A nonlinear reward after-effect of cognitive effort expenditure.

Effort is widely viewed as a cost that diminishes the appeal of rewards, yet people sometimes value outcomes more when they have worked hard to obtain them. This paradox in the reward after-effect of effort expenditure persists because nearly all studies compare only two effort levels, implicitly assuming a monotonic effort-value function. We tested whether this function is nonlinear by recording EEG from 40 healthy adults (20 females) who performed a cognitive effort task with five parametrically varied effort levels to earn monetary rewards. Gain-related neural responses followed a U-shaped function across effort levels, decreasing from low to intermediate effort and rebounding from intermediate to high effort. The P3 and delta oscillations captured the full nonlinear profile, whereas theta oscillations tracked only the descending limb and the reward positivity only the ascending limb. Moreover, individual differences in effort avoidance, independently measured by a demand-selection task, amplified the U-shaped curvature of gain-related delta power, with high-effort-avoidance individuals exhibiting a steeper nonlinear profile. These findings help reconcile the effort paradox, suggesting that effort discounting and effort enhancement are complementary segments of a single nonlinear valuation function and challenging purely monotonic accounts of the reward after-effect of effort expenditure.