Priors for perceived speed combine supramodally for visual-tactile motion but not for audio-visual motion.
Perception combines incoming sensory evidence with priors learned from preceding input. One example is the central tendency effect in which stimulus estimates regress towards a prior based on the mean of the stimulus distribution. Whether such priors are specific to each modality or shared across modalities remains unresolved. Here, we examined this question using speed perception in the visual, auditory and tactile modalities and used central tendency as a behavioral probe. By giving each modality its own speed range, clear predictions emerge when modalities are interleaved: if priors are modality specific, each modality should maintain its own central tendency bias whereas a supramodal prior would exhibit a different mean based on the pooled set of speeds. Moreover, a supra-modal prior should exhibit precision weighting, with the more reliable cue receiving more weight. For vision and touch, speed estimates converged when interleaved: both modalities shifted but the less reliable one shifted more, consistent with a precision-weighted supramodal prior. For vision and audition, the priors remained largely distinct: audition (the less reliable modality) showed a partial shift but did not converge on the precision weighted, supramodal mean. This was not explained by the large audiovisual reliability difference: it persisted when visual precision was reduced. These results show that in the motion domain, visual and tactile signals can combine crossmodally, but auditory and visual signals do not. Supramodal processing of motion therefore depends on which senses are involved and is likely explained by the lack of an early spatial mapping in audition.