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Abstract Decision-makers often process new evidence selectively, depending on their current beliefs about the world. We asked whether such confirmation biases result from biases in the encoding of sensory evidence in the brain, or alternatively in the utilization of encoded evidence for behavior. Human participants estimated the source of a sequence of visual-spatial evidence samples while we measured cortical population activity with magnetoencephalography. Halfway through the sequence, participants were prompted to judge the more likely source category. We find that processing of subsequent evidence depends on its consistency with the previously chosen category. Evidence encoded in parietal cortex contributes more to the estimation report when that evidence is consistent with the previous choice compared to when it contradicts that choice. Our results indicate that information contradicting pre-existing beliefs has little impact on subsequent behavior, despite being precisely encoded in the brain. This provides room for deliberative control to counteract confirmation biases. 

The tilt illusion—a bias in the perceived orientation of a center stimulus induced by an oriented surround—illustrates how context shapes visual perception. Although extensively studied for decades, we still lack a comprehensive account of the illusion that connects its behavioral and neural characteristics. Here, we demonstrate that the tilt illusion originates from dynamic changes in neural coding precision induced by the surround context. We simultaneously obtained psychophysical and functional MRI responses from human subjects while they viewed gratings in the absence and presence of an oriented surround and independently extracted sensory encoding precision from their behavioral and neural data. Both measures show that in the absence of an oriented surround, encoding reflects the natural scene statistics of orientation. However, with an oriented surround, encoding precision is significantly increased for stimuli similar to the surround orientation. This local change in encoding is sufficient to predict the behavioral characteristics of the tilt illusion using a Bayesian observer model. The effect of surround modulation increases along the ventral stream and is localized to the portion of the visual cortex with receptive fields at the center-surround boundary. The pattern of change in coding accuracy reflects the surround-conditioned orientation statistics in natural scenes, but cannot be explained by local stimulus configuration. Our results suggest that the tilt illusion naturally emerges from an adaptive coding strategy that efficiently reallocates neural coding resources based on the current stimulus context.