Diminished orientation-specific surround suppression of visual processing in schizophrenia

Visual perception of a stimulus is a function of the visual con-text inwhich it is displayed. Surround suppression is a specific form of contextual modulation whereby the perceived con-trast of a center stimulus is decreased by a high-contrast sur-round.Recent studieshavedemonstratedthat individualswith schizophrenia are less prone to visual contextual effects, sug-gesting impairmentsincortical lateralconnectivity.Wetested whether altered contextual modulation in schizophrenia is stimulus orientation selective.Participants viewedanannulus consistingof contrast-reversing sinusoidal gratingsanddeter-mined if any one segment of the annulus had lower contrast relative to the other segments. Three stimulus configurations were tested: no surround (NS), parallel surround (PS), and orthogonal surround (OS). In the PS condition, the annulus was embedded in a 100 % contrast grating parallel to the an-nulus gratings. In theOScondition, the surround gratingwas rotated 90! relative to the orientation of the annulus gratings. Themain dependentmeasurewas the suppression index—the change in contrast threshold in theOSandPS conditions rel-ative to the NS condition. There was a group3 condition in-teraction such that patients had significantly lower PS suppression index than controls, but there were no group dif-ferences in the OS suppression index.We conclude that indi-vidualswithschizophreniapossessanabnormality insurround suppression that is specific for stimulus orientation. In con-junctionwith physiological and anatomical evidence fromba-sic and postmortem studies, our results suggest a deficit of inhibition in primary visual cortex in schizophrenia. Key words: schizophrenia/psychophysics/visual processing/contextual modulatio