Color Vision Mechanisms in the Monkey

Abstract
The monkey, Macaca mulatta, was chosen as the experimental animal for the investigation of the physiological mechanisms of color vision for two reasons: there is excellent behavioral evidence that the rhesus monkey has color vision (this is not true for some of the animals which have been used in similar in- vestigations); furthermore, its color vision system is undoubtedly very similar to, if not identical with, that of man. There is good evidence (Walls, 1942) that color vision systems have evolved several separate times, once in the insects, again in fish, reptiles, and birds, and still again within the primate order. If this is correct, it would be little more than a coincidence ff the way in which information about the wavelength of light is encoded in the visual system of monkey and man were the same as the manner in which it is ac- complished in fish or birds. In any case, if one is interested in relating the ways in which various neural elements respond to different wavelengths of light under various conditions to the manner in which these lights are per- ceived, one cannot go wrong in studying an animal as similar as possible to man, for virtually all of the behavioral results from color discrimination have been obtained on the human observer. The m acaque lateral geniculate nucleus (LGN), from which most of the recordings have been obtained, consists of six layers of cells separated by fiber layers. These cells receive connections from the axons of the ganglion cells of the retina and are thus the fourth-order neurons in the visual path- way. Anatomical studies (Glees and Le Gros Clark, 1941) have given little evidence for interconnections at this level; the types of responses recorded here may well be essentially the same as one would obtain from ganglion cells in the retina. However, the highly laminated structure of the LGN, in which the optic projection from each eye splits three ways, presents the pos- sibility of some clues being present as to the nature of the organization of the visual system in the differential responses of cells in the various laminae. For this, as well as for certain technical reasons, the LGN rather than the retina was chosen as the recording site.