Effects of method, orientation, and size of angle on the Ponzo illusion.

The Ponzo figure was used to test predictions based upon Pressey's 1971 concept of attentive fields. Subjects made judgements using either the method of successive reproduction (n = 18) or the method of reproduction (n = 17). Size of the acute angle between the oblique and test lines (30 degrees, 45 degrees, and 60 degrees) and orientation of test figures (apex pointed up, left, right, and down) were varied. It was predicted with the method of successive reproduction that illusions of shrinkage would occur and that the variable of orientation would not affect size of illusion. With the method of reproduction the prediction was made that illusions of expansion would occur when the apex pointed up and that illusions of shrinkage would occur when the apex pointed down. Analysis showed that with the method of successive reproduction illusions of shrinkage occurred when the apex was pointed either up or down. When the apex pointed either to the right or left nonsignificant illusions were found. With the method of reproduction the classical illusion of expansion occurred when the apex pointed up but changed to one of shrinkage when the apex pointed down. Size of expansion illusion diminished as the acute angle formed between the oblique and standard lines decreased. In general, some of the results were consistent with Pressey's concept of attentive fields.