Psychophysically derived visual mechanisms in turtle. II--Spatial properties.

Visual mechanisms isolated in Pseudemys by the two-color threshold technique of Stiles show peak wavelength sensitivities at 650 nm (red light) and 540 nm (green light). Ricco critical areas were measured for the two test wavelengths under three conditions: dark, moderate and intense backgrounds. As expected, critical spatial areas decreased with light adaptation. Under dark adaptation only rods and red-sensitive cones were operative, and one photon per 12 rods was sufficient for green-light threshold, as was one photon per four red-sensitive cones for red-light threshold. Rods apparently pool their information among the several receptors within the threshold area. Under light adaptation, rods were not functional and thresholds were determined by red-sensitive and green-sensitive cones alone. Cones did not share information over many receptors, requiring close to one photon per receptor to function at threshold.