Crossmodal association of auditory and visual material properties in infants.

The human perceptual system enables us to extract visual properties of an object's material from auditory information. In monkeys, the neural basis underlying such multisensory association develops through experience of exposure to a material; material information could be processed in the posterior inferior temporal cortex, progressively from the high-order visual areas. In humans, however, the development of this neural representation remains poorly understood. Here, we demonstrated for the first time the presence of a mapping of the auditory material property with visual material ("Metal" and "Wood") in the right temporal region in preverbal 4- to 8-month-old infants, using near-infrared spectroscopy (NIRS). Furthermore, we found that infants acquired the audio-visual mapping for a property of the "Metal" material later than for the "Wood" material, since infants form the visual property of "Metal" material after approximately 6 months of age. These findings indicate that multisensory processing of material information induces the activation of brain areas related to sound symbolism. Our findings also indicate that the material's familiarity might facilitate the development of multisensory processing during the first year of life.