Audiovisual Speech Integration Does Not Rely on the Motor System: Evidence from Articulatory Suppression, the McGurk Effect, and fMRI

Visual speech influences the perception of heard speech. A classic example of this is the McGurk effect, whereby an auditory /pa/ overlaid onto a visual /ka/ induces the fusion percept of /ta/. Recent behavioral and neuroimaging research has highlighted the importance of both articulatory representations and motor speech regions of the brain, particularly Broca's area, in audiovisual (AV) speech integration. Alternatively, AV speech integration may be accomplished by the sensory system through multisensory integration in the posterior STS. We assessed the claims regarding the involvement of the motor system in AV integration in two experiments: (i) examining the effect of articulatory suppression on the McGurk effect and (ii) determining if motor speech regions show an AV integration profile. The hypothesis regarding experiment (i) is that if the motor system plays a role in McGurk fusion, distracting the motor system through articulatory suppression should result in a reduction of McGurk fusion. The results of experiment (i) showed that articulatory suppression results in no such reduction, suggesting that the motor system is not responsible for the McGurk effect. The hypothesis of experiment (ii) was that if the brain activation to AV speech in motor regions (such as Broca's area) reflects AV integration, the profile of activity should reflect AV integration: AV > AO (auditory only) and AV > VO (visual only). The results of experiment (ii) demonstrate that motor speech regions do not show this integration profile, whereas the posterior STS does. Instead, activity in motor regions is task dependent. The combined results suggest that AV speech integration does not rely on the motor system.