Visual decomposition of colour through motion extrapolation.

The perception of yellow has played a central role in distinguishing two main theories of colour vision. Hering proposed that yellow results from the activation of a distinct retinal-neural mechanism, whereas according to the Young-Helmholtz-Maxwell view, yellow results from the combined activation of red and green cone mechanisms. When red and green images are presented separately to corresponding retinal locations in the two eyes, the resulting sensation is yellow. As the pathways from the two eyes do not converge until the cortex, this suggests that yellow can indeed arise from the central combining of separate red and green channels. I now show that the reverse process can also occur; the visual system can decompose a 'yellow' stimulus into its constituent red and green components. A 'yellow' stimulus was created by optically superimposing a flashed red line onto a moving green bar. If the bar is visible only briefly, the flashed line appears yellow. If the trajectory of the green bar is exposed for sufficient time, however, the line is incorrectly perceived to trail the bar, and appears red. Motion processing occurs in the cortex rather than the retina in primates, and so the ability of motion cues to affect the perception of colour is consistent with the Young-Helmholtz-Maxwell notion of a 'central synthesis' of yellow.