Mental Rotation and Mirror-Image Discrimination

Two experiments involving judgments of same-different were conducted to assess conditions in which mental rotation is used. These two experiments were similar except in the way “different” pairs were constructed. In the first experiment, a “different” pair involved two different polygons; in the second one, a “different” pair involved one polygon and its mirror-image. The strategy used by the 8 subjects was investigated through patterns of reaction times. The first experiment exhibited a flat function of reaction times with orientation, whereas the second one exhibited a linear increase in reaction times with angular departure from upright. These results lead to the conclusion that mental rotation is used when a figure has to be distinguished from its mirror-image.

ANGULAR CORRELATION OF ANNIHILATION RADIATION FROM POSITRONS DECAYING IN SEVERAL SUBSTANCES

Using anthracene scintillation counters in coincidence, the angular correlation between annihilation quanta produced by the decay of positrons in the following materials has been measured: the pure elements Mg, S, Cu, Sn, Pt, and Hg, and the chemical compounds LiCl, KCl, CsCl, KF, and KI.Calculations were performed to determine the effect of the geometry on the shape of the angular correlation curves, and the effects of scattering were investigated.The curves for copper, platinum, and possibly mercury could be approximated to by the expression[Formula: see text], where C(θ) is the coincidence counting rate as a function of θ, the angular departure of the two quanta from collinearity, and K and θ0 are constants. The curves for the other materials are more complicated and suggest that there is a relation between the shape of the curves and the outer electronic structure of the atoms with which the positrons annihilate. In alkali halides the positrons appear to annihilate only against the outer electrons of the negatively charged halogen ions.