Mach Bands

A simple experiment first(My Design):
Focus on the picture, and then push closer or draw further from the picture with constant speed, or otherwise, see what happens! One thing I find, among others, is this: The size of the area in the middle, and the color of the outer area really seem to change, though I know they have not. This is only a phenomenon of the normal optics. But it is obvious in this case than, say, in the case of observing your computer from different distance. In this case, we have
1) sensory phenomenology.It does not appear to be the same.
2)NonSensory phenomenology. It appears to be the same, nevertheless.

The following is from plato.stanford.edu
The thin dark bands along the outer edges of the gradients and the thin light bands along the inner edges of the gradients are optical illusions. The cause of this effect is due to contrast perception; we over process at boundaries and under process where there is constancy. In fact sensory response can even stop with unchanging stimulation. Mach is credited for discovering lateral inhibition in our sense organs, the idea that our senses pre-process information before sending it to the brain.

Since every retinal point perceives itself, so to speak, as above or below the average of its neighbors, there results a characteristic type of perception. Whatever is near the mean of the surroundings becomes effaced, whatever is above or below is disproportionately brought into prominence. One could say that the retina schematizes and caricatures. The teleological significance of this process is clear in itself. It is an analog of abstraction and of the formation of concepts. [Mach 1868, in Ratliff 1965: 306]

Let us first ask how we would find our way in the visual world if we would perceive, not relations of, but only differences in, illumination. One and the same thing in the same surroundings would become immediately unrecognizable with the slightest alteration of light intensity, such as when a cloud covers the sun. Should we still want to find our way, we would then have to habituate ourselves to maintain equal levels of light intensity, for instance by closing and opening our eyes. In fact, if this god of the psychophysical law were non-existent, the organism itself would have to devise it and, if the Darwinian theory is correct, it has devised it. The seeing of light intensity relations, within certain limits, is necessary for the existence of organisms. [Mach 1868 in Ratliff 1965: 300]
From plato.stanford.edu