Do people with schizophrenia see past visual 'surround' masking?
People with schizophrenia were generally worse at spotting faint gratings, but a same-orientation surround hurt them much less than it hurt controls, pointing to weaker inhibition in early visual cortex.
Source
Reduced visual surround suppression in schizophrenia shown by measuring contrast detection thresholds
Study at a glance
- Design
- Cross-sectional — Patient-versus-control comparison of contrast detection thresholds under three surround conditions (none, orthogonal, parallel) in a 4-alternative forced-choice task with Bayesian staircases.
- N
- N=45 · 45 participants: 21 medicated outpatients/inpatients with paranoid schizophrenia and 24 age-matched healthy controls.
- Population
- Adults aged 24 to 58 in Madrid; patients diagnosed with paranoid schizophrenia (DSM-IV, SCID-I) on second-generation antipsychotics, controls without psychiatric history.
- Outcome
- Contrast detection threshold for a peripheral grating, and log-ratio suppression (parallel/no-surround) and facilitation (orthogonal/no-surround) indices.
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What they did
The researchers tested 21 patients with paranoid schizophrenia and 24 age-matched controls on a task where a faint striped patch appeared in one of four positions in the visual periphery. The patch appeared alone, inside a surrounding grating at right angles to it, or inside a surround with the same orientation. An adaptive staircase found the lowest contrast each person needed to locate the patch, and suppression was indexed as the log ratio of thresholds with the parallel surround versus no surround.
What they found
Controls needed less contrast than patients when the patch had no surround or an orthogonal surround, but the groups did not differ with a parallel surround. As a result, the suppression index was significantly smaller in patients, while the small facilitation produced by an orthogonal surround was the same in both groups. When a subset of controls and patients were matched on no-surround thresholds, patients actually detected the patch better than controls in the parallel-surround condition. Thresholds and indices were not related to symptom scores or antipsychotic dose.
The limits
What it doesn't show
The reduced suppression index is driven largely by patients' poorer baseline (no-surround) sensitivity rather than better parallel-surround performance, so it partly reflects a general deficit that could stem from attention or peripheral processing problems. The sample is small and groups were unbalanced on sex (patients mostly male), and all patients were medicated, so medication effects cannot be fully excluded despite null correlations. The link to reduced GABA or NMDA function is an interpretation; no neurochemistry was measured. The matched subsets were deliberately non-random and not representative of either population.
Key terms
- Surround suppression
- A target becomes harder to see when it is embedded in a surrounding pattern of the same orientation and spatial frequency, reflecting inhibition between neighbouring visual neurons.
- Contrast detection threshold
- The minimum difference in light and dark (contrast) needed to detect a stimulus at a set accuracy level; lower thresholds mean higher sensitivity.
- Four-alternative forced choice (4AFC)
- A task where the observer must say which of four locations contained the target on every trial, reducing response bias.
- Bayesian adaptive staircase
- A procedure that picks the next stimulus contrast from the updated probability distribution of the observer's threshold, homing in on it efficiently.
- Suppression index
- Here, the log ratio of thresholds with a parallel surround to thresholds with no surround; larger values mean stronger suppression.
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Quiz yourself
In which condition did patients and controls NOT differ in contrast threshold?
Common questions
If patients had a smaller suppression index, does that mean they saw better overall?
No. Patients had worse sensitivity without a surround; their parallel-surround thresholds were simply similar to controls', so the surround cost them relatively less.
Why test the target in the periphery rather than the centre of vision?
Surround suppression is stronger in the periphery than at the fovea, so placing targets away from fixation makes the effect easier to measure.
Could antipsychotic drugs explain the result?
The authors found no correlation between medication dose and thresholds or indices, but all patients were medicated, so this cannot be ruled out entirely.
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