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Several recent papers have presented data from experimental investigations of the human ivsual system (HVS) which support the general hypothesis that the HVS is composed of spatial frequency channels. It has been pointed out, however, that a linear systems analysis of the entire system is not valid. Furthermore, a nonlinear model consisting of a log-bandpass filter produced some experimental results with deviations at high spatial frequencies. A new structure for a nonlinear mathematical model which is easily quantifiable, produces results which compare with experimental data, and has a physiological correlate is presented. The significance of this model is that the bandwidth of the visual system decreases as contrast increases. Thus the system appears to maximize the signal to noise ratio while attempting to maintain a constant " perceptual" spatial-frequency fidelity.