Yebra, M.; Courellis, H.; Cheng, S.; Mosher, C.; Fu, Z.; Tagliati, M.; Mamelak, A.; Rutishauser, U.

Midbrain circuits are thought to govern reward-guided behavior and momentary happiness, yet causal evidence in humans remains scarce. Here, we show that focal microstimulation of the substantia nigra pars compacta alters both decision making and momentary happiness in patients undergoing awake deep-brain stimulation surgery for Parkinson's disease. During a gambling task with interleaved momentary happiness ratings, stimulation delivered at outcome reduced risk-seeking in the loss domain without increasing overall gambling probability. Stimulation improved choice efficiency, guiding behavior toward gambles with higher expected value and thereby increasing cumulative earnings. Computational modeling revealed selective changes in loss processing, with reduced loss weighting and more linear loss utility, while gain processing remained unchanged. In parallel, stimulation did not tonically elevate happiness but selectively amplified outcome-related happiness changes, scaling with decision value and reward prediction errors. These results provide causal evidence that localized perturbation of human midbrain circuits reshapes valuation, choice, and momentary well-being.