Burst deep brain stimulation for Parkinson's disease: A systematic review of efficacy and safety.

Two different systems of adaptation operate in the retina when the eye adapts for seeing in light and darkness, the photochemical adaptation and the neuronal adaptational one. The photochemical process is well known, a very slow working system that is related to the concentration of photopigments in the photoreceptors in the retina. The human eye will need about 30 min to adapt for seeing in the dark. Bleaching and regeneration of photopigments was first discovered (in the frog) at the end of the 19th century and described by Boll () and a couple of years later by Kuhne (). More than 50 years afterwards, Wald () and Hecht & Wald () could identify the substance as rhodopsin and the importance of Vitamin A in vision. About 30 years later, Dowling (, ) quantitatively established that the photochemical adaptation is indeed characterized by bleaching and regeneration of photopigments (rhodopsin). Dowling () and Dowling & Hubbard () identified and described the two processes of adaptation and the differences between them. In strong background light intensities that bleached measurable concentrations of rhodopsin, there was a log‐linear relationship between visual sensitivity measured electrophysiologically and the content of rhodopsin in the rod‐dominated retina (of the rat). However, in weak background light that bleached non‐measurable concentrations of rhodopsin, there was found not to be such a relationship. In such case, the retina also rapidly regained its visual sensitivity in darkness. This adaptation process—fast and not depending on the content of visual pigments in the retina—implying on a swiftly working system and relying on neuronal activity and/or a network of neurons was consequently termed ‘network’ or neuronal adaptation.