A moving edge or contrast between equiluminant colours produces a weaker motion percept than that defined by luminance contrast. However, the specific contrasts that determine chromatic motion have not been explored systematically. Here, rivalrous chromatic motion displays were produced by superimposing two gratings, one drifting from left to right and the other in the opposite direction. Each grating oscillated between equiluminant endpoints chosen from a pool of 16 colours. The rivalrous ambiguity resolved spontaneously for each display as the more-dissimilar colour pair dominated the less-dissimilar pair, and produced a percept of motion in the corresponding direction. These dissimilarity judgements were analysed with multidimensional scaling to represent 'motion salience' as distances in a colour map, to gauge whether chromatic motion is enhanced or weakened if the oscillation aligns with particular directions in the colour plane. Judgements were compared with other control judgements involving standard subjective dissimilarities between the same stimuli. Notably, chromatic motion was strongest when grating endpoints were separated along an orange-blue direction. This does not coincide with either cardinal axis of cone space, (L-M) or S(0) , but rather is a direction that would arise if motion is computed from a combination of the (L-M) and S(0) signals.
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