The Uncanny Valley
The Uncanny Valley was introduced by Japanese roboticist Masahiro Mori in 1970, as a term referring to the hypothetical state in which a robot, avatar, or other non-human humanoid facsimile is at first received in an increasingly positive and empathic way from interacting humans. However, as it becomes even more realistic, it hits a 'valley', a point where positive response tails off dramatically, and responses of unease, revulsion, and even fright are exhibited instead.
Uncanny valley draws heavily from an essay by Sigmund Freud entitled "The Uncanny". This is in turn based upon Ernst Jentsch's concept of the uncanny, identified in a 1906 essay, "On the Psychology of the Uncanny."
This extract from The Uncanny delves to the root of the problem: An impression of automation or of not quite real is developed in the mind of the viewer. A more modern example would be the zombie, where the undead's face and bodily movement is slightly off, and thus not quite 'real'. The additional effort spent in working out why something is not quite real, done entirely unconsciously, is what sparks the fear or revulsion it provokes.
When a face or a body pushes past the uncanny valley, and becomes as realistic as the unconscious mind perceives it should be, these negative emotions rapidly vanish and become positive and emotive once again.
A related condition to uncanny valley exists for the auditory sense. This is known as the McGurk Effect. First described by Harry McGurk in 1976, the McGurk Effect is a visual/auditory phenomenon that occurs when watching lips move and hearing sounds that sound like speech.
A strange feedback loop is created whereby if the speech does not match the lip movement, even if the speech is perfectly clear, the brain will combine the lip movement and sound, and come up with sounds halfway in between what is heard and the lip movement seen.
So for example, if you hear the sound "BA" and see someone's lips move to pronounciate "GA", if you are like most people, you will 'hear' "DA".
Sigmund Freud's "The Uncanny"
VWN Virtual Dictionary: Uncanny Valley
VWN Virtual Dictionary: McGurk Effect