Prestimulus Periodic and Aperiodic Neural Activity Shapes McGurk Perception.

Previous studies emphasize the importance of prestimulus neural oscillations in shaping endogenous brain states that substantially impact perceptual outcomes. However, what features in such oscillations drive perception remains unknown. Furthermore, research has shown that non-oscillatory activity is also important for cognitive processing. However, their interaction prior to perceiving a multisensory stimulus remains unexplored. In this human EEG study (n=18, 10 males and 8 females), we investigated the role of prestimulus periodic power and aperiodic activity in modulating perception of the widely studied McGurk illusion on a trial-by-trial basis. Using logistic mixed-effect models, we reveal that the illusion perception is associated with reduced prestimulus alpha (8-12 Hz) and beta (15-30 Hz) power over frontal and occipital regions, increased theta (4-7 Hz) power in parietal, central, and occipital regions, and increased gamma (31-45 Hz) power across the scalp. Furthermore, lower aperiodic offset and exponent values in central, parietal, and occipital regions also predicted illusory responses. Our logistic mixed interaction models revealed that the aperiodic exponent and periodic power jointly influence the perception of upcoming McGurk stimuli. Specifically, a decrease in occipital theta and global beta power and an increase in occipital and parietal gamma power were associated with a steeper slope. We conclude that the predominant source of variations in the prestimulus state is the aperiodic activity and that fluctuations in both periodic and aperiodic activity account for inter-trial variability in the perception of the McGurk illusion.Significance Statement Prestimulus brain oscillations and aperiodic activity are fundamental to understanding individual perceptual and cognitive processing during multisensory speech perception. However, during multisensory integration between auditory and visual streams, how periodic and aperiodic activity sculpts inter-individual and inter-trial differences in multisensory perception remains largely unknown. In this EEG study, we discovered that lower aperiodic offset and exponent (slope) values in central, parietal, and occipital regions predicted illusory responses. Using statistical interaction models, we further show that the mechanisms of susceptibility to illusory speech perception arise from the significant interplay between aperiodic background activity and oscillatory features. This interplay between periodic and aperiodic activity accounts for inter-trial variability in the perception of the McGurk illusion.