The role of converging lines on the perception of vertical lines: A Ponzo illusion variant

An experiment was performed to examine the effect of the angle of converging contours on the subjective midpoints of vertical lines. According to the inappropriate size-constancy scaling theory of Gregory (1963) increasing the angle of converging contours should increase the apparent depth of the figure and therefore exert a measureable influence on the subjective midpoints of vertical lines enclosed by the contours. However, this manipulation was not found to affect the subjective midpoints of test lines indicating that an explanation of the Ponzo illusion based on the operation of inappropriate size-constancy scaling must be restricted to certain test configurations.