Distortion in the perception of real movement.

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Abstract
The starting point for the study is the phenomenon which consists in the fact that an illuminated arc of 36[degree], rotating at less than fusion speed, and observed with fixed eyes, appears to shrink to a fraction of its actual length. This phenomenon is linked to related observations in the literature. Seven expts. were conducted eliciting the judgments of 8 to 12 observers for each, varying the stimulus and the speed of movement, and in one case substituting apparent for real movement. It was found that shrinkage increases with speed; that shrinkage is due to telescoping rather than a dropping out of parts of the stimulus; that a sine curve and an angle side shrink less than an arc; that greater surface of the stimulus does not necessarily mean less shrinkage, a triangle tending to shrink more than one of its sides; that Gestalt laws are operative in the phenomenon. Shrinkage is shown to be a function of the degree of overlap of retinal stimulation, provided the existence of visual pulsations is assumed. This assumption is supported by the work of Bartley, and would carry the Gestalt theory of motion perception one step further, namely, that stroboscopic motion perception is physiologically more elementary than "real" motion perception. The duration of one pulsation was calculated from data to be 46-61 ms., and was compared to similar values previously found significant in the field of visual perception.

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