Abstract
Working memory (WM) function has traditionally been investigated in terms of two dimensions: within-individual effects of WM load, and between-individual differences in task performance. In human neuroimaging studies, the N-back task has frequently been used to study both. A reliable finding is that activation in frontoparietal regions exhibits an inverted-U pattern, such that activity tends to decrease at high load levels. Yet it is not known whether such U-shaped patterns are a key individual differences factor that can predict load-related changes in task performance. The current study investigated this question by manipulating load levels across a much wider range than explored previously (N = 1–6), and providing a more comprehensive examination of brain-behavior relationships. In a sample of healthy young adults (n = 57), the analysis focused on a distinct region of left lateral prefrontal cortex (LPFC) identified in prior work to show a unique relationship with task performance and WM function. In this region it was the linear slope of load-related activity, rather than the U-shaped pattern, that was positively associated with individual differences in target accuracy. Comprehensive supplemental analyses revealed the brain-wide selectivity of this pattern. Target accuracy was also independently predicted by the global resting-state connectivity of this LPFC region. These effects were robust, as demonstrated by cross-validation analyses and out-of-sample prediction, and also critically, were primarily driven by the high-load conditions. Together, the results highlight the utility of high-load conditions for investigating individual differences in WM function.
Highlights
Understanding the neural basis of working memory and executive control (WM/EC) functions has been a major aim of cognitive neuroscience research
We focused on a functional connectivity (FC) measure which can be computed for specific, focal brain regions, and which has been related to N-back performance in prior work: the global brain connectivity (GBC) index
The N-back is one of the most widely used paradigms to study working memory and executive control, there is still a poor understanding of how brain activity varies by load and how load-related activity patterns relate to task performance
Summary
Understanding the neural basis of working memory and executive control (WM/EC) functions has been a major aim of cognitive neuroscience research. WM load can be varied in an incremental, parametric fashion by increasing the value of N (Braver et al, 1997). This is a critical component of the paradigm, since as N-back levels increase, task performance shows a reliable decrement, while the subjective experience of cognitive effort and task difficulty increases (Ewing and Fairclough, 2010; Otto et al, 2014; Westbrook et al, 2013; Westbrook et al, n.d.) The concomitant increase in task difficulty and drop in performance is useful psychometrically as it drives variability among participants, and the potential to reveal the neural correlates of individual differences
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