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Robert (his real name) was having an especially trying week. Hospitalized for a heart attack, early one morning he started hearing puppies barking outside his door. A few minutes later, he heard puppies inside his room. Finally he saw dozens of cute, cuddly, white puppies floating around his bed, frolicking in the air. He felt them nipping at his toes.
Robert didn't add a bowl of dog food to the egg white omelet on his breakfast order. He called his nurse and asked if he could be taken off morphine. When the morphine drip stopped. the cute, cuddly, floating puppies abruptly disappeared a few hours later.
Robert's experience was not unique. Many kinds of intoxication, illness, and injury can lead to hallucinations of sight and sound and even hallucinations involving taste, smell, and touch. But when is seeing, hearing, smelling, tasting, or feeling things that aren't there a sign of mental illness, and when is a hallucinatory experience a sign something is going on in the brain?
Hallucinations versus Delusions
Simply sensing something that is not objectively, physically present is a hallucination. A hallucination is not necessarily the sign of a mental illness.
People who experience hallucinations but who retain insight, such as Robert, for instance, who did not believe puppies were actually floating through his hospital room, know that their hallucinations are not real and act accordingly.
People who experience delusions, however, take their hallucinations seriously. Had Robert been delusional, for example, he might have ordered that bowl of dog food to feed his new canine friends. Or he may asked if the floating puppies had had their rabies shots. Or he might have demanded a dog-free room.
People who suffer hallucinations might, for instance, see a tiny man on their shoulder (something beyond Robert's experience with morphine). People who suffer delusions might take orders from the tiny man or try to buy him a tiny suit or a tiny pipe for his tobacco.
Hallucinations are an unexpected sensory experience. Delusions are an indicator of mental illness.
Hallucinations versus Imagination
Hallucinations can also be distinguished from imagination. The identifying characteristic of imagination is that is expected. Imagination is self-directed. If you mourn the death of your grandmother, for example, and when you intentionally think about her, even if you "see" your deceased grandmother, the activity is more likely to be imagination than hallucination.
If, on the other hand, you come home after work and find your deceased grandmother and her deceased best friends playing pinochle, you are likely suffering a hallucination. And if you order them a pizza, you are likely to be delusional.