A research conducted at the Basque Center on Cognition, Brain and Language –recentlly published in JNeurosci– found that brain waves synchronize with lip-reading as it does with sound waves.
Our auditory cortex activates when listening a sound such as speech to synchronize with the rhythm of incoming sound waves. In the case of comprehending unintelligible speech through lip-reading, it is still not known how the brain process sound.
Mathieu Bourguignon and colleagues from the Basque Center on Cognition, Brain and Language conducted a study in healthy adults. The volunteers watched a silent video of a woman speaking or they listened a story, while their brain activity was measured through magnetoencephalography. The results showed that their auditory cortices synchronized with the speech produced by the woman in the video, even though they could not hear it.
The waves synchronized similarly in those who watch the soundless video and those who listened the story. Therefore, the brain can obtain auditory information from the visual information available through lip-reading.
The Spanish team think the visual cortex may synchronize with lip movement; and then, that signal is sent to other brain areas where it is translate into sound information, creating the sound wave synchronization.
Source: Science Daily