Human amnesia and the medial temporal lobe illuminated by neuropsychological and neurohistological findings for patient E.P

Is implicit memory associated with the hippocampus?

According to the traditional memory-systems view, the hippocampus is critical during explicit (conscious) long-term memory, whereas other brain regions support implicit (nonconscious) memory. In the last two decades, some fMRI studies have reported hippocampal activity during implicit memory tasks. The aim of the present discussion paper was to identify whether any implicit memory fMRI studies have provided convincing evidence that the hippocampus is associated with nonconscious processes without being confounded by conscious processes. Experimental protocol and analysis parameters included the stimulus type(s), task(s), measures of subjective awareness, explicit memory accuracy, the relevant fMRI contrast(s) or analysis, and confound(s). A systematic review was conducted to identify implicit memory studies that reported fMRI activity in the hippocampus. After applying exclusion criteria, 13 articles remained for analysis. We found that there were no implicit memory fMRI studies where subjective awareness was absent, explicit memory performance was at chance, and there were no confounds that could have driven the observed hippocampal activity. The confounds included explicit memory (including false memory), imbalanced attentional states between conditions (yielding activation of the default-mode network), imbalanced stimuli between conditions, and differential novelty. As such, not a single fMRI study provided convincing evidence that implicit memory was associated with the hippocampus. Neuropsychological evidence was also considered, and implicit memory deficits were caused by factors known to disrupt brain regions beyond the hippocampus, such that the behavioral effects could not be attributed to this region. The present results indicate that implicit memory is not associated with the hippocampus.

A case of severe anterograde amnesia in the era of smartphone technology

A.V. is a young herpes simplex encephalitis (HSE) survivor who suffered extensive bilateral damage to the medial temporal lobe (MTL) leading to a severe and pervasive form of anterograde amnesia. Structural Magnetic Resonance Imaging (MRI) revealed lesions that encompass the hippocampus and amygdala in both hemispheres and that extend more laterally in the right temporal lobe. At the same time, detailed neuropsychological testing showed that the disparity between A.V.'s preserved intellectual functioning (Full Scale IQ: 115) and severe memory deficit (Delayed Memory Index: 42) is one of the largest on record. Despite this deficit, A.V. has regained a higher level of functioning and autonomy compared to previously documented amnesic cases with major bilateral MTL lesions. As a millennial, one advantage which A.V. has over prior amnesic cases is fluency with digital technology - particularly the smartphone. The analysis of his phone and specific app usage showed a pattern that is consistent with the strategy to offload cognitive tasks that would normally be supported by the MTL. A.V.'s behavior is significant in terms of rehabilitation and may have broader implications at the societal level and for public health given the ubiquity of smartphone technology and its potential to become integrated with neural mnemonic functions.

Spatial navigation and memory: A review of the similarities and differences relevant to brain models and age

Spatial navigation and memory are often seen as heavily intertwined at the cognitive and neural levels of analysis. We review models that hypothesize a central role for the medial temporal lobes, including the hippocampus, in both navigation and aspects of memory, particularly allocentric navigation and episodic memory. While these models have explanatory power in instances in which they overlap, they are limited in explaining functional and neuroanatomical differences. Focusing on human cognition, we explore the idea of navigation as a dynamically acquired skill and memory as an internally driven process, which may better account for the differences between the two. We also review network models of navigation and memory, which place a greater emphasis on connections rather than the functions of focal brain regions. These models, in turn, may have greater explanatory power for the differences between navigation and memory and the differing effects of brain lesions and age.

Ex vivo, in situ perfusion protocol for human brain fixation compatible with microscopy, MRI techniques, and anatomical studies

We present a method for human brain fixation based on simultaneous perfusion of 4% paraformaldehyde through carotids after a flush with saline. The left carotid cannula is used to perfuse the body with 10% formalin, to allow further use of the body for anatomical research or teaching. The aim of our method is to develop a vascular fixation protocol for the human brain, by adapting protocols that are commonly used in experimental animal studies. We show that a variety of histological procedures can be carried out (cyto- and myeloarchitectonics, histochemistry, immunohistochemistry, intracellular cell injection, and electron microscopy). In addition, ex vivo, ex situ high-resolution MRI (9.4T) can be obtained in the same specimens. This procedure resulted in similar morphological features to those obtained by intravascular perfusion in experimental animals, provided that the postmortem interval was under 10 h for several of the techniques used and under 4 h in the case of intracellular injections and electron microscopy. The use of intravascular fixation of the brain inside the skull provides a fixed whole human brain, perfectly fitted to the skull, with negligible deformation compared to conventional techniques. Given this characteristic of ex vivo, in situ fixation, this procedure can probably be considered the most suitable one available for ex vivo MRI scans of the brain. We describe the compatibility of the method proposed for intravascular fixation of the human brain and fixation of the donor's body for anatomical purposes. Thus, body donor programs can provide human brain tissue, while the remainder of the body can also be fixed for anatomical studies. Therefore, this method of human brain fixation through the carotid system optimizes the procurement of human brain tissue, allowing a greater understanding of human neurological diseases, while benefiting anatomy departments by making the remainder of the body available for teaching purposes.

Physically Active Lifestyle Is Associated With Attenuation of Hippocampal Dysfunction in Cognitively Intact Older Adults

Alterations in hippocampal function have been shown in older adults, which are expressed as changes in hippocampal activity and connectivity. While hippocampal activation during memory demands has been demonstrated to decrease with age, some older individuals present increased activity, or hyperactivity, of the hippocampus which is associated with increased neuropathology and poor memory function. In addition, lower functional coherence between the hippocampus and core hubs of the default mode network (DMN), namely, the posteromedial and medial prefrontal cortices, as well as increased local intrahippocampal connectivity, were also demonstrated in cognitively intact older adults. Aerobic exercise has been shown to elicit neuroprotective effects on hippocampal structure and vasculature in aging, and improvements in cardiorespiratory fitness have been suggested to mediate these exercise-related effects. However, how these lifestyle factors relate to hippocampal function is not clear. Fifty-two cognitively intact older adults (aged 65-80 years) have been recruited and divided into physically active (n = 29) or non-active (n = 23) groups based on their aerobic activity lifestyle habits. Participants underwent resting-state and task-based fMRI experiments which included an associative memory encoding paradigm followed by a post-scan memory recognition test. In addition, 44 participants also performed cardiopulmonary exercise tests to evaluate cardiorespiratory fitness by measuring peak oxygen consumption (Vo2peak). While both groups demonstrated increased anterior hippocampal activation during memory encoding, a physically active lifestyle was associated with significantly lower activity level and higher memory performance in the recognition task. In addition, the physically active group also demonstrated higher functional connectivity of the anterior and posterior hippocampi with the core hubs of the DMN and lower local intra-hippocampal connectivity within and between hemispheres. Vo2peak was negatively associated with the hippocampal activation level and demonstrated a positive correlation with hippocampal-DMN connectivity. According to these findings, an aerobically active lifestyle may be associated with attenuation of hippocampal dysfunction in cognitively intact older adults.

Living with global amnesia: self-established compensation strategies of a patient with severe memory impairment - a narrative report

We report the case of C.H., a 48-year-old patient with global amnesia caused by herpes simplex encephalitis at the age of 20 and subsequent extensive bilateral temporal lobe lesions. Neuropsychological examinations performed at various intervals found persistent dense explicit memory impairment and limited vocabulary, yet intact procedural memory. Despite these limitations, C.H. self-developed and acquired a variety of effective strategies. As a result, C.H. achieved a high level of autonomy in everyday life. Her remarkable case is an encouraging and helpful example for successful implementation of creative methods and procedures to compensate and alleviate cognitive limitation, even if extensive.

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.

Neuropathology of a remarkable case of memory impairment informs human memory

Kent Cochrane (K.C.) has been investigated by researchers for nearly three decades after intracranial trauma from a motorcycle accident at age 30 resulted in a striking profile of amnesia. K.C. suffered severe anterograde amnesia in both verbal and non-verbal domains which was accompanied by selective retrograde amnesia for personal events experienced prior to the time of his injury (episodic memory), with relative preservation of memory for personal and world facts (semantic memory), and of implicit memory. This pattern of spared and impaired memory extended to spatial memory for large-scale environments and beyond memory to future imagining and decision-making. Post-mortem brain findings at age 62 included moderate diffuse atrophy, left orbitofrontal contusion, left posterior cerebral artery infarct, and left anterior frontal watershed infarct. Notably, there was severe neuronal loss and gliosis of the hippocampi bilaterally. The left hippocampus was severely affected anteriorly and posteriorly, but CA2, CA4, and the dentate gyrus (DG) were focally spared. There was associated degeneration of the left fornix. The right hippocampus showed near complete destruction anteriorly, with relative preservation posteriorly, mainly of CA4 and DG. Bilateral parahippocampal gyri and left anterior thalamus also showed neuron loss and gliosis. There was no evidence of co-existing neurodegenerative phenomena on beta-amyloid, phosphorylated tau, or TDP-43 immunostaining. The extent of damage to medial temporal lobe structures is in keeping with K.C.'s profound anterograde and retrograde amnesia, with the exception of the unexpected finding of preserved CA2/CA4 and DG. K.C.'s case demonstrates that relatively clean functional dissociations are still possible following widespread brain damage, with structurally compromised brain regions unlikely to be critical to cognitive functions found to be intact. In this way, the findings presented here add to K.C.'s significant contributions to our understanding of clinical-anatomical relationships in memory.

Precision, binding, and the hippocampus: Precisely what are we talking about?

Endel Tulving's proposal that episodic memory is distinct from other memory systems like semantic memory remains an extremely influential idea in cognitive neuroscience research. As originally suggested by Tulving, episodic memory involves three key components that differentiate it from all other memory systems: spatiotemporal binding, mental time travel, and autonoetic consciousness. Here, we focus on the idea of spatiotemporal binding in episodic memory and, in particular, how consideration of the precision of spatiotemporal context helps expand our understanding of episodic memory. Precision also helps shed light on another key issue in cognitive neuroscience, the role of the hippocampus outside of episodic memory in perception, attention, and working memory. By considering precision alongside item-context bindings, we attempt to shed new light on both the nature of how we represent context and what roles the hippocampus plays in episodic memory and beyond.

Preserved capacity for learning statistical regularities and directing selective attention after hippocampal lesions

Prior knowledge about the probabilistic structure of visual environments is necessary to resolve ambiguous information about objects in the world. Expectations based on stimulus regularities exert a powerful influence on human perception and decision making by improving the efficiency of information processing. Another type of prior knowledge, termed top-down attention, can also improve perceptual performance by facilitating the selective processing of relevant over irrelevant information. While much is known about attention, the mechanisms that support expectations about statistical regularities are not well-understood. The hippocampus has been implicated as a key structure involved in or perhaps necessary for the learning of statistical regularities, consistent with its role in various kinds of learning and memory. Here, we tested this hypothesis using a motion discrimination task in which we manipulated the most likely direction of motion, the degree of attention afforded to the relevant stimulus, and the amount of available sensory evidence. We tested memory-impaired patients with bilateral damage to the hippocampus and compared their performance with controls. Despite a modest slowing in response initiation across all task conditions, patients performed similar to controls. Like controls, patients exhibited a tendency to respond faster and more accurately when the motion direction was more probable, the stimulus was better attended, and more sensory evidence was available. Together, these findings demonstrate a robust, hippocampus-independent capacity for learning statistical regularities in the sensory environment in order to improve information processing.

Rehearsal initiates systems memory consolidation, sleep makes it last

After encoding, memories undergo a transitional process termed systems memory consolidation. It allows fast acquisition of new information by the hippocampus, as well as stable storage in neocortical long-term networks, where memory is protected from interference. Whereas this process is generally thought to occur slowly over time and sleep, we recently found a rapid memory systems transition from hippocampus to posterior parietal cortex (PPC) that occurs over repeated rehearsal within one study session. Here, we use fMRI to demonstrate that this transition is stabilized over sleep, whereas wakefulness leads to a reset to naïve responses, such as observed during early encoding. The role of sleep therefore seems to go beyond providing additional rehearsal through memory trace reactivation, as previously thought. We conclude that repeated study induces systems consolidation, while sleep ensures that these transformations become stable and long lasting. Thus, sleep and repeated rehearsal jointly contribute to long-term memory consolidation.

The nature of recollection across months and years and after medial temporal lobe damage

We studied the narrative recollections of memory-impaired patients with medial temporal lobe (MTL) damage who took a 25-min guided walk during which 11 planned events occurred. The recollections of the patients, recorded directly after the walk, were compared with the recollections of controls tested directly after the walk (C1), after one month (C2), or after 2.6 years (C3). With respect to memory for the walk, the narrative recollections of the patients were impoverished compared with C1 but resembled the recollections of volunteers tested after long delays (C2 and C3). In addition, how language was used by the patients in their recollections resembled how language was used by groups C2 and C3 (higher-frequency words, less concrete words, fewer nouns, more adverbs, more pronouns, and more indefinite articles). These findings appear to reflect how individuals, either memory-impaired patients or controls, typically speak about the past when memory is weak and lacks detail and need not have special implications about language use and MTL function beyond the domain of memory. A notable exception to the similarity between patient narratives and the narratives of C2 and C3 was that the control groups reported the events of the walk in correct chronological order, whereas the order in which patients reported events bore no relationship to the order in which events occurred. We suggest that the MTL is especially important for accessing global information about events and the relationships among their elements.

The role of hippocampus in the retrieval of autobiographical memories in patients with amnestic Mild Cognitive Impairment due to Alzheimer's disease

The role of the hippocampus and neocortical areas in the retrieval of past memories in pre-dementia Alzheimer's disease (AD) patients was investigated. The aim was to assess whether the hippocampus has a temporary role in memory trace formation, according to the Cortical Reallocation Theory (CRT), or whether it continuously updates and enriches memories, according to the Multiple Trace Theory. According to the former theory, hippocampal damage should affect more recent memories, whereas the association cortex is expected to affect memories of the entire lifespan. In the second case, damage to either the hippocampus or the association cortices should affect memories of the entire lifespan. Seventeen patients with amnestic Mild Cognitive Impairment due to AD were submitted to autobiographical (i.e., episodic and semantic personal) memory assessment. Patients underwent MRI for the acquisition of T1-weighted brain volumes. Voxel-based morphometry was used to assess correlations between grey matter (GM) volumes and autobiographical memory. Correlation analyses revealed a strict association between GM volumes in the hippocampus and patients' ability to retrieve the most recent but not the oldest autobiographical memories in both aspects, episodic and semantic. Moreover, patients' GM volumes in the pre-frontal and temporal polar areas were associated with recollection of episodic and semantic events, respectively. Finally, GM volumes in the precuneus and occipital cortex were associated with retrieval of the most recent episodic events. These findings indicate that the hippocampus has a specific time-dependent role; thus, they support the CRT.

Neural correlates of free recall of "famous events" in a "hypermnestic" individual as compared to an age- and education-matched reference group

BACKGROUND

Memory performance of an individual (within the age range: 50-55 years old) showing superior memory abilities (protagonist PR) was compared to an age- and education-matched reference group in a historical facts ("famous events") retrieval task.

RESULTS

Contrasting task versus baseline performance both PR and the reference group showed fMRI activation patterns in parietal and occipital brain regions. The reference group additionally demonstrated activation patterns in cingulate gyrus, whereas PR showed additional widespread activation patterns comprising frontal and cerebellar brain regions. The direct comparison between PR and the reference group revealed larger fMRI contrasts for PR in right frontal, superior temporal and cerebellar brain regions.

CONCLUSIONS

It was concluded that PR generally recruits brain regions as normal memory performers do, but in a more elaborate way, and furthermore, that he applied a memory-strategy that potentially includes executively driven multi-modal transcoding of information and recruitment of implicit memory resources.

Spared perception of object geometry and object components after hippocampal damage

We tested the proposal that medial temporal lobe (MTL) structures support not just memory but also high-level object perception. In one task, participants decided whether a line drawing could represent an object in three-dimensional space and, in another task, they saw the components of an object and decided what object could be formed if the components were assembled. Patients with hippocampal lesions were intact, indicating that the hippocampus is not needed for perceiving the structural coherence of objects or appreciating the relations among object parts. Patients with large MTL lesions were moderately impaired, likely due to damage outside the MTL.

Magnetic Resonance Imaging and Anatomical Correlation of Human Temporal Lobe Landmarks, in 3D Euclidean Space: A Study of Control and Alzheimer's Disease Subjects

BACKGROUND

The medial temporal lobe (MTL), and in particular the hippocampal formation, is essential in the processing and consolidation of declarative memory. The 3D environment of the anatomical structures contained in the MTL is an important issue.

OBJECTIVE

Our aim was to explore the spatial relationship of the anatomical structures of the MTL and changes in aging and/or Alzheimer's disease (AD).

METHODS

MTL anatomical landmarks are identified and registered to create a 3D network. The brain network is quantitatively described as a plane, rostrocaudally-oriented, and presenting Euclidean/real distances. Correspondence between 1.5T RM, 3T RM, and histological sections were assessed to determine the most important recognizable changes in AD, based on statistical significance.

RESULTS

In both 1.5T and 3T RM images and histology, inter-rater reliability was high. Sex and hemisphere had no influence on network pattern. Minor changes were found in relation to aging. Distances from the temporal pole to the dentate gyrus showed the most significant differences when comparing control and AD groups. The best discriminative distance between control and AD cases was found in the temporal pole/dentate gyrus rostrocaudal length in histological sections. Moreover, more distances between landmarks were required to obtain 100% discrimination between control (divided into <65 years or >65 years) and AD cases.

DISCUSSION

Changes in the distance between MTL anatomical landmarks can successfully be detected by using measurements of 3D network patterns in control and AD cases.

Current Topics Regarding the Function of the Medial Temporal Lobe Memory System

The first clear insight that the medial temporal lobe of the human brain was in fact a system of anatomically connected structures that were organized into a memory system came in 1957 from the observations by Brenda Milner of the noted amnesic patient H.M. Subsequent work in humans, monkeys, and rodents has identified all of the components of the medial temporal lobe (MTL) that formed the memory system. Currently, work is ongoing to identify the specific contributions each structure in the medial temporal lobe makes towards the formation and storage of long-term declarative memory. The historical background of this work is described including what insights the study of noted neurologic patients H.M. and E.P. provided for understanding the function of the medial temporal lobe. The development of an animal model of medial temporal lobe function is described. Additionally, the insights that lead to the understanding that the brain contains multiple, anatomically discrete, memory systems are described. Finally, three current topics of debate are addressed: First, does the perirhinal cortex exclusively support memory, or does it support both memory and higher order visual perception? Second, is there an anatomical separation between recollection and familiarity ? Third, is the organization of spatial memory different between humans and rats, or perhaps the difference is between the working memory capacities of the two species?

Current Topics Regarding the Function of the Medial Temporal Lobe Memory System

The first clear insight that the medial temporal lobe of the human brain was in fact a system of anatomically connected structures that were organized into a memory system came in 1957 from the observations by Brenda Milner of the noted amnesic patient H.M. Subsequent work in humans, monkeys, and rodents has identified all of the components of the medial temporal lobe (MTL) that formed the memory system. Currently, work is ongoing to identify the specific contributions each structure in the medial temporal lobe makes towards the formation and storage of long-term declarative memory. The historical background of this work is described including what insights the study of noted neurologic patients H.M. and E.P. provided for understanding the function of the medial temporal lobe. The development of an animal model of medial temporal lobe function is described. Additionally, the insights that lead to the understanding that the brain contains multiple, anatomically discrete, memory systems are described. Finally, three current topics of debate are addressed: First, does the perirhinal cortex exclusively support memory, or does it support both memory and higher order visual perception? Second, is there an anatomical separation between recollection and familiarity? Third, is the organization of spatial memory different between humans and rats, or perhaps the difference is between the working memory capacities of the two species?

Three Scientific Controversies to Engage Students in Reading Primary Literature

In this review, three sets of papers are presented. Each of the sets presents a historical or active controversy in neuroscience ranging from cell biology and cell signaling, to developmental neuroscience, to cognitive neuroscience. The first set captures a historical controversy about whether the beta/gamma subunit of G-proteins can be active in opening ion channels. The second set represents a modern instantiation of the oldest debate in neuroscience: are our minds and brains the product of innate factors or environmental influences. This debate plays out in a series of papers on the development of the visual system. The third set contrasts the view that the hippocampus and surrounding structures primarily function to represent our location in space (a position for which the 2014 Nobel Prize in Physiology or Medicine was awarded to three investigators) with the perspective that the hippocampus is a general-purpose structure for declarative memories, spatial or non-spatial. The first and third controversies feature publications of virtually identical experiments that show opposing results. All three controversies are discussed in regards to the individual scientists who did the experiments and debated directly with each other. The first (beta/gamma subunits) emphasizes the value of reproducibility in scientific research, the second (visual cortex development) emphasizes the value of new techniques and updating scientific models, and the third (hippocampus) exposes students to an ongoing, albeit under discussed, debate.

Accelerated Long-Term Forgetting Limited to the Recollection Component of Recognition Memory in a Temporal Lobe Epileptic Patient

Accelerated long-term forgetting (ALF) is a frequent finding in patients with temporal lobe epilepsy (TLE). Here we report the case of a TLE patient who complained of marked difficulties in remembering personal events and information even though repeated neuropsychological assessments had failed to detect any deficit on common laboratory memory tests. The patient underwent an experimental investigation that involved estimating recollection and familiarity processes in the performance on verbal and visual recognition tests, over intervals ranging from 10 minutes to 7 days. Results showed accelerated forgetting confined to the recollection component only, which was particularly evident in the verbal test.

When eye movements express memory for old and new scenes in the absence of awareness and independent of hippocampus

Eye movements can reflect memory. For example, participants make fewer fixations and sample fewer regions when viewing old versus new scenes (the repetition effect). It is unclear whether the repetition effect requires that participants have knowledge (awareness) of the old–new status of the scenes or if it can occur independent of knowledge about old–new status. It is also unclear whether the repetition effect is hippocampus-dependent or hippocampus-independent. A complication is that testing conscious memory for the scenes might interfere with the expression of unconscious (unaware), experience-dependent eye movements. In experiment 1, 75 volunteers freely viewed old and new scenes without knowledge that memory for the scenes would later be tested. Participants then made memory judgments and confidence judgments for each scene during a surprise recognition memory test. Participants exhibited the repetition effect regardless of the accuracy or confidence associated with their memory judgments (i.e., the repetition effect was independent of their awareness of the old–new status of each scene). In experiment 2, five memory-impaired patients with medial temporal lobe damage and six controls also viewed old and new scenes without expectation of memory testing. Both groups exhibited the repetition effect, even though the patients were impaired at recognizing which scenes were old and which were new. Thus, when participants viewed scenes without expectation of memory testing, eye movements associated with old and new scenes reflected unconscious, hippocampus-independent memory. These findings are consistent with the formulation that, when memory is expressed independent of awareness, memory is hippocampus-independent.

Map reading, navigating from maps, and the medial temporal lobe

We administered map-reading tasks in which participants navigated an array of marks on the floor by following paths on hand-held maps that made up to nine turns. The burden on memory was minimal because the map was always available. Nevertheless, because the map was held in a fixed position in relation to the body, spatial computations were continually needed to transform map coordinates into geographical coordinates as participants followed the maps. Patients with lesions limited to the hippocampus (n = 5) performed similar to controls at all path lengths (experiment 1). They were also intact at executing single moves to an adjacent location, even when trials began by facing in a direction that put the map coordinates and geographical coordinates into conflict (experiment 2). By contrast, one patient with large medial temporal lobe (MTL) lesions performed poorly overall in experiment 1 and poorly in experiment 2 when trials began by facing in the direction that placed the map coordinates and geographical coordinates in maximal conflict. Directly after testing, all patients were impaired at remembering factual details about the task. The findings suggest that the hippocampus is not needed to carry out the spatial computations needed for map reading and navigating from maps. The impairment in map reading associated with large MTL lesions may depend on damage in or near the parahippocampal cortex.

Non-alcoholic Korsakoff syndrome in psychiatric patients with a history of undiagnosed Wernicke's encephalopathy

Wernicke's encephalopathy is often undiagnosed, particularly in non-alcoholics. There are very few reports of non-alcoholic patients diagnosed with Korsakoff syndrome in the absence of a prior diagnosis of Wernicke's encephalopathy and no studies of diffusion tensor imaging in non-alcoholic Korsakoff syndrome. We report on three non-alcoholic psychiatric patients (all women) with long-term non-progressive memory impairment that developed after malnutrition accompanied by at least one of the three Wernicke's encephalopathy manifestations: ocular abnormalities, ataxia or unsteadiness, and an altered mental state or mild memory impairment. In neuropsychological examination, all patients had memory impairment, including intrusions. One patient had mild cerebellar vermis atrophy in MRI taken after the second episode of Wernicke's encephalopathy. The same patient had mild hypometabolism in the lateral cortex of the temporal lobes. Another patient had mild symmetrical atrophy and hypometabolism of the superior frontal lobes. Two patients were examined with diffusion tensor imaging. Reduced fractional anisotropy values were found in the corona radiata in two patients, and the uncinate fasciculus and the inferior longitudinal fasciculus in one patient. Our results suggest that non-alcoholic Korsakoff syndrome is underdiagnosed. Psychiatric patients with long-term memory impairment may have Korsakoff syndrome and, therefore, they should be evaluated for a history of previously undiagnosed Wernicke's encephalopathy.

Functional (dissociative) retrograde amnesia

Retrograde amnesia is described as condition which can occur after direct brain damage, but which occurs more frequently as a result of a psychiatric illness. In order to understand the amnesic condition, content-based divisions of memory are defined. The measurement of retrograde memory is discussed and the dichotomy between "organic" and "psychogenic" retrograde amnesia is questioned. Briefly, brain damage-related etiologies of retrograde amnesia are mentioned. The major portion of the review is devoted to dissociative amnesia (also named psychogenic or functional amnesia) and to the discussion of an overlap between psychogenic and "brain organic" forms of amnesia. The "inability of access hypothesis" is proposed to account for most of both the organic and psychogenic (dissociative) patients with primarily retrograde amnesia. Questions such as why recovery from retrograde amnesia can occur in retrograde (dissociative) amnesia, and why long-term new learning of episodic-autobiographic episodes is possible, are addressed. It is concluded that research on retrograde amnesia research is still in its infancy, as the neural correlates of memory storage are still unknown. It is argued that the recollection of episodic-autobiographic episodes most likely involves frontotemporal regions of the right hemisphere, a region which appears to be hypometabolic in patients with dissociative amnesia.

Postmortem examination of patient H.M.'s brain based on histological sectioning and digital 3D reconstruction

Modern scientific knowledge of how memory functions are organized in the human brain originated from the case of Henry G. Molaison (H.M.), an epileptic patient whose amnesia ensued unexpectedly following a bilateral surgical ablation of medial temporal lobe structures, including the hippocampus. The neuroanatomical extent of the 1953 operation could not be assessed definitively during H.M.'s life. Here we describe the results of a procedure designed to reconstruct a microscopic anatomical model of the whole brain and conduct detailed 3D measurements in the medial temporal lobe region. This approach, combined with cellular-level imaging of stained histological slices, demonstrates a significant amount of residual hippocampal tissue with distinctive cytoarchitecture. Our study also reveals diffuse pathology in the deep white matter and a small, circumscribed lesion in the left orbitofrontal cortex. The findings constitute new evidence that may help elucidate the consequences of H.M.'s operation in the context of the brain's overall pathology.

Do age and sex impact on the absolute cell numbers of human brain regions?

What is the influence of sex and age on the quantitative cell composition of the human brain? By using the isotropic fractionator to estimate absolute cell numbers in selected brain regions, we looked for sex- and age-related differences in 32 medial temporal lobes (comprised basically by the hippocampal formation, amygdala and parahippocampal gyrus), sixteen male (29-92 years) and sixteen female (25-82); and 31 cerebella, seventeen male (29-92 years) and fourteen female (25-82). These regions were dissected from the brain, fixed and homogenized, and then labeled with a DNA-marker (to count all nuclei) and with a neuron-specific nuclear marker (to estimate neuron number). Total number of cells in the medial temporal lobe was found to be 1.91 billion in men, and 1.47 billion in women, a difference of 23 %. This region showed 34 % more neurons in men than in women: 525.1 million against 347.4 million. In contrast, no sex differences were found in the cerebellum. Regarding the influence of age, a quadratic correlation was found between neuronal numbers and age in the female medial temporal lobe, suggesting an early increase followed by slight decline after age 50. The cerebellum showed numerical stability along aging for both neurons and non-neuronal cells. In sum, results indicate a sex-related regional difference in total and neuronal cell numbers in the medial temporal lobe, but not in the cerebellum. On the other hand, aging was found to impact on cell numbers in the medial temporal lobe, while the cerebellum proved resilient to neuronal losses in the course of life.

20 years after “The ontogeny of human memory: A cognitive neuroscience perspective,” where are we?

In 1995, Nelson published a paper describing a model of memory development during the first years of life. The current article seeks to provide an update on the original work published 20 years ago. Specifically, we review our current knowledge on the relation between the emergence of explicit memory functions throughout development and the maturation of associated brain regions. It is now well established that the brain regions subserving explicit memory functions (i.e. the hippocampal formation) are far from mature at birth, and exhibit important and gradual structural changes during childhood and beyond. Accordingly, explicit memory functions develop progressively. While some functions are present shortly after birth (formerly proposed as pre-explicit memory), others exhibit protracted developmental profiles during the first years of life. We examine the link between the emergence of different memory functions and the maturation of specific hippocampal circuits.

Structural and functional correlates of behavioral pattern separation in the hippocampus and medial temporal lobe

Structures of the medial temporal lobe (MTL) are known to be involved in declarative memory processes. However, little is known about how age-related changes in MTL structures, white matter integrity, and functional connectivity affect pattern separation processes in the MTL. In this study, we used magnetic resonance imaging (MRI) to measure the volumes of MTL regions of interest, including hippocampal subfields (dentate gyrus, CA3, CA1, and subiculum) in healthy older and younger adults. Additionally, we used diffusion tensor imaging to measure white matter integrity for both groups. Finally, we used functional MRI to acquire resting functional connectivity measures for both groups. We show that, along with age, the volume of left CA3/dentate gyrus predicts memory performance. Differences in fractional anisotropy and the strength of resting functional connections between the hippocampus and other cortical structures implicated in memory processing were not significant predictors of performance. As previous studies have only hinted, it seems that the size of left CA3/dentate gyrus contributes more to successful discrimination between similar mnemonic representations than other hippocampal sub-fields, MTL structures, and other neuroimaging correlates. Accordingly, the implications of aging and atrophy on lure discrimination capacities are discussed.

H.M.'s contributions to neuroscience: a review and autopsy studies

H.M., Henry Molaison, was one of the world's most famous amnesic patients. His amnesia was caused by an experimental brain operation, bilateral medial temporal lobe resection, carried out in 1953 to relieve intractable epilepsy. He died on December 2, 2008, and that night we conducted a wide variety of in situ MRI scans in a 3 T scanner at the Massachusetts General Hospital (Mass General) Athinoula A. Martinos Center for Biomedical Imaging. For the in situ experiments, we acquired a full set of standard clinical scans, 1 mm isotropic anatomical scans, and multiple averages of 440 μm isotropic anatomical scans. The next morning, H.M.'s body was transported to the Mass General Morgue for autopsy. The photographs taken at that time provided the first documentation of H.M.'s lesions in his physical brain. After tissue fixation, we obtained ex vivo structural data at ultra-high resolution using 3 T and 7 T magnets. For the ex vivo acquisitions, the highest resolution images were 210 μm isotropic. Based on the MRI data, the anatomical areas removed during H.M.'s experimental operation were the medial temporopolar cortex, piriform cortex, virtually all of the entorhinal cortex, most of the perirhinal cortex and subiculum, the amygdala (except parts of the dorsal-most nuclei-central and medial), anterior half of the hippocampus, and the dentate gyrus (posterior head and body). The posterior parahippocampal gyrus and medial temporal stem were partially damaged. Spared medial temporal lobe tissue included the dorsal-most amygdala, the hippocampal-amygdalo-transition-area, ∼2 cm of the tail of the hippocampus, a small part of perirhinal cortex, a small portion of medial hippocampal tissue, and ∼2 cm of posterior parahippocampal gyrus. H.M.'s impact on the field of memory has been remarkable, and his contributions to neuroscience continue with a unique dataset that includes in vivo, in situ, and ex vivo high-resolution MRI.

New learning of music after bilateral medial temporal lobe damage: evidence from an amnesic patient

Damage to the hippocampus impairs the ability to acquire new declarative memories, but not the ability to learn simple motor tasks. An unresolved question is whether hippocampal damage affects learning for music performance, which requires motor processes, but in a cognitively complex context. We studied learning of novel musical pieces by sight-reading in a newly identified amnesic, LSJ, who was a skilled amateur violist prior to contracting herpes simplex encephalitis. LSJ has suffered virtually complete destruction of the hippocampus bilaterally, as well as extensive damage to other medial temporal lobe structures and the left anterior temporal lobe. Because of LSJ's rare combination of musical training and near-complete hippocampal destruction, her case provides a unique opportunity to investigate the role of the hippocampus for complex motor learning processes specifically related to music performance. Three novel pieces of viola music were composed and closely matched for factors contributing to a piece's musical complexity. LSJ practiced playing two of the pieces, one in each of the two sessions during the same day. Relative to a third unpracticed control piece, LSJ showed significant pre- to post-training improvement for the two practiced pieces. Learning effects were observed both with detailed analyses of correctly played notes, and with subjective whole-piece performance evaluations by string instrument players. The learning effects were evident immediately after practice and 14 days later. The observed learning stands in sharp contrast to LSJ's complete lack of awareness that the same pieces were being presented repeatedly, and to the profound impairments she exhibits in other learning tasks. Although learning in simple motor tasks has been previously observed in amnesic patients, our results demonstrate that non-hippocampal structures can support complex learning of novel musical sequences for music performance.

Comparative neuroanatomical parcellation of the human and nonhuman primate temporal pole

The temporal pole is unique to nonhuman and human primates, although other species also present temporal cortex. A clear distinction is made between the gross anatomical, macroscopic temporal pole, located at the tip of the temporal lobe, and the temporal polar cortex, which is a general term that encompasses all reported divisions and can be applied to both nonhuman and human primates. In the 19th century early neuroanatomists identified the temporal polar cortex as a different entity, independent from the remainder of the temporal lobe. More recently, the temporal polar cortex has been subdivided into different fields. The analysis of the different portions that make up the temporal polar cortex was first described in the nonhuman primate, and later on in humans. In this review we examine the historical course of the concepts about the extension, structure, and main cytoarchitectonic areas. The different descriptions are presented in chronological order and their relevance is discussed. In general, some common features arise in nonhuman and human primates temporopolar cortex, namely, an increased thickness of this cortical area, a predominant dysgranular type of cortex, and a more restricted extension in humans relative to earlier accounts. A common pattern of cytoarchitectonic areas results from the criteria of previous authors, although the sulcal anatomy of the human temporal pole has great variability. The understanding of the extension, composition, and limits of the temporal polar cortex is crucial for identification of separate regions in neuroimaging studies. J. Comp. Neurol. 521:4163-4176, 2013. © 2013 Wiley Periodicals, Inc.

Comparison of explicit and incidental learning strategies in memory-impaired patients

Declarative memory for rapidly learned, novel associations is thought to depend on structures in the medial temporal lobe (MTL), whereas associations learned more gradually can sometimes be supported by nondeclarative memory and by structures outside the MTL. A recent study suggested that even rapidly learned associations can be supported by structures outside the MTL when an incidental encoding procedure termed "fast mapping" (FM) is used. We tested six memory-impaired patients with bilateral damage to hippocampus and one patient with large bilateral lesions of the MTL. Participants saw photographs and names of animals, plants, and foods that were previously unfamiliar (e.g., mangosteen). Instead of asking participants to study name-object pairings for a later memory test (as with traditional memory instructions), participants answered questions that allowed them to infer which object corresponded to a particular name. In a second condition, participants learned name-object associations of unfamiliar items by using standard, explicit encoding instructions (e.g., remember the mangosteen). In FM and explicit encoding conditions, patients were impaired (and performed no better than a group that was given the same tests but had not previously studied the material). The same results were obtained in a second experiment that used the same procedures with modifications to allow for more robust learning and more reliable measures of performance. Thus, our results with the FM procedure and memory-impaired patients yielded the same deficits in learning and memory that have been obtained by using other more traditional paradigms.

High field structural MRI reveals specific episodic memory correlates in the subfields of the hippocampus

The involvement of the hippocampus (HC) in episodic memory is well accepted; however it is unclear how each subfield within the HC contributes to memory function. Recent magnetic resonance imaging (MRI) studies suggest differential involvement of hippocampal subfields and subregions in episodic memory. However, most structural MRI studies have examined the HC subfields within a single subregion of the HC and used specialised experimental memory paradigms. The purpose of the present study was to determine the association between volumes of HC subfields throughout the entire HC structure and performance on standard neuropsychological memory tests in a young, healthy population. We recruited 34 healthy participants under the age of 50. MRI data was acquired with a fast spin echo (FSE) sequence yielding a 0.52×0.68×1.0 mm(3) native resolution. The HC subfields - the cornu ammonis 1-3 (CA), dentate gyrus (DG), and subiculum (SUB) - were segmented manually within three hippocampal subregions using a previously defined protocol. Participants were administered the Wechsler Memory Scale, 4th edition (WMS-IV) to assess performance in episodic memory using verbal (Logical Memory, LM) and visual (Designs, DE; visual-spatial memory, DE-Spatial; visual-content memory, DE-Content) memory subtests. Working memory subtests (Spatial Addition, SA; and Symbol Span, SSP) were included as well. Working memory was not associated with any HC volumes. Volumes of the DG were correlated with verbal memory (LM) and visual-spatial memory (DE-Spatial). Posterior CA volumes correlated with both visual-spatial and visual-object memory (DE-Spatial, DE-Content). In general, anterior subregion volumes (HC head) correlated with verbal memory, while some anterior and many posterior HC subregion volumes (body and tail) correlated with visual memory scores (DE-Spatial, DE-Content). In addition, while verbal memory showed left-lateralized associations with HC volumes, visual memory was associated with HC volumes bilaterally. This the first study to examine the associations between hippocampal subfield volumes across the entire hippocampal formation with performance in a set of standard memory tasks.

A pencil rescues impaired performance on a visual discrimination task in patients with medial temporal lobe lesions

We tested proposals that medial temporal lobe (MTL) structures support not just memory but certain kinds of visual perception as well. Patients with hippocampal lesions or larger MTL lesions attempted to identify the unique object among twin pairs of objects that had a high degree of feature overlap. Patients were markedly impaired under the more difficult task conditions. However, the deficit was fully rescued when patients used a pencil to draw lines between the twin pairs, thereby eliminating the need to hold material in memory as they worked at each display. The perceptual demands of the task were presumably the same with or without this memory aid. Accordingly, the results suggest that the deficit on this and similar tasks, which involve comparisons across stimuli, are better understood in terms of impaired memory rather than impaired perception.

The nature of anterograde and retrograde memory impairment after damage to the medial temporal lobe

The study of anterograde and retrograde amnesia (AA and RA) in the laboratory and the clinic has provided important information about the structure and organization of memory. The severity of AA is usually correlated with the severity of RA. Nevertheless, variations in the expression of AA and RA have been reported, which presumably reflect variation in the locus and extent of brain damage. The relationship between AA and RA has rarely been described quantitatively in groups of patients where detailed anatomical information is available. We have quantified the severity of AA and RA for factual information in 11 memory-impaired patients with bilateral medial temporal lobe lesions, including 5 for whom detailed post-mortem neurohistological information was available. The findings describe an orderly relationship between AA and RA, such that patients with more severe AA also had more extensive RA. In addition, RA was measurable only after AA reached a substantial level of severity. This relationship between AA and RA in patients with identified medial temporal lobe lesions appears to describe a general principle, which applies to a range of etiologies, including traumatic amnesia, where the locus and extent of brain damage is less well understood. Whenever patients deviate substantially from the relationship described here, one should be alert to the likelihood that significant damage has occurred outside or in addition to the structures in the medial temporal lobe.