Recent research suggests that people who have psychotic experiences, but no diagnosis of psychotic illness, have altered cognitive functioning compared with people without psychotic experiences.
A substantial minority of the general population, around six percent, experiences subclinical psychotic experiences, report MSc student Josephine Mollon of King’s College London, UK, and colleagues in the journal JAMA Psychiatry.
“Evidence suggests that subclinical psychotic experiences may lie on a continuum with clinically significant psychotic symptoms, and therefore be informative for research into the cause of psychotic illness,” they write.
Both disorders share risk factors such as low IQ, childhood maltreatment, and stressful life events, as well as similar brain scan results such as deficits in grey and white matter.
The researchers looked at neuropsychological functioning and psychotic experiences in adults, taking into account sociodemographic characteristics and age. They used information gathered from household surveys covering 1,677 people aged 16 years or older, living in two areas of London, UK. Average age was 40 years.
Participants’ psychotic experiences were measured using the Psychosis Screening Questionnaire, which is administered by an interviewer. It assesses psychotic experiences in the previous year, covering thought disorder, paranoia, strange experiences, and hallucinations. The tool also covers hypomania, a mild form of mania, marked by elation and hyperactivity, but this was not assessed as the focus was on psychosis.
Cognitive functioning was measured with a series of tests looking at verbal knowledge (using a reading test), working memory, general memory, and cognitive processing speed. From this, an overall IQ score was calculated.
One in ten of the participants had previously had psychotic experiences. This group was not significantly different from those without psychotic experiences on overall IQ or processing speed. But they scored less highly on verbal knowledge, working memory, and general memory.
Medium to large impairments in cognitive functioning were seen among participants aged 50 years and older with psychotic experiences. These differences remained once socioeconomic status, cannabis use, and common mental disorders were taken into account.
The team writes, “The profile of cognitive impairment in adults with psychotic experiences differed from that seen in adults with psychotic disorders, suggesting important differences between subclinical and clinical psychosis.”
Commenting on the study, researcher Josephine Mollon says, “Psychotic symptoms, such as hallucinations and delusions, are core features of psychotic disorders. A significant minority of the general population also reports subclinical psychotic experiences.
“We used population-based survey data to characterize cognitive functioning in adults with psychotic experiences while adjusting for important sociodemographic characteristics and investigating the effect of age.”
She continues, “Those with subclinical psychotic experiences did not show an impairment in processing speed, which is severely compromised in psychotic patients, suggesting that processing speed deficits indicate vulnerability to psychosis.
“Moreover, psychotic experiences, together with cognitive deficits, may be most challenging in those aged 50 years and older. Even mild, subclinical psychotic experiences, when combined with the effects of aging, may strain cognitive reserves and lead to large, burdensome cognitive deficits.”
In conclusion, Mollon adds, “Our findings suggest a continuum of psychotic experiences and cognitive deficits in a much larger proportion of the population than that seen in clinical practice. Effective treatment of such deficits could be helpful for many individuals.”
She recommends that future research on the topic should involve long-term studies “to elucidate how psychotic experiences interact with cognitive deficits throughout the life course and to identify risk and resiliency factors.”
This study is the first to investigate the effect of age on cognitive impairment associated with psychotic experiences in adults. Some previous studies suggest that these experiences are most prevalent in adolescence and old age, while others have not found significant age differences. Among the participants in this study, psychotic experiences were more likely in the youngest group but remained sizable in the other age groups.
Because the data in this study came from household surveys, the researchers could look for possible mechanisms behind the links they found with psychotic experiences and cognition.
They say, “First-degree relatives were significantly impaired on verbal knowledge, whereas unrelated cohabitants showed no impairment. Our findings suggest that a complex interplay of genetic, biological, and psychosocial factors lies behind the association between psychotic experiences and neuropsychological impairment.
“This pattern of verbal knowledge impairment suggests common genetic and/or family environmental factors.”
Mollon, J. et al. Psychotic Experiences and Neuropsychological Functioning in a Population-based Sample. JAMA Psychiatry, 30 December 2015 doi: 10.1001/jamapsychiatry.2015.2551