Using fMRI to understand event construction in developmental amnesia

Early midlife ovarian removal is associated with lower posterior hippocampal function

INTRODUCTION

Women with early bilateral salpingo-oophorectomy (BSO) have greater Alzheimer's disease (AD) risk than women with spontaneous menopause (SM), but the pathway toward this risk is understudied. Considering associative memory deficits may reflect early signs of AD, we studied how BSO affected brain activity underlying associative memory.

METHODS

Early midlife women with BSO (with and without 17β-estradiol therapy [ET]) and age-matched controls (AMCs) with intact ovaries completed a face-name associative memory task during functional magnetic resonance imaging. Hippocampal activity along the anteroposterior axis during associative encoding and retrieval was compared among three groups (BSO [n = 28], BSO+ET [n = 35], AMCs [n = 40]).

RESULTS

Both BSO groups (with and without ET) showed lower posterior hippocampal activation during encoding compared to the AMC group. However, this difference in activation was not significantly correlated with associative memory task performance.

DISCUSSION

Early 17β-estradiol loss may influence posterior hippocampal activity during associative encoding, possibly presaging late-life AD.

HIGHLIGHTS

After ovarian removal, changes in hippocampal function may affect dementia risk. Midlife ovarian removal is associated with less activation in the posterior hippocampus. Estradiol therapy may ameliorate alterations in brain function during learning.

Anatomo-functional changes in neural substrates of cognitive memory in developmental amnesia: Insights from automated and manual MRI examinations

Despite bilateral hippocampal damage dating to perinatal or early-childhood period, and severely-impaired episodic memory that unfolds in later childhood, patients with developmental amnesia continue to exhibit well-developed semantic memory across the developmental trajectory. Detailed information on the extent and focality of brain damage in these patients is needed to hypothesize about the neural substrate that supports their remarkable capacity for encoding and retrieval of semantic memory. In particular, we need to assess whether the residual hippocampal tissue is involved in this preservation, or whether the surrounding cortical areas reorganise to rescue aspects of these critical cognitive memory processes after early injury. We used voxel-based morphometry (VBM) analysis, automatic (FreeSurfer) and manual segmentation to characterize structural changes in the brain of an exceptionally large cohort of 23 patients with developmental amnesia in comparison with 32 control subjects. Both the VBM and the FreeSurfer analyses revealed severe structural alterations in the hippocampus and thalamus of patients with developmental amnesia. Milder damage was found in the amygdala, caudate and parahippocampal gyrus. Manual segmentation demonstrated differences in the degree of atrophy of the hippocampal subregions in patients. The level of atrophy in CA-DG subregions and subicular complex was more than 40% while the atrophy of the uncus was moderate (-23%). Anatomo-functional correlations were observed between the volumes of residual hippocampal subregions in patients and selective aspects of their cognitive performance viz, intelligence, working memory, and verbal and visuospatial recall. Our findings suggest that in patients with developmental amnesia, cognitive processing is compromised as a function of the extent of atrophy in hippocampal subregions, such that the greater the damage, the more likely it is that surrounding cortical areas will be recruited to rescue the putative functions of the damaged subregions. Our findings document for the first time not only the extent, but also the limits of circuit reorganization occurring in the young brain after early bilateral hippocampal damage.

Semantic memory before episodic memory: How memory research can inform knowledge and belief representations

Knowledge and belief attribution are discussed in the context of episodic and semantic memory theory and research, with reference to patient-lesion and developmental studies under naturalistic conditions. Consideration of how episodic and semantic memory relate to each other and intersect in the real world, including how they fail, can illuminate the approach to studying how people represent others' minds.

Representation of viewed and recalled film clips in patterns of brain activity in a person with developmental amnesia

As clear memories transport us back into the past, the brain also revives prior patterns of neural activity, a phenomenon known as neural reactivation. While growing evidence indicates a link between neural reactivation and typical variations in memory performance in healthy individuals, it is unclear how and to what extent reactivation is disrupted by a memory disorder. The current study characterizes neural reactivation in a case of amnesia using Multivoxel Pattern Analysis (MVPA). We tested NC, an individual with developmental amnesia linked to a diencephalic stroke, and 19 young adult controls on a functional magnetic resonance imaging (fMRI) task during which participants viewed and recalled short videos multiple times. An encoding classifier trained and tested to identify videos based on brain activity patterns elicited at perception revealed superior classification in NC. The enhanced consistency in stimulus representation we observed in NC at encoding was accompanied by an absence of multivariate repetition suppression, which occurred over repeated viewing in the controls. Another recall classifier trained and tested to identify videos during mental replay indicated normal levels of classification in NC, despite his poor memory for stimulus content. However, a cross-condition classifier trained on perception trials and tested on mental replay trials-a strict test of reactivation-revealed significantly poorer classification in NC. Thus, while NC's brain activity was consistent and stimulus-specific during mental replay, this specificity did not reflect the reactivation of patterns elicited at perception to the same extent as controls. Fittingly, we identified brain regions for which activity supported stimulus representation during mental replay to a greater extent in NC than in controls. This activity was not modeled on perception, suggesting that compensatory patterns of representation based on generic knowledge can support consistent mental constructs when memory is faulty. Our results reveal several ways in which amnesia impacts distributed patterns of stimulus representation during encoding and retrieval.

Role of the hippocampus in the spacing effect during memory retrieval

It is well known that distributed learning (DL) leads to improved memory performance compared with massed learning (ML) (i.e., spacing effect). However, the extent to which the hippocampus is involved in the spacing effect at shorter and longer retention intervals remains unclear. To address this issue, two groups of participants were asked to encode face-scene pairs at 20-min, 1-day, and 1-month intervals before they were scanned using fMRI during an associative recognition task. The pairs were repeated six times in either a massed (i.e., six times in 1 day) or a distributed (i.e., six times over 3 days, twice per day) manner. The results showed that compared with that in the ML group, the activation of the left hippocampus was stronger in the DL group when the participants retrieved old pairs correctly and rejected new pairs correctly at different retention intervals. In addition, the posterior hippocampus was more strongly activated when the new associations were rejected correctly after DL than ML, especially at the 1-month interval. Hence, our results provide evidence that the hippocampus is involved in better memory performance after DL compared to ML at both shorter and longer retention intervals.

Gradual learning and inflexible strategy use in amnesia: Evidence from case H.C

The value of case studies in informing our understanding of dissociations and interactions in memory was recognized early on by Endel Tulving, whose comprehensive work with the amnesic case K.C. helped to confirm distinctions between episodic and semantic memory. Following in this tradition, we examined memory and the use of cognitive strategies in the developmental amnesic case H.C., a young woman with structural abnormalities in the extended hippocampal system (Rosenbaum et al., 2014). H.C. was tested on two tasks, transitivity and transverse patterning, that each required learning the relations among items, and for the former, also examined the ability to make inferences across sets of relations. H.C. was tested across multiple sessions and demonstrated two seemingly contradictory patterns of performance: evidence of gradual learning, yet an inability to flexibly switch to a cognitive strategy that may otherwise benefit performance. Specifically, on the transitivity task, H.C. showed gradual learning of novel relations that led to successful inferential performance. On transverse patterning, H.C. showed some gradual learning of the relations among the objects across sessions, and expressed knowledge that the task followed 'rock-paper-scissors' rules. However, H.C. did not benefit from a unitization strategy, which had shown previous success with other amnesic cases (D'Angelo et al., 2015; Ryan, Moses, Barense, & Rosenbaum, 2013). H.C.'s over-reliance on 'rock-paper-scissors' rules, even in the face of alternate strategies, is suggestive of an inability to enact cognitive flexibility. Poor performance thus may have resulted from interference from the experimentally presented strategy on her self-imposed strategy. The present findings echo work reported by Tulving in case K.C. (Tulving, Hayman, & Macdonald, 1991). Whereas neurologically intact individuals may rely on the functions of the hippocampal system to rapidly learn new information and resolve interference, some individuals with hippocampal amnesia may learn information gradually, but such learning is particularly prone to interference, resulting in an inability to flexibly adapt to changes in the learning conditions in order to optimize performance.

Ameliorating Episodic Memory Deficits in a Young Adult With Developmental (Congenital) Amnesia

OBJECTIVES

Although the spacing effect has been investigated extensively in a variety of populations, few studies have focused on individuals with hippocampal amnesia and none, to our knowledge, have investigated differences in performance as a function of spacing schedule in these cases. In the current study, we investigated the benefit of expanding and equal-interval, compared to massed, spacing schedules in a developmental amnesic person, H.C., who shows congenitally based abnormal development of the hippocampal memory system.

METHODS

Given the possibility of plasticity and reorganization in the developing brain, we investigated whether H.C. would benefit more from an expanding versus equal-interval schedule using a continuous recognition paradigm, even though this task has been shown to recruit structures within the medial temporal lobe, including the hippocampus.

RESULTS

H.C. and matched controls both showed a clear spacing effect, although neither group benefited more from an equal-interval or expanding spacing schedule.

CONCLUSIONS

The results of the current study show that the spacing effect is an effective and clinically meaningful memory intervention technique that may be applied to clinical conditions known to affect hippocampal function and episodic memory early in life. (JINS, 2018, 24, 1003-1012).

Comparing and Contrasting the Cognitive Effects of Hippocampal and Ventromedial Prefrontal Cortex Damage: A Review of Human Lesion Studies

The hippocampus and ventromedial prefrontal cortex (vmPFC) are closely connected brain regions whose functions are still debated. In order to offer a fresh perspective on understanding the contributions of these two brain regions to cognition, in this review we considered cognitive tasks that usually elicit deficits in hippocampal-damaged patients (e.g., autobiographical memory retrieval), and examined the performance of vmPFC-lesioned patients on these tasks. We then took cognitive tasks where performance is typically compromised following vmPFC damage (e.g., decision making), and looked at how these are affected by hippocampal lesions. Three salient motifs emerged. First, there are surprising gaps in our knowledge about how hippocampal and vmPFC patients perform on tasks typically associated with the other group. Second, while hippocampal or vmPFC damage seems to adversely affect performance on so-called hippocampal tasks, the performance of hippocampal and vmPFC patients clearly diverges on classic vmPFC tasks. Third, although performance appears analogous on hippocampal tasks, on closer inspection, there are significant disparities between hippocampal and vmPFC patients. Based on these findings, we suggest a tentative hierarchical model to explain the functions of the hippocampus and vmPFC. We propose that the vmPFC initiates the construction of mental scenes by coordinating the curation of relevant elements from neocortical areas, which are then funneled into the hippocampus to build a scene. The vmPFC then engages in iterative re-initiation via feedback loops with neocortex and hippocampus to facilitate the flow and integration of the multiple scenes that comprise the coherent unfolding of an extended mental event.

Different neural routes to autobiographical memory recall in healthy people and individuals with left medial temporal lobe epilepsy

Individuals with medial temporal lobe epilepsy (mTLE) are poor at recalling vivid details from autobiographical memories (AM), instead retrieving gist-like schematic memories. Recent research has suggested that this impoverished recall in comparison to controls may reflect (1) differential engagement of anterior vs posterior regions of the hippocampus (HC) and/or (2) differences between the engagement of the HC vs the ventromedial prefrontal cortex (vmPFC). Here we examined these hypotheses by comparing connectivity amongst hippocampal regions and between vmPFC and other brain regions during construction (retrieval of a particular event) vs elaboration (retrieval of perceptual detail) phases of AM recall in 12 individuals with left mTLE and 12 matched controls. Whereas functional connectivity amongst hippocampal regions changed from AM construction to elaboration in controls, the pattern of intra-hippocampal connectivity was unvarying in patients. Furthermore, patterns of connectivity from the vmPFC differed between phases in distinct ways in the two groups of participants. In patients, vmPFC activation was correlated with other prefrontal and lateral temporal cortices during construction and with visual-perceptual cortices during elaboration. While controls did not show a difference in whole-brain connectivity, they did uniquely show a dynamic shift from vmPFC connectivity to anterior HC during construction and to posterior HC during elaboration. Together, these findings suggest that impoverished AM recall in mTLE is a consequence of reduced activation and flexibility of bilateral hippocampal networks and greater reliance on neocortical contributions to memory retrieval.