Effects of instruction and practice on the length-reproduction task using the Müller-Lyer figure

Two experiments were conducted to investigate the effects of instruction and practice on the length-reproduction task using the Müller-Lyer figure. The task consisted of reproducing visually presented lines by positioning the arm. In Exp. 1, 20 subjects were asked to move a stylus exactly the same length as the line presented in the illusory figure. Instructed subjects were told to ignore the visual illusion, while uninstructed subjects were told nothing. No differences in groups' constant errors were observed; however, subjects in both groups reproduced longer or shorter lines in accordance with the visual illusion. In Exp. 2, we investigated the effect of visual illusion in relation to learned movement. After practicing a length reproduction task 200 times, subjects performed a test session in which illusory figures were among the presented lines. Subjects were instructed to ignore any visual illusions, but this again had no effect on the reproduced length, that is, longer or shorter lines were reproduced in accord with the illusion. The findings show that visual illusion still had an effect after subjects were instructed to ignore it or had practiced the task extensively.