Mirror, mirror on the wall, who I see is not me at all.
When you look into a mirror, who do you see? Yourself?

Not if you have Mirrored-self misidentification, a delusional belief that your reflection in a mirror belongs to a stranger’s. The stranger just happens to look like you.
The disorder might be because of mirror agnosia, an inability to properly interact with a mirror. There might also be a deficit in facial reading and identification involved.

What is really interesting is that this delusion can be induced in a laboratory via hypnosis. One study took participants and hypnotized them, urging them that when they looked into a mirror, they would either see a stranger’s reflection or not be able to recognize the person. Half of the participants received the suggestion when hypnotized, and the other half received it when fully awake. Needless to say, those hypnotized with the suggestions suffered from the mirror delusion later.

One patient, TH, was known to also have mirror agnosia. That is, he didn’t understand how mirrors worked. For example, when he looked into a mirror and an object was held up behind him and reflected in the mirror, TH was asked to touch the object.
Guess what he did?
He put his hand to the mirror, and then put his hand behind the mirror, in an attempt to ‘touch the object’. He never attempted to reach behind his shoulder.

TH said his reflection was a “dead ringer” for himself. He even attempted to speak to the reflection, as if it were another person. Since the reflection never replied, TH assumed the reflection had lost his voice.

TH was then asked about the reflection’s personality. He said that the person didn’t give him any reason to be weary of him. And when asked where the reflection loved, TH said he lived in an apartment next to his own apartment.

Another patient, FE, suffered from Mirrored-self misidentification, but not mirror agnosia. He did however have facial processing deficits. When he looked into a mirror, his perception of his face was no different from his memory of his face that he was unable to belief that the reflection was his.

Interestingly, both TH and FE were found to have developed dementia. It seems that Mirrored-self misidentification is found in a number of dementia cases. A study indicated that a number of dementia cases were unable to fulfill the reach-over-the-shoulder task assigned to patient TH. However, not all Mirrored-self misidentification patients are dementia cases. Schizophrenia and stroke patients can also exhibit the delusion. But even these patients are not most of the delusion cases.

In the end, it was found that to TH, a mirror was just a window, and since anyone you see through a window is not yourself and is apart from yourself. The person is a part of the world, separate from you. Therefore, the reflection is not you; it just cannot be.