Automatic and effortful control of interference in working memory can be distinguished by unique behavioral and functional brain representations

Hippocampal and prefrontal GABA and glutamate concentration contribute to component processes of working memory in aging

Both animal and human studies indicate that individual variation in the neurometabolites gamma-aminobutyric acid and glutamate is linked to cognitive function. Age-related differences in these neurometabolites could potentially explain lower cognitive ability in older age. Working memory-the capacity to hold a limited amount of information online for a short period-has a central role in cognition, and this ability is also impaired in older individuals. Here, we investigated the relationship between gamma-aminobutyric acid (GABA+) levels and a composite measure of glutamate/glutamine (Glx) in the hippocampus and inferior frontal gyrus (IFG) and how these neurochemical markers relate to working memory in younger and older adults. Across age groups, we found a significant positive association between working memory accuracy and Glx in the IFG, as well as a significant negative association between GABA+ in this region and proactive interference. Age-stratified analyses demonstrated significant positive associations between components of working memory and hippocampal/IFG Glx, as well as a significant negative association between IFG GABA+ and proactive interference in older adults only. These results provide novel evidence for a specific involvement of excitatory Glx and working memory accuracy as well as inhibitory GABA+ for control of proactive interference in working memory, and how these effects are differentially affected by age.

Age Differences in Brain Functional Connectivity Underlying Proactive Interference in Working Memory

Aging is typically accompanied by a decline in working memory (WM) capacity, even in the absence of pathology. Proficient WM requires cognitive control processes that can retain goal-relevant information for easy retrieval and resolve interference from irrelevant information. Aging has been associated with a reduced ability to resolve proactive interference (PI) in WM, leading to impaired retrieval of goal-relevant information. It remains unclear how age-related differences in the ability to resolve PI in WM are related to patterns of resting-state functional connectivity (rsFC) in the brain. Here, we investigated the association between PI in WM and rsFC cross-sectionally (n = 237) and 5 years longitudinally (n = 134) across the adult life span by employing both seed-based and data-driven approaches. Results revealed that the ability to resolve PI was associated with differential patterns of inferior frontal gyrus (IFG) rsFC in younger/middle-aged adults (25-60 years) and older adults (65-80 years) in two clusters centered in the vermis and caudate. Specifically, more PI was associated with stronger inferior frontal gyrus-vermis connectivity and weaker inferior frontal gyrus-caudate connectivity in older adults, while younger/middle-aged adults showed associations in the opposite directions with the identified clusters. Longitudinal analyses revealed that a reduced ability to control PI was associated with reduced inferior frontal gyrus-insula and inferior frontal gyrus-anterior cingulate cortex connectivity in older adults, while younger/middle-aged adults showed associations in the opposite direction with these clusters. Whole brain multivariate pattern analyses showed age-differential patterns of rsFC indicative of age-related structural decline and age-related compensation. The current results show that rsFC is associated with the ability to control PI in WM and that these associations are modulated by age.

The role of the hippocampus in working memory and word reading: Novel neural correlates of reading among youth living in the context of economic disadvantage

A left-lateralized cortical reading circuit underlies successful reading and fails to engage in individuals with reading problems. Studies identifying this circuit included youth from economically advantaged backgrounds and focused on cortical, not subcortical, structures. However, among youth with low scores on reading tests who are living in the context of economic disadvantage, this brain network is actively engaged during reading, despite persistent reading problems. This finding suggests that other brain circuits may underlie reading in these youth. A hippocampal circuit is one likely candidate, as it has recently been shown to support domain-general processes like working memory (WM) that are also associated with reading. Given age-related increases in hippocampal volume, WM, and reading, and known associations between WM and reading, we hypothesized that hippocampal volume would be associated with reading via WM processes. Using a cross-sectional developmental design, we explored this in middle childhood (average age at MRI scan ∼10; N = 50) and adolescence (average age at MRI scan ∼17; N = 175). Results suggest that the hippocampus is a critical contributor to word reading in adolescents living in economically disadvantaged contexts, and that this operates through working memory processes. Such findings point to new targets for reading intervention in adolescents.

Encoding-related Brain Activity Predicts Subsequent Trial-level Control of Proactive Interference in Working Memory

Proactive interference (PI) appears when familiar information interferes with newly acquired information and is a major cause of forgetting in working memory. It has been proposed that encoding of item-context associations might help mitigate familiarity-based PI. Here, we investigate whether encoding-related brain activation could predict subsequent level of PI at retrieval using trial-specific parametric modulation. Participants were scanned with event-related fMRI while performing a 2-back working memory task with embedded 3-back lures designed to induce PI. We found that the ability to control interference in working memory was modulated by level of activation in the left inferior frontal gyrus, left hippocampus, and bilateral caudate nucleus during encoding. These results provide insight to the processes underlying control of PI in working memory and suggest that encoding of temporal context details support subsequent interference control.

Stimulus shapes strategy: Effects of stimulus characteristics and individual differences in academic achievement on the neural mechanisms engaged during the N-back task

This fMRI study of 126 youth explored whether the neural mechanisms underlying the N-back task, commonly used to examine executive control over the contents of working memory, are associated with individual differences in academic achievement in reading and math. Moreover, the study explored whether these relationships occur regardless of the nature of the stimulus being manipulated in working memory (letters, numbers, nonsense shapes) or whether these relationships are specific to achievement domain and stimulus type (i.e., letters for reading and numbers for math). The results indicated that higher academic achievement in each of reading and math was associated with greater activation of dorsolateral prefrontal cortex in the N-back task regardless of stimulus type (i.e., did not differ for letters and numbers), suggesting that at least some aspects of the neural mechanisms underlying these academic domains are executive in nature. In addition, regardless of level of academic achievement, prefrontal regions were engaged to a greater degree for letters than numbers than nonsense shapes. In contrast, nonsense shapes yielded greater hippocampal activation than letters and numbers. Potential reasons for this pattern of findings are discussed.

Metacognition in working memory: Confidence judgments during an n-back task

Metacognition in working memory (WM) has received less attention than episodic memory, and few studies have investigated confidence judgements while carrying out a verbal WM task. The present study investigated whether individuals are aware of their own level of performance while carrying out an ongoing verbal WM task, and whether judgments of confidence are sensitive to factors that determine WM performance. A verbal n-back task was adapted to obtain confidence judgments on a trial-by-trial basis. Memory load and lure interference were manipulated. Results showed that metacognition judgments were affected by memory load and levels of interference just as performance accuracy. Even when judgments were sensitive to memory factors, participants were overconfident and generally showed poor metacognitive accuracy at discriminating between erroneous and accurate responses. Results are discussed in terms of possible cues contributing to metacognitive judgements during an ongoing WM task and reasons for WM metacognitive accuracy.