Probabilistic neural code supports metacognition in value-based decisions in the human brain

Subjective value underlies many everyday decisions, yet the neural representations supporting these valuations are inherently noisy and inevitably subject to uncertainty. Whether humans have metacognitive access to this uncertainty, and how such access is supported neurally, remains unclear. Here, we provide converging behavioral and neural evidence that subjective value is represented probabilistically in the human brain. Behaviorally, participants' explicit uncertainty reports tracked trial-by-trial fluctuations in valuation variability. Neurally, we decoded full probability distributions over value from fMRI population activity in ventromedial prefrontal cortex (vmPFC) and superior parietal lobe and intraparietal sulcus (SPL-IPS). Decoded neural uncertainty predicted behavioral reported uncertainty at the trial, item, and individual levels. These findings indicate that neural populations jointly encode value and uncertainty, extending probabilistic population coding from perception and working memory to subjective value, and providing a neural basis for metacognitive access to uncertainty in value-based decisions.