Dissociative Effects of Age on Neural Differentiation at the Category and Item Levels

Mnemonic brain state engagement is diminished in healthy aging

Healthy older adults typically show impaired episodic memory - memory for when and where an event occurred. This selective episodic memory deficit may arise from differential engagement in the retrieval state, a brain state in which attention is focused internally in an attempt to access prior knowledge, and the encoding state, a brain state which supports the formation of new memories and that trades off with the retrieval state. We hypothesize that older adults are biased toward a retrieval state. We recorded scalp electroencephalography while young, middle-aged and older adults performed a memory task in which they were explicitly directed to either encode or retrieve on a given trial. We used multivariate pattern analysis of spectral activity to decode retrieval vs. encoding state engagement. We find that whereas all age groups can follow task demands to selectively engage in encoding or retrieval, mnemonic brain state engagement is diminished for older adults relative to young and middle-aged adults. These findings suggest that differential mnemonic state engagement may underlie age-related memory changes.

Healthy aging delays and dedifferentiates high-level visual representations

Healthy aging impacts visual information processing with consequences for subsequent high-level cognition and everyday behavior, but the underlying neural changes in visual representations remain unknown. Here, we investigate the nature of representations underlying object recognition in older compared to younger adults by tracking them in time using electroencephalography (EEG), across space using functional magnetic resonance imaging (fMRI), and by probing their behavioral relevance using similarity judgments. Applying a multivariate analysis framework to combine experimental assessments, four key findings about how brain aging impacts object recognition emerge. First, aging selectively delays the formation of object representations, profoundly changing the chronometry of visual processing. Second, the delay in the formation of object representations emerges in high-level rather than low- and mid-level ventral visual cortex, supporting the theory that brain areas developing last deteriorate first. Third, aging reduces content selectivity in the high-level ventral visual cortex, indicating age-related neural dedifferentiation as the mechanism of representational change. Finally, we demonstrate that the identified representations of the aging brain are behaviorally relevant, ascertaining ecological relevance. Together, our results reveal the impact of healthy aging on the visual brain.

The precision of hippocampal representations predicts incremental value-learning across the adult lifespan

Correctly assigning value to different options and leveraging this information to guide choice is a cornerstone of adaptive decision-making. Reinforcement learning (RL) has provided a computational framework to study this process, and neural signals linked to RL have been identified in the striatum and medial prefrontal cortex. More recently, hippocampal contributions to this kind of value-learning have been proposed, at least under some conditions. Here, we test whether the hippocampus provides a signal of the option's identity that aids in credit assignment when learning about several perceptually similar items, and evaluate how this process differs across the lifespan. A sample of 251 younger and older adults, including a subset (n = 76) with simultaneous fMRI, completed an RL task in which they learned the value of four houses through trial-and-error. Older adults showed decreased choice accuracy, accompanied by reduced neural signaling of value at choice but not feedback. Using representational similarity analysis, we found that the precision with which choice options were represented in the posterior hippocampus during choice predicted accurate decisions across age groups. Interestingly, despite previous evidence for neural de-differentiation in older adults, we found no support for a "blurring" of these stimulus representations in older adults. Rather, we observed reduced connectivity between the posterior hippocampus and the medial PFC in older adults, and this connectivity correlated with choice consistency. Taken together, these findings identify a hippocampal contribution to incremental value learning, and that reductions in incremental value learning in older adults are associated with the reduced transfer of information between the hippocampus and mPFC, rather than the precision of the information in the hippocampus itself.

Moderating effects of cortical thickness, volume, and memory performance on age differences in neural reinstatement of scene information

The strength of neural reinstatement, a correlate of episodic memory retrieval, reportedly reflects the amount and fidelity of mnemonic content and is weaker in older than younger adults, especially for scene memoranda. Given evidence that age-related declines in cortical thickness and volume contribute to age-related cognitive decline, we analyzed fMRI data acquired from healthy young and older adults to examine relationships between cortical thickness, cortical volume, age, and scene- related reinstatement in the parahippocampal place area (PPA) and medial place area (MPA), two cortical regions implicated in scene processing. A 'reinstatement index' was estimated from fMRI data collected during tests of source memory for scene images, and multiple regression analyses were employed to examine the effects of the variables of interest on scene reinstatement. There were robust age differences in reinstatement, cortical thickness, and cortical volume. In both regions of interest, cortical volume fully mediated the effects of age on reinstatement. Additionally, PPA reinstatement strength predicted source memory performance independently of cortical volume or age. These findings suggest that age differences in scene reinstatement are mediated by cortical volume and that memory performance and cortical volume are associated with unique components of variance in reinstatement strength.

The unity/diversity framework of executive functions: behavioral and neural evidence in older adults

Executive functions (EFs), encompassing inhibition, shifting, and updating as three fundamental subdomains, are typically characterized by a unity/diversity construct. However, given the dedifferentiation trend observed in aging, it remains controversial whether the construct of EFs in older adults becomes unidimensional or maintains unity/diversity. This study aims to explore and validate the construct of EFs in older adults. At the behavioral level, we conducted confirmatory factor analysis on data from 222 older adults who completed six tasks specifically targeting inhibition, shifting, and updating. One unidimensional model and six unity/diversity models of EFs were evaluated. Our results indicated that the EFs of older adults demonstrated greater congruence with the unity/diversity construct. At neural level, thirty older adults completed three thematically consistent fMRI tasks, targeting three subdomains of EFs respectively. Multivariate pattern analysis showed that rostromedial prefrontal cortex robustly showed similar neural representation across different tasks (unity). Meanwhile, the three EF domains were encoded by distinct global neural representation and the lateral prefrontal cortex play a crucial role in classification (diversity). These findings underscore the unity/diversity framework of EFs in older adults and offer important insights for designing interventions aimed at improving EFs in this population.

Early visual cortices reveal interrelated item and category representations in aging

Neural dedifferentiation, the finding that neural representations tend to be less distinct in older adults compared with younger adults, has been associated with age-related declines in memory performance. Most studies assessing the relation between memory and neural dedifferentiation have evaluated how age impacts the distinctiveness of neural representations for different visual categories (e.g., scenes and objects). However, how age impacts the quality of neural representations at the level of individual items is still an open question. Here, we present data from an age-comparative fMRI study that aimed to understand how the distinctiveness of neural representations for individual stimuli differs between younger and older adults and relates to memory outcomes. Pattern similarity searchlight analyses yielded indicators of neural dedifferentiation at the level of individual items as well as at the category level in posterior occipital cortices. We asked whether age differences in neural distinctiveness at each representational level were associated with inter- and/or intraindividual variability in memory performance. While age-related dedifferentiation at both the item and category level related to between-person differences in memory, neural distinctiveness at the category level also tracked within-person variability in memory performance. Concurrently, neural distinctiveness at the item level was strongly associated with neural distinctiveness at the category level both within and across participants, elucidating a potential representational mechanism linking item- and category-level distinctiveness. In sum, we provide evidence that age-related neural dedifferentiation co-exists across multiple representational levels and is related to memory performance.Significance Statement Age-related memory decline has been associated with neural dedifferentiation, the finding that older adults have less distinctive neural representations than younger adults. This has been mostly shown for category information, while evidence for age differences in the specificity of item representations is meager. We used pattern similarity searchlight analyses to find indicators of neural dedifferentiation at both levels of representation (category and item) and linked distinctiveness to memory performance. Both item- and category-level dedifferentiation in the calcarine cortex were related to interindividual differences in memory performance, while category-level distinctiveness further tracked intraindividual variability. Crucially, neural distinctiveness was strongly tied between the item and category levels, indicating that intersecting representational properties of posterior occipital cortices reflect both individual exemplars and categories.