Decrement in magnitude of constant error to the Miiller-Lyer figure was reported to occur as a function of trials early in this century (3,5,6,11). Recently interest in this phenomenon has been revived (7,8,9). Kohler and Fishback (7,8) reported: an inverse relationship between number of trials per day and magnitude of decrement; greater decrement with spaced trials; additional decrement during overnight rest periods. Preliminary research (12) failed to verify these relationships and suggested alternative hypotheses. Decrement in magnitude of a response as a function of trials has long been referred to in the field of learning as "habituation" (4). If the change in response to the Muller-Lyer figure is an habituatory decrement, it is related to experimental variables in the same manner as 1 This paper is based upon a dissertation submitted to the Graduate School of Indiana University in partial fulfillment of the requirements for the Ph.D. degree. The author is indebted to J. R. Kantor, W. K. Estes, and H. G. Yamaguchi for many helpful suggestions during the course of this study.
Two experiments were conducted to test the hypothesis that a change in vestibular stimulation can result in apparent discrepancies in the perceived size and distance of two targets of equal size placed at identical distances, but in different planes of space. This effect could be offered as a reasonable explanation, at least in part, for the moon illusion phenomenon.
Twenty Ss were required to match the size and distance of equal-size disks in different planes ofspace. Head position was shown to be signi[icantly related to the Ss' inability to make these discriminations. Evidence was also given to support the hypothesis of a vestibular contribution to space perception.
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