The Effect of Illuminant Position on Perceived Curvature

In shaded scenes surface features can appear either concave or convex, depending upon the viewers judment about the direction of the prevailing illuminant. If other curvature cues are added to the image this ambiguity can be removed. However, it is not clear to what extent, if any, illuminant positin exerts an influence on the perceived magnitude of surface curvature. Subjects were presented with pairs of spherical surface patches in a curavture matching task. The patches were defined by shading and texture cues. The percevied curvature of a standard patch was measured as a function of light source position. We found a clear effect of light source position on apparent curvature. Perceived curvature decreased as light source tilt increased and as light source slant decreased. We also found that the strength of this effect is determined partly by a surface's reflectance function and partly by the relative weight of the texture cue. When a specular component was added to the stimuli, the effect of light source orientation was weakened. The weight of the texture cue was manipulated by disrupting the regular distribution of texture elements. We found an inverse relationship between the strength of the effecct and the weight of the texture cue: lowering the texture cue weight resulted in an enhancement of the illuminant position effect.

Identification of tone duration, line length and letter position: An experimental approach to timing and working memory deficits in schizophrenia.

Patients with schizophrenia display numerous cognitive deficits, including problems in working memory, time estimation, and absolute identification of stimuli. Research in these fields has traditionally been conducted independently. We examined these cognitive processes using tasks that are structurally similar and that yield rich error data. Relative to healthy control participants (n = 20), patients with schizophrenia (n = 20) were impaired on a duration identification task and a probed-recall memory task but not on a line-length identification task. These findings do not support the notion of a global impairment in absolute identification in schizophrenia. However, the authors suggest that some aspect of temporal information processing is indeed disturbed in schizophrenia.