Gaze patterns reveal attention-based modulation of lure discrimination.

Lure discrimination in the Mnemonic Similarity Task (MST) has been widely used to measure pattern separation. However, the classic index of lure discrimination in the MST has arbitrary assumptions with limited supporting evidence. The present study has thus developed several models with different assumptions on the process underlying MST as well as the different model-derived indices of lure discrimination. Furthermore, we have assessed and compared these models in a measurement-based approach. We found that the model for the classic lure discrimination index fails to accurately predict the responses in an MST from > 150 participants. Instead, a new index based on the unidimensional signal detection model provides the best fits of the empirical dataset. This work highlights the value of model-based approaches in measuring lure discriminability in the MST.

Lean Body Mass, but Not Fat Mass, Is Associated with Hippocampal Memory Performance (P14-011-19)

Older adults often have difficulty remembering the details of recently encountered objects. We previously found this with the Mnemonic Similarity Task (MST). Surprisingly, the older adults' MST Lure Discrimination Index (LDI) was significantly correlated with visual acuity but not with memory or executive function. Here we ran a replication with new, larger samples of young (N = 45) and older adults (N = 70). We then combined the original and replication older adult samples (N = 108) to critically examine the relative contributions of visual acuity, memory, and executive function composite scores to LDI performance using dominance analysis. This provided, to our knowledge, the first direct statistical comparison of all 3 of these factors and their interactions on LDI. Participants completed the MST and a battery assessing visual acuity, memory, and executive function. We examined age group differences on MST performance in the new (i.e., replication) young and older adult samples and performed multiple regression and dominance analysis on the combined older adult sample. Consistent with previous findings, the older adults showed significantly poorer LDI but preserved item recognition. LDI was significantly correlated with both memory and executive function but not with visual acuity. In the combined older adult sample, all 3 composites predicted LDI, but dominance analysis indicated that executive function was the most important predictor. Older adults' MST LDI difficulty may be predicted by their executive function and visual acuity. These factors should be considered when interpreting older adults' MST performance.

Stability of age-related deficits in the mnemonic similarity task across task variations.

Using the Mnemonic Similarity Task (MST), we have demonstrated an age-related impairment in lure discrimination, or the ability to recognize an item as distinct from one that was similar, but not identical to one viewed earlier. A growing body of evidence links these behavioral changes to age-related alterations in the hippocampus. In this study, we sought to evaluate a novel version of this task, utilizing scenes that might emphasize the role of the hippocampus in contextual and spatial processing. In addition, we investigated whether, by utilizing two stimulus classes (scenes and objects), we could also interrogate the roles of the PRC and PHC in aging. Thus, we evaluated differential contributions to these tasks by relating performance on objects versus scenes to volumes of the hippocampus and surrounding medial temporal lobe structures. We found that while there was an age-related impairment on lure discrimination performance for both objects and scenes, relationships to brain volumes and other measure of memory performance were stronger when using objects. In particular, lure discrimination performance for objects showed a positive relationship with the volume of the hippocampus, specifically the combined dentate gyrus (DG) and CA3 subfields, and the subiculum. We conclude that though using scenes was effective in detecting age-related lure discrimination impairments, it does not provide as strong a brain-behavior relationship as using objects.

Study ObjectivesThe impact of sleep loss on memory encoding is well described in adults, yet less understood in youth, despite the prevalence and educational relevance of adolescent sleep loss. Here, we implement at-home sleep restriction in youth ages 10–14 and a well-validated hippocampus-dependent learning task to elucidate how real-world levels of sleep loss affect distinct memory encoding processes at this young age.MethodsA within-subject cross-over design involved five nights of at-home sleep restriction (7.5 h in bed) compared to sleep optimization (10 h in bed). Restriction was achieved by delaying bedtime and advancing risetime equally. All sleep was monitored with wrist actigraphy, sleep diaries, and daily calls to the laboratory. Testing involved the validated Mnemonic Similarity Task (MST), which can distinguish between two components of successful memory encoding: general memory recognition for old items and “lure discrimination,” a hippocampus-dependent ability to distinguish similar yet distinct items.ResultsAs estimated by actigraphy, our manipulation reduced sleep by 1.4 ± 0.48 h per night for five nights. This reduction resulted in a selective deficit in MST-indexed memory encoding; we observed a decrease in lure discrimination (i.e. the ability to distinguish highly similar items), but no impact on recognition of old items.ConclusionsWe present evidence that low levels of sleep loss for five nights (typical of a school week) are sufficient to alter memory encoding in youth. We interpret these data in the context of classroom-based learning and speculate that reduced lure discrimination may yield memory that is less capable of distinguishing closely related facts and concepts.Statement of SignificanceSleep loss impacts learning; however, most of this literature stems from adults. Despite the sleep loss experienced by youth as developmental pressures mix with early school-start-times, we know relatively little about how insufficient sleep at this age impacts learning. Here, we present a repeated-measures experiment in 10–14-year-old youth using an at-home sleep manipulation. We show five nights of sleep restriction (mirroring a typical school week by reducing time-in-bed from 10 to 7.5 h) alters hippocampus-dependent memory encoding with selectivity for discerning similar items from one another. We discuss these data in the context of a sleep and memory literature rooted in the career of Prof. Robert Stickgold and offer educational implications for this vulnerable time of life.

Current developmental psychopathology models indicate that schizophrenia can be understood as the most extreme expression of a multidimensional continuum of symptoms and impairment referred to as schizotypy. In nondisordered adults, schizotypy predicts risk for developing schizophrenia-spectrum psychopathology. Schizophrenia is associated with disruptions in detecting subtle differences between objects, which is linked to hippocampal dysfunction. These disruptions have been shown in the Mnemonic Similarity Task (MST) when patients are less likely to reject lures that are similar but not identical to studied objects, and instead mistake them for studied items. This pattern of errors may be a behavioral manifestation of impaired pattern separation, a key episodic memory ability associated with hippocampal integrity and overreliance on pattern completion. We examined whether multidimensional schizotypy is associated with such deficits in nondisordered young adults. Participants (n = 230) were assessed for positive, negative, and disorganized schizotypy and completed the MST and a perceptual discrimination task. MST performance showed that a combination of elevated negative and disorganized schizotypy was associated with decreased rejections of similar lures because they were mistakenly identified as studied items. These deficits were not observed in traditional recognition measures within the same task, nor in perceptual discrimination, suggesting that mnemonic discrimination deficits assessed by MST were selective and did not reflect generalized deficits. These findings extend the results obtained in schizophrenia patients and support a multidimensional model of schizophrenia-spectrum psychopathology.

In the present study we investigated the long-standing question whether and why emotionally arousing memories are more distinct as compared to neutral experiences. We assumed that memory benefits from the distinctiveness of emotional information, and that emotions affect encoding by reducing interference among overlapping memory representations. Since pattern separation is the process which minimizes interference between memory representations with similar features, we examined the behavioral manifestation of putative neural mechanisms enabling pattern separation (i.e. mnemonic discrimination) for emotionally arousing materials using the Mnemonic Similarity Task with negative, positive, and neutral images as stimuli. Immediately after incidental encoding, subjects were presented with stimuli they had seen at encoding and also with new items. Crucially, participants were also presented with lure images that were visually similar to ones they had seen before. Response options were old, new, and similar. Our results showed that individuals were better in discriminating between similar, emotionally arousing memories, when compared to the neutral stimuli. Moreover, this so-called lure discrimination performance was better for the negative images, than it was for the positive stimuli. Finally, we showed that the high arousing negative stimuli were better separated than the low arousing negative stimuli, and a similar pattern of results was found for the positive items. Altogether, these findings suggest that lure discrimination is modulated by arousal and not by valence. We argue that noradrenergic activity might facilitate interference resolution among memory representations with similar features, and that superior pattern separation might play a key role in memory enhancement for emotional experiences.

Emotional modulation of memorability in mnemonic discrimination

Remembering specific memories with precision relies on the differentiation of similar memory contents - a process commonly referred to as pattern separation and behaviorally operationalized as lure discrimination in the mnemonic similarity task. Although pattern separation is typically investigated in the context of long-term memory (LTM), recent research extends these findings to short-term memory (STM) within a mixture model framework, emphasizing the distinction between memory quality and quantity. According to this framework, pattern separation is associated with memory precision across STM and LTM, regardless of the overall memory likelihood. However, these associations among memory quality measures may persist without the mixture model assumption. In an alternative model, a unitary memory strength measure quantified as a discrimination score (d') may also capture the association between pattern separation and memory quality, as pattern separation has been previously linked with strength-based memory performance. We tested these possibilities based on individual differences among 132 participants who underwent tasks measuring mnemonic pattern separation and STM/LTM quality. We found that behavioral estimates of pattern separation are significantly correlated with STM and LTM precision, irrespective of the likelihood of STM/LTM recall success. However, these associations are absent when considering the correlation between pattern separation and memory strength under a unitary model framework. By leveraging individual differences to constrain our understanding of cognitive models, our data unravel the intricate relationship between pattern separation and memory quality across timescales. These findings may therefore contribute to identifying sensitive behavioral measures for detecting subtle memory deficits in older adults or clinical populations.

The effects of perceived stress and anhedonic depression on mnemonic similarity task performance

BackgroundThe Mnemonic Similarity Task (MST) is a behavioral tool designed to tax pattern separation, a critical hippocampal function involving the transformation of similar experiences into discrete, non‐overlapping memory representations. Our pilot study aimed to assess the feasibility of MST administration among the oldest old, individuals 80 years and older, explore the association of MST performance metrics with established global cognitive measures, and evaluate potential age effects.MethodsThirty‐one participants aged 80 years and older from the UCI Alzheimer's Disease Research Center (ADRC) and the 90+ Study underwent MST as part of their cognitive assessment. Participants categorized 64 images as “indoor” or “outdoor” during the encoding phase and later classified 96 images as “old,” “similar,” or “new” in a recognition phase. Images were categorized as targets (old), lures (similar), or foils (novel). Key outcome measures included recognition memory (REC) and the lure discrimination index (LDI). Linear regression analyses were conducted to evaluate the associations between MST scores, age, and cognitive measures, i.e. the Mini‐Mental State Examination (MMSE).ResultsMean age was 92.7± 5.6 and 61.3% were female (Table 1). MST was successfully completed by all participants. Regression analyses revealed no significant association between MST scores and age (REC: p = 0.12; LDI: p = 0.09) (Table 2). A significant positive correlation was identified between MMSE and REC scores (β = 0.36, p = 0.02). No significant association was found between MMSE and LDI (β = 0.15, p = 0.28). Scatterplots indicated variability in individual MST scores with no discernible age‐related decline. (Figure 1)ConclusionThe MST is a feasible and effective task for assessing pattern separation and recognition memory in the oldest old. The observed association between MMSE and REC, but not LDI, deserves further investigation. The direct relationship between REC and MMSE underscores the utility of MST as a cognitive assessment tool. Further research with expanded cognitive measures and imaging biomarkers is warranted to elucidate the underlying mechanisms driving MST performance and its role as a digital biomarker for cognitive health.

Objective:Pattern separation (PS) is the ability to represent similar experiences as separate, non-overlapping representations. It is usually assessed via the Mnemonic Similarity Task – Object Version (MST-O) which, however, assesses PS performance without taking behavioral context discrimination into account, since it is based on pictures of everyday simple objects on a white background. We here present a validation study for a new task, the Mnemonic Similarity Task – Context Version (MST-C), which is designed to measure PS while taking behavioral context discrimination into account by using real-life context photographs.Methods:Fifty healthy subjects underwent the two MST tasks to assess convergent evidence. Instruments assessing memory and attention were also administered to study discriminant evidence. The test-retest reliability of MST-C was analyzed.Results:Weak evidence supports convergent validity between the MST-C task and the MST-O as measures of PS (rs = 0.464; p < 0.01); PS performance assessed via the MST-C did not correlate with memory or attention; a moderate test-retest reliability was found (rs = 0.595; p < 0.01).Conclusion:The MST-C seems useful for assessing PS performance conceptualized as the ability to discriminate complex and realistic spatial contexts. Future studies are welcome to evaluate the validity of the MST-C task as a measure of PS in clinical populations.

It has not been clarified if attention influences perception of targets in visual masking. Three forms of common masks (random pattern, para-/metacontrast, and four dots) were thus chosen in the present study and presented with character targets in three temporal sequences (forward, backward, and sandwiched mask or forward-backward mask combination). In order to pinpoint the level of processing where masking arises, character targets were varied in depth of processing from random arrangements of strokes up to real Chinese characters. The attentional influence was examined under perceptual discrimination and lexical decision tasks, respectively. The results revealed significant interactions among four factors (mask form, temporal sequence, depth of processing, and task). Identification of character targets in each form of mask sequence varied with task demand, with greater suppression in the perceptual discrimination task. These findings suggested that attentional demand can bias processing in favor of task-related information in visual masking. Variations in masking effects may be contributed by both attentional demand and spatio-temporal interaction.

Not only memory: Investigating the sensitivity and specificity of the Mnemonic Similarity Task in older adults