Oliver Sacks,M.D.writes that hallucinations are “percepts arising in the absence of any external reality – seeing things or hearing things that are not there.”  They tend to be startling, mostly beyond conscious control; often detailed, colorful, bizarre, exotic, and meaningless.  Some hallucinations are fleeting and some people live with hallucinations all their lives.  People experiencing continuing hallucinations can usually accommodate them and differentiate them from reality, though hallucinations can interfere with activities such as driving.

The book excludes schizophrenia; that is a separate topic.  These hallucinations are commonly associated with sensory deprivation (loss of sight or hearing) or monotony (long-distance truck drivers, sailors, and pilots; or marathon athletes); “mystagogic” drugs; or with illnesses such as epilepsy or fever.  Psychotropic drugs can relieve hallucinations or bring them on.  Sacks notes that many people try to induce hallucinations through, for example, sleep deprivation, darkness, solitude, or meditation.  Resulting hallucinations may have religious or personal significance.

Sacks says hallucinations are more common than generally believed.  In one study, 80% of elderly patients with vision loss had simple hallucinations (shapes, colors, and patterns) and 15% had complex hallucinations (people, animals, and complete scenes.)  Anyone can experience transient hallucinations and many of us will at some point in our lives.  Sufferers tend to hide their experiences, fearing others will think they are crazy or demented.  “Hearing voices” is especially associated by the public with madness, but that is not usually true.

The book presents many detailed descriptions of individuals’ hallucinated sights, voices, music, and even smells.  After a while, these descriptions can get tedious, but it does demonstrate the wide variety of experiences.  The lack of interaction, plot, or meaning to most hallucinations reminds me strongly of “real life” ghost stories, which also seldom make sense.

Studies using brain scans made during hallucinations confirm that the brain is doing something different than when it “imagines” an event.  Hallucinations may be all in your head, but they are real events in the brain.  It is interesting to learn that different parts of the brain seem quite specialized to recognizing, for example, faces, text, or music.

Chapters have descriptive titles, such as visual migraines, altered states, Doppelgangers, Parkinsonism, and phantoms.  You can skip around to the aspects of hallucinations that most interest you.

The book is easy to read, with only minimal technical jargon.  It presents an important message.  If you or someone you know has hallucinations, they are probably not crazy.  At least, not because of hallucinations.