Threshold summation indices were determined in a study of interrelations among Stiles π mechanisms. Test wavelengths of 630, 520, 475, and 420 nm were presented upon a 555-nm adapting field. The 630–520-nm test-stimulus combination showed very small summation indices compared to the index derived on the assumption of independence of the π4 and π5 mechanisms. We suggest that π5 is not a single or unitary mechanism, but is comprised of two components, π5r and π5g, that may inhibit or cancel each other’s effects in producing the net response of mechanism π5. The hypothesis also explains fairly well the observed small index for the combination 630–475 nm at low field luminance. The mechanisms π1, π4, and the composite π5 seem to be independent, as shown by good agreement between the calculated and experimental indices at high field luminance for the combination 630–475 nm, and over the entire range of field luminance for the combination 520–475 nm. Complete summation is observed for the combination 475–420 nm at high field luminance, with which mechanism π1 should be almost exclusively involved.
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