A Hippocampal Marker of Recollection Memory Ability among Healthy Young Adults: Contributions of Posterior and Anterior Segments

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.

Neural correlates of cognitive remediation in patients with mood disorders

Little is known about the brain changes that mediate improvement following cognitive remediation. We used neuropsychological tests and functional magnetic resonance imaging to study working memory and recollection memory in patients with mood disorders, before (PRE) and after (POST) 10 weeks of cognitive remediation. Thirty-eight patients completed a recollection memory task at PRE (28 had complete PRE and POST scans) and 35 patients completed an n-back working memory task at PRE (23 had complete PRE and POST scans). We also compared patients at PRE with two groups of healthy controls subjects (n=18 for the recollection memory task and n=15 for the working memory task). At PRE, compared to controls, patients had (i) poorer backward digit span scores, (ii) lower accuracy scores and weaker frontopolar activation during the 2-back condition, and (iii) poorer recollection scores and altered medial temporal activation on the recollection memory task. Following remediation, patients (i) improved on the backward digit span, (ii) activation increased in lateral and medial prefrontal, superior temporal, and lateral parietal regions in the 2-back condition, and (iii) recollection-related activation increased in the bilateral hippocampus. Improvements in 2-back accuracy correlated with activation increases in lateral and medial prefrontal and lateral parietal regions, and improved recollection scores correlated with activation increases in the left hippocampus. PRE-POST improvements on the backward digit span correlated with PRE-POST improvements in 2-back task accuracy; however, there was no direct association between improvement on the backward digit span following training and change in functional activation. These findings suggest that cognitive remediation may lead to behavioural improvements on tests of working memory. The relation between behavioural change and changes in functional activation following remediation requires further study.

Functional and effective hippocampal-neocortical connectivity during construction and elaboration of autobiographical memory retrieval

Autobiographical memory (AM) provides the opportunity to study interactions among brain areas that support the search for a specific episodic memory (construction), and the later experience of mentally reliving it (elaboration). While the hippocampus supports both construction and elaboration, it is unclear how hippocampal-neocortical connectivity differs between these stages, and how this connectivity involves the anterior and posterior segments of the hippocampus, as these have been considered to support the retrieval of general concepts and recollection processes, respectively. We acquired fMRI data in 18 healthy participants during an AM retrieval task in which participants were asked to access a specific AM (construction) and then to recollect it by recovering as many episodic details as possible (elaboration). Using multivariate analytic techniques, we examined changes in functional and effective connectivity of hippocampal-neocortical interactions during these phases of AM retrieval. We found that the left anterior hippocampus interacted with frontal areas during construction and bilateral posterior hippocampi with visual perceptual areas during elaboration, indicating key roles for both hippocampi in coordinating transient neocortical networks at both AM stages. Our findings demonstrate the importance of direct interrogation of hippocampal-neocortical interactions to better illuminate the neural dynamics underlying complex cognitive tasks such as AM retrieval.

Age differences in hippocampus-cortex connectivity during true and false memory retrieval

The present functional magnetic resonance imaging (fMRI) study investigated developmental differences in functional connectivity associated with true and false memory retrieval. A sample of 8- to 9-year-olds and adults (N = 31) was assessed with the Deese/Roediger-McDermott (DRM) paradigm, known to induce high levels of false recognition of lures that are semantically associated with studied items. The strength of semantic association among list items was manipulated. Relative to children, adults correctly recognized more studied items and falsely recognized more critical lures. High-association lists resulted in higher recognition of both studied items and critical lures. Functional connectivity analysis revealed that, overall, true recognition was supported by coupling within two hippocampal-temporal and fronto-parietal set of regions; in contrast, coupling among more distributed hippocampal-temporal-parietal-frontal regions was observed during false recognition. Critically, adults, compared to children, exhibited stronger hippocampal/parietal coupling and stronger hippocampal/dorsolateral prefrontal cortex (PFC) coupling for veridical recognition of high-associative strength items. In contrast, children, compared to adults, exhibited stronger hippocampus/ventrolateral PFC coupling and stronger bilateral middle-temporal gyrus/ventrolateral PFC coupling for high-associative strength critical lures. Our results underscored a role for the anterior hippocampus in true and false recognition, showing different functional patterns as a function of age and association strength.

Representations of recent and remote autobiographical memories in hippocampal subfields

The hippocampus has long been implicated in supporting autobiographical memories, but little is known about how they are instantiated in hippocampal subfields. Using high-resolution functional magnetic resonance imaging (fMRI) combined with multivoxel pattern analysis we found that it was possible to detect representations of specific autobiographical memories in individual hippocampal subfields. Moreover, while subfields in the anterior hippocampus contained information about both recent (2 weeks old) and remote (10 years old) autobiographical memories, posterior CA3 and DG only contained information about the remote memories. Thus, the hippocampal subfields are differentially involved in the representation of recent and remote autobiographical memories during vivid recall.

Histology-derived volumetric annotation of the human hippocampal subfields in postmortem MRI

Recently, there has been a growing effort to analyze the morphometry of hippocampal subfields using both in vivo and postmortem magnetic resonance imaging (MRI). However, given that boundaries between subregions of the hippocampal formation (HF) are conventionally defined on the basis of microscopic features that often lack discernible signature in MRI, subfield delineation in MRI literature has largely relied on heuristic geometric rules, the validity of which with respect to the underlying anatomy is largely unknown. The development and evaluation of such rules are challenged by the limited availability of data linking MRI appearance to microscopic hippocampal anatomy, particularly in three dimensions (3D). The present paper, for the first time, demonstrates the feasibility of labeling hippocampal subfields in a high resolution volumetric MRI dataset based directly on microscopic features extracted from histology. It uses a combination of computational techniques and manual post-processing to map subfield boundaries from a stack of histology images (obtained with 200μm spacing and 5μm slice thickness; stained using the Kluver-Barrera method) onto a postmortem 9.4Tesla MRI scan of the intact, whole hippocampal formation acquired with 160μm isotropic resolution. The histology reconstruction procedure consists of sequential application of a graph-theoretic slice stacking algorithm that mitigates the effects of distorted slices, followed by iterative affine and diffeomorphic co-registration to postmortem MRI scans of approximately 1cm-thick tissue sub-blocks acquired with 200μm isotropic resolution. These 1cm blocks are subsequently co-registered to the MRI of the whole HF. Reconstruction accuracy is evaluated as the average displacement error between boundaries manually delineated in both the histology and MRI following the sequential stages of reconstruction. The methods presented and evaluated in this single-subject study can potentially be applied to multiple hippocampal tissue samples in order to construct a histologically informed MRI atlas of the hippocampal formation.

Distinct familiarity-based response patterns for faces and buildings in perirhinal and parahippocampal cortex

An unresolved question in our understanding of the medial temporal lobes is how functional differences between structures pertaining to stimulus category relate to the distinction between item-based and contextually based recognition-memory processes. Specifically, it remains unclear whether perirhinal cortex (PrC) supports item-based familiarity signals for all stimulus categories or whether parahippocampal cortex (PhC) may also play a role for stimulus categories that are known to engage this structure in other task contexts. Here, we used multivoxel pattern analyses of fMRI data to compare patterns of activity in humans that are associated with the perceived familiarity of faces, buildings, and chairs. During scanning, participants judged the familiarity of previously studied and novel items from all three categories. Instances in which recognition was based on recollection were removed from all analyses. In right PrC, we found patterns of activity that distinguished familiar from novel faces. By contrast, in right PhC, we observed such patterns for buildings. Familiarity signals for chairs were present in both structures but shared little overlap with the patterns observed for faces and buildings on a more fine-grained scale. In the hippocampus, we found no evidence for familiarity signals for any object category. Our findings show that both PrC and PhC contribute to the assessment of item familiarity. They suggest that PhC does not only represent episodic context but can also represent item information for some object categories in recognition-memory decisions. In turn, our findings also indicate that the involvement of PrC in representing item familiarity is not ubiquitous.

Hippocampal network connectivity and activation differentiates post-traumatic stress disorder from generalized anxiety disorder

Anxiety disorders are a diverse group of clinical states. Post-traumatic stress disorder (PTSD) and generalized anxiety disorder (GAD), eg, share elevated anxiety symptoms, but differ with respect to fear-related memory dysregulation. As the hippocampus is implicated in both general anxiety and fear memory, it may be an important brain locus for mapping the similarities and differences among anxiety disorders. Anxiety and fear also functionally associate with different subdivisions of the hippocampus along its longitudinal axis: the human posterior (rodent dorsal) hippocampus is involved in memory, through connectivity with the medial prefrontal-medial parietal default-mode network, whereas the anterior (rodent ventral) hippocampus is involved in anxiety, through connectivity with limbic-prefrontal circuits. We examined whether differential hippocampal network functioning may help account for similarities and differences in symptoms in PTSD and GAD. Network-sensitive functional magnetic resonance imaging-based resting-state intrinsic connectivity methods, along with task-based assessment of posterior hippocampal/default-mode network function, were used. As predicted, in healthy subjects resting-state connectivity dissociated between posterior hippocampal connectivity with the default-mode network, and anterior hippocampal connectivity to limbic-prefrontal circuitry. The posterior hippocampus and the associated default-mode network, across both resting-state connectivity and task-based measures, were perturbed in PTSD relative to each of the other groups. By contrast, we found only modest support for similarly blunted anterior hippocampal connectivity across both patient groups. These findings provide new insights into the neural circuit-level dysfunctions that account for similar vs different features of two major anxiety disorders, through a translational framework built on animal work and carefully selected clinical disorders.

Structural development of the hippocampus and episodic memory: developmental differences along the anterior/posterior axis

The hippocampus is critically involved in episodic memory, yet relatively little is known about how the development of this structure contributes to the development of episodic memory during middle to late childhood. Previous research has inconsistently reported associations between hippocampal volume and episodic memory performance during this period. We argue that this inconsistency may be due to assessing the hippocampus as a whole, and propose to examine associations separately for subregions along the longitudinal axis of the hippocampus. In the present study, we examined age-related differences in volumes of the hippocampal head, body, and tail, and collected episodic memory measures in children ages 8-11 years and young adults (N = 62). We found that adults had a smaller right hippocampal head, larger hippocampal body bilaterally, and smaller right hippocampal tail compared with children. In adults, but not in children, better episodic memory performance was associated with smaller right hippocampal head and larger hippocampal body. In children, but not in adults, better episodic memory was associated with larger left hippocampal tail. Overall, the results suggest that protracted development of hippocampal subregions contribute to age-related differences in episodic memory.

Spatial cognition and the hippocampus: the anterior-posterior axis

We discuss the question of differentiation along the anterior-posterior longitudinal axis of the hippocampus. Data from a recent fMRI study are reanalyzed to determine whether activations in these hippocampal regions are affected by the nature of the information being accessed during a scanning session in which participants thought about episodes from their lives. Retrieving detailed spatial relational information preferentially activated the posterior hippocampus, whereas retrieving information about locales (or contexts) preferentially activated the anterior hippocampus. These data support the view that there is functional differentiation along the longitudinal axis in humans that matches what has been seen in rats, namely, that the posterior (dorsal) hippocampus is crucial for precise spatial behavior, and the anterior (ventral) hippocampus is crucial for context coding.

The parahippocampal gyrus links the default-mode cortical network with the medial temporal lobe memory system

The default-mode network (DMN) is a distributed functional-anatomic network implicated in supporting memory. Current resting-state functional connectivity studies in humans remain divided on the exact involvement of medial temporal lobe (MTL) in this network at rest. Notably, it is unclear to what extent the MTL regions involved in successful memory encoding are connected to the cortical nodes of the DMN during resting state. Our findings using functional connectivity MRI analyses of resting-state data indicate that the parahippocampal gyrus (PHG) is the primary hub of the DMN in the MTL during resting state. Also, connectivity of the PHG is distinct from connectivity of hippocampal regions identified by an associative memory-encoding task. We confirmed that several hippocampal encoding regions lack significant functional connectivity with cortical DMN nodes during resting state. Additionally, a mediation analysis showed that resting-state connectivity between the hippocampus and posterior cingulate cortex--a major hub of the DMN--is indirect and mediated by the PHG. Our findings support the hypothesis that the MTL memory system represents a functional subnetwork that relates to the cortical nodes of the DMN through parahippocampal functional connections.

Detecting representations of recent and remote autobiographical memories in vmPFC and hippocampus

How autobiographical memories are represented in the human brain and whether this changes with time are questions central to memory neuroscience. Two regions in particular have been consistently implicated, the ventromedial prefrontal cortex (vmPFC) and the hippocampus, although their precise contributions are still contested. The key question in this debate, when reduced to its simplest form, concerns where information about specific autobiographical memories is located. Here, we availed ourselves of the opportunity afforded by multivoxel pattern analysis to provide an alternative to conventional neuropsychological and fMRI approaches, by detecting representations of individual autobiographical memories in patterns of fMRI activity. We examined whether information about specific recent (two weeks old) and remote (10 years old) autobiographical memories was represented in vmPFC and hippocampus, and other medial temporal and neocortical regions. vmPFC contained information about recent and remote autobiographical memories, although remote memories were more readily detected there, indicating that consolidation or a change of some kind had occurred. Information about both types of memory was also present in the hippocampus, suggesting it plays a role in the retrieval of vivid autobiographical memories regardless of remoteness. Interestingly, we also found that while recent and remote memories were both represented within anterior and posterior hippocampus, the latter nevertheless contained more information about remote memories. Thus, like vmPFC, the hippocampus too respected the distinction between recent and remote memories. Overall, these findings clarify and extend our view of vmPFC and hippocampus while also informing systems-level consolidation and providing clear targets for future studies.

Bigger is better! Hippocampal volume and declarative memory performance in healthy young men

The importance of the hippocampus for declarative memory processes is firmly established. Nevertheless, the issue of a correlation between declarative memory performance and hippocampal volume in healthy subjects still remains controversial. The aim of the present study was to investigate this relationship in more detail. For this purpose, 50 healthy young male participants performed the California Verbal Learning Test. Hippocampal volume was assessed by manual segmentation of high-resolution 3D magnetic resonance images. We found a significant positive correlation between putatively hippocampus-dependent memory measures like short-delay retention, long-delay retention and discriminability and percent hippocampal volume. No significant correlation with measures related to executive processes was found. In addition, percent amygdala volume was not related to any of these measures. Our data advance previous findings reported in studies of brain-damaged individuals in a large and homogeneous young healthy sample and are important for theories on the neural basis of episodic memory.

The hippocampus extrapolates beyond the view in scenes: an fMRI study of boundary extension

Boundary extension (BE) is a pervasive phenomenon whereby people remember seeing more of a scene than was present in the physical input, because they extrapolate beyond the borders of the original stimulus. This automatic embedding of a scene into a wider context supports our experience of a continuous and coherent world, and is therefore highly adaptive. BE, whilst occurring rapidly, is nevertheless thought to comprise two stages. The first involves the active extrapolation of the scene beyond its physical boundaries, and is constructive in nature. The second phase occurs at retrieval, where the initial extrapolation beyond the original scene borders is revealed by a subsequent memory error. The brain regions associated with the initial, and crucial, extrapolation of a scene beyond the view have never been investigated. Here, using functional MRI (fMRI) and a classic BE paradigm, we found that this extrapolation of scenes occurred rapidly around the time a scene was first viewed, and was associated with engagement of the hippocampus (HC) and parahippocampal cortex (PHC). Using connectivity analyses we determined that the HC in particular seemed to drive the BE effect, exerting top–down influence on PHC and indeed as far back down the processing stream as early visual cortex (VC). These cortical regions subsequently displayed activity profiles that tracked the trial-by-trial subjective perception of the scenes, rather than physical reality, thereby reflecting the behavioural expression of the BE error. Together our results show that the HC is involved in the active extrapolation of scenes beyond their physical borders. This information is then automatically and rapidly channelled through the scene processing hierarchy as far back as early VC. This suggests that the anticipation and construction of scenes is a pervasive and important aspect of our online perception, with the HC playing a central role.

Multi-voxel pattern analysis in human hippocampal subfields

A complete understanding of the hippocampus depends on elucidating the representations and computations that exist in its anatomically distinct subfields. High-resolution structural and functional MRI scanning is starting to permit insights into hippocampal subfields in humans. In parallel, such scanning has facilitated the use of multi-voxel pattern analysis (MVPA) to examine information present in the distributed pattern of activity across voxels. The aim of this study was to combine these two relatively new innovations and deploy MVPA in the hippocampal subfields. Delineating subregions of the human hippocampus, a prerequisite for our study, remains a significant challenge, with extant methods often only examining part of the hippocampus, or being unable to differentiate CA3 and dentate gyrus (DG). We therefore devised a new high-resolution anatomical scanning and subfield segmentation protocol that allowed us to overcome these issues, and separately identify CA1, CA3, DG, and subiculum (SUB) across the whole hippocampus using a standard 3T MRI scanner. We then used MVPA to examine fMRI data associated with a decision-making paradigm involving highly similar scenes that had relevance for the computations that occur in hippocampal subfields. Intra- and inter-rater scores for subfield identification using our procedure confirmed its reliability. Moreover, we found that decoding of information within hippocampal subfields was possible using MVPA, with findings that included differential effects for CA3 and DG. We suggest that MVPA in human hippocampal subfields may open up new opportunities to examine how different types of information are represented and processed at this fundamental level.

Neural changes underlying the development of episodic memory during middle childhood

Episodic memory is central to the human experience. In typically developing children, episodic memory improves rapidly during middle childhood. While the developmental cognitive neuroscience of episodic memory remains largely uncharted, recent research has begun to provide important insights. It has long been assumed that hippocampus-dependent binding mechanisms are in place by early childhood, and that improvements in episodic memory observed during middle childhood result from the protracted development of the prefrontal cortex. We revisit the notion that binding mechanisms are age-invariant, and propose that changes in the hippocampus and its projections to cortical regions also contribute to the development of episodic memory. We further review the role of developmental changes in lateral prefrontal and parietal cortices in this development. Finally, we discuss changes in white matter tracts connecting brain regions that are critical for episodic memory. Overall, we argue that changes in episodic memory emerge from the concerted effort of a network of relevant brain structures.

Mechanisms supporting superior source memory for familiar items: a multi-voxel pattern analysis study

Recent cognitive research has revealed better source memory performance for familiar relative to novel stimuli. Here we consider two possible explanations for this finding. The source memory advantage for familiar stimuli could arise because stimulus novelty induces attention to stimulus features at the expense of contextual processing, resulting in diminished overall levels of contextual processing at study for novel (vs. familiar) stimuli. Another possibility is that stimulus information retrieved from long-term memory (LTM) provides scaffolding that facilitates the formation of item-context associations. If contextual features are indeed more effectively bound to familiar (vs. novel) items, the relationship between contextual processing at study and subsequent source memory should be stronger for familiar items. We tested these possibilities by applying multi-voxel pattern analysis (MVPA) to a recently collected functional magnetic resonance imaging (fMRI) dataset, with the goal of measuring contextual processing at study and relating it to subsequent source memory performance. Participants were scanned with fMRI while viewing novel proverbs, repeated proverbs (previously novel proverbs that were shown in a pre-study phase), and previously known proverbs in the context of one of two experimental tasks. After scanning was complete, we evaluated participants' source memory for the task associated with each proverb. Drawing upon fMRI data from the study phase, we trained a classifier to detect on-task processing (i.e., how strongly was the correct task set activated). On-task processing was greater for previously known than novel proverbs and similar for repeated and novel proverbs. However, both within and across participants, the relationship between on-task processing and subsequent source memory was stronger for repeated than novel proverbs and similar for previously known and novel proverbs. Finally, focusing on the repeated condition, we found that higher levels of hippocampal activity during the pre-study phase, which we used as an index of episodic encoding, led to a stronger relationship between on-task processing at study and subsequent memory. Together, these findings suggest different mechanisms may be primarily responsible for superior source memory for repeated and previously known stimuli. Specifically, they suggest that prior stimulus knowledge enhances memory by boosting the overall level of contextual processing, whereas stimulus repetition enhances the probability that contextual features will be successfully bound to item features. Several possible theoretical explanations for this pattern are discussed.

Factors affecting medial temporal lobe engagement for past and future episodic events: an ALE meta-analysis of neuroimaging studies

Remembering the past and envisioning the future are at the core of one's sense of identity. Neuroimaging studies investigating the neural substrates underlying past and future episodic events have been growing in number. However, the experimental paradigms used to select and elicit episodic events vary greatly, leading to disparate results, especially with respect to the laterality and antero-posterior localization of hippocampal and adjacent medial temporal activations (i.e., parahippocampal, entorhinal and perirhinal cortices, amygdala). Although a central concern in today's literature, the issue of hippocampal and medial temporal lobe laterality and antero-posterior segregation in past and future episodic events has not yet been addressed extensively. Using the activation likelihood estimation (ALE) procedure (Turkeltaub, Eden, Jones, & Zeffiro, 2002), we performed a meta-analysis of hippocampal and adjacent medial temporal coordinates extracted from neuroimaging studies examining past remembering and future envisioning. We questioned whether methodological choices could influence the laterality of activations, namely (1) the type of cue used (generic vs. specific), (2) the type of task performed (recognition vs. recall/imagine), (3) the nature of the information retrieved (episodic vs. "strictly" episodic events) and (4) the age of participants. We consider "strictly" episodic events as events which are not only spatio-temporally unique and personal like episodic events, but are also associated with contextual and phenomenological details. These four factors were compared two-by-two, generating eight whole-brain statistical maps. Results indicate that (1) specific cues tend to activate more the right anterior hippocampus compared to the use of generic cues, (2) recall/imagine tasks tend to recruit more the left posterior parahippocampal gyrus compared to recognition tasks, (3) (re/pre)experiencing strictly episodic events tends to activate more the bilateral posterior hippocampus compared to episodic events and (4) older subjects tend to activate more the right anterior hippocampus compared to younger subjects. Importantly, our results stress that strictly episodic events triggered by specific cues elicit greater left posterior hippocampal activation than episodic events triggered by specific cues. These findings suggest that such basic methodological choices have an impact on the conclusions reached regarding past and future (re/pre)experiencing and their neural substrates.