The technique of subthreshold addition of sinusoidal gratings was used to analyse the visual system of man during the perception of edges, lines, and bars. The experimentally obtained sensitivity function varies in close relationship to the test pattern, and can be factorized into the conjugate complex spectrum of the test pattern at threshold and a pattern-invariant function of spatial frequency. Interpreting the sensitivity function as transfer function, which is possible under certain conditions, we can describe the visual system as a matched filter which extracts an input signal contaminated with noise of specified spectral energy density. Questions discussed refer to the spatial operations occurring in matched filters, the relationship between the modulation transfer function for sine-wave gratings and the pattern-invariant transfer component, the exact determination of elements within the theoretical concept, and the realization of matched filters by the nervous system.