The term "mirror sign" has been applied to the phenomenon of misrecognition of self as another when seen in a mirror. It may be classified with the delusional misidentification syndromes. This may occur in Alzheimer’s disease and frontotemporal dementia, and is associated with impaired cognition, confabulation, and prefrontal dysfunction. It may lead to a patient complaint of an intruder or a stranger living in the house ("phantom boarder" syndrome).
Some authors believe "the phenomenon of the mirror" to be an extreme example of prosopagnosia, but other studies have not found an association.
Caixeta LF, Caramelli P, Bahia V, Buchpiguel CA, Nitrini R. Clinical and neuroanatomical correlates of the mirror sign in frontotemporal dementia and Alzheimer’s disease. Neurobiology of Aging 2000; 21(suppl1): S217 (abstract 988)