Continuous change of illuminance over retinal area in accordance with the sinusoidal function was studied as a stimulus for the human visual system. Its efficiency in controlling pursuit eye movements was compared with that of a stepwise luminance function (square wave). Such distributions of luminance were generated on a cathode ray screen (wavelength at the eye 9° and 3°) and were given a small translatory motion (2° – 12′). Ss were instructed to follow the moving pattern with pursuit eye movements. There is no difference in performance between the two types of brightness distributions. A stimulus motion of 24′ was sufficient to produce full evidence of eye tracking in all Ss also from the contour-free sinusoidal pattern. This means that the brightness change in every point of the CRT screen was far below the retinal sensitivity threshold at the illuminance level used. Thus a summation effect occurs. This was taken as a support for an hypothesis about “ordinal” stimulation. Arguments from modern neurophysiology are introduced and yield further support for the conclusion.
