The contributions of transient and sustained response codes to audiovisual integration

Multisensory events in our natural environment unfold at multiple temporal scales over extended periods of time. This functional magnetic resonance imaging study investigated whether the brain uses transient (onset, offset) or sustained temporal codes to effectively integrate incoming visual and auditory signals within the cortical hierarchy. Subjects were presented with 1) velocity-modulated radial motion, 2) amplitude-modulated sound, or 3) an in phase combination of both in blocks of variable durations to dissociate transient and sustained blood oxygen level-dependent responses. Audiovisual interactions emerged primarily for transient onset and offset responses highlighting the importance of rapid stimulus transitions for multisensory integration. Strikingly, audiovisual interactions for onset and offset transients were dissociable at the functional and anatomical level. Low-level sensory areas integrated audiovisual inputs at stimulus onset in a superadditive fashion to enhance stimulus salience. In contrast, higher order association areas showed subadditive integration profiles at stimulus offset possibly reflecting the formation of higher order representations. In conclusion, multisensory integration emerges at multiple levels of the cortical hierarchy using different temporal codes and integration profiles. From a methodological perspective, these results highlight the limitations of conventional event related or block designs that cannot characterize these rich dynamics of audiovisual integration.