Scientific American has an interesting article on the hallucinations that people experience when they mourn the death of a loved one. Apparently such hallucinations are the most common form of psychosis seen in otherwise mentally normal people.
Mourning seems to be a time when hallucinations are particularly common, to the point where feeling the presence of the deceased is the norm rather than the exception. One study, by the researcher Agneta Grimby at the University of Goteborg, found that over 80 percent of elderly people experience hallucinations associated with their dead partner one month after bereavement, as if their perception had yet to catch up with the knowledge of their beloved’s passing. As a marker of how vivid such visions can seem, almost a third of the people reported that they spoke in response to their experiences. In other words, these weren’t just peripheral illusions: they could evoke the very essence of the deceased.