Ghost Stories: Visits from the Deceased: Scientific American

The dead stay with us, that much is clear. They remain in our hearts and minds, of course, but for many people they also linger in our senses—as sights, sounds, smells, touches or presences. Grief hallucinations are a normal reaction to bereavement but are rarely discussed, because people fear they might be considered insane or mentally destabilised by their loss. As a society we tend to associate hallucinations with things like drugs and mental illness, but we now know that hallucinations are common in sober healthy people and that they are more likely during times of stress.