Fig. 3. Aulhorn's curves show the influence of background luminance on the differential light threshold. As the background luminance increases (lower curves) and the retina becomes light adapted (i.e., photopic adaptation), the overall retinal sensitivity decreases and the curve peaks centrally at the fovea. As the background luminance decreases (upper curves) and the eye becomes dark adapted (i.e., scotopic adaptation), overall retinal sensitivity increases and the curves develop a central depression. (Lynn JR, Fellman RL, Starita RJ: Exploring the normal visual field. In Ritch R, Shields MD, Krupin T [eds]: The Glaucomas. Vol 1. St Louis: CV Mosby, 1989.)