Voices in your head? Visions of things that aren't there? You don't have to have schizophrenia or take LSD to have a hallucination, and they don't always have to be scary either.
Hallucinations are actually fairly common.
"It turns out that many everyday high functioning people occasionally do have what technically is a hallucination," said Professor John McGrath from the Queensland Brain Institute.
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McGrath recently found that nearly 1 in 20 of the general population report hearing or seeing things - when fully awake - that others don't.
So what is a hallucination? It's a "false perception" of reality and it can occur with a whole range of senses, but the most common ones are visual and auditory hallucinations, said McGrath.
Normally our brain is good at distinguishing between a sound or image that is occurring in the outside world, and one that is just a product of our mind. But occasionally something can go awry.
One major theory is that hallucinations are caused when something goes wrong in the relationship between the brain's frontal lobe and the sensory cortex, said neuropsychologist Flavie Waters from the University of Western Australia.
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For example, research suggests auditory hallucinations experienced by people with schizophrenia involve an overactive auditory cortex, the part of the brain that processes sound, said Waters.
This results in random sounds and speech fragments being generated.
Similarly, people with Parkinson's disease appear to have an overactive visual cortex, which results in images being generated in their brain of things that aren't actually there.
Psychoactive drugs could also upset the relationship between the sense processing parts of the brain and the frontal lobe in a similar way, said Waters.
"It allows the processing of images and sound that would normally be inhibited," she said.
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The big question is whether the same kind of processes are responsible for less extreme hallucinations.
Hallucinations aren't always intrusive, negative and scary, even in conditions like schizophrenia.
About 70 percent of healthy people experience benign hallucinations when they are falling asleep, said Waters. This includes hearing their name being called, the phone ringing or seeing someone sitting at the end of their bed.
Research into this kind of hallucination is in its very early days, said Waters.
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"In the past 100 years it's always been about schizophrenia in the past couple of years we've suddenly ramped up investigations outside of schizophrenia," she said.
"We're still trying to understand whether there are different forms of hallucinations or whether there is only one type that takes different shapes. And what makes a hallucination distressing in some situations and not in others?"
Waters' best guess is that "everyday" hallucinations may share common mechanisms with more serious hallucinations.
She said factors including lack of sleep, stress, grief, and trauma could make the brain more vulnerable to hallucinations by upsetting the relationship between the sensory cortex and the frontal lobe.
"When your brain works well, your frontal lobe is the driver of the car; it decides what's going to happen and is in control of the rest of the brain," she said.
"But when we have lack of sleep and stress and grief, then our frontal lobe just goes on holiday a little bit and doesn't have that supervisory capacity anymore, and it lets the sensory cortex just do what it wants."
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Interestingly, certain hallucinations are seen as part of normal life, and indeed encouraged in some cultures, said Waters.
"In some cultures it's acceptable, for example, to hear the voices of your dead relatives," she said.
"Some cultures are more likely to hear the voice of God or the voice of the Devil," he said.
He said young people were more prone to hallucinations and this could be because their brain circuitry was less robust.
"They may say ‘I hear voices' at 14 and then you ask them at 21 and they don't have them anymore."
This originally appeared on ABC Science Online.