Impaired visual and odor recognition memory span in patients with hippocampal lesions

Differential role of NMDA receptors in hippocampal-dependent spatial memory and plasticity in juvenile male and female rats

Early life, or juvenility, stands out as the most pivotal phase in neurodevelopment due to its profound impact over the long-term cognition. During this period, significant changes are made in the brain's connections both within and between different areas, particularly in tandem with the development of more intricate behaviors. The hippocampus is among the brain regions that undergo significant postnatal remodeling, including dendritic arborization, synaptogenesis, the formation of complex spines and neuron proliferation. Given the crucial role of the hippocampus in spatial memory processing, it has been observed that spatial memory abilities continue to develop as the hippocampus matures, particularly before puberty. The N-methyl-d-aspartate (NMDA) type of glutamate receptor channel is crucial for the induction of activity-dependent synaptic plasticity and spatial memory formation in both rodents and humans. Although extensive evidence shows the role of NMDA receptors (NMDAr) in spatial memory and synaptic plasticity, the studies addressing the role of NMDAr in spatial memory of juveniles are sparse and mostly limited to adult males. In the present study, we, therefore, aimed to investigate the effects of systemic NMDAr blockade by the MK-801 on spatial memory (novel object location memory, OLM) and hippocampal plasticity in the form of long-term potentiation (LTP) of both male and female juvenile rats. Our results show the sex-dimorphic role of NMDAr in spatial memory and plasticity during juvenility, as systemic NMDAr blockade impairs the OLM and LTP in juvenile males without an effect on juvenile females. Taken together, our results demonstrate that spatial memory and hippocampal plasticity are NMDAr-dependent in juvenile males and NMDAr-independent in juvenile females. These sex-specific differences in the mechanisms of spatial memory and plasticity may imply gender-specific treatment for spatial memory disorders even in children.

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.

Olfactory Dysfunction in Schizophrenia: Evaluating Olfactory Abilities Across Species

Though understudied relative to perturbations in the auditory and visual domains, olfactory dysfunction is a common symptom of schizophrenia. Over the past two decades, the availability of standardized assessments to quantify human olfactory abilities, and enhance understanding of the neurophysiology supporting olfaction, has increased, enabling a more thorough characterization of these deficits. In contrast to other psychiatric conditions for which olfactory dysfunction has been observed (e.g., major depressive disorder, bipolar disorder, Alzheimer's disease), the impairments observed in schizophrenia are particularly global and profound. At this level, such deficits in olfactory abilities likely impact the enjoyment of food, detection of environmental hazards, and influence social relationships. More broadly, the study of olfactory phenotypes in schizophrenia presents new avenues for detection of those at-risk for the condition, identification of therapeutic targets for treatment development, and for the characterization of novel animal models relevant to schizophrenia and psychosis. This review will consider the olfactory performance of individuals with schizophrenia in domains for which standardized assessments are available (odor sensitivity, discrimination, identification, and memory). Paradigms available for assessing these abilities in rodents will also be discussed with the aim of facilitating translation. Thus, future studies will be able to include cross-species translation of mechanisms relevant to olfactory function and cognition, what has gone awry in the disease state, and test potential therapeutics.

The rodent medial prefrontal cortex and associated circuits in orchestrating adaptive behavior under variable demands

Emerging evidence implicates rodent medial prefrontal cortex (mPFC) in tasks requiring adaptation of behavior to changing information from external and internal sources. However, the computations within mPFC and subsequent outputs that determine behavior are incompletely understood. We review the involvement of mPFC subregions, and their projections to the striatum and amygdala in two broad types of tasks in rodents: 1) appetitive and aversive Pavlovian and operant conditioning tasks that engage mPFC-striatum and mPFC-amygdala circuits, and 2) foraging-based tasks that require decision making to optimize reward. We find support for region-specific function of the mPFC, with dorsal mPFC and its projections to the dorsomedial striatum supporting action control with higher cognitive demands, and ventral mPFC engagement in translating affective signals into behavior via discrete projections to the ventral striatum and amygdala. However, we also propose that defined mPFC subdivisions operate as a functional continuum rather than segregated functional units, with crosstalk that allows distinct subregion-specific inputs (e.g., internal, affective) to influence adaptive behavior supported by other subregions.

Neural evidence for recognition of naturalistic videos in monkey hippocampus

The role of the hippocampus in recognition memory has long been a source of debate. Tasks used to study recognition that typically require an explicit probe, where the participant must make a response to prove they remember, yield mixed results on hippocampal involvement. Here, we tasked monkeys to freely view naturalistic videos, and only tested their memory via looking times for two separate novel versus repeat video conditions on each trial. Notably, a large proportion (>30%) of hippocampal neurons differentiated these videos via changes in firing rates time-locked to the duration of their presentation on screen, and not during the delay period between them as would be expected for working memory. Many of these single neurons (>15%) contributed to both retrieval conditions, and differentiated novel from repeat videos across trials with trial-unique content, suggesting they detect familiarity. The majority of neurons contributing to the classifier showed an enhancement in firing rate on repeat compared with novel videos, a pattern which has not previously been shown in hippocampus. These results suggest the hippocampus contributes to recognition memory via familiarity during free-viewing.

A novel paradigm for assessing olfactory working memory capacity in mice

A decline in working memory (WM) capacity is suggested to be one of the earliest symptoms observed in Alzheimer's disease (AD). Although WM capacity is widely studied in healthy subjects and neuropsychiatric patients, few tasks are developed to measure this variation in rodents. The present study describes a novel olfactory working memory capacity (OWMC) task, which assesses the ability of mice to remember multiple odours. The task was divided into five phases: context adaptation, digging training, rule-learning for non-matching to a single-sample odour (NMSS), rule-learning for non-matching to multiple sample odours (NMMS) and capacity testing. During the capacity-testing phase, the WM capacity (number of odours that the mice could remember) remained stable (average capacity ranged from 6.11 to 7.00) across different testing sessions in C57 mice. As the memory load increased, the average errors of each capacity level increased and the percent correct gradually declined to chance level, which suggested a limited OWMC in C57 mice. Then, we assessed the OWMC of 5 × FAD transgenic mice, an animal model of AD. We found that the performance displayed no significant differences between young adult (3-month-old) 5 × FAD mice and wild-type (WT) mice during the NMSS phase and NMMS phase; however, during the capacity test with increasing load, we found that the OWMC of young adult 5 × FAD mice was significantly decreased compared with WT mice, and the average error was significantly increased while the percent correct was significantly reduced, which indicated an impairment of WM capacity at the early stage of AD in the 5 × FAD mice model. Finally, we found that FOS protein levels in the medial prefrontal cortex and entorhinal cortex after the capacity test were significantly lower in 5 × FAD than WT mice. In conclusion, we developed a novel paradigm to assess the capacity of olfactory WM in mice, and we found that OWMC was impaired in the early stage of AD.

Effect of Worry Level on Recall Memory for Odors in ApoE-ε4 Carriers and Non-Carriers

OBJECTIVE

Increased levels of worry, age, and presence of the apolipoprotein-E (ApoE)-ε4 allele are associated with the risk of developing cognitive declines and Alzheimer's disease (AD). Odor memory performance has been shown to vary as a function of age and ApoE genotype, and odor memory tests are sensitive to preclinical AD. Worry is known to influence verbal memory; however, its effects on odor memory are unknown. This study aimed to assess the relationships between worry, age, and ε4 status on odor memory.

METHOD

Worry was evaluated for young (n = 53) and older (n = 45) adults using the Penn State Worry Questionnaire. Odor memory was assessed using the California Odor Learning Test, an olfactory analogue to the California Verbal Learning Test.

RESULTS

A significant main effect of worry on long-delay free recall was found, such that increasing worry was associated with better recall across age and ε4 status. A significant interaction effect between ε4 status and worry on both short-and long-delay cued recall was found, such that across age, higher worry was associated with increased cued recall scores among ε4-negative adults, and decreased scores among ε4-positive adults.

CONCLUSIONS

Findings demonstrated that worry influences odor memory and exerts a particular effect on cued recall among ε4 carriers who are at a greater risk of developing AD. Worry is a modifiable predictor of cognitive decline and risk of dementia in aging. Future studies on the effects of treatments aimed at reducing worry (e.g., cognitive behavioral therapies for anxiety) on changes in cognitive functioning are warranted.

Validation of the human odor span task: effects of nicotine

RATIONALE

Amongst non-smokers, nicotine generally enhances performance on tasks of attention, with limited effect on working memory. In contrast, nicotine has been shown to produce robust enhancements of working memory in non-humans.

OBJECTIVES

To address this gap, the present study investigated the effects of nicotine on the performance of non-smokers on a cognitive battery which included a working memory task reverse-translated from use with rodents (the odor span task, OST). Nicotine has been reported to enhance OST performance in rats and the present study assessed whether this effect generalizes to human performance.

METHODS

Thirty non-smokers were tested on three occasions after consuming either placebo, 2 mg, or 4 mg nicotine gum. On each occasion, participants completed a battery of clinical and experimental tasks of working memory and attention.

RESULTS

Nicotine was associated with dose-dependent enhancements in sustained attention, as evidenced by increased hit accuracy on the rapid visual information processing (RVIP) task. However, nicotine failed to produce main effects on OST performance or on alternative measures of working memory (digit span, spatial span, letter-number sequencing, 2-back) or attention (digits forward, 0-back). Interestingly, enhancement of RVIP performance occurred concomitant to significant reductions in self-reported attention/concentration. Human OST performance was significantly related to N-back performance, and as in rodents, OST accuracy declined with increasing memory load.

CONCLUSIONS

Given the similarity of human and rodent OST performance under baseline conditions and the strong association between OST and visual 0-back accuracy, the OST may be particular useful in the study of conditions characterized by inattention.

Influence of pharmacological manipulations of NMDA and cholinergic receptors on working versus reference memory in a dual component odor span task

Developed as a tool to assess working memory capacity in rodents, the odor span task (OST) has significant potential to advance drug discovery in animal models of psychiatric disorders. Prior investigations indicate OST performance is impaired by systemic administration of N-methyl-d-aspartate receptor (NMDA-r) antagonists and is sensitive to cholinergic manipulations. The present study sought to determine whether an impairment in OST performance can be produced by systemic administration of the competitive NMDA-r antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP; 3, 10, 17 mg/kg i.p.) in a unique dual-component variant of the OST, and whether this impairment is ameliorated by nicotine (0.75 mg/kg i.p.). Male Sprague-Dawley rats were trained to asymptotic level of performance on a 24-trial two-comparison incrementing nonmatching to sample OST. In addition, rats were administered a two-comparison olfactory reference memory (RM) task, which was integrated into the OST. The RM task provided an assessment of the effects of drug administration on global behavioral measures, long-term memory and motivation. Several measures of working memory (span, longest run, and accuracy) were dose dependently impaired by CPP without adversely affecting RM. Analysis of drug effects across trial blocks demonstrated a significant impairment of performance even at low memory loads, suggesting a CPP-induced deficit of olfactory short-term memory that is not load-dependent. Although nicotine did not ameliorate CPP-induced impairments in span or accuracy, it did block the impairment in longest run produced by the 10 mg/kg dose of CPP. Overall, our results indicate that performance in our 24 odor two-comparison OST is capacity dependent and that CPP impaired OST working, but not reference, memory.

Recognition memory span in autopsy-confirmed Dementia with Lewy Bodies and Alzheimer's Disease

Evidence from patients with amnesia suggests that recognition memory span tasks engage both long-term memory (i.e., secondary memory) processes mediated by the diencephalic-medial temporal lobe memory system and working memory processes mediated by fronto-striatal systems. Thus, the recognition memory span task may be particularly effective for detecting memory deficits in disorders that disrupt both memory systems. The presence of unique pathology in fronto-striatal circuits in Dementia with Lewy Bodies (DLB) compared to AD suggests that performance on the recognition memory span task might be differentially affected in the two disorders even though they have quantitatively similar deficits in secondary memory. In the present study, patients with autopsy-confirmed DLB or AD, and Normal Control (NC) participants, were tested on separate recognition memory span tasks that required them to retain increasing amounts of verbal, spatial, or visual object (i.e., faces) information across trials. Results showed that recognition memory spans for verbal and spatial stimuli, but not face stimuli, were lower in patients with DLB than in those with AD, and more impaired relative to NC performance. This was despite similar deficits in the two patient groups on independent measures of secondary memory such as the total number of words recalled from long-term storage on the Buschke Selective Reminding Test. The disproportionate vulnerability of recognition memory span task performance in DLB compared to AD may be due to greater fronto-striatal involvement in DLB and a corresponding decrement in cooperative interaction between working memory and secondary memory processes. Assessment of recognition memory span may contribute to the ability to distinguish between DLB and AD relatively early in the course of disease.

Developing treatments for cognitive deficits in schizophrenia: the challenge of translation

Schizophrenia is a life-long debilitating mental disorder affecting tens of millions of people worldwide. The serendipitous discovery of antipsychotics focused pharmaceutical research on developing a better antipsychotic. Our understanding of the disorder has advanced however, with the knowledge that cognitive enhancers are required for patients in order to improve their everyday lives. While antipsychotics treat psychosis, they do not enhance cognition and hence are not antischizophrenics. Developing pro-cognitive therapeutics has been extremely difficult, however, especially when no approved treatment exists. In lieu of stumbling on an efficacious treatment, developing targeted compounds can be facilitated by understanding the neural mechanisms underlying altered cognitive functioning in patients. Equally importantly, these cognitive domains will need to be measured similarly in animals and humans so that novel targets can be tested prior to conducting expensive clinical trials. To date, the limited similarity of testing across species has resulted in a translational bottleneck. In this review, we emphasize that schizophrenia is a disorder characterized by abnormal cognitive behavior. Quantifying these abnormalities using tasks having cross-species validity would enable the quantification of comparable processes in rodents. This approach would increase the likelihood that the neural substrates underlying relevant behaviors will be conserved across species. Hence, we detail cross-species tasks which can be used to test the effects of manipulations relevant to schizophrenia and putative therapeutics. Such tasks offer the hope of providing a bridge between non-clinical and clinical testing that will eventually lead to treatments developed specifically for patients with deficient cognition.

Cerebrolysin improves cognitive performance in rats after mild traumatic brain injury

OBJECT

Long-term memory deficits occur after mild traumatic brain injuries (mTBIs), and effective treatment modalities are currently unavailable. Cerebrolysin, a peptide preparation mimicking the action of neurotrophic factors, has beneficial effects on neurodegenerative diseases and brain injuries. The present study investigated the long-term effects of Cerebrolysin treatment on cognitive function in rats after mTBI.

METHODS

Rats subjected to closed-head mTBI were treated with saline (n = 11) or Cerebrolysin (2.5 ml/kg, n = 11) starting 24 hours after injury and then daily for 28 days. Sham animals underwent surgery without injury (n = 8). To evaluate cognitive function, the modified Morris water maze (MWM) test and a social odor-based novelty recognition task were performed after mTBI. All rats were killed on Day 90 after mTBI, and brain sections were immunostained for histological analyses of amyloid precursor protein (APP), astrogliosis, neuroblasts, and neurogenesis.

RESULTS

Mild TBI caused long-lasting cognitive memory deficits in the MWM and social odor recognition tests up to 90 days after injury. Compared with saline treatment, Cerebrolysin treatment significantly improved both long-term spatial learning and memory in the MWM test and nonspatial recognition memory in the social odor recognition task up to 90 days after mTBI (p < 0.05). Cerebrolysin significantly increased the number of neuroblasts and promoted neurogenesis in the dentate gyrus, and it reduced APP levels and astrogliosis in the corpus callosum, cortex, dentate gyrus, CA1, and CA3 regions (p < 0.05).

CONCLUSIONS

These results indicate that Cerebrolysin treatment of mTBI improves long-term cognitive function, and this improvement may be partially related to decreased brain APP accumulation and astrogliosis as well as increased neuroblasts and neurogenesis.

Improved hippocampal dose with reduced margin radiotherapy for glioblastoma multiforme

BACKGROUND

To dosimetrically evaluate the effect of reduced margin radiotherapy on hippocampal dose for glioblastoma multiforme (GBM) patients.

METHODS

GBM patients enrolled on the Radiation Therapy Oncology Group (RTOG) 0825 trial at our institution were identified. Standard RTOG 0825 expansions were 2 cm + 3-5 mm from the gross tumor volume (GTV) to the clinical tumor volume (CTV) and from the CTV to the planning tumor volume (PTV), respectively. These same patients also had reduced margin tumor volumes generated with 8 mm (GTV to CTV) + 3 mm (CTV to PTV) expansions. Individual plans were created for both standard and reduced margin structures. The dose-volume histograms were statistically compared with a paired, two-tailed Student's t-test with a significance level of p < 0.05.

RESULTS

A total of 16 patients were enrolled on RTOG 0825. The reduced margins resulted in statistically significant reductions in hippocampal dose at all evaluated endpoints. The hippocampal Dmax was reduced from a mean of 61.4 Gy to 56.1 Gy (8.7%), D40% was reduced from 49.9 Gy to 36.5 Gy (26.9%), D60% was reduced from 32.7 Gy to 18.7 Gy (42.9%) and the D80% was reduced from 27.3 Gy to 15.3 Gy (44%).

CONCLUSIONS

The use of reduced margin PTV expansions in the treatment of GBM patients results in significant reductions in hippocampal dose. Though the exact clinical benefit of this reduction is currently unclear, this study does provide support for a future prospective trial evaluating the neurocognitive benefits of reduced margin tumor volumes in the treatment of GBM patients.

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.

Implicit recognition based on lateralized perceptual fluency

In some circumstances, accurate recognition of repeated images in an explicit memory test is driven by implicit memory. We propose that this “implicit recognition” results from perceptual fluency that influences responding without awareness of memory retrieval. Here we examined whether recognition would vary if images appeared in the same or different visual hemifield during learning and testing. Kaleidoscope images were briefly presented left or right of fixation during divided-attention encoding. Presentation in the same visual hemifield at test produced higher recognition accuracy than presentation in the opposite visual hemifield, but only for guess responses. These correct guesses likely reflect a contribution from implicit recognition, given that when the stimulated visual hemifield was the same at study and test, recognition accuracy was higher for guess responses than for responses with any level of confidence. The dramatic difference in guessing accuracy as a function of lateralized perceptual overlap between study and test suggests that implicit recognition arises from memory storage in visual cortical networks that mediate repetition-induced fluency increments.

Why avoid the hippocampus? A comprehensive review

In this review article, we provide a detailed and comprehensive discussion of the rationale for using modern IMRT techniques to spare the subgranular zone of the hippocampus during cranial irradiation. We review the literature on neurocognitive effects of cranial irradiation; discuss clinical and preclinical data associating damage to neural progrenitor cells located in subgranular zone of the hippocampus with radiation-induced neurocognitive decline, specifically in terms of short-term memory formation and recall; and present a review of our pilot investigations into the feasibility and risks of sparing the subgranular zone of the hippocampus during whole-brain radiotherapy for brain metastases. We also introduce our phase II cooperative group clinical trial (RTOG 0933) designed to prospectively evaluate the postulated neurocognitive benefit of hippocampal subgranular zone sparing and scheduled to open in 2010.

Brain atrophy and cognitive impairment in survivors of acute respiratory distress syndrome

PRIMARY OBJECTIVES

Acute Respiratory Distress Syndrome (ARDS) is characterized by severe acute lung injury, hypoxemia and is associated with neurological and cognitive impairments. This study assessed quantitative brain and ventricular volumes in survivors of ARDS with brain computed tomography (CT) scans compared to normal controls. It also compared the medical and cognitive outcome data of patients with ARDS with and without CT scans.

RESEARCH DESIGN

Observational cohort study.

METHODS

Sixty-six consecutive acute respiratory distress patients, of which 15 patients with ARDS underwent brain CT and 51 patients had no brain imaging. Brain CT scans from 15 survivors of ARDS were compared to age- and sex-matched normal controls. Clinical radiological findings and ventricular volumes, brain volume and generalized brain atrophy.

RESULTS

The patients with ARDS and brain imaging had cognitive impairments, significant brain atrophy, ventricular enlargement and 53% had atrophy or lesions by radiological report.

CONCLUSIONS

Clinicians need to be aware that ARDS can cause significant long-term brain-related morbidity manifest by brain atrophy, lesions and neurocognitive impairments.

Differential effects of normal aging on memory for odor-place and object-place associations

Odor-place and object-place associative memory were compared in healthy older (over the age of 65) and young (18 to 25 years of age) adults. Twelve spatial locations were defined on a tabletop board. Either six odors or six objects were presented one at a time and each was paired with a location on the board. The participant then was presented with each stimulus individually and asked to place it in its paired location. Older adults showed impaired odor-place associative memory but unimpaired object-place memory compared to young adults. Item recognition memory for the individual stimuli or locations used on the associative memory task was similar in both groups. The results suggest that odor-place associative memory is particularly affected by age-related brain changes.

Progressive impairment in olfactory working memory in a mouse model of Mild Cognitive Impairment

Patients with Mild Cognitive Impairment (MCI), exhibiting both working memory and olfactory deficits are likely to progress to Alzheimer's disease (AD). Targeting this pre-clinical AD population with disease modifying agents or cognitive enhancers represents the best strategy for halting or delaying the impact of this pernicious disease. However, there is a paucity of animal models of MCI with which to assess putative therapeutic strategies. We describe an odour span task which assesses the ability of mice to remember lists of odours, and report subtle cognitive deficits in human amyloid over-expressing (Tg2576) mice, at an age prior to plaque deposition. Four-month-old Tg2576 mice exhibited normal acquisition and performance in the standard 12-span task, but were significantly impaired when memory load was increased to 22 odours. By 8-months, a performance deficit was apparent in the 12-span task and by 1-year mice also exhibited significant acquisition deficits. Thus, by assessing olfactory working memory in Tg2576 mice we can model aspects of MCI in rodents and aid development of future therapeutic strategies for AD.

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.

Quantifying medial temporal lobe damage in memory-impaired patients

Studies of memory-impaired patients will be most useful when quantitative neuroanatomical information is available about the patients being studied. Toward that end, in the case of medial temporal lobe amnesia, protocols have been developed from histological material that identify the boundaries of relevant structures on magnetic resonance images. Because the size of these structures varies considerably in the normal population, some correction for overall brain size is usually employed when calculating volume measurements. Although different correction procedures have been used to normalize for brain size, there has been little study of how well different methods reduce variability and which methods might be most useful. We measured the volume of the hippocampal region (hippocampus proper, dentate gyrus, and subicular complex) and the volumes of the temporopolar, entorhinal, perirhinal, and parahippocampal cortices in five memory-impaired patients and 30 controls. We then compared three different methods for normalizing the volume measurements: normalization by intracranial volume, normalization by aligning the brain to a standard atlas, and normalization by brain area at the level of the anterior commissure. Normalization by intracranial volume reduced variability in the volume measurements of nearly all brain regions to a greater extent than did normalization by other methods. When normalized by intracranial volume, the patients exhibited a mean reduction in hippocampal volume of about 40% and negligible reductions in the volumes of other medial temporal lobe structures. On the basis of earlier histological analysis of two other patients (L.M. and W.H.), who also had reductions in hippocampal size of about 40%, we suggest that a volume reduction in this range likely indicates a nearly complete loss of hippocampal neurons.

Impaired odor recognition memory in patients with hippocampal lesions

In humans, impaired recognition memory following lesions thought to be limited to the hippocampal region has been demonstrated for a wide variety of tasks. However, the importance of the human hippocampus for olfactory recognition memory has scarcely been explored. We evaluated the ability of memory-impaired patients with damage thought to be limited to the hippocampal region to recognize a list of odors. The patients were significantly impaired after a retention delay of 1 h. Olfactory sensitivity was intact. This finding is in agreement with earlier reports that rats with hippocampal lesions exhibited memory impairment on an odor delayed nonmatching to sample task (after 30 min and 1 h) and that patients with damage thought to be limited to the hippocampal region were impaired on an odor span memory task. Olfactory recognition memory, similar to recognition memory in other sensory modalities, depends on the integrity of the hippocampal region.

THE MEDIAL TEMPORAL LOBE

The medial temporal lobe includes a system of anatomically related structures that are essential for declarative memory (conscious memory for facts and events). The system consists of the hippocampal region (CA fields, dentate gyrus, and subicular complex) and the adjacent perirhinal, entorhinal, and parahippocampal cortices. Here, we review findings from humans, monkeys, and rodents that illuminate the function of these structures. Our analysis draws on studies of human memory impairment and animal models of memory impairment, as well as neurophysiological and neuroimaging data, to show that this system (a) is principally concerned with memory, (b) operates with neocortex to establish and maintain long-term memory, and (c) ultimately, through a process of consolidation, becomes independent of long-term memory, though questions remain about the role of perirhinal and parahippocampal cortices in this process and about spatial memory in rodents. Data from neurophysiology, neuroimaging, and neuroanatomy point to a division of labor within the medial temporal lobe. However, the available data do not support simple dichotomies between the functions of the hippocampus and the adjacent medial temporal cortex, such as associative versus nonassociative memory, episodic versus semantic memory, and recollection versus familiarity.

Sequences assessed by declarative and procedural tests of memory in amnesic patients with hippocampal damage

Previous research indicates that amnesic subjects tested on sequential learning or serial reaction time tasks can learn a repeated procedural sequence but are unable to explicitly recall the correct sequence when asked to generate the sequence. Rats with hippocampal lesions are also able to learn and remember procedural or implicit sequences but were impaired for declarative sequences. We used analogous procedures used in rats to assess the role of the hippocampus in the acquisition of declarative and procedural sequences in amnesic and control participants. Amnesic participants with damage restricted to the hippocampus and control participants were administered analogous tasks of declarative and procedural sequential learning using a computer version of the radial arm maze. The amnesic participants had slower response times during the acquisition of procedural sequences, but were not impaired compared to controls when switched to a random sequence, suggesting that both groups learned the sequence. Alternatively, the amnesic but not control participants were significantly impaired in the declarative sequence task. Our findings provide support for evolutionary continuity in cognitive function of the hippocampus in rats and humans and the dissociation between the declarative and procedural sequential learning. The performance differences on the two sequence learning tasks are likely due to the use of different strategies associated with learning sequences based on procedural versus declarative knowledge.