Hallucinations, by Oliver Sacks


In Hallucinations, neurologist and prolific author Oliver Sacks weaves his own case histories and those of other physicians into essays about the various neurological conditions that can give rise to hallucinations. The term “hallucinations,” as used here, is not limited to the visual realm, but includes all of the senses—the usual five plus the sense of proprioception, which is the awareness of one’s own body, its position, what it’s doing, etc.

A hallucination differs from a true or accurate perception, a misinterpretation of a perception (e.g., an optical illusion), or something that occurs only in the mind and that one is aware occurs only in the mind (e.g., something imagined or dreamt). Thus a hallucination is a pseudo-perception created in the mind that does not correspond to anything outside the mind, and yet is experienced by the hallucinator as if it were a normal perception that corresponds to and is caused by something outside the mind.

The conditions Sacks discusses include Charles Bonnet syndrome (which is a tendency of blind people and people with visual impairment to have visual hallucinations), sensory deprivation, conditions that give rise to hallucinatory smells, conditions that give rise to hearing voices or other auditory hallucinations, Parkinson’s syndrome, drugs like mescaline or opium that can give rise to hallucinations, migraines, epilepsy, hemianopia (damage to the part of the brain that processes vision, caused by, for instance, a stroke), delirium, hypnogogic and hypnopompic hallucinations (hallucinations that occur when you’re half-awake, either falling asleep or awakening), narcolepsy, flashbacks or compulsive returns to something emotionally powerful in one’s past, out-of-body experiences and other conditions where one thinks one can perceive oneself or a doppelgänger of oneself from the outside, and phantom limb syndrome and other hallucinations of the body image.

It’s refreshing that Sacks sticks to neurological explanations rather than humoring the more fantasy-prone of his readers by suggesting that hallucinations might be accurate perceptions after all, of divine, supernatural, or extraterrestrial phenomena. (He notes that hallucinations have been interpreted in these ways, and he doesn’t claim that it can somehow be proven that such interpretations are always false, but he doesn’t state or imply that they need to be taken seriously as realistic possibilities.)

I get the impression from the book that hallucinations are considerably more common than I would have guessed. Phantom limb syndrome is experienced by virtually everyone who has a limb amputated, for instance.

Sacks makes the point that hallucinations tend to be underreported because so many people fear they’ll be regarded as crazy if they mention them. (Though, again, depending on one’s cultural and social circumstances, to proclaim them and insist they’re aliens or something religious could actually be socially advantageous.) This phenomenon is exacerbated by the fact that so many of these conditions occur more frequently in the elderly or disabled, and those populations are especially concerned that they’ll be institutionalized and lose their freedom if they’re regarded as mentally unsound.

So how often you hear about hallucinations is likely much less than how often they actually occur.

As I read through the descriptions of the hallucinations, I picked up on certain patterns: (Note that these are just rules of thumb, just things that are true of most hallucinations of most types. As is explained in the book, there are exceptions.)

• When people are hallucinating, they usually are aware they are doing so; they don’t mistake the hallucinations for reality. (This does not contradict the definition stated earlier. The hallucinations are experienced as if they were perceptions, but the people experiencing them are aware—usually—that they are in fact only in their mind.)

• Unlike in dreams, people hallucinating are typically purely observers rather than participants in what they hallucinate.

• The subject matter in hallucinations typically is not of emotional importance to the person hallucinating, nor, again unlike in dreams, does it relate in any discernible way to recent experiences the person had or to something of likely significance in their unconscious mind. It’s seemingly neutral, random stuff.

• People in hallucinations usually do not in any way seek to interact with or even acknowledge the person having the hallucination. They just go about their business doing whatever it is they’re doing.

• Hallucinations typically are not fuzzy or hard to make out or fading in and out. They tend to be at least as clear, distinct, and unambiguous as ordinary perceptions.

• Unlike when you’re just thinking about something or imagining something, you typically have little or no ability to will hallucinations to start, stop, or change. They come into your mind uninvited and are out of your control.

What are perhaps the most intriguing hallucinations are those exceptional types that the person hallucinating does not in fact recognize as hallucinations but is convinced are normal and accurate perceptions. Or maybe not “normal” since it is these that can be bizarre enough to give rise to religious and paranormal claims about angels, fairies, aliens, ghosts, etc., but accurate anyway.

My impression from Hallucinations is that typically either hallucinations are self-evidently unreliable—where the hallucinators never are fooled, never think the hallucination is real, any more than someone watching TV would think there are really little people inside the box in one’s home—or, less often, the hallucinators are sure the hallucinations are real and their conviction cannot be shaken. I’m surprised that the middle ground—that the hallucinators’ initial impression is that they are real, but then upon reflection they realize that they are not—seems to be rare.

It happens, there are examples of it in the book, but it seems to be very much the exception that people can be “reasoned out” of their hallucinations with evidence.

One example might be the case in the movie A Beautiful Mind, where the protagonist realizes the people he’s been seeing for years can’t be real because “they don’t age.” So they seem real, but other considerations establish for the person having the hallucination that they are not real.

That I can understand. But what’s bizarre is when hallucinators are not amenable to persuasion like that. A particularly striking case Sacks cites here—and which he first wrote about in The Man Who Mistook His Wife for a Hat—is that of the man who “threw himself out of bed.”

The man woke up one night to discover that someone had placed a human leg in his bed next to him, presumably as some kind of practical joke. Maybe it was a leg from a cadaver, maybe it was some very, very lifelike artificial leg, but whatever it was was altogether creepy and he didn’t want it in his bed. So in horror and disgust he grabbed it and flung it away from him, out of bed. Much to his surprise he ended up on the floor with it.

Turns out it was his own leg. That in and of itself is plenty weird, that one’s sense of proprioception could be that out of whack that you wouldn’t recognize your own leg as being part of you, but what seems weirder to me is that he couldn’t be talked out of his hallucination.

So even when Sacks calmly showed him how the leg that gave him the willies was positioned precisely where one would expect his own leg to be, that it was attached to his body, that he had only one other leg, that the leg he perceived as foreign was identical to the leg he acknowledged as his own in terms of size, shape, coloration, hairiness, etc., except that one was a right leg and one was a left leg, he still kept insisting that it absolutely wasn’t his leg.

You’d think his reaction would be, “Well yeah, obviously that’s my leg. How very curious it is that it doesn’t feel at all like it’s my leg.” But no, he remained utterly convinced despite the evidence that that wasn’t his leg.

I’m somewhat less puzzled by the people who won’t give up their fanciful interpretations of their experience. A person typically can’t be shown in nearly as definitive a way as in the leg case that he or she wasn’t visited last night by a ghost, or an angel, or a little gray alien. You can explain to the person about hypnogogic or hypnopompic hallucinations in the abstract, but that’s not like being able to show them that their leg that they can examine for themselves is obviously their leg. So I’m not surprised that people who maybe have some emotional need or preference for supernatural explanations will cling to their interpretation that God spoke to them or whatever.

Another thing that that makes me think about, though, is if someone were indeed fully committed to rationality and to going with whatever beliefs were best supported by the evidence, when would it ever be justified to believe one had had direct experience of the supernatural?

I’m used to people believing in paranormal stuff either because they believe what others say about it, or because they had a genuine experience that they misinterpreted (e.g., they saw a light in the sky and inferred it was an extraterrestrial craft of some kind, or they heard the creaks of their house settling and inferred there was a ghost walking about). But a hallucination is different. As noted, a hallucination isn’t akin to an optical illusion, like the light in the sky or creaking house examples. It’s not a misinterpretation of something real; it’s not real.

But how then could you distinguish between a hallucination and something paranormal and real?

You’re lying in bed one night, and you look up and a figure very much like Jesus is depicted in books materializes from thin air and speaks directly to you, identifying himself.

Assuming he doesn’t make it easy, like by mentioning in conversation with you the solution to some mathematical puzzle that’s never been solved, how would you know if you’d been visited by Jesus? And if you did conclude that the experience was a hallucination, how could you be sure? If there really were a supernatural realm, and the real Jesus did periodically reveal himself to people, how would what happened to you not match that?

I assume it’s not that puzzling to someone used to the scientific method. There wasn’t anyone else in your bedroom who could confirm the perception of Jesus, and Jesus’ appearance hasn’t been replicated subsequently, so the most justified conclusion is that it never happened.

I suppose I’ve always thought of that more as the proper way to reason it out if you had not had the experience yourself and you had to assess someone else’s claim that they had. But given the prevalence of hallucinations, I guess the reasoning applies just as well if you’re the one who had the experience. It’s just a little harder to swallow in that case. I can much more readily relinquish a belief in the face of superior evidence if we’re talking about something I believe based on what I’ve been told, but a part of me finds it rather disconcerting that even if I seemingly have a direct experience of something I may not be right in accepting it.

Maybe the way to look at it is that nothing is truly a direct, uninterpreted experience. Even the basic causal hypothesis that the perception you have of something is caused by the real version of that something outside of you is itself a (fallible) inference from your experience.

Sometimes seeing is not believing, or shouldn’t be anyway.


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