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Structure determines function of the retina, a neural center. 4. The 'duplex' nature of vision.

Part 4 reveals how the differences in the quality of vision at high and low illumination are explained by synaptic interaction between the neurons in the outer plexiform layer changing with illumination. Analysis of data available in the literature revealed that rod vision is active over the entire range of light intensities generating visual responses. It is revealed that the input from the retina to brain centers involves information regarding a black and white image based on rod vision, which is improved with respect to spatial brightness contrast and darkness contrast by cone interference. Cones add color to the image, further increasing image contrast. The duplicity theory is shown to be based on erratic assumptions regarding how rods and cones function. The theory lacks an experimental basis and is not required to explain differences in the quality of the perceived images at high and low illumination.