Partial additivity of rod signals with M- and L-cone signals in increment detection

Test additivity experiments revealed the combination rules for increment detection by rods and either M- or L-cone-dominated mechanisms isolated by means of chromatic adaptation (Stiles' pi 4 and pi 5, respectively). Increment thresholds were measured for single test wavelengths detected by each mechanism. Pairs of test wavelengths were then superimposed, and increment thresholds were measured for simultaneous detection by both rod and cone mechanisms. The observed degree of additivity was corrected (reduced) to compensate for the partial detection by each mechanism of both test wavelengths in the combined stimuli. We find that subthreshold rod signals are partially additive with subthreshold signals from both M- and L-cones. The degree of additivity is high and similar for both M- and L-cones: less than the ideal prediction of linear addition, but greater than that predicted by either probability summation of independent mechanisms or orthogonal vector addition.