A method for testing the visual system to detect the presence of disease, and to distinguish disease which has degraded the linear visual pathway from disease degrading the nonlinear visual pathway in a manner which is substantially independent of the stimulus applied to the eye and the connection of the instrumentation to the patient being tested. Three light sources having their amplitude varied at different frequencies stimulate the eye. The response evoked in the brain is detected and Fourier analyzed. The amplitude of selected Fourier components are detected and used to compute a ratio which removes the dependence upon the factors stated above. The Fourier component amplitude factors are selected so that the number of factors in the numerator of the ratio is equal to the number in the denominator and the sum of the orders of the factors in the numerator are equal to the sum of the orders in the denominator. This provides a ration of the response of the linear system to the response of a nonlinear system, which can be compared to a data bank of such test results to determine whether a particular patient being tested deviates enough from the normal response to suspect disease, and the direction and magnitude of that deviation so that the type and extent of disease can be indicated.
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