Studies of the angular function of a Duncker-type induced motion illusion

Duncker (1929/1955, Source Book of Gestalt Psychology, pp 161-172) demonstrated a laboratory version of induced motion. He showed that, when a stationary spot of light in a dark laboratory is enclosed in an oscillating rectangular frame, the frame is perceived as stationary and the dot appears to move in the direction opposite the true motion of the frame. Zivotofsky (2004, Investigative Ophthalmology & Visual Science 45 2867-2872) studied a more complex variant of the Duncker illusion, in which both the inducing and the test stimuli moved: a single red test dot moved horizontally left or right while a dense background set of black dots on a white background moved vertically up or down. When the background inducing dots moved up (down), the truly horizontally translating test dot appeared to drift at an angle down (up) from the horizontal. In experiment 1, we used two methods to measure the complete angular function of the Zivotofsky effect and found it to peak with an inducer-test direction separation of approximately 30°, similar to the inducing angle that has been found to maximise other direction illusions. Experiment 2 tested and confirmed predictions regarding the effects of relative test and inducer speeds based on the vectorial subtraction of the inducing velocity from the test velocity.