Advances in Physiology Education, Vol 267, Issue 6 54-S64, Copyright © 1994 by American Physiological Society
Psychophysical determination of visual processing time by comparing depth seen in Pulfrich and Mach-Dvorak illusions
D. Mojon, W. Zhang, M. Oetliker and H. Oetliker
Physiologisches Institut, Universitat Bern, Switzerland.
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.
