The summation-index technique was applied to the study of rod–cone interrelation. Adaptation to 500 nm and parafoveal presentation of the test stimuli provides a condition where the first test flash, 440 nm, stimulates only rods and the second test flash, beyond 580 nm, mostly cones. For that combination, the summation index took values around 0.17, implying rod–cone independence or only a very small positive summation between them. When the second flash also stimulated rods alone, the index took a value of 0.30 implying complete energy summation within the rod system. The summation index was also obtained for a dark-adapted eye, and the same conclusion was drawn as for the 500-nm adapted eye. Inhibition between two systems, which would be shown by an index less than 0.11, was never found.
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