Reduced depth illusions in schizophrenia: The state of the illness matters but the kind of object may not

Abstract

Purpose. People with schizophrenia are less susceptible to illusions in which pseudoscopically-presented concave objects appear as convex. Here, we ask: Does this reduced illusion also occur with physical objects? If so, does it depend either on the state of the illness or on abnormal processing of specific visual attributes? Method. To address these questions, 19 healthy controls and 19 persons with schizophrenia made convexity judgments about 5 physical objects. Two objects were concave scenes ("reverspectives"); two were concave faces; and one was a convex face (catch stimulus). A reverspective and a concave face were painted with life-like texture; the remaining objects were uniformly colored. All five objects were viewed twice—once monocularly with the subject moving left-right (parallel to the objects, to create motion parallax) and once stereoscopically with the subject stationary. For each of the ten viewings, subjects were probed every 12 seconds for 2 minutes on whether the object appeared convex or concave. Results. The two subject groups were equally adept at identifying the catch stimulus as convex (p>.25). For concave objects, patients gave more accurate (veridical) responses than controls (p<.05). This group difference increased with illness severity (p=.02), as measured by the number of months since the last hospitalization (ranging from <3 to >24 months). Across all subjects, the illusion weakened when stereoscopic cues were present (p<.001) or when texture cues were absent (p<.001). The group difference did not depend on texture, object-type or viewing condition (ps>.24). Conclusions. We conclude that 1) physically concave objects produce fewer depth illusions in schizophrenia; 2) veridical perception increases with illness severity; and 3) the group difference is constant across different object types and viewing conditions. This last conclusion implies that reduced illusions in schizophrenia may not be stimulus- or attribute-specific but instead may owe to a weakened convexity prior.

Meeting abstract presented at VSS 2012