Different types of associative encoding evoke differential processing in both younger and older adults: Evidence from univariate and multivariate analyses

Examining the neural basis of unitization: A review

Associative memory refers to the ability to form and remember associations between individual pieces of information rather than memory for a single object or word. Encoding associations in memory tends to be a more difficult task than item (only) encoding, because associative memory requires encoding multiple items as well as the specific links amongst the items. Accordingly, researchers have worked to identify interventions and strategies to reduce the effort and neural resources required for successful associative memory processing. Unitization is one such strategy that has traditionally been defined as the process by which two or more discrete items are processed, or encoded, such that they are perceived as a single ensemble. The current review explores the neural research on unitization while considering the behavioral benefits that accompany the process.

Neural distinctiveness and discriminability underlying unitization and associative memory in aging

Previous work has suggested unitized pairs behave as a single unit and more critically, are processed neurally different than those of associative memories. The current works examines the neural differences between unitization and non-unitized memory using fMRI and multivoxel analyses. Specifically, we examined the differences across face-occupation pairings as a function of whether the pairing was viewed as a person performing the given job (unitized binding) or a person saying they knew someone who had a particular job (non-unitized binding). The results show that at encoding and retrieval, the angular gyrus can discriminate between unitized and non-unitized target trials. Additionally, during encoding, the medial temporal lobe (hippocampus and perirhinal cortex), frontal parietal regions (angular gyrus and medial frontal gyrus) and visual regions (middle occipital cortex) exhibit distinct neural patterns to recollected unitized and non-unitized targets. Furthermore, the perirhinal cortex and medial frontal gyrus show greater neural similarity within subsequently recollected unitized trials compared to non-unitized trials. We conclude that an encoding based strategy to elicit unitization can produce greater associative memory compared to non-unitized trials in older adults. Additionally, when unitized trials are subsequently recollected in the perirhinal cortex older adults show greater neural similarity within unitized trials compared to non-unitized trials.

Understanding associative false memories in aging using multivariate analyses

Age-related declines in associative memory are ubiquitous, with decreases in behavioral discriminability largely arising from increases in false memories for recombined lures. Using representational similarity analyses to examine the neural basis of associative false memories in aging, the current study found that neural pattern similarity between Hits and FAs and Hits and CRs differed as a function of age in occipital ROIs, such that older adults exhibited a smaller difference between the two similarity metrics than did younger adults. Additionally, greater Hit-FA representational similarity correlated with increases in associative FAs across several ROIs. Results suggest that while neural representations underlying targets may not differ across ages, greater pattern similarity between the neural representation of targets and lures may reflect reduced distinctiveness of the information encoded in memory, such that old and new items are more difficult to discriminate, leading to more false alarms.

Neural Mechanisms of Perceiving and Subsequently Recollecting Emotional Facial Expressions in Young and Older Adults

It is known that emotional facial expressions modulate the perception and subsequent recollection of faces and that aging alters these modulatory effects. Yet, the underlying neural mechanisms are not well understood, and they were the focus of the current fMRI study. We scanned healthy young and older adults while perceiving happy, neutral, or angry faces paired with names. Participants were then provided with the names of the faces and asked to recall the facial expression of each face. fMRI analyses focused on the fusiform face area (FFA), the posterior superior temporal sulcus (pSTS), the OFC, the amygdala, and the hippocampus (HC). Univariate activity, multivariate pattern (MVPA), and functional connectivity analyses were performed. The study yielded two main sets of findings. First, in pSTS and the amygdala, univariate activity and MVPA discrimination during the processing of facial expressions were similar in young and older adults, whereas in FFA and OFC, MVPA discriminated facial expressions less accurately in older than young adults. These findings suggest that facial expression representations in FFA and OFC reflect age-related dedifferentiation and positivity effect. Second, HC-OFC connectivity showed subsequent memory effects (SMEs) for happy expressions in both age groups, HC-FFA connectivity exhibited SMEs for happy and neutral expressions in young adults, and HC-pSTS interactions displayed SMEs for happy expressions in older adults. These results could be related to compensatory mechanisms and positivity effects in older adults. Taken together, the results clarify the effects of aging on the neural mechanisms in perceiving and encoding facial expressions.

Generalization of memory-related brain function in asymptomatic older women with a family history of late onset Alzheimer's Disease: Results from the PREVENT-AD Cohort

Late-onset Alzheimer's disease (AD) disproportionately affects women compared to men. Episodic memory decline is one of the earliest and most pronounced deficits observed in AD. However, it remains unclear whether sex influences episodic memory-related brain function in cognitively intact older adults at risk of developing AD. Here we used task-based multivariate partial least squares analysis to examine sex differences in episodic memory-related brain activity and brain activity-behavior correlations in a matched sample of cognitively intact older women and men with a family history of AD from the PREVENT-AD cohort study in Montreal, Canada (M_age=63.03±3.78; M_education=15.41±3.40). We observed sex differences in task-related brain activity and brain activity-behavior correlations during the encoding of object-location associative memories and object-only item memory, and the retrieval of object only item memories. Our findings suggest a generalization of episodic memory-related brain activation and performance in women compared to men. Follow up analyses should test for sex differences in the relationship between brain activity patterns and performance longitudinally, in association with risk factors for AD development. This article is part of the Virtual Special Issue titled COGNITIVE NEUROSCIENCE OF HEALTHY AND PATHOLOGICAL AGING. The full issue can be found on ScienceDirect at https://www.sciencedirect.com/journal/neurobiology-of-aging/special-issue/105379XPWJP.

Age differences in Neural Activation to Face Trustworthiness: Voxel Pattern and Activation Level Assessments

Judgment of trustworthiness is an important social ability. Many studies show neural activation differences to variations in face trustworthiness in brain reward regions. A previously published analysis of the present fMRI data showed that older adults' (OA) reward region activation responded significantly to trustworthiness in a set of older and younger faces, whereas younger adults' (YA) activation did not-a finding inconsistent with studies that used only younger faces. We hypothesized that voxel pattern analyses would be more sensitive to YA neural responses to trustworthiness in our set of faces, replicating YA neural discrimination in prior literature. Based on evidence for OA neural dedifferentiation, we also hypothesized that voxel pattern analyses would more accurately classify YA than OA neural responses to face trustworthiness. We reanalyzed the data with two pattern classification models and evaluated the models' performance with permutation testing. Voxel patterns discriminated face trustworthiness levels in both YA and OA reward regions, while allowing better classification of face trustworthiness for YA than OA, the reverse of previous results for neural activation levels. The moderation of age differences by analytic method shines a light on the possibility that voxel patterns uniquely index neural representations of the stimulus information content, consistent with findings of impaired representation with age.

Effects of age on goal-dependent modulation of episodic memory retrieval

Retrieval gating refers to the ability to modulate the retrieval of features of a single memory episode according to behavioral goals. Recent findings demonstrate that younger adults engage retrieval gating by attenuating the representation of task-irrelevant features of an episode. Here, we examine whether retrieval gating varies with age. Younger and older adults incidentally encoded words superimposed over scenes or scrambled backgrounds that were displayed in one of three spatial locations. Participants subsequently underwent fMRI as they completed two memory tasks: the background task, which tested memory for the word's background, and the location task, testing memory for the word's location. Employing univariate and multivariate approaches, we demonstrated that younger, but not older adults, exhibited attenuated reinstatement of scene information when it was goal-irrelevant (during the location task). Additionally, in younger adults only, the strength of scene reinstatement in the parahippocampal place area during the background task was related to item and source memory performance. Together, these findings point to an age-related decline in the ability to engage retrieval gating.

Age Differences In Retrieval-Related Reinstatement Reflect Age-Related Dedifferentiation At Encoding

Age-related reductions in neural selectivity have been linked to cognitive decline. We examined whether age differences in the strength of retrieval-related cortical reinstatement could be explained by analogous differences in neural selectivity at encoding, and whether reinstatement was associated with memory performance in an age-dependent or an age-independent manner. Young and older adults underwent fMRI as they encoded words paired with images of faces or scenes. During a subsequent scanned memory test participants judged whether test words were studied or unstudied and, for words judged studied, also made a source memory judgment about the associated image category. Using multi-voxel pattern similarity analyses, we identified robust evidence for reduced scene reinstatement in older relative to younger adults. This decline was however largely explained by age differences in neural differentiation at encoding; moreover, a similar relationship between neural selectivity at encoding and retrieval was evident in young participants. The results suggest that, regardless of age, the selectivity with which events are neurally processed at the time of encoding can determine the strength of retrieval-related cortical reinstatement.

Examining the Neural Basis of Congruent and Incongruent Configural Contexts during Associative Retrieval

Disrupting the configural context, or relative organization and orientation of paired stimuli, between encoding and retrieval negatively impacts memory. Using univariate and multivariate fMRI analyses, we examined the effect of retaining and manipulating the configural context on neural mechanisms supporting associative retrieval. Behavioral results showed participants had significantly higher hit rates for recollecting pairs in a contextually congruent, versus incongruent, configuration. In addition, contextual congruency between memory phases was a critical determinant to characterizing both the magnitude and patterns of neural activation within visual and parietal cortices. Regions within visual cortices also exhibited higher correlations between patterns of activity at encoding and retrieval when configural context was congruent across memory phases than incongruent. Collectively, these findings shed light on how manipulating configural context between encoding and retrieval affects associative recognition, with changes in the configural context leading to reductions in information transfer and increases in task difficulty.

Multi-level analyses of associative recognition memory: the whole is greater than the sum of its parts

Associative recognition memory depends on the integration of information concerning an item and the spatio-temporal context in which it was encountered. Such an integration depends on dynamic interactions across a brain-wide memory network. Here we discuss evidence from multiple levels of analysis, behavioural, cellular and synaptic which demonstrating the existence of multiple overlapping, subnetworks embedded within these large-scale networks. Recent advances have revealed that of these subnetworks, a distinct hippocampal-prefrontal networks are engaged by different representations (object-spatial or object temporal). Other subnetworks are recruited by distinct processing demands, such as encoding and retrieval which are supported by distinct cellular and synaptic processes. One challenge to multi-level investigations of memory continues to be that conclusions are drawn from correlations of effects rather than from direct evidence of causation.