Effect of intershaft wing removal on the Mueller-Lyer illusion as a function of judgment task.

Two methods were used to measure the Mueller-Lyer illusion. In the comparison procedure, subjects showed the difference between the lengths of the shafts; in the independent-judgment procedure they showed the length of each shaft individually. With both methods, one part of the figure was placed above the other and wings were removed from one part, either between or outside the two shafts. Removing the wings in the intershaft space reduced the illusion more than removing the wings outside the two parts of the figure with the comparison procedure but not with the independent-judgment procedure. The wings in the intershaft space contribute to the illusion, but only when the figure is considered as a whole.