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.

A gene encoding the major beta tubulin of the mitotic spindle in Physarum polycephalum plasmodia.

The multinucleate plasmodium of Physarum polycephalum is unusual among eucaryotic cells in that it uses tubulins only in mitotic-spindle microtubules; cytoskeletal, flagellar, and centriolar microtubules are absent in this cell type. We have identified a beta-tubulin cDNA clone, beta 105, which is shown to correspond to the transcript of the betC beta-tubulin locus and to encode beta 2 tubulin, the beta tubulin expressed specifically in the plasmodium and used exclusively in the mitotic spindle. Physarum amoebae utilize tubulins in the cytoskeleton, centrioles, and flagella, in addition to the mitotic spindle. Sequence analysis shows that beta 2 tubulin is only 83% identical to the two beta tubulins expressed in amoebae. This compares with 70 to 83% identity between Physarum beta 2 tubulin and the beta tubulins of yeasts, fungi, alga, trypanosome, fruit fly, chicken, and mouse. On the other hand, Physarum beta 2 tubulin is no more similar to, for example, Aspergillus beta tubulins than it is to those of Drosophila melanogaster or mammals. Several eucaryotes express at least one widely diverged beta tubulin as well as one or more beta tubulins that conform more closely to a consensus beta-tubulin sequence. We suggest that beta-tubulins diverge more when their expression pattern is restricted, especially when this restriction results in their use in fewer functions. This divergence among beta tubulins could have resulted through neutral drift. For example, exclusive use of Physarum beta 2 tubulin in the spindle may have allowed more amino acid substitutions than would be functionally tolerable in the beta tubulins that are utilized in multiple microtubular organelles. Alternatively, restricted use of beta tubulins may allow positive selection to operate more freely to refine beta-tubulin function.