Multisensory Integration is Independent of Perceived Simultaneity

The importance of multisensory integration for perception and action has long been recognised. Integrating information from individual senses increases the chance of survival by reducing the variability in the incoming signals, thus allowing us to respond more rapidly. Reaction times (RTs) are fastest when the components of the multisensory signals are simultaneous. This response facilitation is traditionally attributed to multisensory integration. However, it is unclear if facilitation of RTs occurs when stimuli are perceived as synchronous or are actually physically synchronous. Repeated exposure to audiovisual asynchrony can change the delay at which multisensory stimuli are perceived as simultaneous, thus changing the delay at which the stimuli are integrated-perceptually. Here we set out to determine how such changes in multisensory integration for perception affect our ability to respond to multisensory events. If stimuli perceived as simultaneous were reacted to most rapidly, it would suggest a common system for multisensory integration for perception and action. If not, it would suggest separate systems. We measured RTs to auditory, visual, and audiovisual stimuli following exposure to audiovisual asynchrony. Exposure affected the variability of the unisensory RT distributions; in particular, the slowest RTs were either speed up or slowed down (in the direction predicted from shifts in perceived simultaneity). Additionally, the multisensory facilitation of RTs (beyond statistical summation) only occurred when audiovisual onsets were physically synchronous, rather than when they appeared simultaneous. We conclude that the perception of synchrony is therefore independent of multisensory integration and suggest a division between multisensory processes that are fast (automatic and unaffected by temporal adaptation) and those that are slow (perceptually driven and adaptable).