The intricate relationship between multisensory integration and attention has been extensively researched in the multisensory field; however, the necessity of attention for the binding of multisensory stimuli remains contested. In the current study, we investigated whether diverting attention from well-known multisensory tasks would disrupt integration and whether the complexity of the stimulus and task modulated this interaction. A secondary objective of this study was to investigate individual differences in the interaction of attention and multisensory integration. Participants completed a simple audiovisual speeded detection task and McGurk task under various perceptual load conditions: no load (multisensory task while visual distractors present), low load (multisensory task while detecting the presence of a yellow letter in the visual distractors), and high load (multisensory task while detecting the presence of a number in the visual distractors). Consistent with prior studies, we found that increased perceptual load led to decreased reports of the McGurk illusion, thus confirming the necessity of attention for the integration of speech stimuli. Although increased perceptual load led to longer response times for all stimuli in the speeded detection task, participants responded faster on multisensory trials than unisensory trials. However, the increase in multisensory response times violated the race model for no and low perceptual load conditions only. Additionally, a geometric measure of Miller’s inequality showed a decrease in multisensory integration for the speeded detection task with increasing perceptual load. Surprisingly, we found diverging changes in multisensory integration with increasing load for participants who did not show integration for the no load condition: no changes in integration for the McGurk task with increasing load but increases in integration for the detection task. The results of this study indicate that attention plays a crucial role in multisensory integration for both highly complex and simple multisensory tasks and that attention may interact differently with multisensory processing in individuals who do not strongly integrate multisensory information.

The ability to synthesize information across multiple senses is known as multisensory integration and is essential to our understanding of the world around us. Sensory stimuli that occur close in time are likely to be integrated, and the accuracy of this integration is dependent on our ability to precisely discriminate the relative timing of unisensory stimuli (crossmodal temporal acuity). Previous research has shown that multisensory integration is modulated by both bottom-up stimulus features, such as the temporal structure of unisensory stimuli, and top-down processes such as attention. However, it is currently uncertain how attention alters crossmodal temporal acuity. The present study investigated whether increasing attentional load would decrease crossmodal temporal acuity by utilizing a dual-task paradigm. In this study, participants were asked to judge the temporal order of a flash and beep presented at various temporal offsets (crossmodal temporal order judgment (CTOJ) task) while also directing their attention to a secondary distractor task in which they detected a target stimulus within a stream visual or auditory distractors. We found decreased performance on the CTOJ task as well as increases in both the positive and negative just noticeable difference with increasing load for both the auditory and visual distractor tasks. This strongly suggests that attention promotes greater crossmodal temporal acuity and that reducing the attentional capacity to process multisensory stimuli results in detriments to multisensory temporal processing. Our study is the first to demonstrate changes in multisensory temporal processing with decreased attentional capacity using a dual task paradigm and has strong implications for developmental disorders such as autism spectrum disorders and developmental dyslexia which are associated with alterations in both multisensory temporal processing and attention.