The Effect of Practice on the Reduction of the Muller-Lyer Illusion in Specified Planes

CHAPTER I INTRODUCTION Investigations as to the cause of the reduction of the magnitude of the Muller-Lyer illusion have yielded inconclusive evidence. Work initiated around the turn of the century attributed the reduction in the magnitude of the illusion to the gradual assumption by the observer of a more objective attitude in judging the two parts of the Muller-Lyer figure. However, recently, other investigators (7,8) have attrib* uted the reduction in the magnitude of the illusion to the more rapid development of satiation on the underestimated side of the figure which caused this side to increase in apparent length in relation to the other side. An early investigator, Judd (5) performed an experiment using the Muller-Lyer figure on two subjects. One of these subjects was Judd himself who was familiar with the nature of the illusion. The other subject was not familiar with the illusion. Measurements were obtained upon several forms of the figure, i.e., there were variations in the length of the standard and the angles of the obliques. As a result of this investigation, Judd concluded that a mere change in the length of the lines of the standard, or even in the more marked changes in the obliquity of the added lines and in the position of the whole figure provided no hindrance to the transfer of the effects of practice. However, when the figure was reversed, right to left, the illusion reappeared in full strength, but the curve of practice fell more rapidly than previously. According to Judd, an illusion is a case of confused perception in which there is no fully developed mode of synthesis. He attributed the reduction in the magnitude of the illusion to the gradual assumption by the subject of a more objective attitude in judging the length of the two halves of the illusion. However, as Judd noted, the method employed in his experiment was somewhat irregular and that his work should provide an impetus for further research in this field