Spatial factors of brightness illusion in the Ehrenstein figure.

The strength of brightness illusion in an Ehrenstein figure has been examined as a function of two variables--inducing line length and gap size--by a two-alternative forced-choice procedure. The results show an interaction between the two variables; the length of the inducers and gap size are both involved in the formation of the brightness illusion, with gap size as the stronger factor. A spatial limit (corresponding to a gap size of 2.4 deg) was found, below which the illusion is always present regardless of the length of the inducers. An area ratio, defined as the ratio of the area of the ring formed by the four inducing lines of an Ehrenstein figure to the area of the illusory surface, takes into account the different spatial factors studied in the experiment and the global size of the Ehrenstein figure. The strength of the illusion was found to increase linearly with increasing values of the area ratio.