Control stimuli in investigations of the acute-angled and obtuse-angled Müller-Lyer illusions.

Over the past decade, attention has been drawn to the importance of intertip distance (the distance between the tips of the arrowheads or featherheads) as a possible determinant of the Müller-Lyer illusion. Investigation of this stimulus property for the arrowhead and featherhead components of the Müller-Lyer figure has indicated that the two components exhibit a lack of symmetry, a conclusion supported by other studies using rather different approaches to the investigation of symmetry. This article argues that the control stimuli used in previous investigations are inadequate. It is proposed that in addition to the conventional straight-line control stimulus, in which the standard shaft is stripped of the oblique inducing lines. H-shaped figures in which the inducing lines are vertical should be included. Results are reported of an experiment in which both types of control stimuli are used to investigate the effects of intertip distance, and the spatial separation of that distance from the standard shaft, upon the arrowhead and featherhead figures.