Pi-4: adaptation of more than one class of cone

The Pi-4 color mechanism was isolated in three observers with a 500-nm, 200-msec, l-deg foveal test flash. Stiles's field displacement law was tested with increment thresholds upon monochromatic adapting fields of several wave-lengths. The data of all three observers reject the displacement law. Data that use a 10-msec-duration test flash likewise reject the displacement law. We conclude that the Pi-4 branch represents light adaptation controlled by more than one class of cone. A model of the Pi-4 detection pathway is proposed that quantitatively describes the increment-threshold data.