Subjective Cognitive Effort Indexes Sub-Criticality in the Brain

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Project Summary Cognitive effort is subjectively costly. It can cause people to discount valuable goals and avoid thoughtful planning and careful deliberation. Effort avoidance can thus be problematic, especially in disorders like depression and schizophrenia where excessive cost sensitivity undermines cognitive motivation. To address cost sensitivity, we first need to understand what the brain treats as costly. Traditional indices, like lateral frontal functional magnetic resonance imaging signals, and parietal alpha desynchronization in electroencephalography data, track cognitive load to a point, but often plateau or decline, even when subjective effort continues to rise. This project will examine a promising candidate mechanism underlying the experience of subjective cognitive effort: divergence from criticality. Criticality characterizes cortical dynamics at rest, and prior studies have shown that brains become increasingly sub-critical under the very conditions known to increase subjective effort: increasing working memory load, fatigue, sleep deprivation, novelty, and cognitive aging. In the Applicant’s K99 training phase, they found that brain dynamics diverge from a critical point, the harder that tasks become. Most importantly, however, people whos’ brains diverge the most from that critical point experience tasks as more subjectively effortful than people whos’ brains more closely approximate criticality. It is hypothesized that proximity to a critical point depends on the balance of cortical excitatory versus inhibitory signaling. Consistent with this hypothesis, the Applicant also found that artificially perturbing cortical excitability using transcranial magnetic stimulation also pushes brains farther away from a critical point and increases subjective effort on a subsequent cognitive task. The direction of the effect of transcranial magnetic stimulation was surprising, however, opening doors to new hypotheses about the effects of transcranial magnetic stimulation on brain function from the perspective of critical dynamics. This project will test the hypothesis that the cortical excitatory inhibitory balance regulates how closely to a critical state the brain operates, and thereby how sensitive people are to effort costs. In so doing, the project will also re-examine the effects of common transcranial magnetic stimulation protocols on cortical excitability with respect to critical, sub-critical, and super-critical dynamics. Thus, Aim 1 will combine electroencephalography and transcranial magnetic stimulation to test hypothesized mechanisms which may underlie cognitive effort, working memory, and cognitive motivation and also re-assess longstanding assumptions about the effects of transcranial magnetic stimulation on cortical plasticity.