I. The Problem
Can apparent motion occur when the stimulus objects are of different form ?
This question opens the door to the experimental investigation of phenomenal topological invariance under kinematic conditions.
II. Historical Background
LINKV~2, 3, seems to have been the pioneer in pointing out that apparent visual movement can take place even though the stimulus objects differ in form.
NnFF 4 (p. 35) states: "Since LI~JtE, it has been common knowledge that the two stimulus-patterns need not be identical in order to arouse a perception of movement. The limits of difference, however, have not been put to experimental test, and a full description of what occurs under widely unlike stimulus-patterns would be of extreme value".
In the half century that has elapsed since LI~I;E's work, no experimental study--to our best knowledge--has appeared on the subject. LI~xE himself did not have too much to say about the problem. His service consists primarily in showing that apparent movement can occur under the condition of form differences* Lic~:]s's main conclusion is that movement will be perceived if, and only if, the two phases seen are identified as referring to one and the same stimulus object. This is his fundamental position, reemphasized in the Grund/ragen of 19182.
Today, we would prefer to speak of "invariance" rather than of "identity", thus using the language of topology. When LI~J~ found that a disc would change into an ellipse, that a triangle could change into * The same is true for some short remarks by M. Wg~T~IM~ 5 (pp. 185 to 189) and by Ts.~vs s (pp. 128--131, 133f.).