Effort is costly: given a choice, we tend to avoid it¹. However, in many cases, effort adds value to the ensuing rewards². From ants³ to humans⁴, individuals prefer rewards that had been harder to achieve. This counterintuitive process may promote reward seeking even in resource-poor environments, thus enhancing evolutionary fitness⁵. Despite its ubiquity, the neural mechanisms supporting this behavioural effect are poorly understood. Here we show that effort amplifies the dopamine response to an otherwise identical reward, and this amplification depends on local modulation of dopamine axons by acetylcholine. High-effort rewards evoke rapid acetylcholine release from local interneurons in the nucleus accumbens. Acetylcholine then binds to nicotinic receptors on dopamine axon terminals to augment dopamine release when reward is delivered. Blocking the cholinergic modulation blunts dopamine release selectively in high-effort contexts, impairing effortful behaviour while leaving low-effort reward consumption intact. These results reconcile in vitro studies, which have long demonstrated that acetylcholine can trigger dopamine release directly through dopamine axons^{6,7,8,9,10,11}, with in vivo studies that failed to observe such modulation^12,13,14, but did not examine high-effort contexts. Our findings uncover a mechanism that drives effortful behaviour through context-dependent local interactions between acetylcholine and dopamine axons.

BACKGROUND: Recent studies suggest that socioeconomic status (SES) influences outcomes after spinal fusion. The influence of SES on postoperative outcomes is increasingly relevant as rates of lumbar fusion rise. OBJECTIVE: To determine the influence of SES variables including race, education, net worth, and homeownership on postoperative outcomes. METHODS: Optum's deidentified Clinformatics Data Mart Database was used to conduct a retrospective review of SES variables for patients undergoing first-time, inpatient lumbar fusion from 2003 to 2021. Primary outcomes included hospital length of stay (LOS) and 30-day reoperation, readmission, and postoperative complication rates. Secondary outcomes included postoperative emergency room visits, discharge status, and total hospital charges. RESULTS: In total, 217 204 patients were identified. On multivariate analysis, Asian, Black, and Hispanic races were associated with increased LOS (Coeff. [coefficient] 0.92, 95% CI 0.68-1.15; Coeff. 0.61, 95% CI 0.51-0.71; Coeff. 0.43, 95% CI 0.32-0.55). Less than 12th grade education (vs greater than a bachelor's degree) was associated with increased odds of reoperation (OR [odds ratio] 1.88, 95% CI 1.03-3.42). Decreased net worth was associated with increased odds of readmission (OR 1.32, 95% CI 1.25-1.40) and complication (OR 1.14, 95% CI 1.10-1.20). Renting a home (vs homeownership) was associated with increased LOS, readmissions, and total charges (Coeff. 0.30, 95% CI 0.17-0.43; OR 1.19, 95% CI 1.11-1.30; Coeff. 13 200, 95% CI 9000-17 000). CONCLUSION: Black race, less than 12th grade education, <$25K net worth, and lack of homeownership were associated with poorer postoperative outcomes and increased costs. Increasing perioperative support for patients with these sociodemographic risk factors may improve postoperative outcomes.