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Psychophysical determination of visual processing time by comparing depth seen in Pulfrich and Mach-Dvorak illusions.

A practical course for preclinical medical students was developed to illustrate aspects of binocular vision and mechanisms of primary visual transduction. It is based on a graphic analysis of two optical illusions, the Pulfrich and the Mach-Dvorak phenomena. A pendulum swinging in a plane perpendicular to the direction of observation appears to follow an elliptical path when viewed binocularly with a filter in front of one eye (Pulfrich illusion) or with alternating occlusion of the right and left eye above a critical frequency (Mach-Dvorak illusion). The Pulfrich phenomenon permits us to determine the relationship between perceived illusory depth and filter density. Analyzing the Mach-Dvorak phenomenon allows us to determine the dependence of illusory depth on interocular delay. Comparison of both determinations (depth against transmission and depth against time) permits us to establish, without complex calculations, the effect of luminescence on visual processing time. In addition, this course illustrates a general methodological concept mentioned by Popper: students make an unexpected observation, put forward a testable hypothesis, and try to falsify it.