Apparent movements induced by stroboscopic illumination of stabilized images.

Stroboscopic illumination of stabilized images causes habituation effects and illusory phenomena which, at low stimulation frequencies, show up as brighter patches. When a line is used as a stimulus, these patches are observed as brighter spots in the line surrounded by dimmer patches in the background. These locally brighter parts function as sources from which brightness spreads in the direction of the line at light-on and as sinks to which the brightness retracts again after light-off. This spread and retraction of the brightness induces the perception of movements. Higher stimulation frequencies or a diminished stimulus-background contrast enables the brightness to spread also in the direction perpendicular to the line contour, i.e., into the background. Again a perception of movement is induced, local displacements of a part of the field are observed as a result of the brightness spread. Tiny pinpoints of light, the smallest foveal perceptive elements, visible in lines narrower than 10 min arc (Gerrits, 1978), are also observed when these lines are illuminated with stroboscopic light. These tiny elements do not spread their brightnesses and so also no movement is induced. The results are discussed in relation to the properties of the perceptive elements and the cells activating them (habituation, barriers, filling-in).