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Hallucinations and Illusions


It’s psychopathology time again, here on planet Frontier Psychiatrist.  It’s been in the family as my brother recently  attempted the Fourteen Peaks Challenge will testify.  This is his story:

‘In June 2008 my friends and me decided to attempt the 14 Peaks Challenge. This involves scaling all 14 peaks in Wales which are in excess of 3000 ft and is a very long walk, taking up to 24 hours to complete.

After driving up from London, our party started walking at about 2am.  By around 4pm the next day I was starting to experience some quite peculiar visual effects.  Every time I looked at a stone, I could immediately pick out an image of a face or the shape of an animal (typically a crocodile).  Whereas normally it would take a conscious effort to see a pattern in an inert object, the comparisons were coming to me thick and fast.  At one point I picked up a piece of quartz convinced that it had the shape of an ancient Egyptian head.

I gave up the walk at around 6pm, having not slept for around 36 hours.  My tougher companions pushed on and walked for another 10 hours.  Their visual hallucinations apparently became much more vivid than mine with objects becoming actual animals and not just resembling them.’

Depending on your bent in life, this either sounds pretty cool, or pretty scary.  Plenty of people pay good money for similar nights out.

Psychiatrists have spent a lot of time classifying abnormal experiences; psychopathology is the study of this.  This is a big subject, so I’m going to gloss over a few bits.

As human beings we have a number of senses and sense organs and the brain interprets the sensory input from these.  Thus, perception consists of two parts – sensation (visual, auditory, tactile, gustatory, olfactory, kinasthetic and proprioceptive) and interpretation (the cognitive element).

But things can go wrong:

1. The stimulus can be perceived as the corresponding object, but not accurately.  For example an object could be perceived as being the wrong size; this is called micropsia or macropsia.

2. The stimulus is perceived as an object, but not corresponding to the source.  That is to say, both the stimulus and object are present, but different from each other.  This is an illusion

3. There is no stimulus, but a perception occurs.  This is a hallucination. 

4. There is a stimulus, but no perception occurs.  This is a negative hallucination.

So, my brother wasn’t hallucinating, but was seeing an illusion.  There are three major types of illusions:

1. Affect illusions:  here the person’s emotional state leads to misperceptions – perhaps being scared leads to the incorrect interpretation of a shadow.

2. Pareidolia: here a person perceives formed objects from ambiguous stimuli, for example seeing Elvis’s head in a cloud. 

3. Completion illusion: here, due to inattention, an incomplete object is perceived as complete.  For example, CCOK might be read as COOK.


Hallucinations have several important qualities.  They take place in the same space and at the same time as other real perceptions – this is different from a fantasy or imagery, which take place in subjective space, or a dream, which has no real component;  they are experienced as sensations and have all the qualities of a real object from which they are indistinguishable. They are involuntary, so unlike imagery, they are not under conscious control.

Hallucinations can occur in any modality and there are many different types:

Elementary hallucinations are the simplest kind and they are unstructured hallucinations and bear no relation to anything in the natural world.  An example of this is whirring noises in the auditory modality.  In the visual modality, a person with elementary hallucinations might see multicoloured spots.  


Auditory hallucinations often occur with psychiatric illness, and auditory hallucinations of voices are one of the first rank symptoms of schizophreniaVisual hallucinations on the other hand are much more common with organic illness and are very uncommon in schizophrenia.  Organic causes for hallucinations include occipital lobe tumours, post concussional states, hepatic failure and dementia.  

Elderly patients with normal consciousness and no brain pathology, but with reduced visual acuity due to ocular problems experience vivid, distinct formed hallucinations, often of men wearing hats.  This is called Charles Bonnet syndromeLilliputian hallucinations involve seeing tiny people or animals.  These can occur with alcohol withdrawal.  

Other sorts of hallucinations: 

Autoscopic hallucinations are the experience of seeing oneself.  This is different from an ‘out of body’ experience, as with the latter the person sees the world and his own body from a vantage point that is other than his physical body.  In autoscopy, the person ‘remains’ in their own body.

Extracampine hallucinations occur outside the field of normal perception.  An example of this would be hearing someone discussing you down the shops which are a mile away.   

Functional Hallucinations is where an external stimulus provokes hallucination, but both hallucination and stimulus are in the same modality but individually perceived.  An example of this would be hearing a voice when the tap is running.  On the other hand, Reflex hallucinations are when hallucinations in one modality are provoked by a stimulus in another modality.  An example would be seeing a elf whenever listening to music.

Formication is a type of haptic hallucination where there is the sensation of animals crawling under the skin.  This is seen in cocaine intoxication.  A character in the beginning of the film ‘A Scanner Darkly’ has a similar problem.


A pseudohallucination is like a hallucination, but lacks the quality of a perception.  It is a form of vivid imagery.  If someone feels that they are hearing voices in their head, this is a pseudohallucination as it does not have the same qualities as a normal perception. 

Synaesthesia is the perceiving of a stimulus in one modality in a different modality, for example, ‘hearing’ the colour red.  This can happen on taking LSD

Hypnagogic and hypnopompic hallucinations are hallucinations on falling asleep and waking up, respectively.  They may be normal phenomena and are particularly seen in narcolepsy 

Further reading:

Symptoms of the Mind by Femi Oyebode (Buy from Waterstones Amazon)

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  1. Awww! That picture hurts. Does everyone see squiggly multicoloured lines coming out from the centre?

    Susan, I watched that video. It was interesting, but I don’t really see what it has to do with illness. Presumably, whilst Omega 3 can improve normal funtioning it’s fairly unlikely to sort out some poor individual who is hallucinating?

    Although if the individual’s hallucinations made them suddenly fancy a dip in some cold water I’m sure a good dose of Omega 3 would help them swim to shore in record time, and utilising the fastest possible route.

    We should give it to cross channel swimmers.

  2. I think that Susan’s post is actually spam, I tried to delete it after moderating it, which doesn’t seem to have worked. The film makes an unjustified leap of logic to my mind…..

  3. The leap of logic is obvious, it’s a very poorly designed experiment.

    It isn’t really helped by the fact that rat number two looks clearly disorientated. It keeps on attempting to swim out of the basin and consequently hits the rim at full speed, face first. It looks to me like it has been spun around before entering the water. Surely if that rat was fully orientated (even if deficient in Omega 3s) it would have stopped hurting itself after one or two attemps? It’s co-ordination is bizarre.

  4. Actually I wasn’t thinking of that, although that may be the case.

    Let’s assume that the rat having eaten the omega-3 less diet actually does find it more difficult to do the task. It does not necessarily follow that the same holds for humans. Also we’re only talking about one rat here and the experiment would have to be repeated many times to make the finding robust.

    I should say though that I don’t know much about this sort of thing, but I do know that the gold standard way of establishing whether omega-3 rich diets are beneficial would be to do a control trial of some sort, which to the best of my knowledge has not been done.

  5. It should have been done with Gerbils for reasons which are deeply buried within the FP’s subconscious!

  6. Ha, ha person I’ve clearly never met.

  7. A very clear and comprehensive summary.

    In fact the only other bit of language I use in talking ’bout perceptual abnormalities is an oneiroid state of a dreamy state when there’s a propensity for abnormal percepts.

  8. One thing which, as a patient, I find difficult about categories such as this is that the boundaries between different experiences as deliniated by the terminology don’t match the boundaries between my experiences as it feels natural to me to categorise them. Differences between different ways of categorising experiences makes communication difficult.

  9. Nutty – your comments are always interesting – can you tell us a bit more about how you would categorize your experiences? My experience of talking to patients in crisis is that their symptoms very rarely fit into categorical boxes.

  10. Great site, really. Hi there preople. The reason I ever visited it was that I’m looking something up and I can’t find it really. This night I was reading an old book from ’73 (I am from eastern Europe, otherwise) and I came across a strange thing (new to me, though somewhat old, from the days of the Cold War), called: polygonal hallucination. Well I hope someone of you people could make it clear to me. If so, please drop me a line at
    Thanks in advance,

  11. Hi
    Interesting site.
    I have been experiencing illusions and sometimes hallucinations for about 7 years. I see inanimate objects wobbling and shifting, distorting and melting, particularly when anxious and stressed. Sometimes I see flashes of colour. Occasionally I hear a voice saying just one random word, or whispering my name. There have been other things but I won’t go into too much detail.
    I have had an MRI scan and things are fine.I have M.E/CFS and a depressive ‘illness’. I see a psychiatrist.
    I know it would be difficult for you to know without speaking to me in person and knowing my medical circumstances, but do you know of any reason as to why I experience these things?
    From this page I would say I am experiencing, Affect Illusions and Elementary hallucinations.