Visual adaptation--a reinterpretation: discussion.

This discussion paper seeks to reshape the contemporary understanding of visual adaptation. Received wisdom says that input luminance is scaled down in the retina. There is, first, a near-logarithmic compression described by the Naka-Rushton equation and, second, a control of gain (better attenuation) by feedback from the output of each ganglion cell that is equivalent to modifying the half-saturation constant in the Naka-Rushton equation. The reinterpretation proposed here asserts the following instead: (a) the scaling down in the retina is accomplished by receptive fields of different areas, which function over different ranges of luminance, ranges inversely proportional to the area of the receptive field. (b) The visual pathway is differentially coupled to the physical stimulus, so that the maintained discharge increases only as the square root of the luminance. (c) The Naka-Rushton equation describes merely the saturation of neural response as input increases; when a neuron is overloaded, output tends to regularity and onward transmission is blocked by a subsequent stage of differential coupling. Three existing studies of the relation between input to and output from retinal ganglion cells are reinterpreted in the light of this alternative view of visual adaptation.