Color discrimination at threshold: the approach through increment threshold sensitivity.

A close correlation between Stiles's increment threshold (pi-mechanism) approach and color discrimination at threshold was found: a sharp transition from detection of two lights via two pi-mechanisms (pi 1 and pi 4) to detection via only one pi-mechanism (pi 1) was paralleled by a sharp decline from excellent discrimination between those two lights (even with the rods bleached) to no discrimination. This parallel between detection and discrimination suggests that Stiles's approach can isolate single visual pathways with distinct perceptual correlates.