Speech motor system may mediate visual information to auditory cortex during silent speech reading
While the sense of hearing is clearly the dominant channel for speech perception, humans are surprisingly good at reading the lips of one’s conversation partners, a phenomenon referred to as speech reading. This ability has been demonstrated already in early psychophysics studies to significantly enhance speech perception, especially when speech is to be perceived under noisy conditions. There is a fairly good amount of neuroimaging literature on the underlying neural mechanisms. In these studies, visual speech stimuli (i.e., articulatory gestures) have been reported to modulate auditory cortical processing, with some evidence pointing to speech motor system first being activated by visual speech and then influencing auditory-cortical processing via an efference copy.
In their recent study, Chu et al. (2013) studied the neural basis of speech reading by presenting 19 healthy volunteers with silent videoclips of a person articulating vowels during event-related functional magnetic resonance imaging (fMRI). Speech reading activated a wide range of occipital, temporal, and prefrontal cortical areas. The authors used structural equation modeling to estimate information flow during speech reading between the activated areas. The results suggested that there is parallel information flow from extrastriate areas to anterior prefrontal areas and, further, feedback information flow from the anterior prefrontal areas to posterior-superior temporal lobe auditory areas. These effective connectivity estimates thus support the model wherein speech reading influences auditory-cortical areas via prefrontal speech motor areas, possibly in the form of an efference copy that might facilitate speech perception.
Reference: Chu Y-H, Lin F-H, Chou Y-J, Tsai K W-K, Kuo W-J, Jaaskelainen IP. Effective cerebral connectivity during silent speech reading revealed by functional magnetic resonance imaging. PLoS ONE (2013) 8: e80265. http://dx.doi.org/10.1371/journal.pone.0080265