The phenomenon that a person is not able to recognize faces anymore, but only a nose, eyes or mouth, is called prosopagnosia. Often, people with prosopagnosia are only able to recognize certain characteristics like bushy eyebrows, beard and mustache, or jewelry, fragrance, voice or typical walk etc. Sometimes even a face is not even recognized as such.
The opposite case is that someone will recognize a known person, but misses the feeling of familiarity or the feeling that you know someone.
For example, a person believes that a person is replaced by a double. This latter phenomenon may also occur if there is no brain damage but a mental illness; the syndrome of Cargas.
The problem with prosopagnosia is based on a defect in the winding: fusiform gyrus (see drawing) at the bottom of the brains.