White matter microstructure predicts effort and reward sensitivity
From rodents to humans, animals constantly face a central question: is the reward worth the effort? Effort and reward sensitivity in such situations vary substantially across individuals and ultimately shape goal-directed behavior. Yet, the brain mechanisms underlying this variability across individuals remain unclear. Here, we combined computational modeling of effort and reward sensitivity during decision-making with whole-brain diffusion MRI in 45 healthy participants to identify white matter substrates of individual sensitivity. A data-driven, cluster-based analysis of fractional anisotropy and mean diffusivity revealed 12 clusters: five linked to effort sensitivity, all within tracts connected to major frontal valuation nodes (e.g., supplementary motor area [SMA], dorsal anterior cingulate cortex [dACC], orbitofrontal cortex [OFC]), and seven linked to reward sensitivity, spanning frontal valuation, fronto-parietal, and sensorimotor networks. The strongest associations involved two SMA-connected clusters, one shared across effort and reward sensitivity and another consistent across both microstructural metrics. Critically, microstructural features from the five effort-related and seven reward-related clusters reliably predicted individual effort and reward sensitivity in out-of-sample machine learning analyses, respectively, whereas randomly sampled clusters did not. SMA-connected tracts were the dominant predictors in these decoding analyses, with additional contributions from fronto-parietal and sensorimotor pathways for reward sensitivity. These findings reveal a distributed white matter architecture underlying inter-individual differences in effort and reward sensitivity, with SMA pathways emerging as central hubs. They demonstrate that localized white matter microstructure can robustly predict these individual differences, offering a framework to forecast the impact of lesions or interventions on goal-directed behavior, including apathy and impulsivity. SIGNIFICANCE STATEMENTWhy do some people give up easily when faced with high effort demands, while others persist even when rewards are small? Such differences in effort and reward sensitivity shape goal-directed behavior, yet their neural basis is unclear. Using diffusion MRI and computational modeling, we show that white matter microstructure in specific pathways reliably predicts individual differences in these sensitivities. Tracts connected to the supplementary motor area emerged as central hubs, with additional contributions from fronto-parietal and sensorimotor networks. These results demonstrate that variability in effort and reward sensitivity is rooted not only in brain activity but also in structural connectivity, providing a framework to anticipate how white matter lesions or interventions may alter goal-directed behavior, including apathy and impulsivity.