Abstract
While it is widely recognized that thinking is somehow costly, involving cognitive effort and producing mental fatigue, these costs have alternatively been assumed to exist, treated as the brain's assessment of lost opportunities, or suggested to be metabolic but with implausible biological bases. We present a model of cognitive cost based on the novel idea that the brain senses and plans for longer-term allocation of metabolic resources by purposively conserving brain activity. We identify several distinct ways the brain might control its metabolic output, and show how a control-theoretic model that models decision-making with an energy budget can explain cognitive effort avoidance in terms of an optimal allocation of limited energetic resources. The model accounts for both subject responsiveness to reward and the detrimental effects of hypoglycemia on cognitive function. A critical component of the model is using astrocytic glycogen as a plausible basis for limited energetic reserves. Glycogen acts as an energy buffer that can temporarily support high neural activity beyond the rate supported by blood glucose supply. The published dynamics of glycogen depletion and repletion are consonant with a broad array of phenomena associated with cognitive cost. Our model thus subsumes both the “cost/benefit” and “limited resource” models of cognitive cost while retaining valuable contributions of each. We discuss how the rational control of metabolic resources could underpin the control of attention, working memory, cognitive look ahead, and model-free vs. model-based policy learning.
Highlights
Cognitive processes that require vigilance, model-based lookahead, or extensive utilization of attention or working memory are said to incur a cost (Kool et al, 2010), described as aversive (McGuire and Botvinick, 2010), or characterized as computationally expensive (Redish, 2013)
Direct measurement of blood sugar levels (Fairclough and Houston, 2004; Gailliot et al, 2007) and metabolic rate (Huang et al, 2012) during cognitively demanding tasks show effects that are too small to account for the detrimental effects on performance that occur during a hypoglycemic state. We suggest that this apparent contradiction can be explained by accounting for the energy storage and buffer mechanism provided by astrocytic glycogen
The model we introduce in the present work is intended to represent neural dynamics on the time scale of roughly 20 min to several hours, as that is the time scale of glycogen depletion and subjective mental fatigue
Summary
Cognitive processes that require vigilance, model-based lookahead, or extensive utilization of attention or working memory are said to incur a cost (Kool et al, 2010), described as aversive (McGuire and Botvinick, 2010), or characterized as computationally expensive (Redish, 2013). As the blood sugar level of an individual falls, precisely those processes that are considered costly are first affected (see Feldman and Barshi, 2007) Another influential approach links cognitive costs to the utilization of a limited resource, purportedly blood glucose (Gailliot et al, 2007). Using an optimal control theory framework allows and requires the modeler to be explicit about the dynamics of the system being modeled, the objective function to be optimized, and the planning horizon of the agent From this perspective, assumptions of limited resources and cost/benefit tradeoffs are not diametrically opposed, as some have suggested (Kurzban et al, 2013), but are different components in a more general framework. We discuss the implications of our model and suggest directions for future work
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