Assimilation and contrast in geometrical illusions: a theoretical analysis

It is argued that an unwarranted dichotomization of geometrical illusions has produced theories whose explanatory scope is limited to either assimilation or contrast. Since recent attempts at integration lack precision, a 1977 unified model of the perception of extent by Brigner was revised to predict accurately both assimilative and contrastive phases of the parallel lines illusion. Application of the revised model to other geometrical illusions was discussed and a means of parsimoniously accounting for variations in assimilative illusions occurring with age and with lightness of the stimulus figure was suggested.