Perceived path of oblique motion: horizontal-vertical and stimulus-orientation effects.

Subjects adjusted the path of moving stimuli to produce apparent slopes of 45 degrees with respect to horizontal. The stimulus was either a single moving dot or a vertical or horizontal bar. In separate experiments either the stimuli were tracked or fixation was maintained on a stationary fixation target positioned 8 deg to the right of the center of stimulus motion. In both experiments the selected path slopes were in general more horizontal than 45 degrees. This pattern indicates that subjects overestimate the vertical component of motion along an oblique path, and is interpreted as a manifestation of the spatial anisometropy generally termed the 'horizontal-vertical illusion'. Additionally, paths selected for horizontal bars were more vertical than those for vertical bars. This finding is interpreted in the context of a previous report of the influence of stimulus orientation on perceived velocity.