The Contribution of Relational Factors to Line-Length Matches

Both Künnapas and Rock and Ebenholtz investigated the effects of surrounding-frame size upon line-length judgments. Whereas Künnapas obtained errors of the order of 10–15%, typical of those which occur in other geometrical illusion figures, Rock and Ebenholtz reported errors closer to 100%. Rock and Ebenholtz claimed that perceived size is largely relationally determined and that this fact was obscured in Künnapas' experiment by allowing observers to compare the test lines directly with each other and within a common framework provided by the wall of the room.

On the contrary, the experiments reported here suggested that errors in the Rock and Ebenholtz study may have been inflated by their use of nondirective instructions and by the confounded effects of other variables. When the Rock and Ebenholtz experiments were repeated with adequate controls over these variables, the effect of surrounding frames on line-length matches were of the order of 15–19%, similar to those of Künnapas but much smaller than those reported by Rock and Ebenholtz. The complexity of the stimuli in the Rock and Ebenholtz type of experiment is such, however, that the effects of a number of variables and possible cue conflicts remain to be investigated.