Quantitative Effects of Visual Stimuli Upon Pigmentation

Abstract

It is known that continued action of dark or light backgrounds upon fishes leads first to rather rapid "physiological" color adaptations, due to the rearrangement of pigment already present; later to actual increases or losses of the pigment ("morphological" color change). These responses have been shown to be controlled not by the absolute amt. of light reaching the eyes of the fish from the background, but to the specific reflectivity or albedo of this background. In other words, the fish''s pigmentary responses are controlled by the ratio between the incident light (which is ordinarily from overhead) and the light reflected from the surfaces below and surrounding the animal. This fact implies that the upper and lower portions of the visual field (i.e., the lower and upper halves of the retina) have a different physiological significance in relation to chromatophore responses. For this there is now considerable exptl. evidence. Respecting the quantitative relations between visual stimuli and the resulting pigmental changes, the A. and his collaborator, Peter Doudoroff, give conclusive evidence that the number of melanophores (or quantity of melanin) varies inversely as the logarithm of the albedo of the background. The correspondence is pointed out between this generalization and the Weber-Fechner law, in its application to human brightness-discrimination. On the contrary, the absolute degree of illumination plays a minor part in the result.