Mirrored-self misidentification: Two cases of focal onset dementia

We present two patients in whom the mirror sign, the inability to recognize one's own reflected image, was a stable and persisting symptom signalling the onset of a progressive dementing illness. Extensive neuropsychological testing was conducted with both patients, with particular emphasis on face processing and the understanding of reflected space. Both patients were also investigated with structural imaging techniques (computed tomography and magnetic resonance imaging). Although the neuroimaging results were not strongly lateralizing for either patient, neuropsychological testing revealed striking right hemisphere dysfunction with relatively intact left hemisphere cognitive function in both patients. Of particular interest was the patients' dissociation on tests of face processing; one patient, FE, had significant face processing deficits while the other patient, TH, had relatively intact face processing. Further testing with TH revealed striking deficits in his ability to interpret reflected space. The results of the face processing tests are discussed in the context of current models of normal face processing, with particular emphasis on the affective component in face recognition. We propose that a combination of cognitive deficits underlie the mirror sign delusion, including perceptual, affective and reasoning impairments, and also discuss the contributions of cortical and subcortical lesions in these two patients and in delusions in general.