Guided by Expectations: Overweighted Semantic Priors in Schizotypy and their Links to Glutamate

An imbalance in the weighting of prior beliefs and sensory evidence are thought to contribute to the development of psychotic symptoms, such as hallucinations and delusions. We investigated how much individuals with schizotypal traits, a subclinical expression of psychosis-proneness, use high-level priors or sensory evidence to understand noise-degraded language, potentially resulting in task-based hallucinations – perceptions that match expectations but not the inputs. In a language comprehension task, we manipulated semantic predictability, sensory degradation and surprisal to estimate prior weights using a Bayesian Belief updating model. Study 1 (n=109) shows that high-level priors were overweighted with increasing schizotypy. Study 2 (n=55) replicated this effect and revealed that an overweighting of priors was associated with increased cingulate glutamate, providing a neurobiological basis for over-reliance on top-down predictions. These results offer a mechanistic and neurobiological understanding of how predictive coding alterations contribute to symptoms along the psychosis spectrum.