Modulation thresholds for sinusoidal light distributions on the retina

In an optical system, the 'contrast transfer function' describes the relationship between the contrast of test objects whose intensity varies sinusoidally with distance and the contrast of their corresponding images at various spatial frequencies (lines/unit distance). It conveys more information about the characteristics of the system than does the specification of only the resolving power. This approach, widely applied to electrical systems, is strengthened by the formal identity of the transformation equations in diffraction theory between objects and images with those of Fourier analysis and synthesis (Hopkins, 1953). The method can also be applied to the response of a receiving layer, such as photographic film. When a sequence of optical and receiving systems is used in series, the total response of the whole system is deducible from those of its components, provided the principle of linearity holds. In a physical system when carrying out a sine-wave analysis, one determines the output magnitude for a constant input at various frequencies, but this is not readily feasible in the case of the visual system. Instead, one may reverse the procedure and measure the input required for a constant output, for example, a threshold response. In the present paper data will be presented of the application of such an approach to the retinal and central stages of the visual process, excluding the purely optical ones of the eye.