Audiovisual integration in the human perception of materials.

Interest in the perception of the material of objects has been growing. While material perception is a critical ability for animals to properly regulate behavioral interactions with surrounding objects (e.g., eating), little is known about its underlying processing. Vision and audition provide useful information for material perception; using only its visual appearance or impact sound, we can infer what an object is made from. However, what material is perceived when the visual appearance of one material is combined with the impact sound of another, and what are the rules that govern cross-modal integration of material information? We addressed these questions by asking 16 human participants to rate how likely it was that audiovisual stimuli (48 combinations of visual appearances of six materials and impact sounds of eight materials) along with visual-only stimuli and auditory-only stimuli fell into each of 13 material categories. The results indicated strong interactions between audiovisual material perceptions; for example, the appearance of glass paired with a pepper sound is perceived as transparent plastic. Rating material-category likelihoods follow a multiplicative integration rule in that the categories judged to be likely are consistent with both visual and auditory stimuli. On the other hand, rating-material properties, such as roughness and hardness, follow a weighted average rule. Despite a difference in their integration calculations, both rules can be interpreted as optimal Bayesian integration of independent audiovisual estimations for the two types of material judgment, respectively.