Perceptual distortions are common in population

pretty_colours.jpgResearchers from Cardiff University report that anomalies of sensation and perception are common in the general population, with more than 1 in 10 reporting higher levels than the average of patients diagnosed with psychosis.

The research project was inspired by a need for a comprehensive measure of anomalous sensory experience and perceptual distortion, as the majority of existing measures are derived from psychiatric assessment techniques.

Consequently, they often focus on specific forms of perceptual distortion, such as ‘visions’ or ‘voices’, and do not always cover other types of anomalous experience.

To tackle this problem the researchers designed, tested and validated, a new measure of anomalous perceptual experience that specifically uses non-clinical language to ask about a wide range of phenomena, including unusual touch sensations, changes in time perception and being unable to distinguish one sensation from another.

Sensory distortions are traditionally associated with mental or neurological illness, although recent work is now suggesting that unusual experiences are distributed throughout the population (this is known as the ‘continuum model of psychosis’).

Perhaps unsurprisingly, when patients with psychosis completed the measure high levels of unusual experience were reported.

It is not clear, however, why some people with high levels of unusual experiences become distressed and impaired by their experiences, often leading to a diagnosis of mental illness, while others are able to function and remain untroubled by them.

One possibility is that there might be different sources for different types of unusual experience. When the types of experiences reported by healthy individuals in the study were analysed for how they clustered together, three themes emerged.

One cluster was associated with relatively benign smell and taste experiences, another with experiences potentially related to temporal lobe disturbance and another with experiences traditionally linked to psychosis.

This suggests that the distribution of perceptual distortions found in the population may be driven by a number of underlying processes, all which might contribute to producing strange experiences in the individual.

The research is published as an open-access paper in the journal Schizophrenia Bulletin.

Disclaimer: This paper is from my own research group.

5 thoughts on “Perceptual distortions are common in population”

  1. Regarding this bit: “It is not clear, however, why some people with high levels of unusual experiences become distressed and impaired by their experiences, often leading to a diagnosis of mental illness, while others are able to function and remain untroubled by them”, the papers published in the latest issue of Acta Psychiatrica Scandinavica (the subject of a previous post) argue that it is a previous traumatic experience(s), especially in childhood, that renders people vulnerable to finding such unusual sensory/perceptual experiences distressing, and thus likely to lead to illness. Look out for a forthcoming report on one of these studies at the BPS Research Digest (

  2. John Read and others’ work, although highlighting the important role of childhood trauma in psychosis, often doesn’t directly address whether this contributes to perceptual distortions or perceptual distortions that become distressing and / or impairing.
    This work, like much other work on psychosis, looks at the history or features of patients diagnosed with psychosis. To be diagnosed, patients have at some point turned up in front of a psychiatrist, usually because either they’ve become distressed or other people are worried about them.
    This is a sample bias, in that it ignores all the people who may have very unusual experiences, perhaps that would otherwise be diagnosed as psychotic, were it not for the fact that they never get distressed and impaired by them.
    In other words, there may be people who suffer childhood trauma, who also have intense perceptual distortions as adults, but remain functional and outside the mental health system as a result. Very little research has been done on this population, so it is very hard to say whether childhood trauma uniquely contributes to the disabling aspect of these experiences or just the presence of the experiences themselves.
    My ex-colleague from Cardiff, Liz Andrews, was working with clairvoyants, mediums and other people who experienced voices, visions and so on. These people were generally not distressed or impaired by their experiences. Interestingly, Liz found higher levels of childhood trauma also in this population, suggesting trauma may not specifically be linked to adult disability and diagnosis, only to unusual experiences.

  3. Hi Vaughan
    I don’t mean to be an all out apologist for the work of John Read and others, but one of their new papers (that I report on in a forthcoming issue of the BPS Research Digest) is a prospective study in the general population and does investigate the link between psychotic experiences and a history of trauma in people who have not visited a psychiatrist. Of thousands of people interviewed three years earlier, those who went on to develop psychotic symptoms were more likely to find them distressing if they had been traumatised as a child. That’s not to say the study is without problems! The paper I’m referring to is Bak, M. et al. (2005). Early trauma may increase the risk for psychotic experiences by impacting on emotional response and perception of control. Acta Psychiatrica Scandinavica, 112, 360-366.

  4. Hi Christian,
    This sounds like a fantastic paper and one that addresses exactly the problem I mentioned earlier. I was totally unaware of it, so I’ll look forward to reading it as anything that tells us more about how unusual experiences become distressing is very valuable.
    Thanks for the pointer!

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