Fig. 11. Classic dark adaptation curves. The rod-cone break occurs between 6 and 12 minutes. Many parameters affect cone plateau level, the time of the cone-rod break, and the final (scotopic) sensitivity. In this example, fields were centrally fixated and the size of the field was varied. With 2 degree fields, little appreciable change in threshold is noted after an initial decline. This is due to the central rod-free zone. Large test fields stimulate rods in the periphery resulting in a lowering of threshold, Log I at a time when rod sensitivity has begun to exceed cone sensitivity. By 30 minutes, threshold to detect the largest adapting field has begun to asymptote. The larger field does not benefit from stimulation of the center because the light levels are far below threshold for detection by the cones in the central retina. (Hecht S, Haig C, Wald G: The dark adaptation of retinal fields of different size and location. J Gen Physiol 19:321, 1935.)