The Effect of display type on direction repulsion - a role for motion constancy?

M any previous studies concerning the perception and discrimination of motion direction have utilised “transparent motion” displays, in which two or more sets of dots drift in different directions. Under these conditions, observers commonly misperceive either direction as being further from the other than it actually is – a phenomenon known as “direction repulsion”. This is often interpreted to reveal elements of how relative motion is computed by the visual system. However, using a display that more closely followed real-world conditions, we found that a single “object” moving across a drifting “background” of dots produced much larger errors in direction judgement, errors which peaked when background motion was orthogonal to object motion, indicating that higher-order motion constancy mechanisms may be involved in relative motion judgements. A separate experiment found smaller errors under aftereffect conditions, and these errors peaked at 60 rather than 90 degrees of direction separation, suggesting that direction aftereffects and illusions may have different mechanisms.