A theory of the Π₁ and Π₃ color mechanisms of stiles

A unified account of the Π₁ and Π₃ color mechanisms of Stiles is proposed. We hypothesize that under the conditions that isolate these branches of the two-color threshold a signal originating with the photon absorptions of the short-wavelength cones passes through two sites of attenuation. The first site's gain is controlled by the short-wavelength cones alone; the second site's gain is determined by a net "blue/yellow" opponent signal. The two sites are distinguished by spectral sensitivity, absolute field sensitivity and dynamic properties. The theory, formalized in four equations, provides a good account of these features of the increment thresholds: (1) super-additivity of the effects of Π₁-equated long- and short-wavelength fields; (2) upward deviation of the Π₁ threshold from the Weber line on bright blue backgrounds; (3) cancellative subadditivity of fields whose mixture is in approximate blue/yellow equilibrium: (4) the "limited conditioning effect" of long-wavelength fields; (5) transient "tritanopia".