Reduced binocular depth inversion in schizophrenic patients
Introduction
Symptoms of schizophrenia include hallucinations, delusions, thought disorders and negative symptoms. To explain psychotic symptoms like hallucinations and illusions, Broadbent (1958) proposed a model of information processing in perception, according to which the data transmission system has a limited capacity for the transmittable amount of information. He furthermore suggested the existence of a “filter”, which limits the information flow from the sensory input to the “limited capacity channel (p-system)”. According to the current view, perception is not one singular process; rather it is assumed to result from interaction between different subsystems. Two components of the sensory system have to be considered — one “bottom–up” component and an additional “top–down” component (Wallbott and Ricci-Bitti, 1993, Cauller and Layer, 1995).
Humans acquire information about a 3D shape from a different number of sources. Binocular vision, motion parallax, texture gradients, outline contour, lighting direction, and shading all contribute information about 3D shape (Ramachandran, 1988, Hill and Bruce, 1993, Hill and Bruce, 1994). The visual information available from scenes of the natural world is encapsulated, to a large degree, by the geometrical relationship between 3D solid objects and their projections into the two retinal images. The above-mentioned perceptual cues for 3D shape are used by human observers. These cues are created by transformation of the geometrical information by spatial or temporal displacement of the vantage point or the viewed object in the cases of motion and stereo and by the link between surface properties and geometry in the cases of shading, texture, and specularities. The interaction between these different cues for 3D shape is still an unsolved problem. Wheatstone (1838) first identified the binocular disparity as a source of information used by humans to obtain a 3D shape. It relies on the horizontal retinal disparity between the images of an object projected on the two retinas. Yellot (1981) demonstrated stereoscopic visual experience to be the result of a process in which the brain tests hypotheses about the 3D shape of objects against the evidence provided from their retinal representations.
Binocular depth inversion represents a well-known model of illusionary perception. It occurs frequently with hollow 3D objects. Especially, when hollow faces are displayed, such inversion takes place and the hollow faces are perceived as normal faces. Binocular depth inversion has been understood as a process of generating hypotheses on the 3D shape of objects by interpreting the bottom–up signals from the retinas using perceptual and conceptual knowledge (top–down) as well as general rules of perception (Schneider et al., 1996, Emrich et al., 1997, Sternemann et al., 1997). The resistance to reversal of depth has been attributed to familiarity with the shape of objects (Gregory, 1973). Hill and Bruce (1994) have shown experimentally that both the familiarity of faces and a general preference for convexity tend to favour the illusory, face-like interpretation of the hollow mask.
The current hypothesis was that the “equilibrium” between the above-mentioned perceptual components may be disturbed in schizophrenic patients and that an improvement occurs due to normalization of psychopathological symptoms during therapy.
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Subjects and methods
Twenty patients (aged 38.45±12.54, 12 females and 8 males), 15 healthy volunteers (aged 31.51±9.74, 8 females and 7 males) and 10 patients with depression (aged 34.56±12.19, 6 females and 4 males) participated in the study. All schizophrenic patients fulfilled DSM-IV and ICD 10 criteria for schizophrenia and received neuroleptics. The patients with depression fulfilled DSM-IV and ICD 10 criteria for major depression and four of these patients had a comorbid disorder (personality
Results
Twenty patients started the study. Three patients dropped out after three weeks and four patients could not be investigated before discharge. The main reason for discontinuation was non-cooperation.
The demographic and disease characteristics of the patients are described in Table 1 (age, sex, duration of illness, severity of illness, duration of inpatient treatment). BPRS total score, Positive PANSS subscale, Negative PANSS subscale, and PANSS total score are shown in Table 2. The improvement
Discussion
Perception arises as a result of an interaction between sensory input and prior knowledge (Frith and Dolan, 1997). Illusionary perceptions have already been used in investigations of psychopathology and abnormalities of perception in schizophrenic patients. Binocular depth inversion represents a well-known model of illusionary perception. Recently, the underlying mechanisms of binocular depth inversion have been revealed in more detail (Yellott, 1981, Hill and Bruce, 1993, Hill and Bruce, 1994,
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Present address. Department of Psychiatry, Heinrich Heine University, Bergische Landstr. 2, D-40629 Duesseldorf, Germany.