Abstract
We expand the anecdotic report by Johansson that back-and-forth linear harmonic motions appear uniform. Six experiments explore the role of shape and spatial orientation of the trajectory of a point-light target in the perceptual judgment of uniform motion. In Experiment 1, the target oscillated back-and-forth along a circular arc around an invisible pivot. The imaginary segment from the pivot to the midpoint of the trajectory could be oriented vertically downward (consistent with an upright pendulum), horizontally leftward, or vertically upward (upside-down). In Experiments 2 to 5, the target moved uni-directionally. The effect of suppressing the alternation of movement directions was tested with curvilinear (Experiment 2 and 3) or rectilinear (Experiment 4 and 5) paths. Experiment 6 replicated the upright condition of Experiment 1, but participants were asked to hold the gaze on a fixation point. When some features of the trajectory evoked the motion of either a simple pendulum or a mass-spring system, observers identified as uniform the kinematic profiles close to harmonic motion. The bias towards harmonic motion was most consistent in the upright orientation of Experiment 1 and 6. The bias disappeared when the stimuli were incompatible with both pendulum and mass-spring models (Experiments 3 to 5). The results are compatible with the hypothesis that the perception of dynamic stimuli is biased by the laws of motion obeyed by natural events, so that only natural motions appear uniform.
Highlights
Humans make striking perceptual mistakes in judging even the simplest kinematics of visual stimuli
Responses were scattered over a wider range of kinematic profiles (IQR = 6) than in Experiment 1
Summary and conclusions Taken together, the results of all our experiments are compatible with the hypothesis put forth by Runeson [3] that the perception of dynamic stimuli is biased by the laws of motion obeyed by natural events, so that only natural motions appear uniform
Summary
Humans make striking perceptual mistakes in judging even the simplest kinematics of visual stimuli. A spot moving at constant velocity along a rectilinear path is perceived as moving fast upon entering the visual field and decelerating to a constant velocity [1,2,3]. Runeson [3] showed that the motion is perceived as uniform when the velocity is mildly increasing at the onset and levels off. Johansson [4] asked to describe qualitatively the motion of a target moving back-andforth sinusoidally. Aside from a slight deceleration at trajectory endpoints, velocity looked constant
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