Capgras' Syndrome

Second in an occasional series on bizarre neurological disorders...

Capgras' syndrome is a neurological syndrome where the patient will typically identify people close to them as being imposters - identical in every possible way, but identical replicas. Classically, the patient will accept living with these imposters but will secretly "know" that they are not the people they claim to be. When Capgras' occurs secondary to other disorders such as the schizophrenias, the experience can take on a frightening dimension along the lines of "Invasion of The Body snatchers".

Neurologically speaking, Capgras' Syndrome occurs when the kinesthetic component of facial recognition is damaged. The result being that the strategy of facial recognition no longer has the internal kinesthetic on which to exit. Thus the patient gets caught up in an eternal loop in his strategy and needs to form an explanation (the delusion) in order to understand why this experience should occur.

More information can be found here