KINETICS OF CONE PIGMENTS MEASURED OBJECTIVELY ON THE LIVING HUMAN FOVEA

When the human retina is viewed in an ophthalmoscope, the light entering the observer’s eye has passed twice through the retina and hence has suffered absorption by the retinal pigments there. Since retinal pigments become nearly transparent upon photolysis, that is, “bleaching,” the amount of pigment originally present can be measured by receiving the reflected light, not upon the observer’s eye, but upon a photo cell, the output of which can be measured accurately. The increase in output after bleaching can be brought back to the initial value by interposing a suitable density in the light path, and the density that must be added will obviously be the same as the density of the pigment-for double passage-that has been bleached away. When the region of the retina so examined is the parafovea, the chief pigment measured is rhodopsin. Previous publications by Rushton et al. (195.5)’ Campbell and Rushton (1955), and Rushton (1956) describe the equipment and give rather strong evidence that rhodopsin density is what is measured. When the retinal region examined is restricted to the fovea, a visual pigment is also seen, but the pigment here is not rhodopsin; instead, it appears to be from the cones. The kinetics of this supposed cone pigment forms the subject of this paper. The first step is to present some grounds for the belief that cone pigments can be measured in this way.