What Are the Causes of Hallucinations?

A picture of a face superimposed over a brain.
It has been said that eyes are a window to the soul. They are also where the light of the outside world directly meets the nervous system. Getty Images

Picture this.

You are walking from the kitchen to the bedroom, maybe passing through the living room. The windows are open and there is a gentle breeze disturbing the stillness of the place. The curtains, the chandelier, the leaves of your indoor plants and maybe even your hair are all moving in unison. Suddenly, as you are about to step into the hallway, a shadow catches the corner of your eye and you turn around.

The breeze has settled; everything is perfectly immobile. But on the other side of the room, where there was nothing but wind a moment ago, a girl in a green sweater is playing with a red balloon. The sight is unexpected and yet, you don’t seem to be surprised. She even smiles at you before resuming her recreational task. You smile and keep walking towards your bedroom. Three dogs, a cat, and two hummingbirds pass you by before you reach your destination. A moment ago, you had no pets.

If you were seventy years old and diagnosed with a type of dementia called Lewy body, this could be happening to you. A hallucination is the experience of a sensation in the absence of a provoking stimulus. The hallucinated sensation can be visual, auditory, tactile and sometimes olfactory or gustatory. For example, a tactile hallucination is when you feel something crawling on your skin but nothing is there.

This is not to be confused with an illusion, which is the distortion or misinterpretation of a real perception: if you thought the plant in your living room was a girl in a green sweater, for example. A hallucination is usually very vivid and feels real, almost like a dream that takes place while you are awake.

While some hallucinations can be pleasant, others can be very scary and disruptive.

Hallucinations can occur in three main settings:

  1. Diseases of the eye
  2. Diseases of the brain
  3. Adverse effect of drugs

Diseases of the Eye

In 1760, Charles Bonnet, a Swiss naturalist and philosopher, first described the intriguing case of his 87-year-old grandfather who suffered from severe cataracts. His father still had his full mental capacities, yet he saw people, birds, animals and buildings while being nearly blind in both eyes! He gave his name to the syndrome Charles Bonnet Syndrome, which describes the presence of visual hallucinations (and only visual with no other sensory modality affected) in elderly people with various eye diseases: retinal detachment, macular degeneration, cataracts and damage to the optic nerve and pathways. The mechanism is not well understood. Some scientists have proposed that there is a “release” of the brain areas that normally process images. The visual stimuli sent from our retina to our brain usually inhibits our brain from processing any image other than the one that is currently in front of our eyes.

For example, if you were bored and daydreaming at work, you would still see the screen of your computer in front of you as opposed to the beach that you can only attempt to visualize. When the eye is sick, visual stimulation is absent and this control is lost, hence “releasing” the brain from the confinement of reality.

Diseases of the Brain

Hallucinations are the manifestations of many diseases of the brain (and the mind, if you are Cartesian about it), although their mechanism is poorly understood:

1. Psychiatric illnesses, in particular schizophrenia, are probably one of the conditions most commonly associated with hallucinations in general. The hallucinations of schizophrenia tend to be of the auditory type, although visual hallucinations can certainly occur.

2. Delirium is a constellation of symptoms defined as the inability to sustain attention accompanied by changes in consciousness. It can occur in various medical conditions, including your run-of-the-mill infection. Alcohol withdrawal can also result in delirium accompanied by abnormal movement (delirium tremens). Roughly a third of people with delirium can have visual hallucinations.

3. Lewy body disease is a type of dementia defined as cognitive loss accompanied by movement symptoms resembling those of Parkinson’s disease, visual hallucinations, and a fluctuating course. In this case, insight is usually preserved and the hallucinations are complex and colorful, but generally not scary. Hallucinations can also occur in other types of dementia, including Alzheimer’s disease.

4. Visual hallucinations can result from strokes that occur either in the visual centers of the brain located in the occipital (Latin for "back of the head") lobes, or in the brainstem. The mechanism of the latter is related to a “release” phenomenon similar to the one postulated for the Charles Bonnet syndrome. Auditory hallucinations can also occur in strokes affecting auditory centers in the brain located in the temporal lobes.

5. Migraines can be accompanied by hallucinations, such as flickering zigzag lines in their simplest forms. These can occur before a headache, or by themselves without any concomitant pain. A more sophisticated manifestation of migraine hallucinations is the Alice-in-Wonderland syndrome, so called because it affects the perception of size. Objects, people, buildings or your own limbs can appear to shrink or enlarge, just like the effect of the beverage, cake, and mushrooms that Carroll’s heroine ingests in his nineteenth-century masterpiece.

6. Hypnagogic (hypnos: sleep and agogos: inducing) and hypnopompic (pompe: sending away) hallucinations can occur during sleep onset or awakening, respectively. They can be visual or auditory and are usually bizarre. They can be associated with sleep disorders such as narcolepsy.

7. Seizures can result in various hallucinations (including olfactory and gustatory) depending on their location in the brain. They are usually brief and can be followed by loss of consciousness from a more generalized seizure. When they are olfactory, they invoke an unpleasant smell, often described as burning rubber.

Adverse Effect of Drugs

Hallucinogenic drugs, including LSD (lysergic acid diethylamide) and PCP (phencyclidine), act on a chemical receptor in the brain to induce altered perceptions and sometimes frank hallucinations. In addition, many drugs that are available in the market have side effects that include hallucinations. These drugs can affect various chemical systems in the brain, including the regulation of serotonin, dopamine or acetylcholine (all three of which are crucial chemicals for normal brain function). For example, drugs used to treat Parkinson’s disease are meant to boost the dopaminergic network, which puts one at risk for hallucinations. Interestingly, medications to treat hallucinations often act by decreasing the effect of dopamine.

Whether an image, a sound, or a voice is real or unreal, it is important to understand that all these sensations, which we take for granted as truth, are in fact fabricated by our own natural brain wiring. We only “see” because we have an entire brain network that specializes in processing light signals. The slightest change to this predetermined machinery and our entire world of “truth” would crumble. Just imagine if your brain was meant to process light as resulting in smells and vice-versa: you’ll then know paintings as perfumes and deodorants as rays of light. And that, then, would be “truth.”

Sources:

Schadlu AP, Schadlu R Shepherd JB 3rd. Charles Bonnet syndrome: a review. Current Opinions in Ophthalmology; 2009, 20(3):219-222.

Teeple RC, Caplan JP, Stern TA. Visual Hallucinations: Differential Diagnosis and Treatment. Primary Care Companion to the Journal of Clinical Psychiatry; 2009, 11(1):26-32.

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