Wings in the intershaft space contribute to the Mueller-Lyer illusion.

The interference of the wings of the wings-in part of the Mueller-Lyer figure was examined for the version of the illusion in which one part of the figure is placed above the other. Wings were removed in pairs from either above or below the shaft of one of the two parts of the figure. Subjects indicated the apparent difference between the lengths of the shafts of the two parts of the figure. Removal of the wings between the shafts of the wings-in part of the figure reduced the amount of the illusion more than removal of the wings from outside the shafts. Removing wings from the wings-out part of the figure reduced the amount of illusion, but it made no difference whether the wing removal occurred between or outside the shafts.