Long-range temporal correlations are suppressed during cognitive tasks, suggesting that higher-order cognitive processing entails sub-criticality. We hypothesize that subjective cognitive effort reflects the degree to which brain dynamics diverge from criticality during task performance. To test our hypothesis, we collected EEG data while 85 healthy young adults performed levels N = 1—4 of the N-back working memory task, and analyzed within-trial dynamics of the branching statistic. Our analysis revealed 1) super-critical branching during inter-trial intervals requiring working memory maintenance, 2) a sharp peak immediately after stimulus presentation, when working memory gating is required, followed by 3) a protracted dip to sub-critical branching and a slow recovery to inter-trial levels. We furthermore find that these features are largely amplified for higher load levels, for errors versus correct trials, and for target versus non-target trials, supporting the hypothesis that more intensive cognitive processing is associated with greater within-trial divergence from criticality. To test for a relationship with subjective effort, participants performed a cognitive effort discounting task in which subjective effort is quantified by the amount of money participants give up to avoid more- versus less-demanding N-back levels. Importantly, participants who find the tasks to be more effort-costly diverge more from critical branching on demanding trials, suggesting that subjective effort reflects the degree to which cognitive processing entails divergence from criticality.
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