Spatiotemporal Filtering and the Interpolation Effect in Apparent Motion

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Abstract
A target moving in discrete spatial steps with an appropriate interstep interval (ISI) can appear visually as if it is in continuous motion. The momentary spatial position of such a target is interpolated by the observer between its real physical positions. The extent of this interpolation was measured by a vernier alignment technique, and was found to decrease as the ISI was lengthened. A discretely moving target may be described as a continuously moving target on which is superimposed a periodic modulation of spatial position. It is shown that the traditional ‘staircase’ stimulus for apparent motion can be generalized to include other kinds of periodic modulation. With the use of various analog-filtered and digitally filtered versions of staircase stimuli with different ISIs, it was shown that the phenomenal interpolation of a periodically modulated moving target was affected only when the frequencies of modulation were less than about 25 Hz. The spatial amplitude of the modulation also has some effect.