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 McGurk effect arises during phonetic processing because the integration of audio and visual information happens early in speech perception. The McGurk effect is very robust; that is, knowledge about it seems to have little effect on one's perception of it. This is different from certain optical illusions, which break down once one 'sees through' them. 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. With the exception of people who can identify most of what is being said from speech-reading alone, most people are quite limited in their ability to identify speech from visual-only signals. A more extensive phenomenon is the ability of visual speech to increase the intelligibility of heard speech in a noisy environment. Visible speech can also alter the perception of perfectly audible speech sounds when the visual speech stimuli are mismatched with the auditory speech. 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. Speech is perceived by all of the senses working together (seeing, touching, and listening to a face move). The brain is often unaware of the separate sensory contributions of what it perceives. Therefore, when it comes to recognizing speech the brain cannot differentiate whether it is seeing or hearing the incoming information.