Background. Schizophrenia is associated with perceptual organization deficits and abnormal neuronal connectivity has been described in early visual areas. The purpose of this study was to investigate the functional integrity of lateral connections in early visual areas of patients with schizophrenia and type I bipolar disorder with a history of psychosis.

Method. Twenty-four out-patients with schizophrenia, 22 out-patients with bipolar disorder, and 20 healthy control subjects participated in the study. Using a computer-assisted psychophysical test, contrast thresholds were measured for centrally presented target stimuli (Gabor patches), which were surrounded by two collinear flankers. Target-to-flanker distances were 0, 1, 2, 3, 4, 6, 9 and 12λ. Psychophysical measures were contrast threshold changes at each target-to-flanker distance compared with baseline thresholds determined for isolated targets with no flankers. Clinical measures included IQ, positive, negative, and depressive symptoms.

Results. In patients with schizophrenia, flankers did not facilitate contrast detection for target stimuli at 2–6λ distances compared to controls [effect size (Cohen's d): 1·25–1·42]. The inhibitory effect of flankers (0 and 1λ) and contrast thresholds in the absence of flankers were spared. Patients with bipolar disorder did not differ from the controls. Medicated and non-medicated patients displayed similar performances. Positive and negative symptoms and depression did not correlate with contrast threshold values.

Conclusions. Excitatory lateral connections in early visual cortex are specifically impaired in patients with schizophrenia, which may contribute to perceptual disorders such as unclear seeing, partial or skewed sight, disrupted rectilinearity, and abnormal figure-ground segregation.