Hallucinations – beyond psychiatric disease

To live on a day-to-day basis is insufficient for human beings; we need to transcend, transport, escape; we need meaning, understanding, and explanation; we need to see overall patterns in our lives. We need hope, the sense of a future. And we need freedom (or at least the illusion of freedom) to get beyond ourselves, whether with telescopes and microscopes and our ever-burgeoning technology or in states of mind which allow us to travel to other worlds, to transcend our immediate surroundings. We need detachment of this sort as much as we need engagement in our lives.

Oliver Sacks, Hallucinations

An anecdote: As a medical student, I’ve had the opportunity to go through a psychiatry rotation. One patient that perhaps stood out was a very polite 60-year-old man who spoke with admirable patience to all 20 students about his hardship ever since his wife had passed away. Following a set of common screening questions, he was asked whether he recalled ever seeing anything bizarre, or that wasn’t actually there.

Reluctantly, he admitted it had happened once. He recounted how, a few years prior, he was in the hospital just waking up after a surgical procedure. As he stood up at the edge of the bed, still a bit dizzy from the anesthetic, he looked down and saw several bugs crawling on the floor.

He tried stepping on them, nervous, until his roommate caught his attention and asked what was wrong. “Can’t you see, the bugs, they’re everywhere! And in a hospital, this is unacceptable...” he continued muttering. A nurse was called, and about 20 minutes after administering a sedative, all the bugs were gone. All that was left was the intricate mosaic on the floor.

So, what was wrong? Was this person mentally ill? Why was he not immediately admitted to a psychiatric ward, and, in fact, why was no one too alarmed by his hallucinations? Simply put, because hallucinations mean more than psychiatric disease and hallucinogenic substances. In fact, various studies evaluating the prevalence of hallucinations bring out extremely varied results, with some suggesting that up to 30% of the general population may experience hallucinatory events of some form during their lifetime. 2,3,4 These intriguing neuro-psychiatric phenomena may be challenging to understand, but, ultimately, they have to have some sort of underlying mechanism. A mechanism that can, in turn, be triggered by various incentives – psychiatric disease, of course; hallucinogens, also. But what else? And how, exactly?

One book of reference in tackling hallucinations rather as a phenomenon, a syndrome, and not just a corollary of psychosis and schizophrenia, is Oliver Sacks’ Hallucinations. Let’s dissect a few of his views and most illustrative cases:

In general, people are afraid to acknowledge hallucinations because they immediately see them as a sign of something awful happening to the brain, whereas in most cases they’re not.

Oliver Sacks, Hallucinations

What the previously described patient was experiencing was a potential side-effect of suffering major surgery. 

Postoperative delirium, a not-so-rare occurrence, especially for the elderly, is an alteration of the mental status following surgery. Some patients may only experience a slight and short-lived lack of attention or comprehension, while others may become agitated and hallucinate. 5

Major surgery is by no means the only stressful medical situation that can trigger hallucinations. The term delirium is, in fact, a fluid notion meant to encompass all the organic, understandable conditions that may affect awareness and attention. Simply put, an altered state of mind with an obvious (and often treatable) cause.6

Patients with delirium were almost always on medical or surgical wards, not on neurological or psychiatric wards, for delirium generally indicates a medical problem, a consequence of something affecting the whole body, including the brain, and it disappears as soon as the medical problem has been righted. 

Oliver Sacks, Hallucinations

Delirium is said to appear in one third of hospitalized patients over 70. It’s viewed as a failure of the mechanisms we count on to keep us grounded, which can be brought about by any significant disease or procedure in an already vulnerable patient. Hallucinating in such a context does not seem so unreasonable. Maybe we don’t even think of delirious patients as mentally ill anymore, just temporarily imbalanced. 6

The world of hallucinations is, however, as diverse as it is fascinating. Many other, often unexpected, contexts may give rise to this peculiar manifestation.

According to psychiatric guidelines, a hallucination is a perception of something that is not actually there – be it an image, a sound, or even a smell or a taste. 7

One does not see with the eyes; one sees with the brain, which has dozens of different systems for analyzing the input from the eyes. And seeing with the brain is often called imagination. And we are familiar with the landscapes of our own imagination, our inscapes. We've lived with them all our lives. But there are also hallucinations as well, and hallucinations are completely different. They don't seem to be of our creation. They don't seem to be under our control. They seem to come from the outside, and to mimic perception. 

Oliver Sacks, Hallucinations

By definition, a blind person claiming to see, for example, must be hallucinating. Even more so when he/she may claim to see antique figures walking around the house, or animals that are not there.

I went in to see her. It was evident straight away that she was perfectly sane and lucid and of good intelligence, but she'd been very startled and very bewildered, because she'd been seeing things. And she told me -- the nurses hadn't mentioned this -- that she was blind, that she had been completely blind from macular degeneration for five years. […] So I said, "What sort of things?", and she said, "People in Eastern dress, in drapes, walking up and down stairs. A man who turns towards me and smiles. But he has huge teeth on one side of his mouth. Animals too." 

Oliver Sacks, Hallucinations

This complication that may arise following vision loss is called Charles Bonnet Syndrome, or visual release hallucinations. After being blind for a while, as if confused or bored by the sudden lack of visual stimulation, the brain begins to “create” its own perceptions, resulting in remarkably vivid and complex imagery. Throughout the process, the person is lucid and often aware that this is not normal, that they can’t see and that what they are experiencing is, in fact, a hallucination, making the experience even more stressful. While the condition does not have a specific cure, it usually improves on its own after a few months or years. 8

In small studies, similar hallucinogenic effects have been described by artificially abolishing external stimuli – say, in a sensory deprivation chamber. 9,10

It seems that not just blindness, but loss as a general principle can trigger hallucinations. It is more common than we’d think for people to experience hallucinations after a traumatic loss of a loved one.

Especially common are hallucinations engendered by loss and grief – particularly following the death of a spouse after decades of togetherness and marriage. Losing a parent, a spouse, or a child is losing a part of oneself; and bereavement causes a sudden hole in one’s life, a hole which – somehow – must be filled.

Oliver Sacks, Hallucinations

Assessing such a phenomenon is not an easy feat, mainly due to people’s reluctance to report having hallucinations. A recent study, however, estimated that more than half of bereaved people experience some form of hallucination following their loss. 11

A more familiar form of loss-associated hallucinations is phantom limbs. Once considered a psychiatric issue, phantom limb pain, reported in as many as 80% of patients post-amputation 12, is now recognized as a real sensation originating in the spinal cord and brain.13

Almost as if to contrast hallucinations arising due to lack of activity, such manifestations may also appear in states traditionally associated with excessive stimulation. More often, simple hallucinations, in the form of luminous lines, shapes, and patterns, may come right before a migraine episode. This is referred to as an aura, and it characterizes about a third of all migraineurs. 14

There is an increasing feeling among neuroscientists that self-organizing activity in vast populations of visual neurons is a prerequisite of visual perception – that this is how seeing begins. Spontaneous self-organization is not restricted to living systems; one may see it in the formation of snow crystals, in the roilings and eddies of turbulent water, in certain oscillating chemical reactions. Here, too, self-organization can produce geometries and patterns in space and time very similar to what one may see in a migraine aura. In this sense, the geometrical hallucinations of migraine allow us to experience in ourselves not only a universal of neural functioning but a universal of nature itself.

Oliver Sacks, Hallucinations

Hallucinations can also appear as part of epileptic seizures, with a specific that is dictated by the area the seizure occurs in – generating particular images, sounds, smells or even sensations such as joy or fear that arise out of thin air. 1

Last but not least, it is not uncommon to hallucinate just before falling asleep (hypnagogic hallucinations) or just before waking up (hypnopompic hallucinations). These states, on the edge of consciousness, seem particularly predisposed to alterations in perception. One study found that, out of almost 5000 interviewed people, 37% reported hypnagogic hallucinations, and 12.5% reported hypnopompic hallucinations. Although they are more frequent in patients with mental illnesses and especially narcolepsy, they were also found in people with insomnia or those who were overworked, blurring the line between pathological phenomenon and normal reaction.15

All things considered, we can no longer view hallucinations as just a symptom of mental illness or specific drugs. They represent a not-so-rare process that may well occur in an otherwise healthy individual under certain circumstances. Unraveling the elusive mechanism underlying this ill-perceived symptom is a major task for current neuropsychiatry. We will tackle current theories regarding what is taking place in a hallucinating brain in the next article.


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  2. Ohayon, M. M. Prevalence of hallucinations and their pathological associations in the general population. Psychiatry Res. 97, 153–164 (2000).
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