The Basis of the Dependence of Visual Acuity on Illumination

Abstract

It is a well-known fact that visual acuity, or the resolving power of the eye, varies with light intensity. Many speculations have been made regarding the cause of this variation. Particularly interesting is the theory recently put forward by Selig Hecht.' This theory not only attempts to provide a quantitative basis for the relation between visual acuity and illumination, but has certain implications which are of fundamental importance for sensory physiology in general. Hecht assumes that the resolving power of the retina, like that of a photographic plate, must be determined by the "grain" of its surface, in other words, by the number of light-sensitive elements per unit area. "But the number of rods and cones in the retina is fixed anatomically. Therefore, it is necessary to suppose that the number of elements in the retina can be varied functionally." He supposes that the retinal elements vary in their absolute thresholds of response, only a small number being functional at the lowest intensities, and more and more coming progressively into function as the intensity is raised. On the basis of his theory regarding the nature of the photochemical system in the rods and cones, plus certain assumptions regarding the distribution of thresholds among the elements, he arrives at a quantitative deduction of the law relating acuity and intensity. This deduction agrees remarkably well with the empirical data of Konig on the visual acuity of the eye. Hecht suggests that the number of active receptors furnishes the basis not only for visual acuity, but for the visual discrimination of intensities. The perception of a just noticeable difference in light intensity is determined by the addition or subtraction of a single functional receptor per unit retinal area. Number of elements, and not frequency of optic nerve discharge , governs the subjective differentiation of intensities. Carrying out an analysis of the data on intensity discrimination, and a comparison with the data on visual acuity, he finds quantitative confirmation of this additional hypothesis. Houstoun2 has lately developed a theory of visual acuity and of intensity discrimination which is essentially a modified form of the theory of Hecht. In view of the far-reaching importance of Hecht's conceptions, it seemed desirable to make a careful study of the variations of visual acuity with illumination, with the purpose of obtaining, if possible, some direct experimental evidence regarding the underlying causes of these variations.