Eigengrau (German: "intrinsic gray" / literally: "own gray"), also called Eigenlicht ("intrinsic light"), dark light, or brain gray, is the uniform dark gray background that many people report seeing in the absence of light. The term dates back to the nineteenth century, but has rarely been used in recent scientific publications. Nowadays, the phenomenon is more commonly referred to as "visual noise" or "background adaptation".
Eigengrau is perceived as lighter than a black object in normal lighting conditions, because contrast is more important to the visual system than absolute brightness. For example, the night sky looks darker than eigengrau because of the contrast provided by the stars.
Researchers noticed early on that the shape of intensity-sensitivity curves could be explained by assuming that an intrinsic source of noise in the retina produces random events indistinguishable from those triggered by real photons. Later experiments on rod cells of cane toads (Bufo marinus) showed that the frequency of these spontaneous events is strongly temperature-dependent, which implies that they are caused by the thermal isomerization of rhodopsin. In human rod cells, these events occur about once every 100 seconds on average, which, taking into account the number of rhodopsin molecules in a rod cell, implies that the half-life of a rhodopsin molecule is about 420 years. The indistinguishability of dark events from photon responses supports this explanation, because rhodopsin is at the input of the transduction chain. On the other hand, processes such as the spontaneous release of neurotransmitters cannot be completely ruled out.
- Ladd, Trumbull (1894). "Direct control of the retinal field.". Psychological Review 1: 351–355. doi:10.1037/h0068980. Retrieved 28 September 2012.
- Hansen RM, Fulton AB (January 2000). "Background adaptation in children with a history of mild retinopathy of prematurity". Invest. Ophthalmol. Vis. Sci. 41 (1): 320–4. PMID 10634637.
- Wallach, Hans (1948). "Brightness Constancy and the Nature of Achromatic Colors". Journal of Experimental Psychology 38 (3): 310–324. doi:10.1037/h0053804. PMID 18865234.
- Barlow, H. B. (1972). "Dark and Light Adaptation: Psychophysics.". Visual Psychophysics. New York: Springer-Verlag. ISBN 0-387-05146-5.
- Barlow, H. B. (1977). "Retinal and Central Factors in Human Vision Limited by Noise". Vertebrate Photoreception. New York: Academic Press. ISBN 0-12-078950-7.
- Baylor, D. A.; Matthews, G; Yau, K.-W. (1980). "Two components of electrical dark noise in toad retinal rod outer segments". Journal of Physiology 309: 591–621. PMC 1274605. PMID 6788941.
- Baylor, Denis A. (1 January 1987). "Photoreceptor Signals and Vision". Investigative Ophthalmology & Visual Science 28 (1): 34–49. PMID 3026986.
- Shapley, Robert; Enroth-Cugell, Christina (1984). "Visual Adaptation and Retinal Gain Controls". Progress in Retinal Research 3: 263–346. doi:10.1016/0278-4327(84)90011-7.