Müller-Lyer Illusions: Their Origin in Processes Facilitating Object Recognition

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Abstract
Perspective theories for the Müller-Lyer illusions have considered them to reflect constancies normally permitting the viewer to compensate for distance in estimating the true size of three-dimensional objects. Our experiments suggest a rather different perspective theory involving object recognition. A novel experimental procedure was used. Each of the separate groups of thirty subjects viewed only one of nineteen experimental drawings based on the Müller-Lyer figures. They judged the overall length of the shaft and divided it into subjectively equal quarters. The major findings were that the effect of each component angle was generally independent of other angles and independent of any overall perspective appearance of the figure. The shortening effect produced by an acute angle was limited to the contiguous quarter, whüe the lengthening effect of an obtuse angle extended undiminished from the contiguous quarter to the next quarter. Perspective photographs are shown, demonstrating that these angle-induced changes in apparent length can compensate selectively for the different perspective distortions occurring for the component parts of a single three-dimensional object. Such expansions and contractions provide no direct help in estimating overall size or distance of the object. But they do make possible recognition of the actual shape of the object and its orientation to the viewer, as well as producing errors when judging the length of Müller-Lyer figures.