Sigmund Exner1846–1926

An eye for movement

Exner studied under Brücke and Helmholtz and developed research interests in motion perception and the structure of the insect eye. One aspect of vision he examined was the facetted eye of insects, and he is shown on the left in an image of such an eye. Exner made important contributions to the study of motion aftereffects – the appearance of motion in a stationary object following adaptation to a moving one. He examined linear motion in two directions simultaneously: horizontal and vertical gratings were moved vertically and horizontally, respectively, behind a circular aperture and he reported that the motion seen during adaptation was in a diagonal direction, with a motion aftereffect in the opposite diagonal. Exner also explored the phenomenon of stroboscopic motion by using light sparks produced by electrical discharges. The time interval and spatial separation between the two sparks could be controlled, and he found that two slightly separated sparks, one appearing more than 50 ms after the other, appeared as a single light moving from one location to the other. On the basis of this observation, Exner contended that motion was a fundamental sensation that did not require combined elements of location and time. This phenomenon was built on by Max Wertheimer, and became one of the building blocks of Gestalt psychology. He developed a model of motion aftereffects in which he is portrayed in the centre. He was also concerned with cortical localization and he is shown on the right in his diagram of numbered brain areas.