Functional MRI and magnetoencephalography (MEG) studis of the neural basis of lipreading
Seeing the articulatory gestures of a speaker significantly enhances speech perception. Recent advances in magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI), such as cortically constrained fMRI-weighted linear inverse estimation methods, are making it possible to study the neural basis of audiovisual interactions in speech perception. Findings from recent MEG studies suggest that visual stimuli influence auditory cortex already ~100 ms from stimulus onset and given that articulatory gestures typically precede speech sounds by a few tens of milliseconds it is possible that visual stimuli modulate auditory processing already at relatively early processing stages. fMRI studies suggest that a network of cortical areas is activated during speech perception, including the primary auditory cortex, posterior superior temporal sulcus, and speech motor areas. Tentatively, it is possible that activation of the speech motor system during lipreading enhance speech perception by tuning, in a top-down fashion, speech-sound processing in the superior aspects of the posterior temporal lobe. Anatomically, the superior-posterior temporal lobe areas receive connections from the auditory, visual, and speech motor cortical areas. Thus, it is possible that neuronal receptive fields are shaped during development to respond to speech-sound features that coincide with visual and motor speech cues, in contrast with the anterior/lateral temporal lobe areas that might process speech sounds predominantly based on acoustic cues. The superior-posterior temporal lobe areas have also been consistently associated with auditory spatial processing and the involvement of these areas in audiovisual speech perception might partly be explained by the spatial processing requirements when associating sounds, seen articulations, and one?s own motor movements. In this talk, empirical findings relating to this hypothesis are reviewed along with a brief overview of combined MEG/fMRI methodology.