![]() ![]() In people with lesions to the left hemisphere of the brain, visual features often play a critical role in speech and language therapy. In people that have had callosotomies done, the McGurk effect is still present but significantly slower. A McGurk response is more likely to occur in right-handed individuals for whom the face has privileged access to the right hemisphere and words to the left hemisphere. They work together to integrate speech information that is received through the auditory and visual senses. Factors Internal Brain damage īoth hemispheres of the brain make a contribution to the McGurk effect. Research into this area can provide information on not only theoretical questions, but also it can provide therapeutic and diagnostic relevance for those with disorders relating to audio and visual integration of speech cues. Not limited to syllables, the effect can occur in whole words and have an effect on daily interactions that people are unaware of. Wareham and Wright's 2005 study showed that inconsistent visual information can change the perception of spoken utterances, suggesting that the McGurk effect may have many influences in everyday perception. It has also been examined in relation to witness testimony. Therefore, when it comes to recognizing speech the brain cannot differentiate whether it is seeing or hearing the incoming information. The brain is often unaware of the separate sensory contributions of what it perceives. Speech is perceived by all of the senses working together (seeing, touching, and listening to a face move). Normally, speech perception is thought to be an auditory process however, our use of information is immediate, automatic, and, to a large degree, unconscious and therefore, despite what is widely accepted as true, speech is not only something we hear. ![]() Visible speech can also alter the perception of perfectly audible speech sounds when the visual speech stimuli are mismatched with the auditory speech. A more extensive phenomenon is the ability of visual speech to increase the intelligibility of heard speech in a noisy environment. With the exception of people who can identify most of what is being said from lip reading alone, most people are quite limited in their ability to identify speech from visual-only signals. Some people, including those that have been researching the phenomenon for more than twenty years, experience the effect even when they are aware that it is taking place. This is different from certain optical illusions, which break down once one "sees through" them. The McGurk effect is very robust that is, knowledge about it seems to have little effect on one's perception of it. The McGurk effect arises during phonetic processing because the integration of audio and visual information happens early in speech perception. Vision is the primary sense for humans, but speech perception is multimodal, which means that it involves information from more than one sensory modality, in particular, audition and vision. The information coming from the eyes and ears is contradictory, and in this instance, the eyes (visual information) have had a greater effect on the brain, and thus the fusion and combination responses have been created. This is the brain's effort to provide the consciousness with its best guess about the incoming information. Two types of illusion in response to incongruent audiovisual stimuli have been observed: fusions ('ba' auditory and 'ga' visual produce 'da') and combinations ('ga' auditory and 'ba' visual produce 'bga'). McGurk and MacDonald originally believed that this resulted from the common phonetic and visual properties of /b/ and /g/. As an example, the syllables /ba-ba/ are spoken over the lip movements of /ga-ga/, and the perception is of /da-da/. Often, the perceived phoneme is a third, intermediate phoneme. This effect may be experienced when a video of one phoneme's production is dubbed with a sound-recording of a different phoneme being spoken. When the video was played back, both researchers heard a third phoneme rather than the one spoken or mouthed in the video. This effect was discovered by accident when McGurk and his research assistant, MacDonald, asked a technician to dub a video with a different phoneme from the one spoken while conducting a study on how infants perceive language at different developmental stages. It was first described in 1976 in a paper by Harry McGurk and John MacDonald, titled "Hearing Lips and Seeing Voices" in Nature (23 December 1976). ![]()
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