Visual Illusion and Figural After-Effect, with and without Fixation

Abstract

An experiment, which was carried out with forty observers to investigate the effects of two conditions of observation, fixation and non-fixation, on the relative magnitudes of figural after-effect and immediate illusion is described. The combined inspection figure and test figure used in investigating after-effect displacements formed the figure used in investigating illusory displacements. An account is given of the apparatus used which enabled quantitative measurements of the displacements to be made by a production method. A second experiment on the magnitude of illusion and after-effect occurring with a slightly different figure and with fixation the only condition of observation is also briefly referred to. The results of the experiments indicate (a) that illusory displacements exceed aftereffect displacements, contrary to Köhler and Wallach's (1944) non-quantitative observation, (b) that virtually no after-effect occurs in the absence of fixation as observation condition, (c) that while fixation is necessary to the production of after-effects, observation without fixation favours a significantly larger illusory effect. It is concluded that, in spite of the similarities between after-effect and illusory displacements, differing mechanism must be postulated as underlying them, and, therefore, that neither Köhler and Wallach's theory nor Osgood and Heyer's (1951) theory of figural after-effect is capable of immediate extension to cover illusions.