Memory in the aging brain: doubly dissociating the contribution of the hippocampus and entorhinal cortex

Recognition memory in amnestic-mild cognitive impairment: insights from event-related potentials

Episodic memory loss is the hallmark cognitive dysfunction associated with Alzheimer’s disease (AD). Amnestic mild cognitive impairment (a-MCI) frequently represents a transitional stage between normal aging and early AD. A better understanding of the qualitative features of memory loss in a-MCI may have important implications for predicting those most likely to harbor AD-related pathology and for disease monitoring. Dual process models of memory argue that recognition memory is subserved by the dissociable processes of recollection and familiarity. Work studying recognition memory in a-MCI from this perspective has been controversial, particularly with regard to the integrity of familiarity. Event-related potentials (ERPs) offer an alternative means for assessing these functions without the associated assumptions of behavioral estimation methods. ERPs were recorded while a-MCI patients and cognitively normal (CN) age-matched adults performed a recognition memory task. When retrieval success was measured (hits versus correct rejections) in which performance was matched by group, a-MCI patients displayed similar neural correlates to that of the CN group, including modulation of the FN400 and the late positive complex (LPC) which are thought to index familiarity and recollection, respectively. Alternatively, when the integrity of these components was measured based on retrieval attempts (studied versus unstudied items), a-MCI patients displayed a reduced FN400 and LPC. Furthermore, modulation of the FN400 correlated with a behavioral estimate of familiarity and the LPC with a behavioral estimate of recollection obtained in a separate experiment in the same individuals, consistent with the proposed mappings of these indices. These results support a global decline of recognition memory in a-MCI, which suggests that the memory loss of prodromal AD may be qualitatively distinct from normal aging.

The hippocampus supports high-resolution binding in the service of perception, working memory and long-term memory

It is well established that the hippocampus plays a critical role in our ability to recollect past events. A number of recent studies have indicated that the hippocampus may also play a critical role in working memory and perception, but these results have been highly controversial because other similar studies have failed to find evidence for hippocampal involvement. Thus, the precise role that the hippocampus plays in cognition is still debated. In the current paper, I propose that the hippocampus supports the generation and utilization of complex high-resolution bindings that link together the qualitative aspects that make up an event; these bindings are essential for recollection, and they can also contribute to performance across a variety of tasks including perception and working memory. An examination of the existing patient literature provides support for this proposal by showing that hippocampal damage leads to impairments on perception and working memory tasks that require complex high-resolution bindings. Conversely, hippocampal damage is much less likely to lead to impairments on tasks that require only low-resolution or simple associations/relations. The current proposal can be distinguished from earlier accounts of hippocampal function, and it generates a number of novel predictions that can be tested in future studies.

Development and assessment of a composite score for memory in the Alzheimer's Disease Neuroimaging Initiative (ADNI)

We sought to develop and evaluate a composite memory score from the neuropsychological battery used in the Alzheimer's Disease (AD) Neuroimaging Initiative (ADNI). We used modern psychometric approaches to analyze longitudinal Rey Auditory Verbal Learning Test (RAVLT, 2 versions), AD Assessment Schedule - Cognition (ADAS-Cog, 3 versions), Mini-Mental State Examination (MMSE), and Logical Memory data to develop ADNI-Mem, a composite memory score. We compared RAVLT and ADAS-Cog versions, and compared ADNI-Mem to RAVLT recall sum scores, four ADAS-Cog-derived scores, the MMSE, and the Clinical Dementia Rating Sum of Boxes. We evaluated rates of decline in normal cognition, mild cognitive impairment (MCI), and AD, ability to predict conversion from MCI to AD, strength of association with selected imaging parameters, and ability to differentiate rates of decline between participants with and without AD cerebrospinal fluid (CSF) signatures. The second version of the RAVLT was harder than the first. The ADAS-Cog versions were of similar difficulty. ADNI-Mem was slightly better at detecting change than total RAVLT recall scores. It was as good as or better than all of the other scores at predicting conversion from MCI to AD. It was associated with all our selected imaging parameters for people with MCI and AD. Participants with MCI with an AD CSF signature had somewhat more rapid decline than did those without. This paper illustrates appropriate methods for addressing the different versions of word lists, and demonstrates the additional power to be gleaned with a psychometrically sound composite memory score.

Distinct medial temporal contributions to different forms of recognition in amnestic mild cognitive impairment and Alzheimer's disease

The simplest expression of episodic memory is the experience of familiarity, the isolated recognition that something has been encountered previously. Brain structures of the medial temporal lobe (MTL) make essential contributions to episodic memory, but the distinct contributions from each MTL structure to familiarity are debatable. Here we used specialized tests to assess recognition impairments and their relationship to MTL integrity in people with amnestic mild cognitive impairment (aMCI, n=19), people with probable Alzheimer's disease (AD; n=10), and age-matched individuals without any neurological disorder (n=20). Recognition of previously presented silhouette objects was tested in two formats-forced-choice recognition with four concurrent choices (one target and three foils) and yes/no recognition with individually presented targets and foils. Every foil was extremely similar to a corresponding target, such that forced-choice recognition could be based on differential familiarity among the choices, whereas yes/no recognition necessitated additional memory and decision factors. Only yes/no recognition was impaired in the aMCI group, whereas both forced-choice and yes/no recognition were impaired in the AD group. Magnetic resonance imaging showed differential brain atrophy, as MTL volume was reduced in the AD group but not in the aMCI group. Pulsed arterial spin-labeled scans demonstrated that MTL blood flow was abnormally increased in aMCI, which could indicate physiological dysfunction prior to the emergence of significant atrophy. Regression analyses with data from all patients revealed that regional patterns of MTL integrity were differentially related to forced-choice and yes/no recognition. Smaller perirhinal cortex volume was associated with lower forced-choice recognition accuracy, but not with lower yes/no recognition accuracy. Instead, smaller hippocampal volumes were associated with lower yes/no recognition accuracy. In sum, familiarity memory can be specifically assessed using the forced-choice recognition test, it declines later than other MTL-dependent memory functions as AD progresses, and it has distinct anatomical substrates.

Vulnerable neural systems and the borderland of brain aging and neurodegeneration

Brain aging is characterized by considerable heterogeneity, including varying degrees of dysfunction in specific brain systems, notably a medial temporal lobe memory system and a frontostriatal executive system. These same systems are also affected by neurodegenerative diseases. Recent work using techniques for presymptomatic detection of disease in cognitively normal older people has shown that some of the late life alterations in cognition, neural structure, and function attributed to aging probably reflect early neurodegeneration. However, it has become clear that these same brain systems are also vulnerable to aging in the absence of even subtle disease. Thus, fundamental systemic limitations appear to confer vulnerability of these neural systems to a variety of insults, including those recognized as typical disease and those that are attributed to age. By focusing on the fundamental causes of neural system vulnerability, the prevention or treatment of a wide range of late-life neural dysfunction might be possible.

Familiarity-based memory as an early cognitive marker of preclinical and prodromal AD

There is great interest in the development of cognitive markers that differentiate "normal" age-associated cognitive change from that of Alzheimer's disease (AD) in its prodromal (i.e., mild cognitive impairment; MCI) or even preclinical stages. Dual process models posit that recognition memory is supported by the dissociable processes of recollection and familiarity. Familiarity-based memory has generally been considered to be spared during normal aging, but it remains controversial whether this type of memory is impaired in early AD. Here, we describe findings of estimates of recollection and familiarity in young adults (YA), cognitively normal older adults (CN), and patients with amnestic-MCI (a-MCI). These measures in the CN and a-MCI patients were then related to a structural imaging biomarker of AD that has previously been demonstrated to be sensitive to preclinical and prodromal AD, the Cortical Signature of AD (ADsig). Consistent with much work in the literature, recollection, but not familiarity, was impaired in CN versus YA. Replicating our prior findings, a-MCI patients displayed impairment in both familiarity and recollection. Finally, the familiarity measure was correlated with the ADsig biomarker across the CN and a-MCI group, as well as within the CN adults alone. No other standard psychometric measure was as highly associated with the ADsig, suggesting that familiarity may be a sensitive biomarker of AD-specific brain changes in preclinical and prodromal AD and that it may offer a qualitatively distinct measure of early AD memory impairment relative to normal age-associated change.

Associative memory in aging: the effect of unitization on source memory

In normal aging, memory for associations declines more than memory for individual items. Unitization is an encoding process defined by creation of a new single entity to represent a new arbitrary association. The current study tested the hypothesis that age-related differences in associative memory can be reduced by encoding instructions that promote unitization. In two experiments, groups of 20 young and 20 older participants learned new associations between a word and a background color under two conditions. In the item detail condition, they had to imagine that the item is the same color as the background-an instruction promoting unitization of the associations. In the context detail condition, which did not promote unitization, they had to imagine that the item interacted with another colored object. At test, they had to retrieve the color that was associated with each word (source memory). In both experiments, the results showed an age-related decrement in source memory performance in the context detail but not in the item detail condition. Moreover, Experiment 2 examined receiver operating characteristics in older participants and indicated that familiarity contributed more to source memory performance in the item detail than in the context detail condition. These findings suggest that unitization of new associations can overcome the associative memory deficit observed in aging, at least for item-color associations.

Recall versus familiarity when recall fails for words and scenes: the differential roles of the hippocampus, perirhinal cortex, and category-specific cortical regions

This fMRI study examined recall and familiarity for words and scenes using the novel recognition without cued recall (RWCR) paradigm. Subjects performed a cued recall task in which half of the test cues resembled studied items (and thus were familiar) and half did not. Subjects also judged the familiarity of the cue itself. RWCR is the finding that, among cues for which recall fails, subjects generally rate cues that resemble studied items as more familiar than cues that do not. For words, left and right hippocampal activity increased when recall succeeded relative to when it failed. When recall failed, right hippocampal activity was decreased for familiar relative to unfamiliar cues. In contrast, right Prc activity increased for familiar cues for which recall failed relative to both familiar cues for which recall succeeded and to unfamiliar cues. For scenes, left hippocampal activity increased when recall succeeded relative to when it failed but did not differentiate familiar from unfamiliar cues when recall failed. In contrast, right Prc activity increased for familiar relative to unfamiliar cues when recall failed. Category-specific cortical regions showed effects unique to their respective stimulus types: The visual word form area (VWFA) showed effects for recall vs. familiarity specific to words, and the parahippocampal place area (PPA) showed effects for recall vs. familiarity specific to scenes. In both cases, these effects were such that there was increased activity occurring during recall relative to when recall failed, and decreased activity occurring for familiar relative to unfamiliar cues when recall failed.

The process-dissociation approach two decades later: convergence, boundary conditions, and new directions

The process-dissociation procedure was developed to separate the controlled and automatic contributions of memory. It has spawned the development of a host of new measurement approaches and has been applied across a broad range of fields in the behavioral sciences, ranging from studies of memory and perception to neuroscience and social psychology. Although it has not been without its shortcomings or critics, its growing influence attests to its utility. In the present article, we briefly review the factors motivating its development, describe some of the early applications of the general method, and review the literature examining its underlying assumptions and boundary conditions. We then highlight some of the specific issues that the methods have been applied to and discuss some of the more recent applications of the procedure, along with future directions.

Remote spatial memory in aging: all is not lost

The ability to acquire and retain spatial memories in order to navigate in new environments is known to decline with age, but little is known about the effect of aging on representations of environments learned long ago, in the remote past. To investigate the status of remote spatial memory in old age, we tested healthy young and older adults on a variety of mental navigation tests based on a large-scale city environment that was very familiar to participants but rarely visited by the older adults in recent years. We show that whereas performance on a route learning test of new spatial learning was significantly worse in older than younger adults, performance was comparable or better in the older adults on mental navigation tests based on a well-known environment learned long ago. An exception was in the older adults' ability to vividly re-experience the well-known environment, and recognize and represent the visual details contained within it. The results are seen as analogous to the pattern of better semantic than episodic memory that has been found to accompany healthy aging.

Measuring recollection and familiarity: Improving the remember/know procedure

The remember/know (RK) procedure is the most widely used method to investigate recollection and familiarity. It uses trial-by-trial reports to determine how much recollection and familiarity contribute to different kinds of recognition. Few other methods provide information about individual memory judgements and no alternative allows such direct indications of recollection and familiarity influences. Here we review how the RK procedure has been and should be used to help resolve theoretical disagreements about the processing and neural bases of components of recognition memory. Emphasis is placed on procedural weaknesses and a possible confound of recollection and familiarity with recognition memory strength. Recommendations are made about how to minimise these problems including using modified versions of the procedure. The proposals here are important for improving behavioural and lesion research, and vital for brain imaging work.

Cognitive training-related changes in hippocampal activity associated with recollection in older adults

Impairments in the ability to recollect specific details of personally experienced events are one of the main cognitive changes associated with aging. Cognitive training can improve older adults' recollection. However, little is currently known regarding the neural correlates of these training-related changes in recollection. Prior research suggests that the hippocampus plays a central role in supporting recollection in young and older adults, and that age-related changes in hippocampal function may lead to age-related changes in recollection. The present study investigated whether cognitive training-related increases in older adults' recollection are associated with changes in their hippocampal activity during memory retrieval. Older adults' hippocampal activity during retrieval was examined before and after they were trained to use semantic encoding strategies to intentionally encode words. Training-related changes in recollection were positively correlated with training-related changes in activity for old words in the hippocampus bilaterally. Positive correlations were also found between training-related changes in activity in prefrontal and left lateral temporal regions associated with self-initiated semantic strategy use during encoding and training-related changes in right hippocampal activity associated with recollection during retrieval. These results suggest that cognitive training-related improvements in older adults' recollection can be supported by changes in their hippocampal activity during retrieval. They also suggest that age differences in cognitive processes engaged during encoding are a significant contributor to age differences in recollection during retrieval.

A medial temporal lobe division of labor: insights from memory in aging and early Alzheimer disease

Dual process theories of recognition memory posit that recollection and familiarity represent dissociable processes. Animal studies and human functional imaging experiments support an anatomic dissociation of these processes in the medial temporal lobes (MTL). By this hypothesis, recollection may be dependent on the hippocampus, while familiarity appears to rely on extrahippocampal MTL (ehMTL) structures, particularly perirhinal and lateral entorhinal cortices. Despite these findings, the dual process model and these anatomic mappings remain controversial, in part because the study of patients with lesions to the MTL has been limited and has revealed predominantly single dissociations. We examined measures of recollection and familiarity in three groups (normal older adults, amnesic-mild cognitive impairment, Alzheimer's disease) in which these memory measures and the relative integrity of MTL structures are variable, thus enhancing our power to detect MTL-memory relationships. Recollection and familiarity and volumes of hippocampus and ehMTL, defined as a region including entorhinal/perirhinal cortices and parahippocampus, were measured. Regression analyses revealed a stronger relationship of recollection with the hippocampus compared to ehMTL, while familiarity was more highly related to ehMTL compared to hippocampus. These results are consistent with a division of labor in the MTL and the dual process model.

Neuroanatomical substrates of age-related cognitive decline

There are many reports of relations between age and cognitive variables and of relations between age and variables representing different aspects of brain structure and a few reports of relations between brain structure variables and cognitive variables. These findings have sometimes led to inferences that the age-related brain changes cause the age-related cognitive changes. Although this conclusion may well be true, it is widely recognized that simple correlations are not sufficient to warrant causal conclusions, and other types of correlational information, such as mediation and correlations between longitudinal brain changes and longitudinal cognitive changes, also have limitations with respect to causal inferences. These issues are discussed, and the existing results on relations of regional volume, white matter hyperintensities, and diffusion tensor imaging measures of white matter integrity to age and to measures of cognitive functioning are reviewed. It is concluded that at the current time the evidence that these aspects of brain structure are neuroanatomical substrates of age-related cognitive decline is weak. The final section contains several suggestions concerning measurement and methodology that may lead to stronger conclusions in the future.

Improved proper name recall in aging after electrical stimulation of the anterior temporal lobes

Evidence from neuroimaging and neuropsychology suggests that portions of the anterior temporal lobes (ATLs) play a critical role in proper name retrieval. We previously found that anodal transcranial direct current stimulation (tDCS) to the ATLs improved retrieval of proper names in young adults (Ross et al., 2010). Here we extend that finding to older adults who tend to experience greater proper-naming deficits than young adults. The task was to look at pictures of famous faces or landmarks and verbally recall the associated proper name. Our results show a numerical improvement in face naming after left or right ATL stimulation, but a statistically significant effect only after left-lateralized stimulation. The magnitude of the enhancing effect was similar in older and younger adults but the lateralization of the effect differed depending on age. The implications of these findings for the use of tDCS as tool for rehabilitation of age-related loss of name recall are discussed.

Entorhinal cortex volume is associated with episodic memory related brain activation in normal aging and amnesic mild cognitive impairment

The present study examined the relationship between entorhinal cortex and hippocampal volume with fMRI activation during episodic memory function in elderly controls with no cognitive impairment and individuals with amnesic mild cognitive impairment (aMCI). Both groups displayed limited evidence for a relationship between hippocampal volume and fMRI activation. Smaller right entorhinal cortex volume was correlated with reduced activation in left and right medial frontal cortex (BA 8) during incidental encoding for both aMCI and elderly controls. However, during recognition, smaller left entorhinal cortex volume correlated with reduced activation in right BA 8 for the control group, but greater activation for the aMCI group. There was no significant relationship between entorhinal cortex volume and activation during intentional encoding in either group. The recognition-related dissociation in structure/function relationships in aMCI paralleled our behavioral findings, where individuals with aMCI displayed poorer performance relative to controls during recognition, but not encoding. Taken together, these results suggest that the relationship between entorhinal cortex volume and fMRI activation during episodic memory function is altered in individuals with aMCI.

Reduced specificity of hippocampal and posterior ventrolateral prefrontal activity during relational retrieval in normal aging

Neuroimaging studies of episodic memory in young adults demonstrate greater functional neural activity in ventrolateral pFC and hippocampus during retrieval of relational information as compared with item information. We tested the hypothesis that healthy older adults--individuals who exhibit behavioral declines in relational memory--would show reduced specificity of ventrolateral prefrontal and hippocampal regions during relational retrieval. At study, participants viewed two nouns and were instructed to covertly generate a sentence that related the words. At retrieval, fMRIs were acquired during item and relational memory tasks. In the relational task, participants indicated whether the two words were previously seen together. In the item task, participants indicated whether both items of a pair were previously seen. In young adults, left posterior ventrolateral pFC and bilateral hippocampal activity was modulated by the extent to which the retrieval task elicited relational processing. In older adults, activity in these regions was equivalent for item and relational memory conditions, suggesting a reduction in ventrolateral pFC and hippocampal specificity with normal aging.

Familiarity and recollection in Williams syndrome

Interest is being shown in a componential analysis of performance on declarative memory tasks that distinguishes two different kinds of access to stored memories, recollection and familiarity. From a developmental perspective, it has been hypothesized that recollection emerges later and shows more developmental changes than familiarity. Nevertheless, the contribution of recollection and familiarity to the recognition performance of individuals with intellectual disabilities (ID) has been rarely examined. The present study was aimed at investigating the qualitative profile of declarative long-term memory in a group of individuals with Williams syndrome (WS). We compared 13 individuals with WS and 13 mental-age-matched typically developing children in two different experimental paradigms to assess the contribution of familiarity and recollection to recognition performance. We adopted a modified version of the process dissociation procedure and a task dissociation procedure, both of which are suited to individuals with ID. Results of both experimental paradigms demonstrated reduced recollection and spared familiarity in the declarative memory performances of individuals with WS. These results provide direct evidence of a dissociation between recollection and familiarity in a neurodevelopmental disorder and are discussed in relation to alternative approaches for explaining abnormal cognition in individuals with ID.

Impact of novelty and type of material on recognition in healthy older adults and persons with mild cognitive impairment

The goal of this study was to assess the effect of novelty on correct recognition (hit minus false alarms) and on recollection and familiarity processes in normal aging and amnestic mild cognitive impairment (MCI). Recognition tasks compared well-known and novel stimuli in the verbal domain (words vs. pseudowords) and in the musical domain (well-known vs. novel melodies). Results indicated that novel materials associated with lower correct recognition and lower recollection, an effect that can be related to its lower amenability to elaborative encoding in comparison with well-known items. Results also indicated that normal aging impairs recognition of well-known items, whereas MCI impairs recognition of novel items only. Healthy older adults showed impaired recollection and familiarity relative to younger controls and individuals with MCI showed impaired recollection relative to healthy older adults. The recollection deficit in healthy older adults and persons with MCI and their impaired recognition of well-known items is compatible with the difficulty both groups have in encoding information in an elaborate manner. In turn, familiarity deficit could be related to impaired frontal functioning. Therefore, novelty of material has a differential impact on recognition in persons with age-related memory disorders.

The role of recollection and familiarity in the functional differentiation of the medial temporal lobes

The components of the medial temporal lobes (MTL) receive different kinds of input. The perirhinal cortex receives primarily object/item information, the parahippocampal cortex receives contextual information, and the hippocampus receives high-level inputs that include object/item, context, and other information. Critically, the perirhinal and parahippocampal cortices have similar cytoarchitectonics, which differ considerably from that of the hippocampus and suggest that these cortices process their inputs differently from the way that the hippocampus processes its inputs. Much evidence indicates that the hippocampus is designed to rapidly bind together pattern-separated representations that support recall/recollection well. In contrast, the newer MTL cortices rapidly create poorly pattern-separated memories that support familiarity well, but recall/recollection very poorly. For over a decade, there has been disagreement about whether recall/recollection is primarily mediated by the hippocampus and familiarity by the evolutionarily newer MTL cortices or whether the MTL mediates these kinds of memory in an integrated, homogeneous fashion. Common misconceptions about familiarity, recollection, item, and associative memory are discussed as are methodological problems with MTL lesion and functional imaging research. The possible confound of familiarity with weaker memory and recollection with stronger memory is discussed and the implications of the Montaldi et al. (2006) functional Magnetic Resonance Imaging (fMRI) study, which matched memory strength between strong familiarity and recollection, finding that only recollection activated the hippocampus, are discussed. A suggestion is made about how the long-running conflict of findings in the human hippocampal lesion literature may be resolved.

Recollection and familiarity: examining controversial assumptions and new directions

It is well accepted that recognition memory reflects the contribution of two separable memory retrieval processes, namely recollection and familiarity. However, fundamental questions remain regarding the functional nature and neural substrates of these processes. In this article, we describe a simple quantitative model of recognition memory (i.e., the dual-process signal detection model) that has been useful in integrating findings from a broad range of cognitive studies, and that is now being applied in a growing number of neuroscientific investigations of memory. The model makes several strong assumptions about the behavioral nature and neural substrates of recollection and familiarity. A review of the literature indicates that these assumptions are generally well supported, but that there are clear boundary conditions in which these assumptions break down. We argue that these findings provide important insights into the operation of the processes underlying recognition. Finally, we consider how the dual-process approach relates to recent neuroanatomical and computational models and how it might be integrated with recent findings concerning the role of medial temporal lobe regions in other cognitive functions such as novelty detection, perception, implicit memory and short-term memory.

Everyday memory: self-perception and structural brain correlates in a healthy elderly population

Mapping the cerebral structural correlates of age-related cognitive decline is a growing area of research. The aim of the present study was to investigate the relationship between healthy elderly's perceived memory functioning in daily life, neuropsychological test performance on a standardized test on verbal memory, and cortical thickness and subcortical volumes in brain regions implicated in memory networks, including the medial temporal lobe (MTL). Eighty-three healthy and cognitively well-functioning volunteers aged 60-85 years underwent MRI scans, Everyday Memory Questionnaire (EMQ), and neuropsychological assessment. Both self-perceived memory in daily life related to attention and executive functions and an objective measure of verbal recall (CVLT) were, independently, associated with thickness of the left MTL. The two cognitive variables were uncorrelated, and including both measures in the model nearly doubled the amount of explained variance on MTL thickness. This suggests that measures of perceived everyday memory might substantially inform and supplement studies investigating the relationships between neuropsychological test performance and brain morphology. The results are consistent with a bigger-is-better relationship in the MTL and suggest that EMQ and neuropsychological test performance have detectable and comparable structural correlates in a region critically involved in memory functions in the well-functioning elderly.

Structural brain changes in aging: courses, causes and cognitive consequences

The structure of the brain is constantly changing from birth throughout the lifetime, meaning that normal aging, free from dementia, is associated with structural brain changes. This paper reviews recent evidence from magnetic resonance imaging (MRI) studies about age-related changes in the brain. The main conclusions are that (1) the brain shrinks in volume and the ventricular system expands in healthy aging. However, the pattern of changes is highly heterogeneous, with the largest changes seen in the frontal and temporal cortex, and in the putamen, thalamus, and accumbens. With modern approaches to analysis of MRI data, changes in cortical thickness and subcortical volume can be tracked over periods as short as one year, with annual reductions of between 0.5% and 1.0% in most brain areas. (2) The volumetric brain reductions in healthy aging are likely only to a minor extent related to neuronal loss. Rather, shrinkage of neurons, reductions of synaptic spines, and lower numbers of synapses probably account for the reductions in grey matter. In addition, the length of myelinated axons is greatly reduced, up to almost 50%. (3) Reductions in specific cognitive abilities--for instance processing speed, executive functions, and episodic memory--are seen in healthy aging. Such reductions are to a substantial degree mediated by neuroanatomical changes, meaning that between 25% and 100% of the differences between young and old participants in selected cognitive functions can be explained by group differences in structural brain characteristics.

Fractionating verbal episodic memory in Alzheimer's disease

The aim of this study was to determine the neural correlates of different stages of episodic memory function and their modulation by Alzheimer's disease (AD). Several decades of work has supported the role of the medial temporal lobes (MTL) in episodic memory function. However, a more recent work, derived in part from functional neuroimaging studies, has suggested that other brain structures make up a large-scale network that appears to support successful encoding and retrieval of episodic memories. Furthermore, controversy exists as to whether dissociable MTL regions support qualitatively different aspects of memory (hippocampus: contextual memory or 'recollection'; perirhinal/lateral entorhinal cortex: item memory or 'familiarity'). There is limited neuropsychological support for these models and most work in AD only has examined free recall memory measures. We studied the relationship between performance on different stages of the Rey Auditory Verbal Learning Test (AVLT), a 15-item word list learning task, and structural MRI measures in mild AD patients. Structural measures included hippocampal volume and cortical thickness of several ROIs known to undergo atrophy in AD. Correlation and multiple regression analyses, controlling for age, education, and gender, were performed in 146 mild AD patients (MMSE 23.3±2.0). To evaluate the robustness of these relationships, similar analyses were performed with additional standardized verbal memory measures. Early immediate recall trials (e.g. Trial 1 of the AVLT) were not associated with the size of MTL regions, but correlated most strongly with inferior parietal, middle frontal gyrus, and temporal pole ROIs. After repeated exposure (e.g. Trial 5 of the AVLT), immediate recall was correlated with both MTL and a similar distribution of isocortical structures, but most strongly the temporal pole. For delayed recall, only the hippocampus correlated with performance. In contrast, for delayed recognition discrimination, the perirhinal/entorhinal cortex correlated more strongly than the hippocampus; no other isocortical regions were strongly associated with performance. Convergent results were found for immediate and delayed trials of other memory tests. The current results suggest that a richer understanding of the memory deficits in AD can be gained by examining multiple measures, which tap different aspects of memory function. Furthermore, the present findings are consistent with models hypothesizing different stages of verbal list learning map onto dissociable brain regions. These data have implications for understanding the anatomic basis of processes underlying episodic memory, particularly related to a division of labor within the medial temporal lobes and within the large-scale MTL-cortical memory network.

ROC in animals: uncovering the neural substrates of recollection and familiarity in episodic recognition memory

It is a consensus that familiarity and recollection contribute to episodic recognition memory. However, it remains controversial whether familiarity and recollection are qualitatively distinct processes supported by different brain regions, or whether they reflect different strengths of the same process and share the same support. In this review, I discuss how adapting standard human recognition memory paradigms to rats, performing circumscribed brain lesions and using receiver operating characteristic (ROC) methods contributed to solve this controversy. First, I describe the validation of the animal ROC paradigms and report evidence that familiarity and recollection are distinct processes in intact rats. Second, I report results from rats with hippocampal dysfunction which confirm this finding and lead to the conclusion that the hippocampus supports recollection but not familiarity. Finally, I describe a recent study focusing on the medial entorhinal cortex (MEC) that investigates the contribution of areas upstream of the hippocampus to recollection and familiarity.

Double dissociation of selective recollection and familiarity impairments following two different surgical treatments for temporal-lobe epilepsy

Research has firmly established that the integrity of the medial temporal lobe (MTL) is critical for recognition memory. This ability is supported by recollection, which involves recovery of contextual details of a past stimulus encounter, and familiarity assessment, which leads to awareness of prior occurrence without such recovery. Dual-process models of MTL organization posit that recollection and familiarity are supported by the hippocampus and perirhinal cortex, respectively. Alternatively, it has been argued that both structures support these recognition processes similarly as part of a more integrated declarative memory system; from this perspective, reported selective recollection impairments with circumscribed hippocampal lesions may reflect differential sensitivity to overall memory strength, rather than a deficit in a distinct recognition process. Findings from past neuropsychological research remain inconsistent and controversial, in part due to biases in patient selection, variability in clinical etiology, and limited lesion documentation. Here, we administered a verbal recognition-memory task in combination with remember-know judgements to 10 individuals who had undergone left- or right-sided stereotactic amygdalo-hippocampotomy as a surgical treatment for intractable temporal-lobe epilepsy. Comparisons with healthy control participants revealed isolated impairments in recollection with preserved familiarity, regardless of hemispheric site of lesion. In addition, we show that this impairment can be observed at a comparable level of memory strength (i.e., overall recognition performance) as the selective familiarity impairment we previously described in N.B.--an individual who underwent a tailored surgical resection of the left anterior temporal lobe with hippocampal sparing for treatment of temporal-lobe epilepsy. By revealing a double dissociation concerning temporal-lobe mechanisms for recollection and familiarity, this evidence argues against a unitary, strength-based account of MTL organization.

Aging effects on recollection and familiarity: the role of white matter hyperintensities

Previous studies have indicated that aging is associated with declines in recollection whereas familiarity-based recognition is left largely unaffected. The brain changes underlying these recollection declines are yet not well understood. In the current study we examined the role of white matter integrity as measured by white matter hyperintensities (WMH) on age-related changes in recollection and familiarity. Recognition was measured using a remember/know procedure (Experiment 1) and a source-memory process-dissociation procedure (Experiment 2). Robust age related declines in recollection were observed, but there was no evidence that white matter damage was related to the observed memory declines. Although future studies with larger samples will be necessary to fully characterize the role of WMH in normal age-related declines in different types of memory, the results suggest that declines in recollection are not strongly related to the brain changes indexed by WMHs.

Recognition memory: adding a response deadline eliminates recollection but spares familiarity

A current controversy in memory research concerns whether recognition is supported by distinct processes of familiarity and recollection, or instead by a single process wherein familiarity and recollection reflect weak and strong memories, respectively. Recent studies using receiver operating characteristic (ROC) analyses in an animal model have shown that manipulations of the memory demands can eliminate the contribution of familiarity while sparing recollection. Here it is shown that a different manipulation, specifically the addition of a response deadline in recognition testing, results in the opposite performance pattern, eliminating the contribution of recollection while sparing that of familiarity. This dissociation, combined with the earlier findings, demonstrates that familiarity and recollection are differentially sensitive to specific memory demands, strongly supporting the dual process view.

Captivity reduces hippocampal volume but not survival of new cells in a food-storing bird

In many naturalistic studies of the hippocampus wild animals are held in captivity. To test if captivity itself affects hippocampal integrity, adult black-capped chickadees (Poecile atricapilla) were caught in the fall, injected with bromodeoxyuridine to mark neurogenesis, and alternately released to the wild or held in captivity. The wild birds were recaptured after 4-6 weeks and perfused simultaneously with their captive counterparts. The hippocampus of captive birds was 23% smaller than wild birds, with no hemispheric differences in volume within groups. Between groups there was no statistically significant difference in the size of the telencephalon, or in the number and density of surviving new cells. Proximate causes of the reduced hippocampal volume could include stress, lack of exercise, diminished social interaction, or limited caching opportunity-a hippocampal-dependent activity. The results suggest the avian hippocampus-a structure essential for rapid, complex relational and spatial learning-is both plastic and sensitive, much as in mammals, including humans.

Effect of disease severity and dopaminergic medication on recollection and familiarity in patients with idiopathic nondementing Parkinson's

The effect of disease severity and dopaminergic medication on the assessment of familiarity and the recollection of episodic details during recognition in nondementing idiopathic Parkinson's is uncertain. Some studies have reported familiarity as deficient in mild Parkinson's yet others have found it intact even in moderate Parkinson's. Recollection has been found to be both preserved and deficient in mild and moderate Parkinson's. The extent to which these conflicting findings are explained by disease severity or dopaminergic medication or a combination of the two is uncertain, as all studies assessed patients in a medicated state, and disease severity has not always been consistently reported. Twelve patients with mild Parkinson's and 11 with moderate Parkinson's (medicated Hoehn and Yahr mean: 2.1 and 3.2, respectively), completed matched versions of a yes/no recognition memory test in a medicated and unmedicated condition (termed ON and OFF, respectively). Twenty-one matched healthy volunteers also completed both memory tasks in 2 separate sessions (termed Blue and Green, respectively). In the ON/Green condition, the moderate Parkinson's recollection performance was significantly poorer than the healthy volunteers and mild Parkinson's. By contrast, recognition memory and familiarity measures in both Parkinson's group were relatively spared. In the OFF/Blue condition, the moderate Parkinson's recollection was impaired, but only in relation to the healthy volunteer set. There were no significant differences in recollection performance between the mild and moderate Parkinson's groups. Again, recognition memory and familiarity measures in both Parkinson's group were relatively spared. Further analyses showed the moderate patients' recollection rates to be significantly poorer ON-medication compared to OFF. These findings are discussed in relation to the staging of disease progression on medial temporal areas which separately support recollection and familiarity, and the putative effects the different classes of dopaminergic drugs may have on these areas.

Linking hippocampal structure and function to memory performance in an aging population

BACKGROUND

Hippocampal atrophy and reductions in basal cerebral blood volume (CBV), a hemodynamic correlate of brain function, occur with cognitive impairment in Alzheimer disease, but whether these are early or late changes remains unclear. Magnetic resonance imaging is used to assess structure and function in the hippocampal formation.

OBJECTIVE

To estimate differences in the associations of hippocampal and entorhinal cortex volumes and CBV with memory function in the early and late stages of cognitive impairment by relating these measures to memory function in persons with and without dementia who underwent detailed brain imaging and neuropsychological assessment.

DESIGN

Multivariate regression analyses were used to relate entorhinal cortex volume, entorhinal cortex CBV, hippocampal volume, and hippocampal CBV to measurements of memory performance. The same measures were related to language function as a reference cognitive domain.

SETTING

Community-based cohort.

PARTICIPANTS

Two hundred thirty-one elderly Medicare recipients (aged > or =65 years) residing in northern Manhattan, New York.

MAIN OUTCOME MEASURES

Values for entorhinal cortex volume, hippocampal volume, entorhinal cortex CBV, and hippocampal CBV and their relation to memory performance.

RESULTS

No association was noted between entorhinal cortex volume or hippocampal CBV and memory. Decreased hippocampal volume was strongly associated with worse performance in total recall, and lower entorhinal cortex CBV was associated with lower performance in delayed recall. Excluding persons with Alzheimer disease, the association of entorhinal cortex CBV with memory measures was stronger, whereas the association between hippocampal volume and total recall became nonsignificant.

CONCLUSIONS

In the early stages of Alzheimer disease or in persons without dementia with worse memory ability, functional and metabolic hippocampal hypofunction contributes to memory impairment, whereas in the later stages, functional and structural changes play a role.

CSF biomarker pathology correlates with a medial temporo-parietal network affected by very mild to moderate Alzheimer's disease but not a fronto-striatal network affected by healthy aging

It is suggested that reductions in a medial temporo-parietal episodic memory network characterize Alzheimer's disease (AD), while changes in a fronto-striatal executive network characterize healthy aging. In the present study, magnetic resonance imaging (MRI) was used to test this directly. MRI scans of 372 participants from two samples were analyzed: Sample 1 consisted of 96 very mild to moderate AD patients, 93 healthy elderly (HE), and 137 young (HY), all with available MR scans, while Sample 2 consisted of 46 MCI patients, with available MR scans and measures of CSF biomarkers Abeta42 and tau protein. Substantial morphometric reductions of the medial temporo-parietal network were found in AD, while the fronto-striatal network was less affected. Both networks were affected by healthy aging, but the fronto-striatal to a greater degree than the medial temporo-parietal. Further exploratory analyses of 49 cortical and subcortical structures indicated no overlap between predictors of AD vs. HE and predictors of HE vs. HY. CSF biomarker pathology correlated with the medial temporo-parietal but not fronto-striatal network. Likewise, the AD-prone structures from the exploratory analyses were related to CSF biomarkers, while the aging-prone structures were not. It is concluded that the pattern of macrostructural brain changes in very mild to moderate AD can be clearly delineated from that of healthy aging.

Source monitoring 15 years later: what have we learned from fMRI about the neural mechanisms of source memory?

Focusing primarily on functional magnetic resonance imaging (fMRI), this article reviews evidence regarding the roles of subregions of the medial temporal lobes, prefrontal cortex, posterior representational areas, and parietal cortex in source memory. In addition to evidence from standard episodic memory tasks assessing accuracy for neutral information, the article considers studies assessing the qualitative characteristics of memories, the encoding and remembering of emotional information, and false memories, as well as evidence from populations that show disrupted source memory (older adults, individuals with depression, posttraumatic stress disorder, or schizophrenia). Although there is still substantial work to be done, fMRI is advancing understanding of source memory and highlighting unresolved issues. A continued 2-way interaction between cognitive theory, as illustrated by the source monitoring framework (M. K. Johnson, S. Hashtroudi, & D. S. Lindsay, 1993), and evidence from cognitive neuroimaging studies should clarify conceptualization of cognitive processes (e.g., feature binding, retrieval, monitoring), prior knowledge (e.g., semantics, schemas), and specific features (e.g., perceptual and emotional information) and of how they combine to create true and false memories.

Aging in rhesus macaques is associated with changes in novelty preference and altered saccade dynamics

Studies demonstrating recognition deficits with aging often use tasks in which subjects have an incentive to correctly encode or retrieve the experimental stimuli. In contrast to these tasks, which may engage strategic encoding and retrieval processes, the visual paired comparison (VPC) task measures spontaneous eye movements made toward a novel as compared with familiar stimulus. In the present study, seven rhesus macaques aged 6 to 30 years exhibited a dramatic age-dependent decline in preference for a novel image compared with one presented seconds earlier. The age effect could not be accounted for by memory deficits alone, because it was present even when familiarization preceded test by 1 second. It also could not be explained by an encoding deficit, because the effect persisted with increased familiarity of the sample stimulus. Reduced novelty preference did correlate with eye movement variables, including reaction time distributions and saccade frequency. At long delay intervals (24 or 48 hours) aging was paradoxically associated with increased novelty preference. Several explanations for the age effect are considered, including the possible role of dopamine.

A specialized odor memory buffer in primary olfactory cortex

Background

The neural substrates of olfactory working memory are unknown. We addressed the questions of whether olfactory working memory involves a verbal representation of the odor, or a sensory image of the odor, or both, and the location of the neural substrates of these processes.

Methodology/Principal Findings

We used functional magnetic resonance imaging to measure activity in the brains of subjects who were remembering either nameable or unnameable odorants. We found a double dissociation whereby remembering nameable odorants was reflected in sustained activity in prefrontal language areas, and remembering unnameable odorants was reflected in sustained activity in primary olfactory cortex.

Conclusions/Significance

These findings suggest a novel dedicated mechanism in primary olfactory cortex, where odor information is maintained in temporary storage to subserve ongoing tasks.

Impaired recollection but spared familiarity in patients with extended hippocampal system damage revealed by 3 convergent methods

To understand recognition memory, the detection of stimulus repetition, it first is necessary to resolve the debate between 2 fundamentally different models of recognition. Contemporary single-process models assume that recognition memory relies solely on the neural system required for the recall of prior events. Dual-process models assume that recognition comprises 2 independent forms of memory: one supports recall, and the other detects repeated stimuli by signaling their familiarity, the feeling of previous occurrence without the recall of any associated information. These 2 models were contrasted in patients who had undergone surgical removal of a colloid cyst, a condition associated with memory loss when accompanied by fornix and/or mammillary body atrophy. Comparisons were made between 2 groups of 9 patients that differed only with respect to the extent of mammillary body atrophy. Only the more atrophied group was impaired on tests of recall, but both groups showed normal recognition levels on a task that equates recall and recognition performance in normal participants. To explore the nature of this spared recognition, we estimated recall-based recognition and familiarity-based recognition using 3 distinct methods: self-report, receiver operating characteristics, and structural equation modeling. All 3 methods showed impaired recall-based recognition accompanied by intact familiarity in the most atrophied group, as predicted only by dual-process models. When structural equation modeling was applied to all 62 colloid cyst patients, the recall/familiarity dual-process model best explained the patients' memory pattern. The convergent evidence that mammillary body atrophy impairs recall but spares familiarity-based recognition appears irreconcilable with single-process models.

Familiarity or Conceptual Priming: Event-related Potentials in Name Recognition

Recent interest has been drawn to the separate components of recognition memory, as studied by event-related potentials (ERPs). In ERPs, recollection is usually accompanied by a late, parietal positive deflection. An earlier, frontal component has been suggested to be a counterpart, accompanying recognition by familiarity. However, this component, the FN400, has alternatively been suggested to reflect a form of implicit memory, conceptual priming. The present study examined the ERP components of recognition memory using an episodic memory task with a stimulus material consisting of names, half of which were famous. Along a different dimension, the names varied in how rare or common they were. These dimensions, frequency and fame, exerted powerful effects on memory accuracy, and dissociated the two recognition processes, such that frequency gave rise to familiarity and fame fostered recollection, when the receiver operating characteristics data were analyzed with Yonelinas' dual-process signal detection model. The ERPs corresponded fully to the behavioral data because frequency affected the frontal component exclusively, and fame affected the parietal component exclusively. Moreover, a separate behavioral experiment showed that conceptual priming was sensitive to fame, but not to frequency. Our data therefore indicate that the FN400 varies jointly with familiarity, but independently of conceptual priming.

Dissociation between recognition and recall in developmental amnesia

Developmental amnesia (DA) is a memory disorder due to hypoxia/ischaemia-induced damage to the hippocampus early in life. To test the hypothesis that this disorder is associated with a disproportionate impairment in recall vis-à-vis recognition, we examined a group of 10 patients with DA on the Doors and People test, which affords a quantitative comparison between measures of the two memory processes. The results supported the hypothesis in that the patients showed a sharp, though not complete, recall-recognition dissociation, exhibiting impairment on both measures relative to their matched controls, but with a far greater loss in recall than in recognition. Whether their relatively spared recognition ability is due to restriction of their medial temporal lobe damage to the hippocampus or whether it is due instead to their early age at injury is still uncertain.

The role of white matter hyperintensities and medial temporal lobe atrophy in age-related executive dysfunctioning

Various studies support an association between white matter hyperintensities (WMH) and deficits in executive function in nondemented ageing. Studies examining executive functions and WMH have generally adopted executive function as a phrase including various functions such as flexibility, inhibition, and working memory. However, these functions include distinctive cognitive processes and not all may be affected as a result of WMH. Furthermore, atrophy of the medial temporal lobe (MTA) is frequently observed in ageing. Nevertheless, in previous studies of nondemented ageing MTA was not considered when examining a relationship between white matter and executive function. The goal of the present study was to examine how WMH and MTA relate to a variety of executive functions, including flexibility, fluency, inhibition, planning, set shifting, and working memory. Strong correlations were observed between WMH and MTA and most of the executive functions. However, only MTA was related to flexibility and set shifting performance. Regression analysis furthermore showed that MTA was the strongest predictor of working memory, after which no further significant association with WMH was noted. Alternatively, both MTA and periventricular hyperintensities independently predicted inhibition performance. These findings emphasize the importance of MTA when examining age-related decline in executive functioning.

The role of the entorhinal cortex in extinction: influences of aging

The entorhinal cortex is perhaps the area of the brain in which neurofibrillary tangles and amyloid plaques are first detectable in old age with or without mild cognitive impairment, and very particularly in Alzheimer's disease. It plays a key role in memory formation, retrieval, and extinction, as part of circuits that include the hippocampus, the amygdaloid nucleus, and several regions of the neocortex, in particular of the prefrontal cortex. Lesions or biochemical impairments of the entorhinal cortex hinder extinction. Microinfusion experiments have shown that glutamate NMDA receptors, calcium and calmodulin-dependent protein kinase II, and protein synthesis in the entorhinal cortex are involved in and required for extinction. Aging also hinders extinction; it is possible that its effect may be in part mediated by the entorhinal cortex.