The effects of common movement and spatial separation on position- and motion-based judgments of relative movement

Abstract

When common movement is superimposed on relative movement (changes in separation between two dots), relative movement thresholds increase nonlinearly as a function of initial dot separation. For large separations (> 2.0 deg), thresholds increase gradually with increased separation. It is shown that this reflects judgments based on perceived relative motion. For small separations (< 2.0 deg), thresholds increase sharply with increased separation. It is shown that this reflects judgments based on perceived changes in relative position. Evidence is presented that superimposed common movement reduces sensitivity to relative movement by reducing sensitivity to relative motion. This provides a “window”, in the range of small dot separations, for relative movement judgments to be based on the perception of changes in relative position, even though motion is perceived for individual dots.