The McGurk Effect is a perceptual illusion in which what you see affects what you hear. When what you see conflicts with what you hear, you may hear a third phoneme that is mixture of the other two phonemes. If the video shows a person saying “ga” and the audio plays “ba” you may hear a third, intermediate phoneme, “da”. This perceived intermediate phoneme is the brain’s way of satisfying the conflicting audio and visual inputs.
This illusion is known to persist even once one learns about it. While in some visual illusions one can “see through” the illusion and the illusion is broken down, one will still perceive the third phoneme if presented with conflicting audio and visual inputs.
There are many examples of the shepard’s tone used in musical compositions before it was even discovered or named. It can create a sense of rising excitement and energy in the music, but can also create an eerie and unsettling tone.
In this musical excerpt from Tchaikovsky’s Symphony No. 6 in B Minor (Pathetique), we can hear an example of the shepard’s tone illusion before it was recognized as such. In the score we can see that only one octave is used in the scale, but when we listen to the music we can hear the illusion that the notes seem to continue above an octave.
(from Leonard Bernstein’s The Infinite Variety of Music)
Passage from Alban Berg’s 1925 opera Wozzeck
“In his 1925 opera Wozzeck, Berg employed a continuously rising scale that was orchestrated in such a way that the upper instruments faded out at the top of their range while the lower instruments faded in at the low end.” (Diana Deutch)