Perception
Are people with schizophrenia less fooled by visual context?
Open access · cc by · source: Europe PMC
People with schizophrenia were less fooled by surrounding context when judging contrast and size, but were affected normally when judging brightness and tilt, so the context deficit is not across the board.
Study at a glance
- Design
- Case-control — Patients versus age-, sex- and IQ-matched controls on four two-interval forced-choice psychophysics tasks (luminance, contrast, orientation, size), each fit with psychometric functions to estimate bias and threshold.
- N
- N=48 · 24 people with schizophrenia and 24 matched healthy controls.
- Population
- Adult inpatients and outpatients with DSM-IV schizophrenia in London and matched controls
- Outcome
- Perceptual bias (strength of surround suppression) and discrimination threshold for each of four visual dimensions
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Key findings
Patients showed significantly weaker suppression than controls for contrast and size judgments, meaning their matches were closer to the true values, with effect sizes of about 0.68 and 0.52 standard deviations. Suppression for brightness and orientation did not differ significantly between groups, although orientation showed a nonsignificant trend in the same direction. Patients also had higher thresholds for contrast. Biases on different tasks did not correlate with one another, and none of the measures correlated with medication dose or symptom severity.
Methodology
The researchers tested 24 people with schizophrenia and 24 controls matched for age, sex and IQ on four versions of a surround suppression illusion. On each trial participants compared a plain patch with a patch embedded in a surround and said which was brighter, higher in contrast, more tilted toward horizontal, or larger. Fitting each person's responses gave a bias (how much the surround distorted perception) and a threshold (how sensitive they were), and these were compared between groups and correlated with medication dose and symptom ratings.
Limitations
With 24 patients per group, the study may have been underpowered to detect smaller differences such as the orientation trend. Patients were medicated, and the dose check using chlorpromazine equivalents is a weak test of medication effects; attention or motivation differences could also contribute, and patients' data were fit less well. The results show where context effects are reduced but do not directly show the neural mechanism, such as reduced GABA-based inhibition, that the authors propose.
How this study connects
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