Equivalent impairment of spatial and nonspatial memory following damage to the human hippocampus

Differential effects of bilateral hippocampal CA3 damage on the implicit learning and recognition of complex event sequences

Learning regularities in the environment is a fundament of human cognition, which is supported by a network of brain regions that include the hippocampus. In two experiments, we assessed the effects of selective bilateral damage to human hippocampal subregion CA3, which was associated with autobiographical episodic amnesia extending ~50 years prior to the damage, on the ability to recognize complex, deterministic event sequences presented either in a spatial or a non-spatial configuration. In contrast to findings from related paradigms, modalities, and homologue species, hippocampal damage did not preclude recognition memory for an event sequence studied and tested at four spatial locations, whereas recognition memory for an event sequence presented at a single location was at chance. In two additional experiments, recognition memory for novel single-items was intact, whereas the ability to recognize novel single-items in a different location from that presented at study was at chance. The results are at variance with a general role of the hippocampus in the learning and recognition of complex event sequences based on non-adjacent spatial and temporal dependencies. We discuss the impact of the results on established theoretical accounts of the hippocampal contributions to implicit sequence learning and episodic memory.

DREADD-inactivation of dorsal CA1 pyramidal neurons in mice impairs retrieval of object and spatial memories

The hippocampus, a medial temporal lobe brain region, is critical for the consolidation of information from short-term memory into long-term episodic memory and for spatial memory that enables navigation. Hippocampal damage in humans has been linked to amnesia and memory loss, characteristic of Alzheimer's disease and other dementias. Numerous studies indicate that the rodent hippocampus contributes significantly to long-term memory for spatial and nonspatial information. For example, muscimol-induced depression of CA1 neuronal activity in the dorsal hippocampus impairs the encoding, consolidation, and retrieval of nonspatial object memory in mice. Here, a chemogenetic designer receptor exclusively activated by designer drugs (DREADDs) approach was used to test the selective involvement of CA1 pyramidal neurons in memory retrieval for objects and for spatial location in a cohort of male C57BL/6J mice. Activation of the inhibitory (hM4Di) DREADDs receptor expressed in CA1 neurons significantly impaired the retrieval of object memory in the spontaneous object recognition task and of spatial memory in the Morris water maze. Silencing of CA1 neuronal activity in hM4Di-expressing mice was confirmed by comparing Fos expression in vehicle- and clozapine-N-oxide-treated mice after exploration of a novel environment. Histological analyses revealed that expression of the hM4Di receptor was limited to CA1 neurons of the dorsal hippocampus. These results suggest that a common subset of CA1 neurons (i.e., those expressing hM4Di receptors) in mouse hippocampus contributed to the retrieval of long-term memory for nonspatial and spatial information. Our findings support the view that the contribution of the rodent hippocampus is like that of the primate hippocampus, specifically essential for global memory. Our results further validate mice as a suitable model system to study the neurobiological mechanisms of human episodic memory, but also in developing treatments and understanding the underlying causes of diseases affecting long-term memory, such as Alzheimer's disease.

Multipotential and systemic effects of traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of disability and mortality of people at all ages. Biochemical, cellular and physiological events that occur during primary injury lead to a delayed and long-term secondary damage that can last from hours to years. Secondary brain injury causes tissue damage in the central nervous system and a subsequent strong and rapid inflammatory response that may lead to persistent inflammation. However, this inflammatory response is not limited to the brain. Inflammatory mediators are transferred from damaged brain tissue to the bloodstream and produce a systemic inflammatory response in peripheral organs, including the cardiovascular, pulmonary, gastrointestinal, renal and endocrine systems. Complications of TBI are associated with its multiple and systemic effects that should be considered in the treatment of TBI patients. Therefore, in this review, an attempt was made to examine the systemic effects of TBI in detail. It is hoped that this review will identify the mechanisms of injury and complications of TBI, and open a window for promising treatment in TBI complications.

Rapamycin Improves Recognition Memory and Normalizes Amino-Acids and Amines Levels in the Hippocampal Dentate Gyrus in Adult Rats Exposed to Ethanol during the Neonatal Period

The mammalian target of rapamycin (mTOR), a serine/ threonine kinase, is implicated in synaptic plasticity by controlling protein synthesis. Research suggests that ethanol exposure during pregnancy alters the mTOR signaling pathway in the fetal hippocampus. Thus, we investigated the influence of pre-treatment with rapamycin, an mTORC1 inhibitor, on the development of recognition memory deficits in adult rats that were neonatally exposed to ethanol. In the study, male and female rat pups received ethanol (5 g/kg/day) by intragastric intubation at postanatal day (PND 4-9), an equivalent to the third trimester of human pregnancy. Rapamycin (3 and 10 mg/kg) was given intraperitoneally before every ethanol administration. Short- and long-term recognition memory was assessed in the novel object recognition (NOR) task in adult (PND 59/60) rats. Locomotor activity and anxiety-like behavior were also evaluated to exclude the influence of such behavior on the outcome of the memory task. Moreover, the effects of rapamycin pre-treatment during neonatal ethanol exposure on the content of amino-acids and amines essential for the proper development of cognitive function in the dentate gyrus (DG) of the hippocampus was evaluated using proton magnetic resonance spectroscopy (1H MRS) in male adult (PND 60) rats. Our results show the deleterious effect of ethanol given to neonatal rats on long-term recognition memory in adults. The effect was more pronounced in male rather than female rats. Rapamycin reversed this ethanol-induced memory impairment and normalized the levels of amino acids and amines in the DG. This suggests the involvement of mTORC1 in the deleterious effect of ethanol on the developing brain.

The effects of hippocampal and area parahippocampalis lesions on the processing and retention of serial-order behavior, autoshaping, and spatial behavior in pigeons

We examined the role of the avian hippocampus and area parahippocampalis in serial-order behavior and a variety of other tasks known to be sensitive to hippocampal damage in mammals. Damage to the hippocampus and area parahippocampalis caused impairments in autoshaping and performance on an analogue of a radial-arm maze task, but had no effect on acquisition of 2-item, 3-item, and 4-item serial-order lists. Additionally, the lesions had no effect on the retention of 3-items lists, or on the ability to perform novel derived lists composed of elements from lists they had previously learned. The impairments in autoshaping and spatial behavior are consistent with the findings in mammals. The absence of impairments on the serial-order task may also be consistent once one considers that damage to the hippocampus in mammals seems to affect more internally-organized rather than externally-organized serial-order tasks. Together, the findings support the view that the avian hippocampal complex serves a function very similar to the mammalian hippocampus, a finding that is interesting given that the architecture of the avian hippocampus differs dramatically from that of the mammalian hippocampus.

Neuropsychological and neuropathological observations of a long-studied case of memory impairment

We report neuropsychological and neuropathological findings for a patient (A.B.), who developed memory impairment after a cardiac arrest at age 39. A.B. was a clinical psychologist who, although unable to return to work, was an active participant in our neuropsychological studies for 24 y. He exhibited a moderately severe and circumscribed impairment in the formation of long-term, declarative memory (anterograde amnesia), together with temporally graded retrograde amnesia covering ∼5 y prior to the cardiac arrest. More remote memory for both facts and autobiographical events was intact. His neuropathology was extensive and involved the medial temporal lobe, the diencephalon, cerebral cortex, basal ganglia, and cerebellum. In the hippocampal formation, there was substantial cell loss in the CA1 and CA3 fields, the hilus of the dentate gyrus (with sparing of granule cells), and the entorhinal cortex. There was also cell loss in the CA2 field, but some remnants remained. The amygdala demonstrated substantial neuronal loss, particularly in its deep nuclei. In the thalamus, there was damage and atrophy of the anterior nuclear complex, the mediodorsal nucleus, and the pulvinar. There was also loss of cells in the medial and lateral mammillary nuclei in the hypothalamus. We suggest that the neuropathology resulted from two separate factors: the initial cardiac arrest (and respiratory distress) and the recurrent seizures that followed, which led to additional damage characteristic of temporal lobe epilepsy.

The Interplay Between Beta-Amyloid 1-42 (Aβ1-42)-Induced Hippocampal Inflammatory Response, p-tau, Vascular Pathology, and Their Synergistic Contributions to Neuronal Death and Behavioral Deficits

Alzheimer's disease (AD), the most common chronic neurodegenerative disorder, has complex neuropathology. The principal neuropathological hallmarks of the disease are the deposition of extracellular β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) comprised of hyperphosphorylated tau (p-tau) protein. These changes occur with neuroinflammation, a compromised blood-brain barrier (BBB) integrity, and neuronal synaptic dysfunction, all of which ultimately lead to neuronal cell loss and cognitive deficits in AD. Aβ1-42 was stereotaxically administered bilaterally into the CA1 region of the hippocampi of 18-month-old male C57BL/6 mice. This study aimed to characterize, utilizing immunohistochemistry and behavioral testing, the spatial and temporal effects of Aβ1-42 on a broad set of parameters characteristic of AD: p-tau, neuroinflammation, vascular pathology, pyramidal cell survival, and behavior. Three days after Aβ1-42 injection and before significant neuronal cell loss was detected, acute neuroinflammatory and vascular responses were observed. These responses included the up-regulation of glial fibrillary acidic protein (GFAP), cell adhesion molecule-1 (PECAM-1, also known as CD31), fibrinogen labeling, and an increased number of activated astrocytes and microglia in the CA1 region of the hippocampus. From day 7, there was significant pyramidal cell loss in the CA1 region of the hippocampus, and by 30 days, significant localized up-regulation of p-tau, GFAP, Iba-1, CD31, and alpha-smooth muscle actin (α-SMA) in the Aβ1-42-injected mice compared with controls. These molecular changes in Aβ1-42-injected mice were accompanied by cognitive deterioration, as demonstrated by long-term spatial memory impairment. This study is reporting a comprehensive examination of a complex set of parameters associated with intrahippocampal administration of Aβ1-42 in mice, their spatiotemporal interactions and combined contribution to the disease progression. We show that a single Aβ injection can reproduce aspects of the inflammatory, vascular, and p-tau induced pathology occurring in the AD human brain that lead to cognitive deficits.

Object Recognition Memory: Distinct Yet Complementary Roles of the Mouse CA1 and Perirhinal Cortex

While the essential contribution of the hippocampus to spatial memory is well established, object recognition memory has been traditionally attributed to the perirhinal cortex (PRh). However, the results of several studies indicate that under specific procedural conditions, temporary or permanent lesions of the hippocampus affect object memory processes as measured in the Spontaneous Object Recognition (SOR) task. The PRh and hippocampus are considered to contribute distinctly to object recognition memory based on memory strength. Allowing mice more, or less, exploration of novel objects during the encoding phase of the task (i.e., sample session), yields stronger, or weaker, object memory, respectively. The current studies employed temporary local inactivation and immunohistochemistry to determine the differential contributions of neuronal activity in PRh and the CA1 region of the hippocampus to strong and weak object memory. Temporary inactivation of the CA1 immediately after the SOR sample session impaired strong object memory but spared weak object memory; while temporary inactivation of PRh post-sample impaired weak object memory but spared strong object memory. Furthermore, mRNA transcription and de novo protein synthesis are required for the consolidation of episodic memory, and activation patterns of immediate early genes (IEGs), such as c-Fos and Arc, are linked to behaviorally triggered neuronal activation and synaptic plasticity. Analyses of c-Fos and Arc protein expression in PRh and CA1 neurons by immunohistochemistry, and of Arc mRNA by qPCR after distinct stages of SOR, provide additional support that strong object memory is dependent on CA1 neuronal activity, while weak object memory is dependent on PRh neuronal activity. Taken together, the results support the view that both PRh and CA1 are required for object memory under distinct conditions. Specifically, our results are consistent with a model that as the mouse begins to explore a novel object, information about it accumulates within PRh, and a weak memory of the object is encoded. If object exploration continues beyond some threshold, strong memory for the event of object exploration is encoded; the consolidation of which is CA1-dependent. These data serve to reconcile the dissension in the literature by demonstrating functional and complementary roles for CA1 and PRh neurons in rodent object memory.

Medial Temporal Lobe Amnesia Is Associated with a Deficit in Recovering Temporal Context

Medial-temporal lobe (MTL) lesions are associated with severe impairments in episodic memory. In the framework of the temporal context model, the hypothesized mechanism for episodic memory is the reinstatement of a prior experienced context (i.e., "jump back in time"), which relies upon the MTL [Howard, M. W., Fotedar, M. S., Datey, A. V., & Hasselmo, M. E. The temporal context model in spatial navigation and relational learning: Toward a common explanation of medial temporal lobe function across domains. Psychological Review, 112, 75-116, 2005]. This hypothesis has proven difficult to test in amnesia because of the floor-level performance by patients in recall tasks. To circumvent this issue, in this study, we used a "looped-list" format, in which a set of verbal stimuli was presented multiple times in a consistent order. This allowed for comparison of statistical properties such as probability of first recall and lag-conditional response probability (lag-CRP) between amnesic patients and healthy controls. Results revealed that the lag-CRP, but not the probability of first recall, is altered in amnesia, suggesting a selective disruption of temporal contiguity. To further characterize the results, we fit a scale-invariant version of the temporal context model [Howard, M. W., Shankar, K. H., Aue, W. R., & Criss, A. H. A distributed representation of internal time. Psychological Review, 122, 24-53, 2015] to the probability of first recall and lag-CRP curves. The modeling results suggested that the deficit in temporal contiguity in amnesia is best described as a failure to recover temporal context. These results provide the first direct evidence for an impairment in a jump-back-in-time mechanism in patients with MTL amnesia.

Long-Term Visuo-Gustatory Appetitive and Aversive Conditioning Potentiate Human Visual Evoked Potentials

Human recognition of foods and beverages are often based on visual cues associated with flavors. The dynamics of neurophysiological plasticity related to acquisition of such long-term associations has only recently become the target of investigation. In the present work, the effects of appetitive and aversive visuo-gustatory conditioning were studied with high density EEG-recordings focusing on late components in the visual evoked potentials (VEPs), specifically the N2-P3 waves. Unfamiliar images were paired with either a pleasant or an unpleasant juice and VEPs evoked by the images were compared before and 1 day after the pairings. In electrodes located over posterior visual cortex areas, the following changes were observed after conditioning: the amplitude from the N2-peak to the P3-peak increased and the N2 peak delay was reduced. The percentage increase of N2-to-P3 amplitudes was asymmetrically distributed over the posterior hemispheres despite the fact that the images were bilaterally symmetrical across the two visual hemifields. The percentage increases of N2-to-P3 amplitudes in each experimental subject correlated with the subject's evaluation of positive or negative hedonic valences of the two juices. The results from 118 scalp electrodes gave surface maps of theta power distributions showing increased power over posterior visual areas after the pairings. Source current distributions calculated from swLORETA revealed that visual evoked currents rose as a result of conditioning in five cortical regions-from primary visual areas and into the inferior temporal gyrus (ITG). These learning-induced changes were seen after both appetitive and aversive training while a sham trained control group showed no changes. It is concluded that long-term visuo-gustatory conditioning potentiated the N2-P3 complex, and it is suggested that the changes are regulated by the perceived hedonic valence of the US.

Pathophysiology and Treatment of Memory Dysfunction After Traumatic Brain Injury

Memory is fundamental to everyday life, and cognitive impairments resulting from traumatic brain injury (TBI) have devastating effects on TBI survivors. A contributing component to memory impairments caused by TBI is alteration in the neural circuits associated with memory function. In this review, we aim to bring together experimental findings that characterize behavioral memory deficits and the underlying pathophysiology of memory-involved circuits after TBI. While there is little doubt that TBI causes memory and cognitive dysfunction, it is difficult to conclude which memory phase, i.e., encoding, maintenance, or retrieval, is specifically altered by TBI. This is most likely due to variation in behavioral protocols and experimental models. Additionally, we review a selection of experimental treatments that hold translational potential to mitigate memory dysfunction following injury.

Intact Artificial Grammar Learning in Amnesia: Dissociation of Classification Learning and Explicit Memory for Specific Instances

The present study investigates whether the ability to classify on the basis of rules can be learned independently of memory for the specific instances used to leach the rules. Thirteen amnesic patients and 14 control subjects studied letter strings generated by an artificial grammar. Subjects were then shown new letter strings and were instructed to classify them as grammatical or nongrammatical. Amnesic patients performed as well as normal subjects. However, amnesic patients performed more poorly than control subjects on a recognition test of the exemplars that had been presented. Amnesic patients also performed more poorly than control subjects when the instructions were to base the classification on explicit comparison with the original exemplars. The results show that classification learning based on exemplars of an artificial grammar can develop normally despite impaired memory for the exemplars themselves. Whereas exemplar memory depends on interactions between neocortex and the limbic system, classification learning may depend on interaction between neocortex and the neostriatum.

The Role of Wnt/β-Catenin Signaling Pathway in Disrupted Hippocampal Neurogenesis of Temporal Lobe Epilepsy: A Potential Therapeutic Target?

Temporal lobe epilepsy is one of the most common clinical neurological disorders. One of the major pathological findings in temporal lobe epilepsy is hippocampal sclerosis, characterized by massive neuronal loss and severe gliosis. The epileptogenesis process of temporal lobe epilepsy usually starts with initial precipitating insults, followed by neurodegeneration, abnormal hippocampus circuitry reorganization, and the formation of hypersynchronicity. Experimental and clinical evidence strongly suggests that dysfunctional neurogenesis is involved in the epileptogenesis. Recent data demonstrate that neurogenesis is induced by acute seizures or precipitating insults, whereas the capacity of neuronal recruitment and proliferation substantially decreases in the chronic phase of epilepsy. Participation of the Wnt/β-catenin signaling pathway in neurogenesis reveals its importance in epileptogenesis; its dysfunction contributes to the structural and functional abnormality of temporal lobe epilepsy, while rescuing this pathway exerts neuroprotective effects. Here, we summarize data supporting the involvement of Wnt/β-catenin signaling in the epileptogenesis of temporal lobe epilepsy. We also propose that the Wnt/β-catenin signaling pathway may serve as a promising therapeutic target for temporal lobe epilepsy treatment.

Augmented Inhibition from Cannabinoid-Sensitive Interneurons Diminishes CA1 Output after Traumatic Brain Injury

The neurological impairments associated with traumatic brain injury include learning and memory deficits and increased risk of seizures. The hippocampus is critically involved in both of these phenomena and highly susceptible to damage by traumatic brain injury. To examine network activity in the hippocampal CA1 region after lateral fluid percussion injury, we used a combination of voltage-sensitive dye, field potential, and patch clamp recording in mouse hippocampal brain slices. When the stratum radiatum (SR) was stimulated in slices from injured mice, we found decreased depolarization in SR and increased hyperpolarization in stratum oriens (SO), together with a decrease in the percentage of pyramidal neurons firing stimulus-evoked action potentials. Increased hyperpolarization in SO persisted when glutamatergic transmission was blocked. However, we found no changes in SO responses when the alveus was stimulated to directly activate SO. These results suggest that the increased SO hyperpolarization evoked by SR stimulation was mediated by interneurons that have cell bodies and/or axons in SR, and form synapses in stratum pyramidale and SO. A low concentration (100 nM) of the synthetic cannabinoid WIN55,212-2, restored CA1 output in slices from injured animals. These findings support the hypothesis that increased GABAergic signaling by cannabinoid-sensitive interneurons contributes to the reduced CA1 output following traumatic brain injury.

The human hippocampus beyond the cognitive map: evidence from a densely amnesic patient

We tested a densely amnesic patient (P9), with bilateral hippocampal damage resulting from an autoimmune disorder, and 12 age- and sex-matched controls on a series of memory tasks designed to characterize allocentric spatial learning and memory abilities. We compared P9's ability to perform spatial memory tasks with her ability to perform non-spatial, color memory tasks. First, P9's performance was impaired as compared to controls even in the simplest versions of an allocentric spatial memory task, in which she had to find repeatedly over 10 trials the same location(s) of one, two or three illuminating foot pad(s) among 23 pads distributed in an open-field arena. In contrast, she performed as well as controls when she had to find repeatedly over 10 trials the same one, two or three pad(s) marked by color cue(s), whose locations varied between trials. Second, P9's performance was severely impaired in working memory tasks, when she had to learn on a trial-unique basis and remember the location(s) or the color(s) of one, two or three pad(s), while performing an interfering task during the 1-min interval separating encoding and retrieval. Without interference during the retention interval of the trial-unique tasks, P9's performance was partially preserved in the color tasks, whereas it remained severely impaired in the allocentric spatial tasks. Detailed behavioral analyses indicate that P9's memory representations are more limited than those of controls both in their precision (metric coding) and in the number of items that can be maintained in memory (capacity). These findings are consistent with the theory that the hippocampus contributes to the integration or binding of multiple items, in order to produce high-resolution/high-capacity representations of spatial and non-spatial information in the service of short-term/working and long-term memory.

Mnemonic dissociations: the power of parameters

Abstract Memory, as reflected in neural plasticity, is certainly not a functionally homogeneous process. Within the context of the mnemonic functions of the hippocampal system, this paper discusses two issues. The first concerns the appropriate conceptual approach to be taken in a neuropsychological analysis of memory. We conclude that a dimensional approach, emphasizing a quantitative analysis of individual mnemonic processes, has a significant advantage over the dichotomous classification of different types of memory. The second issue concerns the most appropriate way to describe the role of the hippocampus in mnemonic processes. On the basis of electrophysiological, neurochemical, behavioral, and computational analyses of hippocampal function, we suggest that its major role is the reduction of interference.

Changes in mouse cognition and hippocampal gene expression observed in a mild physical- and blast-traumatic brain injury

Warfare has long been associated with traumatic brain injury (TBI) in militarized zones. Common forms of TBI can be caused by a physical insult to the head-brain or by the effects of a high velocity blast shock wave generated by the detonation of an explosive device. While both forms of trauma are distinctly different regarding the mechanism of trauma induction, there are striking similarities in the cognitive and emotional status of survivors. Presently, proven effective therapeutics for the treatment of either form of TBI are unavailable. To be able to develop efficacious therapies, studies involving animal models of physical- and blast-TBI are required to identify possible novel or existing medicines that may be of value in the management of clinical events. We examined indices of cognition and anxiety-like behavior and the hippocampal gene transcriptome of mice subjected to both forms of TBI. We identified common behavioral deficits and gene expression regulations, in addition to unique injury-specific forms of gene regulation. Molecular pathways presented a pattern similar to that seen in gene expression. Interestingly, pathways connected to Alzheimer's disease displayed a markedly different form of regulation depending on the type of TBI. While these data highlight similarities in behavioral outcomes after trauma, the divergence in hippocampal transcriptome observed between models suggests that, at the molecular level, the TBIs are quite different. These models may provide tools to help define therapeutic approaches for the treatment of physical- and blast-TBIs. Based upon observations of increasing numbers of personnel displaying TBI related emotional and behavioral changes in militarized zones, the development of efficacious therapies will become a national if not a global priority.

Hippocampal anatomy supports the use of context in object recognition: a computational model

The human hippocampus receives distinct signals via the lateral entorhinal cortex, typically associated with object features, and the medial entorhinal cortex, associated with spatial or contextual information. The existence of these distinct types of information calls for some means by which they can be managed in an appropriate way, by integrating them or keeping them separate as required to improve recognition. We hypothesize that several anatomical features of the hippocampus, including differentiation in connectivity between the superior/inferior blades of DG and the distal/proximal regions of CA3 and CA1, work together to play this information managing role. We construct a set of neural network models with these features and compare their recognition performance when given noisy or partial versions of contexts and their associated objects. We found that the anterior and posterior regions of the hippocampus naturally require different ratios of object and context input for optimal performance, due to the greater number of objects versus contexts. Additionally, we found that having separate processing regions in DG significantly aided recognition in situations where object inputs were degraded. However, split processing in both DG and CA3 resulted in performance tradeoffs, though the actual hippocampus may have ways of mitigating such losses.

Cholinergic degeneration is associated with increased plaque deposition and cognitive impairment in APPswe/PS1dE9 mice

Cholinergic dysfunction and deposition of plaques containing amyloid β-peptides (Aβ) are two of the characteristics of Alzheimer's disease. Here, we combine APPswe/PS1dE9 (APP/PS1) mice with the cholinergic immunotoxin mu p75-saporin (SAP) to integrate partial basal forebrain cholinergic degeneration and the neuropathology of APP/PS1 mice. By 6 months of age, APP/PS1 mice and wild type littermates (Wt) received intracerebroventricular injection of 0.6 μg SAP (lesion) or PBS (sham). Two months following surgery, APP/PS1 mice treated with SAP were significantly impaired compared to sham treated APP/PS1 mice in a behavioural paradigm addressing working memory. Conversely, the performance of Wt mice was unaffected by SAP treatment. Choline acetyltransferase activity was reduced in the hippocampus and frontal cortex following SAP treatment. The selective effect of a mild SAP lesion in APP/PS1 mice was not due to a more extensive cholinergic degeneration since the reduction in choline acetyltransferase activity was similar following SAP treatment in APP/PS1 mice and Wt. Interestingly, plaque load was significantly increased in SAP treated APP/PS1 mice relative to sham lesioned APP/PS1 mice. Additionally, APP/PS1 mice treated with SAP showed a tendency towards an increased level of soluble and insoluble Aβ1-40 and Aβ1-42 measured in brain tissue homogenate. Our results suggest that the combination of cholinergic degeneration and Aβ overexpression in the APP/PS1 mouse model results in cognitive decline and accelerated plaque burden. SAP treated APP/PS1 mice might thus constitute an improved model of Alzheimer's disease-like neuropathology and cognitive deficits compared to the conventional APP/PS1 model without selective removal of basal forebrain cholinergic neurons.

Loss of NGF-TrkA signaling from the CNS is not sufficient to induce cognitive impairments in young adult or intermediate-aged mice

Many molecules expressed in the CNS contribute to cognitive functions either by modulating neuronal activity or by mediating neuronal trophic support and/or connectivity. An ongoing discussion is whether signaling of nerve growth factor (NGF) through its high-affinity receptor TrkA contributes to attention behavior and/or learning and memory, based on its expression in relevant regions of the CNS such as the hippocampus, cerebral cortex, amygdala and basal forebrain. Previous animal models carrying either a null allele or transgenic manipulation of Ngf or Trka have proved difficult in addressing this question. To overcome this problem, we conditionally deleted Ngf or Trka from the CNS. Our findings confirm that NGF-TrkA signaling supports survival of only a small proportion of cholinergic neurons during development; however, this signaling is not required for trophic support or connectivity of the remaining basal forebrain cholinergic neurons. Moreover, comprehensive behavioral analysis of young adult and intermediate-aged mice lacking NGF-TrkA signaling demonstrates that this signaling is dispensable for both attention behavior and various aspects of learning and memory.

Correlation between calbindin expression in granule cells of the resected hippocampal dentate gyrus and verbal memory in temporal lobe epilepsy

Calbindin expression of granule cells of the dentate gyrus is decreased in temporal lobe epilepsy (TLE) regardless of its etiology. In this study, we examined the relation between reduction of calbindin immunoreactivity and the verbal and visuo-spatial memory function of patients with TLE of different etiologies. Significant linear correlation was shown between calbindin expression and short-term and long-term percent retention and retroactive interference in auditory verbal learning test (AVLT) of patients including those with hippocampal sclerosis. In addition, we found significant linear regression between calbindin expression and short-term and long-term percent retention of AVLT in patients whose epilepsy was caused by malformation of cortical development or tumor and when no hippocampal sclerosis and substantial neuronal loss were detected. Together with the role of calbindin in memory established in previous studies on calbindin knock-out mice, our results suggest that reduction of calbindin expression may contribute to memory impairments of patients with TLE, particularly, when neuronal loss is not significant.

Role of the dorsal hippocampus in object memory load

The dorsal hippocampus is crucial for mammalian spatial memory, but its exact role in item memory is still hotly debated. Recent evidence in humans suggested that the hippocampus might be selectively involved in item short-term memory to deal with an increasing memory load. In this study, we sought to test this hypothesis. To this aim we developed a novel behavioral procedure to study object memory load in mice by progressively increasing the stimulus set size in the spontaneous object recognition task. Using this procedure, we demonstrated that naive mice have a memory span, which is the number of elements they can remember for a short-time interval, of about six objects. Then, we showed that excitotoxic selective lesions of the dorsal hippocampus did not impair novel object discrimination in the condition of low memory load. In contrast, the same lesion impaired novel object discrimination in the high memory load condition, and reduced the object memory span to four objects. These results have important heuristic and clinical implications because they open new perspective toward the understanding of the role of the hippocampus in item memory and in memory span deficits occurring in human pathologies, such as Alzheimer's disease and schizophrenia.

4- to 6-week-old adult-born hippocampal neurons influence novelty-evoked exploration and contextual fear conditioning

To explore the role of adult hippocampal neurogenesis in novelty processing, we assessed novel object recognition (NOR) in mice after neurogenesis was arrested using focal x-irradiation of the hippocampus, or a reversible, genetic method in which glial fibrillary acidic protein-positive neural progenitor cells are ablated with ganciclovir. Arresting neurogenesis did not alter general activity or object investigation during four exposures with two constant objects. However, when a novel object replaced a constant object, mice with neurogenesis arrested by either ablation method showed increased exploration of the novel object when compared with control mice. The increased novel object exploration did not manifest until 4-6 weeks after x-irradiation or 6 weeks following a genetic ablation, indicating that exploration of the novel object is increased specifically by the elimination of 4- to 6-week-old adult born neurons. The increased novel object exploration was also observed in older mice, which exhibited a marked reduction in neurogenesis relative to young mice. Mice with neurogenesis arrested by either ablation method were also impaired in one-trial contextual fear conditioning (CFC) at 6 weeks but not at 4 weeks following ablation, further supporting the idea that 4- to 6-week-old adult born neurons are necessary for specific forms of hippocampal-dependent learning, and suggesting that the NOR and CFC effects have a common underlying mechanism. These data suggest that the transient enhancement of plasticity observed in young adult-born neurons contributes to cognitive functions.

Cognition is not modified by large but temporary changes in sex hormones in men

CONTEXT

Little is known about the role of testosterone and estradiol on cognition in healthy older men.

OBJECTIVE

The cognitive effects of increasing or lowering testosterone or estradiol were examined.

DESIGN

Cognition was assessed before and after 6 wk of double-blind placebo-controlled hormone modification.

SETTING

The study was conducted at an academic medical center.

PARTICIPANTS

Healthy older (ages 60-80 yr) and younger men (ages 25-35 yr) were recruited from the community.

INTERVENTION

Men were randomized to one of four treatments: 1) maintain testosterone and estradiol at eugonadal levels for young men (GnRH agonist + testosterone gel); 2) block testosterone's conversion to estradiol (GnRH agonist + testosterone gel + aromatase inhibitor); 3) induce hypogonadism (GnRH agonist alone); and 4) all placebo.

MAIN OUTCOME MEASURES

Measures of executive function, memory, and spatial cognition were obtained before and after treatment. Hormone levels were obtained 10 times over the course of the study.

RESULTS

Counter to expectations, hormone treatment did not affect cognition (P > 0.10). Free testosterone was positively related to spatial cognition in older men after treatment and controlling for age and estradiol level or exclusion of the hypogonadal men (P = 0.02). Estradiol was negatively associated with working memory controlling for the same variables (P = 0.01). Blinding to treatment assignment was maintained, with the exception of the hypogonadal group.

CONCLUSIONS

A significant change in sex hormone status, including complete hypogonadism, does not modify cognition in men. These findings, along with studies that show a risk for neurodegenerative disease in those with low testosterone, suggest that sex hormone status may be important for neuroprotection in aging but not modulation of normal day-to-day cognitive function.

A computational predictor of human episodic memory based on a theta phase precession network

In the rodent hippocampus, a phase precession phenomena of place cell firing with the local field potential (LFP) theta is called “theta phase precession” and is considered to contribute to memory formation with spike time dependent plasticity (STDP). On the other hand, in the primate hippocampus, the existence of theta phase precession is unclear. Our computational studies have demonstrated that theta phase precession dynamics could contribute to primate–hippocampal dependent memory formation, such as object–place association memory. In this paper, we evaluate human theta phase precession by using a theory–experiment combined analysis. Human memory recall of object–place associations was analyzed by an individual hippocampal network simulated by theta phase precession dynamics of human eye movement and EEG data during memory encoding. It was found that the computational recall of the resultant network is significantly correlated with human memory recall performance, while other computational predictors without theta phase precession are not significantly correlated with subsequent memory recall. Moreover the correlation is larger than the correlation between human recall and traditional experimental predictors. These results indicate that theta phase precession dynamics are necessary for the better prediction of human recall performance with eye movement and EEG data. In this analysis, theta phase precession dynamics appear useful for the extraction of memory-dependent components from the spatio–temporal pattern of eye movement and EEG data as an associative network. Theta phase precession may be a common neural dynamic between rodents and humans for the formation of environmental memories.

Trial outcome and associative learning signals in the monkey hippocampus

In tasks of associative learning, animals establish new links between unrelated items by using information about trial outcome to strengthen correct/rewarded associations and modify incorrect/unrewarded ones. To study how hippocampal neurons convey information about reward and trial outcome during new associative learning, we recorded hippocampal neurons as monkeys learned novel object-place associations. A large population of hippocampal neurons (50%) signaled trial outcome by differentiating between correct and error trials during the period after the behavioral response. About half these cells increased their activity following correct trials (correct up cells) while the remaining half fired more following error trials (error up cells). Moreover, correct up cells, but not error up cells, conveyed information about learning by increasing their stimulus-selective response properties with behavioral learning. These findings suggest that information about successful trial outcome conveyed by correct up cells may influence new associative learning through changes in the cell's stimulus-selective response properties.

Reorganization of associative memory in humans with long-standing hippocampal damage

Conflicting theories have been advanced to explain why hippocampal lesions affect distinct memory domains and spare others. Recent findings in monkeys suggest that lesion-induced plasticity may contribute to the seeming preservation of some of these domains. We tested this hypothesis by investigating visuo-spatial associative memory in two patient groups with similar surgical lesions to the right medial temporal lobe, but different preoperative disease courses (benign brain tumours, mean: 1.8 +/- 0.6 years, n = 5, age: 28.2 +/- 4.0 years; hippocampal sclerosis, mean: 16.8 +/- 1.9 years, n = 9, age: 38.9 +/- 4.1 years). Compared to controls (n = 14), tumour patients showed a significant delay-dependent deficit in memory of colour-location associations. No such deficit was observed in hippocampal sclerosis patients, which appeared to benefit from a compensatory mechanism that was inefficient in tumour patients. These results indicate that long-standing hippocampal damage can yield significant functional reorganization of the neural substrate underlying memory in the human brain. We suppose that this process accounts for some of the discrepancies between results from previous lesion studies of the human medial temporal lobe.

The hippocampus is required for short-term topographical memory in humans

The hippocampus plays a crucial role within the neural systems for long-term memory, but little if any role in the short-term retention of some types of stimuli. Nonetheless, the hippocampus may be specialized for allocentric topographical processing, which impacts on short-term memory or even perception. To investigate this we developed performance-matched tests of perception (match-to-sample) and short-term memory (2 s delayed-match-to-sample) for the topography and for the nonspatial aspects of visual scenes. Four patients with focal hippocampal damage and one with more extensive damage, including right parahippocampal gyrus, were tested. All five patients showed impaired topographical memory and spared nonspatial processing in both memory and perception. Topographical perception was profoundly impaired in the patient with parahippocampal damage, mildly impaired in two of the hippocampal cases, and clearly preserved in the other two hippocampal cases (including one with dense amnesia). Our results suggest that the hippocampus supports allocentric topographical processing that is indispensable when appropriately tested after even very short delays, while the presence of the sample scene can allow successful topographical perception without it, possibly via a less flexible parahippocampal representation.

Alterations in right posterior hippocampus in early blind individuals

This study compares hippocampal volumes of early blind and sex/age-matched sighted controls through volumetric and localization analyses. Early blind individuals showed a significantly smaller right posterior hippocampus compared with controls. No differences in total hippocampal volumes were found between groups and there were no within-group differences for left versus right hippocampus. Sex, age and total brain grey matter volume had no effect on hippocampal volumes. Although extensive navigational training results in structural enhancement of the hippocampus for the sighted, the reduction of the posterior hippocampus in early blind individuals suggests the implication of this region in visual spatial memory.

Spatial memory and the human hippocampus

The hippocampus and adjacent medial temporal lobe structures are known to support declarative memory, but there is not consensus about what memory functions the hippocampus might support that are distinct from the functions of the adjacent cortex. One idea is that the hippocampus is specifically important for allocentric spatial memory, e.g., the hippocampus is especially needed to remember object locations when there is a shift in viewpoint between study and test. We tested this proposal in two experiments. Patients with damage limited to the hippocampus were given memory tests for object locations in a virtual environment. In the first experiment, participants studied locations of a variable number of images (one to five) and tried to remember the image locations from either the same viewpoint as during study (shift of 0 degrees) or a different viewpoint (shift of 55 degrees, 85 degrees, or 140 degrees). In each viewpoint condition (shifts of 0 degrees, 55 degrees, 85 degrees, and 140 degrees), patients performed normally when remembering one or two image locations. Further, performance declined to a similar degree in each viewpoint condition as patients tried to remember increasing numbers of image locations. In the second experiment, participants tried to remember four images after viewpoint shifts of 0 degrees, 55 degrees, 85 degrees, or 140 degrees. Patients were mildly impaired at all conditions (shifts of 0 degrees, 55 degrees, 85 degrees, and 140 degrees), and the impairment was no greater when viewpoint shifted. We conclude that damage to the hippocampus does not selectively impair viewpoint-independent spatial memory. Rather, hippocampal damage impairs memory as the memory load increases.

Brain injury impairs dentate gyrus inhibitory efficacy

Every 23 s, a person sustains a traumatic brain injury in the United States leaving many patients with substantial cognitive impairment and epilepsy. Injury-induced alterations in the hippocampus underpin many of these disturbances of neurological function. Abnormalities in the dentate gyrus are likely to play a major role in the observed pathophysiology because this subregion functions as a filter impeding excessive or aberrant activity from propagating further into the circuit and following experimental brain injury, the dentate gyrus becomes more excitable. Although alteration in excitation or inhibition could mediate this effect in the dentate gyrus, we show a key role played by an impairment of GABA(A)ergic inhibition. The efficacy of GABA(A)-mediated inhibition depends on a low [Cl-]i that is maintained by neuronal K-Cl co-transporter 2 (KCC2). Using fluid percussion injury (FPI) in the mouse, we demonstrate significant reductions in KCC2 protein and mRNA expression in the dentate gyrus that causes a depolarizing shift in GABA(A) reversal potential, due to impaired chloride clearance, resulting in reduced inhibitory efficiency. This study elucidates a novel mechanism underlying diminished dentate gyrus inhibitory efficacy and provides an innovative target for the development of potential therapeutics to restore the severe pathological consequences of traumatic brain injury.

Injury-induced alterations in CNS electrophysiology

Mild to moderate cases of traumatic brain injury (TBI) are very common, but are not always associated with the overt pathophysiogical changes seen following severe trauma. While neuronal death has been considered to be a major factor, the pervasive memory, cognitive and motor function deficits suffered by many mild TBI patients do not always correlate with cell loss. Therefore, we assert that functional impairment may result from alterations in surviving neurons. Current research has begun to explore CNS synaptic circuits after traumatic injury. Here we review significant findings made using in vivo and in vitro models of TBI that provide mechanistic insight into injury-induced alterations in synaptic electrophysiology. In the hippocampus, research now suggests that TBI regionally alters the delicate balance between excitatory and inhibitory neurotransmission in surviving neurons, disrupting the normal functioning of synaptic circuits. In another approach, a simplified model of neuronal stretch injury in vitro, has been used to directly explore how injury impacts the physiology and cell biology of neurons in the absence of alterations in blood flow, blood brain barrier integrity, or oxygenation associated with in vivo models of brain injury. This chapter discusses how these two models alter excitatory and inhibitory synaptic transmission at the receptor, cellular and circuit levels and how these alterations contribute to cognitive impairment and a reduction in seizure threshold associated with human concussive brain injury.

Involvement of the anterior cingulate cortex in the expression of remote spatial memory

Although the hippocampus plays a crucial role in the formation of spatial memories, as these memories mature they may become additionally (or even exclusively) dependent on extrahippocampal structures. However, the identity of these extrahippocampal structures that support remote spatial memory is currently not known. Using a Morris water-maze task, we show that the anterior cingulate cortex (ACC) plays a key role in the expression of remote spatial memories in mice. To first evaluate whether the ACC is activated after the recall of spatial memory, we examined the expression of the immediate early gene, c-fos, in the ACC. Fos expression was elevated after expression of a remote (1 month old), but not recent (1 d old), water-maze memory, suggesting that ACC plays an increasingly important role as a function of time. Consistent with the gene expression data, targeted pharmacological inactivation of the ACC with the sodium channel blocker lidocaine blocked expression of remote, but spared recent, spatial memory. In contrast, inactivation of the dorsal hippocampus disrupted expression of spatial memory, regardless of its age. We further showed that inactivation of the ACC blocked expression of remote spatial memory in two different mouse strains, after training with either a hidden or visible platform in a constant location, and using the AMPA receptor antagonist CNQX. Together, our data provide evidence that circuits supporting spatial memory are reorganized in a time-dependent manner, and establish that activity in neurons intrinsic to the ACC is critical for processing remote spatial memories.

Spatial and temporal order memory in Korsakoff patients

This study directly compared how well Korsakoff patients can process spatial and temporal order information in memory under conditions that included presentation of only a single feature (i.e., temporal or spatial information), combined spatiotemporal presentation, and combined spatiotemporal order recall. Korsakoff patients were found to suffer comparable spatial and temporal order recall deficits. Of interest, recall of a single feature was the same when only spatial or temporal information was presented compared to conditions that included combined spatiotemporal, presentation and recall. In contrast, control participants performed worse when they have to recall both spatial and temporal order compared to when they have to recall only one of these features. These findings together indicate that spatial and temporal information are not automatically integrated. Korsakoff patients have profound problems in coding the feature at hand. Moreover, their lower recall of both features at the same time suggests that Korsakoff patients are impaired in binding different contextual attributes together in memory.

Acute and chronic estradiol treatments reduce memory deficits induced by transient global ischemia in female rats

Transient global ischemia induces selective, delayed neuronal death in the hippocampal CA1 and delayed cognitive deficits. Estrogen treatment ameliorates hippocampal injury associated with global ischemia. Although much is known about the impact of estrogen on neuronal survival, relatively little is known about its impact on functional outcome assessed behaviorally. We investigated whether long-term estradiol (21-day pellets implanted 14 days prior to ischemia) or acute estradiol (50 microg infused into the lateral ventricles immediately after ischemia) attenuates ischemia-induced cell loss and improves visual and spatial working memory in ovariectomized female rats. Global ischemia significantly impaired visual and spatial memory, assessed by object recognition and object placement tests at 6-9 days. Global ischemia did not affect locomotion, exploration, or anxiety-related behaviors, assessed by an open-field test at 6 days. Long-term estradiol prevented the ischemia-induced deficit in visual working memory, maintaining normal performance in tests with retention intervals of up to 1 h. Long-term estradiol also prevented ischemia-induced deficits in spatial memory tests with short (1 and 7 min), but not longer (15 min), retention intervals. Acute estradiol significantly improved visual memory assessed with short retention intervals, but did not prevent deficits in spatial memory. Acute estradiol significantly increased the number of surviving CA1 neurons, assessed either at 7 days after ischemia or after the completion of behavioral testing 9 days after ischemia. In contrast, chronic estradiol did not reduce CA1 cell death 9 days after ischemia. Thus, long-term estradiol at near physiological levels and acute estradiol administered after ischemic insult improve functional recovery after global ischemia. These findings have important implications for intervention in the neurological sequellae associated with global ischemia.

Wayfinding in familiar and unfamiliar environments in a case of progressive topographical agnosia

A 71-year-old right-handed man (F.G.) presents with prosopagnosia and with an inability to recognize famous and familiar buildings. Despite his deficit, F.G. obtained normal scores on neuropsychological tests of executive functions, language, praxis and primary visuoperceptual skills. Brain MRI showed atrophy predominantly in the right temporal lobe, particularly in the fusiform gyrus and the parahippocampal cortex. The present study investigated F.G.'s ability to orient himself in familiar and new environments. His wayfinding abilities in a familiar environment (i.e., his hometown) were preserved despite an inability to recognize familiar and famous buildings, monuments and landmarks in this environment. Wayfinding was achieved through a heavy reliance on written indications (e.g., names of restaurants and streets), preservation of a pre-existing cognitive map of this familiar environment, and normal executive functions necessary to plan the execution of a given trajectory. In an unfamiliar environment, F.G.'s topographical agnosia resulted in severe wayfinding difficulties and in the inability to build an adequate cognitive spatial representation. F.G.'s topographical agnosia results from a high-level visuoperceptual deficit, characterized by an inability to access a global configuration of complex visual stimuli such as familiar and famous monuments, and an over-reliance on the processing of local features.

Regional hippocampal alteration associated with cognitive deficit following experimental brain injury: a systems, network and cellular evaluation

Cognitive deficits persist in patients who survive traumatic brain injury (TBI). Lateral fluid percussion brain injury in the mouse, a model of human TBI, results in hippocampal-dependent cognitive impairment, similar to retrograde amnesia often associated with TBI. To identify potential substrates of the cognitive impairment, we evaluated regional neuronal loss, regional hippocampal excitability and inhibitory synaptic transmission. Design-based stereology demonstrated an approximate 40% loss of neurons through all subregions of the hippocampus following injury compared with sham. Input/output curves recorded in slices of injured brain demonstrated increased net synaptic efficacy in the dentate gyrus in concert with decreased net synaptic efficacy and excitatory postsynaptic potential-spike relationship in area CA1 compared with sham slices. Pharmacological agents modulating inhibitory transmission partially restored regional injury-induced alterations in net synaptic efficacy. Both evoked and spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in surviving dentate granule neurons were smaller and less frequent in injured brains than in uninjured brains. Conversely, both evoked and spontaneous mIPSCs recorded in surviving area CA1 pyramidal neurons were larger in injured brains than in uninjured brains. Together, these alterations suggest that regional hippocampal function is altered in the injured brain. This study demonstrates for the first time that brain injury selectively disrupts hippocampal function by causing uniform neuronal loss, inhibitory synaptic dysfunction, and regional, but opposing, shifts in circuit excitability. These changes may contribute to the cognitive impairments that result from brain injury.

Online formation of a hierarchical cognitive map for object-place association by theta phase coding

Object-place associative memory, the storage of object and place conjunctions based on a one-time experience, is hippocampal-dependent in humans. Theta phase precession, a type of neural dynamics observed in the rat hippocampus, has recently been suggested to serve a role in instantaneous memory formation based on a one-time experience, while its functional role in associating distinct types of information (object and place information) is unclear. In this study, we hypothesize that theta phase encoding with theta phase precession contributes to the storage of object-place associations. To examine this hypothesis, we propose a neural network model of the corticohippocampal system, including central-peripheral visual pathways and theta phase coding in the hippocampus. Memory storage computer experiments demonstrate that the hippocampal network successfully stores the object-place associations of a one-time experience. Interestingly, it is also found that a random visual input sequence results in a robust formation of asymmetric connections between objects and scenes instantaneously after a single trial. Furthermore, it is found that scene-object connections and scene-scene connections form a hierarchical network representing the spatial alignment of scenes and objects in the environment. Our findings indicate that the theta phase coding, as observed in the rat hippocampus, can facilitate the online memory storage of complex environments in humans as a hierarchical cognitive map.

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.

Visual presentation of novel objects and new spatial arrangements of objects differentially activates the medial temporal lobe subareas in humans

A number of studies in rodents and monkeys report a distinction between the contributions of the hippocampus and perirhinal cortex to memory, such that the hippocampus is crucial for spatial memory whereas the perirhinal cortex has a pivotal role in perception and memory for visual objects. To determine if there is such a distinction in humans, we conducted a functional magnetic resonance imaging study to compare the medial temporal lobe responses to changes in object identity and spatial configurations of objects. We found evidence for the predicted distinction between hippocampal and perirhinal cortical activations, although part of the hippocampus was also activated by identification of novel objects. Additionally, an anterior-posterior activation gradient emerged inside the hippocampus and parahippocampal cortex. The anterior hippocampus, perirhinal cortex and anterior parahippocampal cortex are involved in perception of contextually novel objects, whereas the posterior hippocampus and posterior parahippocampal cortex are involved in processing of novel arrangements of familiar objects. These results demonstrate that there is a functional dissociation between processing of novel object identities and new spatial locations of objects among the subregions of medial temporal lobe structures in humans also.

Memory for visuospatial location following selective hippocampal sclerosis: the use of different coordinate systems

This study addressed the role of the medial temporal lobe regions and, more specifically, the contribution of the human hippocampus in memory for body-centered (egocentric) and environment-centered (allocentric) spatial location. Twenty-one patients with unilateral atrophy of the hippocampus secondary to long-standing epilepsy (left, n = 7; right, n = 14) and 15 normal control participants underwent 3 tasks measuring recall of egocentric or allocentric spatial location. Patients with left hippocampal sclerosis were consistently impaired in the allocentric conditions of all 3 tasks but not in the egocentric conditions. Patients with right hippocampal sclerosis were impaired to a lesser extent and in only 2 of the 3 tasks. It was concluded that hippocampal structures are crucial for allocentric, but not egocentric, spatial memory.

Memory for faces dissociates from memory for location following anterior temporal lobectomy

It has been suggested that the right and left mesial temporal lobes are specialized for processing different types of information for long-term memory (LTM). Although findings have been consistent in regard to the dominant role of the left mesial temporal lobe (MTL) in verbal memory, the role of the right MTL in non-verbal memory remains debatable. Given the existence of two cortical pathways specialized for processing different types of visuospatial information, we examined whether memory processing for these two types of information might also be differentially localized. The effect of unilateral anterior temporal lobectomy (ATL) was compared for memory for unfamiliar faces and a novel sequence of spatial locations in 86 ATL patients (left ATL (LATL) = 35; right ATL (RATL) = 51) pre- and post-surgery to examine the contributions of right and left MTL structures to LTM for pattern and spatial information. Memory for spatial location was not related to side of ATL or time of testing. On the facial memory task, RATL patients showed impairments relative to LATL patients prior to surgery, which became magnified after surgery. These results extend the proposed dissociation between an occipitotemporal and an occipitoparietal pathway for processing visuospatial information by demonstrating dissociations in LTM systems for these two types of information. Lesions in the right MTL, adjacent to the cortical structures believed to be specialized for facial recognition, specifically impair the memory encoding of new faces, but not spatial locations.

Effects of hippocampal lesions in a food location task in pigeons

This study investigated the role of the hippocampus in pigeons learning of a food-related choice task. The effects of lesions induced by ibotenic acid were analyzed in two experiments. Experiment 1 investigated the effects of hippocampal damage on postoperative memory retrieval and in reversal learning. Experiment 2 investigated the effects of hippocampal lesions on the acquisition and reversal of learning. In both experiments probe tests were used to assess the behavioral strategies underlying the choice. In Experiment 1 hippocampal lesions impaired the preoperative learned performance in terms of choice latency but not choice accuracy. Experiment 2 data showed that, in postoperative learning sessions, latency as well as choice accuracy were impaired by hippocampal damage. The probe tests, in which a curtain was placed around the chamber, revealed behavioral patterns of a non-mapping strategy. This was true in both experiments and groups (experimental and controls). Immediately after training, during the probe tests of both experiments, in which food cups were omitted, the three groups spent more time in the target quadrant. However, immediately after the reversal condition, neither hippocampal damaged nor control pigeons showed a preference for the target quadrant. This may be interpreted as evidence for a hippocampal role in stimulus location learning involving non-mapping strategies.

Encoding activity in anterior medial temporal lobe supports subsequent associative recognition

The ability to bind information together, such as linking a name with a face or a car with a parking space, is a vital process in human episodic memory. To identify the neural bases for this binding process, we measured brain activity during a verbal associative encoding task using event-related functional MRI (fMRI), followed by an associative recognition test for the studied word pairs. Analysis of the encoding data sorted by the associative recognition accuracy allowed us to isolate regions involved in successfully creating associations. We found that encoding activity in bilateral anterior medial temporal lobe (MTL) regions was greater for successfully bound pairs, that is, those later recognized as intact, than for all other pairs. These findings provide evidence that the anterior medial temporal lobes support the successful binding of information in memory.

Cortical control of ocular saccades in humans: a model for motricity

Our knowledge of the cortical control of saccadic eye movements (saccades) in humans has recently progressed mainly thanks to lesion and transcranial magnetic stimulation (TMS) studies, but also to functional imaging. It is now well-known that the frontal eye field is involved in the triggering of intentional saccades, the parietal eye field in that of reflexive saccades, the supplementary eye field (SEF) in the initiation of motor programs comprising saccades, the pre-SEF in learning of these programs, and the dorsolateral prefrontal cortex (DLPFC) in saccade inhibition, prediction and spatial working memory. Saccades may also be used as a convenient model of motricity to study general cognitive processes preparing movements, such as attention, spatial memory and motivation. Visuo-spatial attention appears to be controlled by a bilateral parieto-frontal network comprising different parts of the posterior parietal cortex and the frontal areas involved in saccade control, suggesting that visual attentional shifts and saccades are closely linked. Recently, our understanding of the cortical control of spatial memory has noticeably progressed by using the simple visuo-oculomotor model represented by the memory-guided saccade paradigm, in which a single saccade is made to the remembered position of a unique visual item presented a while before. TMS studies have determined that, after a brief stage of spatial integration in the posterior parietal cortex (inferior to 300 ms), short-term spatial memory (i.e. up to 15-20 s) is controlled by the DLPFC. Behavioral and lesion studies have shown that medium-term spatial memory (between 15-20 s and a few minutes) is specifically controlled by the parahippocampal cortex, before long-term memorization (i.e. after a few minutes) in the hippocampal formation. Lastly, it has been shown that the posterior part of the anterior cingulate cortex, called the cingulate eye field, is involved in motivation and the preparation of all intentional saccades, but not in reflexive saccades. These different but complementary study methods used in humans have thus contributed to a better understanding of both eye movement physiology and general cognitive processes preparing motricity as whole.