Adaptive filter mechanisms in human vision

As a result of our experiments we find that the absolute visibility of simple edge and bar patterns is determined by a cooperation between the maximum amplitude and the difference between the maximum gradients occurring in the patterns. Proceeding from the assumption that the second spatial derivative represents a basic mechanism in our visual system (Mach-bands, effects of masking between narrow lines and sharp edges, existence of even-symmetrical receptive fields in the retina and the LGN), we suggest a nonlinear summation device which produces the difference between gradients as a sum over the second derivative. Expressed in a slightly different manner we assume that the sum over the excitation embodied in a Mach-band acts as criterion during the absolute visibility of edge and bar patterns.