What direction of motion do we see if luminance but not colour contrast is reversed during displacement? Psychophysical evidence for a signed-colour input to motion detection.

To investigate the effects of colour upon motion detection, directional discrimination by human observers was determined using two-frame kinematograms in which the two classes of element composing the pattern could differ either in luminance alone (achromatic condition), or in both colour and luminance (chromatic condition). The elements in the second frame could either have the same colour/luminance as corresponding elements in the first frame, or they could be changed (swapped) in colour and/or luminance. The angular size of the elements was varied by changing the viewing distance. Changing colour between frames disrupted motion detection when the angular size of elements was large (0.9 deg) but not when they were small (0.225 deg), replicating a previous result. Detection of motion with chromatic patterns was generally superior to that with achromatic patterns, particularly with large element size. Luminance swap combined with colour swap produced the "reverse phi" phenomenon: however, when luminance was swapped with colour staying the same between frames, forward motion was seen, suggesting that forward motion based on colour dominated over reversed motion based on luminance. We conclude that signed chromatic information has an input to motion detection at low but less so at high spatial frequencies. Information across colour and luminance is combined in a final common pathway for motion detection, resulting either in enhancement if they are in agreement, or in disruption if they conflict.