Perception
Do people with schizophrenia see past visual 'surround' masking?
Open access · cc by · source: Europe PMC
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.
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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Key findings
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.
Methodology
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.
Limitations
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.
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