Binocular white from red and green, and the three-component theory.

RECENTLY Hurvich and Jameson (1951) have criticized certain experiments designed to show whether yellowness is synthesized from redness and greenness (as the three-component theory of colour vision holds), or is given by the activity of a separate (yellow-blue) receptor in accordance with the opponent-colours (Hering) theory and its derivatives. These were experiments in which a red stimulus was supplied to one retina and a green one to the other. If the observer obtains a binocular yellow sensation, this could be taken to prove that its yellowness (hence, presumably, monocular yellowness also) is created centrally from the activities of redness receptors and greenness receptors. Hurvich and Jameson did not review the whole history of such haploscopic presentation of red and green stimuli, but singled out the particular experiment of Hecht (1928) with filtered lights, and its repetition with modern narrow-pass filters by Prentice (1948). Quite properly, these experiments were considered to prove nothing, since in each case the red stimulus employed afforded a slightly yellowish-red sensation and the " green " stimulus was really a yellow-green one. There being a yellow aspect to the hue of each monocular colour, the presence of yellowness in the binocular experience would be in no way mysterious according to any theory of colour vision. What seemed, however, to call for explanation was the disappearance of the redness and the greenness-if these were not being integrated into yellowness. Hurvich and Jameson speak of this disappearance as the result of a "central cancellation ", the mechanism of which, they consider, " constitutes a real problem ". The problem is real only if the redness and greenness do in fact disappear without having been used up in making yellowness. One may suspect that in the Hecht experiment there is not nearly enough total yellowness in the monocular sensations to account for the magnitude of the binocular yellowness; but to test this suspicion quantitatively would be very difficult to say the least. Easy and genuine proof of mutual cancellation of redness and greenness, as against their synthesis into yellowness, would however come from a situation in which red and green sensations, neither of which had any other hue-aspects, disappeared upon combination without the emergence of any yellowness whatever. Hurvich and Jameson perceived this, and claim to have performed the crucial experiment. For each of the two observers (LMH, DAJ) a mixture of X671 mg and x440 m, was devised which afforded a " pure " red sensation,