Estimation of Lengths and Angles of Kinematically Induced Amodal Rods

Abstract

In previous experiments computer-generated stimuli consisting of two dots moving on semi-elliptical paths, either on inner or on outer halves of ellipses, induced percepts of amodal rods rotating or oscillating in 3-D space, with the two stimulus dots as the visible endpoints. The task was to estimate the length and the angle of inclination of the perceived rod in a specified position. Results of the experiments were only in partial accord with the frontoparallel principle (Johansson and Jansson, 1968 Perception & Psychophysics 4 165 – 170). A geometrical model was proposed to explain the perceived characteristics of the rod. The model was tested in two experiments. In the first experiment the length and the angle of inclination of the perceived rod were varied. Subjects were sensitive to changes of length and angle. Furthermore, when they overestimated the length, they underestimated the angle. In the second experiment the curvature of the semi-elliptical paths was increased. Compared to the first experiment, estimations of lengths increased but estimation of angles were not significantly different.