Assimilation theory and the Ponzo illusion: Quantitative predictions

[Correction Notice: An erratum for this article was reported in Vol 26(2) of Canadian Journal of Psychology Revue Canadienne de Psychologie . A corrected formula is provided.] Predicted that the ponzo illusion would increase, and then decrease as angle of the oblique arms increased, on the basis of assimilation theory. A reversed illusion was predicted when the angle was very obtuse. 4 orientations of the ponzo illusion were employed. In these the apex pointed to the top, bottom, left, and right. A different orientation was used on each of 4 sessions separated by a minimum of 1 day. The order was randomized for each of 9 men and 6 women ss. Both predictions were verified. Data suggest a change in the attentive field postulate of assimilation theory, and a general formula was derived to obtain a numerical estimate of illusion. (french summary)