Highlights d Multisensory integration develops prior to unsupervised crossmodal recalibration d Multisensory integration and crossmodal learning are dissociable processes d Crossmodal recalibration in humans emerges first for short timescales d Crossmodal recalibration requiring accumulation of evidence develops last

According to the Bayesian framework of multisensory integration, audiovisual stimuli associated with a stronger prior belief that they share a common cause (i.e., causal prior) are predicted to result in a greater degree of perceptual binding and therefore greater audiovisual integration. In the present psychophysical study, we systematically manipulated the causal prior while keeping sensory evidence constant. We paired auditory and visual stimuli during an association phase to be spatiotemporally either congruent or incongruent, with the goal of driving the causal prior in opposite directions for different audiovisual pairs. Following this association phase, every pairwise combination of the auditory and visual stimuli was tested in a typical ventriloquism-effect (VE) paradigm. The size of the VE (i.e., the shift of auditory localization towards the spatially discrepant visual stimulus) indicated the degree of multisensory integration. Results showed that exposure to an audiovisual pairing as spatiotemporally congruent compared to incongruent resulted in a larger subsequent VE (Experiment 1). This effect was further confirmed in a second VE paradigm, where the congruent and the incongruent visual stimuli flanked the auditory stimulus, and a VE in the direction of the congruent visual stimulus was shown (Experiment 2). Since the unisensory reliabilities for the auditory or visual components did not change after the association phase, the observed effects are likely due to changes in multisensory binding by association learning. As suggested by Bayesian theories of multisensory processing, our findings support the existence of crossmodal causal priors that are flexibly shaped by experience in a changing world.

Following focal sensory adaptation, the perceived separation between visual stimuli that straddle the adapted region is often exaggerated. For instance, in the tilt aftereffect illusion, adaptation to tilted lines causes subsequently viewed lines with nearby orientations to be perceptually repelled from the adapted orientation. Repulsion illusions in the nonvisual senses have been less studied. Here, we investigated whether adaptation induces a repulsion illusion in tactile spatial perception. In a two-interval forced-choice task, participants compared the perceived separation between two point-stimuli applied on the forearms successively. Separation distance was constant on one arm (the reference) and varied on the other arm (the comparison). In Experiment 1, we took three consecutive baseline measurements, verifying that in the absence of manipulation, participants’ distance perception was unbiased across arms and stable across experimental blocks. In Experiment 2, we vibrated a region of skin on the reference arm, verifying that this focally reduced tactile sensitivity, as indicated by elevated monofilament detection thresholds. In Experiment 3, we applied vibration between the two reference points in our distance perception protocol and discovered that this caused an illusory increase in the separation between the points. We conclude that focal adaptation induces a repulsion aftereffect illusion in tactile spatial perception. The illusion provides clues as to how the tactile system represents spatial information. The analogous repulsion aftereffects caused by adaptation in different stimulus domains and sensory systems may point to fundamentally similar strategies for dynamic sensory coding.

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