Discovered by accident by the psychologist Harry McGurk and his research assistant John MacDonald, the McGurk effect was described for the first time in a paper written in 1976. The effect demonstrates the connection between hearing and vision in speech perception; it is based on the illusion that occurs when a sound is paired with a visual component of another sound, leading the person to perceive a third sound. This effect can be produced by making a video of someone speaking a phoneme and then dubbing it with the recording of another phoneme spoken; for example, when the syllables /ba-ba/ are spoken over the lip movement of/ga-ga/ we could perceive the syllables as /da-da/; the video in this page shows that even the visible speech can also alter the perception of audible speech sounds when the visual speech stimuli are not matched with the auditory speech: if we keep the same original sound, but we change video of the lip movement, the sound will appear different, following the visual perception.
The effect demonstrates that the speech perception is not just an auditory process, but it’s something that takes information from our unconscious. Also the brain is not always aware of the separate sensory contributions of what it perceives, therefore it can’t always differentiate whether it is seeing or hearing the incoming information.
This phenomenon is very peculiar and it gets even more intriguing, because it seems to change for people who suffer from mental disorders: researchers have discovered that people suffering from autism spectrum disorder (ASD), Alzheimer’s disease, schizophrenia, dyslexia or aphasia have exhibited a small McGurk effect. For people with ASD it has been suggested that the weakened effect is due to deficits in identifying both auditory and visual components of speech; people with Alzheimer’s disease often have a reduced size of the corpus callosum that produces a hemisphere disconnection process, this condition minimise the influence on visual stimulus, reason for the lowered effect. The McGurk effect seems to be less pronounced in people who suffer from schizophrenia, phenomenon attributed to the slower development of audio-visual integration that doesn’t allow it to reach its developmental peak; schizophrenics often rely on auditory cues more than visual cues in speech perception.
This is a very interesting research which shows that the McGurk effect is also stronger when the right side of the speaker’s mouth is visible.