Probing the causal role of the frontopolar cortex in cognitive effort based decision-making using transcranial direct current stimulation in healthy participants and Parkinson patients with apathy

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Flexible, goal-directed behavior requires allocation of cognitive control. Yet, the factors that motivate when, why and how humans choose the amount of cognitive resources applied to a task remain elusive. Assuming that exerting cognitive control is effortful, aversive and costly, influential neuroeconomic research suggests that the participants’ willingness to engage in cognitively demanding behavior is based on a cost-benefit decision-making process that calculates the trade-off between the costs of applying cognitive effort against the value of the anticipated rewards. Indeed, individuals tend to discount monetary rewards by the amount of effort required to obtain them. However, critical questions regarding the computational and neural underpinnings of cognitive effort-based decision-making remain unanswered, limiting the generalizability and explanatory power of the field. More specifically, while effort-related discounting of rewards has been mostly studied in choice problems that allow participants to directly compare cost-benefit propositions of two behavioral options, it is still unknown whether effort similarly affects other forms of decision-making that rely on different computational mechanisms, such as learning the average reward rate of an action. Further, which brain regions are involved in integrating effort and reward information is still highly debated. Although promising findings showed that activity in the frontopolar cortex (FPC) is associated with reduced effort aversion in cost-benefit decisions, it is untested whether the FPC may play a general role in invigorating effortful behavior across different classes of choice problems. Finally, as alterations in effort-based decision-making may present an underlying factor in apathy, a syndrome of severe amotivation that is common in many clinical disorders, including Parkinson’s disease (PD), it is crucial to identify if and how effort-related processes and their neural basis are impaired in the conditions. The proposed project sets out to thoroughly address these questions. We will conduct two parallel experiments to investigate how modulating FPC activity with transcranial direct current stimulation (tDCS) causally affects effort-based decision-making in two distinct classes of choice problems (cost-benefit vs. average reward rate-based decisions) in healthy adults and PD patients with apathy. Both parts of the project will make use of a two-day, fully crossed, double-blinded within-subject design in which all participants will be tested under both active and sham stimulation. The results of this project will substantially extend previous finding on cognitive effort by shedding light on how it affects fundamental every-day-decision-making processes and how it is implemented in the brain. Furthermore, it will allow us to understand how malfunctioning of these processes may contribute to apathy in PD as well as whether tDCS may be used to improve apathy symptoms in these patients.