When many of us think about hallucinations, we tend to think about schizophrenia. However, hallucinations are incredibly varied, and can happen for many different reasons, many of which are unrelated to mental illness. In fact, many people experience hallucinations at some point in their life even when there's nothing wrong with them.
Many simple hallucinations, for example, are caused by sensory deprivation. Even when staring at a wall for 20 minutes, perhaps during meditation, one can often see splotches of colour appearing and disappearing on the wall in front of them.
More extensive experiments involving sensory deprivation show a progression of hallucinations from simple lines and colours all the way to cartoonish figures if the deprivation lasts long enough.
There also seem to be social deprivation hallucinations. When someone’s spouse dies, they often experience what it's called a bereavement hallucination, seeing or hearing their deceased spouse for a short time after death.
The “third man” effect also afflicts hikers and people in long conditions of solitude, where they vaguely have a feeling that there's an extra person with them. This was reported long ago by the Antarctic explorer Ernest Shackleton.
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Out-of-body experiences are also likely related to sensory deprivation. The experience of, for example, floating above your bed and seeing yourself often happens when you are immobilised, and perhaps under anaesthesia. People who are more in touch with their bodies, such as dancers and athletes, are much less likely to have these autoscopic hallucinations.
This suggests that hallucinations of this kind are generated by sensory deprivation in a proprioceptive system - your perception of where your body is and how it moves.
Low-level hallucinations like this can also be caused by there being too much activity in the sensory areas of your brain. This is most often caused by seizures. Many people with migraines experience low-level hallucinations in their visual field, such a zig zag lines or spots.
Studies show that these “migraine auras” are accompanied by seizures in the parts of the brain that perceive those things in the world. So, for example, if you have a hallucination of zig zag lines, it tends to be because the seizure is in the part of your brain that can detect the zig zag lines that you’re looking at. As a seizure moves through the perceptual system, the hallucinations change in a predictable way.
One of the characteristics of sensory deprivation hallucinations is that they tend to be relatively meaningless. That is, the hallucinations don't interact with you, and they don't feel important that a personal level. This is in contrast with the hallucinations of schizophrenia described in the 2017 video game Hellblade: Senua’s Sacrifice. Such hallucinations, as well as those caused by temporal lobe epilepsy seizures, tend to be drenched in meaning.
People feel persecuted, paranoid, and a hallucination can be accompanied by delusions that random events perceived are for the benefit of, or caused by, the thoughts of the hallucinator.
Where hallucinations caused by sensory deprivation or overactivity might be mildly annoying, or perhaps even pleasant, hallucinations associated with more serious mental illnesses tend to be terrifying and depressing.
The most common kind of hallucination for people with schizophrenia are voices in the head. There is evidence to suggest that this symptom has something to do with retrieving memories of things said, which are then misinterpreted as heard in the world. That is, without knowing it, the hallucinator is retrieving something from memory, but does not realise that they are doing so.
Knowing that your experience is actually a hallucination, known as “insight', is one of the main things that separates hallucination from other kinds of imagination. There are lots of hallucinations where people have insight. People experiencing migraine auras, for example, know perfectly well what the hallucinations are.
Even imagination can also be confused with reality. In one experiment, the researcher asked people to imagine that the song White Christmas was playing in a nearby room. One out of every 20 of the participants ended up actually believing if the song was being played in the other room, even though they had just been instructed to imagine it!
We might define hallucinations as perceptual experiences that are not caused by what would normally cause such experiences during perception. This is what is common to all hallucinations, but when we look at the underlying reasons why hallucinations happen in the brain, we can see that there are many causes - memory retrieval, hallucinogenic drugs, intrusions from dreaming when you are falling asleep, sensory deprivation or overstimulation, and seizures.
Imagination: The Science of Your Mind’s Greatest Power by Jim Davies is out now(£21.99, Pegasus)
