Are people with schizophrenia less fooled by visual context?
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.
Source
Visual surround suppression in schizophrenia
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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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Surround suppression
- A visual effect in which a strong surrounding pattern reduces the perceived strength of a central target, for example making a patch look lower in contrast.
- Gain control
- Inhibitory processes by which neurons adjust their overall responsiveness based on surrounding activity, helping the brain encode information efficiently.
- Point of subjective equality (bias)
- The test value at which two stimuli look identical to an observer; its distance from the true match measures how much context distorts perception.
- Psychometric function
- A curve relating stimulus strength to the probability of a particular response, whose midpoint gives bias and whose slope gives sensitivity.
- Ebbinghaus illusion
- An illusion in which a circle surrounded by large circles looks smaller than the same circle on its own.
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Quiz yourself
For which judgments were patients less affected by the surround?
Common questions
Were patients actually more accurate than controls?
Yes, on the contrast and size tasks their judgments were less distorted by the surround, so their matches were closer to the true values.
Why test several illusions in the same people?
Earlier studies used different patients and methods for each illusion, so testing one group on a standard battery lets researchers see whether the deficit is general or limited to certain dimensions.
Why does the normal brightness result matter?
Brightness context effects are thought to arise very early, in the retina and thalamus, so a normal result suggests the problem lies in the cortex rather than earlier in the visual pathway.
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