P141 Contribution of the motor system to McGurk effect-event-related fMRI and TMS studies

Question ”McGurk effect” is known as an auditory illusion caused by interference from unmatched visual inputs. For example, a combination of auditory/Pa/ and visual/Ka/ results in the perception of a third syllable of/Ta/. Recent neuroimaging studies proposed that left posterior superior temporal sulcus (pSTS), a hub region of audiovisual integration, is concerned with McGurk effect. However, the mechanisms still remain under debate. Methods 27 right-handed healthy volunteers received audiovisual incongruent (auditory/Ba//Pa/ and visual/Ga//Ka/) and congruent (both/Ba//Pa/) perceptual tasks, and then they were asked the syllables listened to. We investigated activated brain regions in event-related functional magnetic resonance imaging (fMRI) study. In addition, we applied single-pulse transcranial magnetic stimulation (TMS) over the lip or foot area of the primary motor cortex (M1) in an event-related design and we measured McGurk susceptibility. Results Behaviorally, audiovisual incongruent trials resulted in higher illusion susceptibility and longer reaction time than congruent trials. FMRI study revealed brain activations in audiovisual areas, dorsal premotor cortex (dPMC) and inferior frontal gyrus (IFG), bilaterally in both audiovisual stimuli. Activations in dPMC and IFG were more prominent in incongruent task, whereas stronger activations of the pSTS in congruent task. Left IFG activation correlated negatively with McGurk susceptibility. Psychophysiological interaction analysis demonstrated increased effective connectivity between the left IFG and M1 lip area in incongruent task. Event-related TMS over M1 lip area showed significant reduction of McGurk susceptibility when compared with TMS over M1 foot area or the control condition (no TMS). Conclusions We showed that the motor system contributes to recognition of audiovisual inputs and it is more active in incongruent condition, while the pSTS is more active in congruent condition. The left IFG-M1 network plays some roles in detecting and resolving multisensory incompatibility to reduce illusion.