Late Integration of Prior Expectations During Precision-Weighted Perceptual Decisions

Abstract Adaptive perceptual decision-making relies on the ability to combine sensory evidence with prior expectations in a precision-weighted manner. Although Bayesian inference provides a clear normative account of how such integration should occur, the neural mechanisms through which the brain represents and combines priors and likelihoods remain poorly understood. Across two preregistered experiments, we investigated how the precision of prior expectations and sensory likelihoods influences visual motion judgements and associated neural activity patterns during a random-dot motion estimation task. Neurotypical adult participants (N=80, 58 female) reported directions of visual motion stimuli, with motion coherence varying randomly across trials. Prior expectations were manipulated block-wise by varying the probabilities with which different motion directions were presented. Consistent with precision-weighted inference, response accuracy improved as coherence increased with robust response biases toward the expected motion direction. Neural activity measured using electroencephalography (EEG) revealed reliable effects of prior expectation on the univariate central-parietal positivity (CPP), consistent with reduced accumulation of sensory evidence under informative priors. Multivariate analysis using inverted-encoding models revealed robust effects of prior informativeness on motion-specific neural representations, but only late, during response planning stages. Together, these findings demonstrate that precision-weighted inference primarily occurs at late stages of the decision process and challenge predictive-processing accounts that emphasise early sensory processing. Significance Statement Perceptual decisions require combining uncertain sensory information with prior expectations about what is likely to occur. Although behaviour often follows this principle, it remains unclear how the brain integrates prior expectations with incoming sensory evidence. Using a visual motion task and concurrent brain imaging, we show that expectations do not alter early sensory processing but instead influence later stages of decision formation and action planning. Neural representations of sensory evidence primarily reflect stimulus reliability, whereas prior expectations selectively enhance representations during response preparation. These findings challenge influential theories proposing that expectations affect early sensory processing and instead highlight their contribution to later decision-related processes. This work advances understanding of how the brain uses experience to optimise perceptual decisions under uncertainty.