. In agreement with previous reports, we also found altered expression in genes related to inflammation, protein processing, and oxidative stress.

Neural correlates of context memory with real-world events

There has been little evidence for the difference in the retrieval processes of when and where something happened, one of the important factors in understanding episodic memory. We used positron emission tomography (PET) to identify the neural networks associated with temporal and spatial context memory of events experienced under experimental conditions similar to those of everyday life. Before PET, subjects experienced 36 events. The events were divided into four groups of nine each. The subjects experienced the first two groups of events before a 15-min recess and the other two after the recess; they experienced the first and last groups of events in one room, took a recess in another room, and experienced the second and third groups in a different room. During PET, the subjects were scanned under three different retrieval tasks: a time-retrieval task, a place-retrieval task, and a simple recognition task. The results showed that the retrieval of time and space, compared with the simple recognition, was associated with activity in substantially different regions as well as a common region: time retrieval with the posterior part of the right orbitofrontal cortex and left inferior parietal lobule, place retrieval with two regions in right parietal association cortex, right posterior cingulate gyrus, left precentral gyrus, and right cerebellum, and both with the right inferior frontal gyrus. These findings indicate that there are unique areas, in addition to a common area, for retrieving temporal and spatial context.