LIGHT ADAPTATION AND SENSITIVITY CONTROLLING MECHANISMS IN VERTEBRATEPHOTORECEPTORS

The human visual system can discriminate increment and decrement light stimuli over a wide range of ambient illumination; from moonlight to bright sunlight. Several mechanisms contribute to this properly but th e major ones reside in the retina and more specifically within the pho toreceptors themselves. Numerous studies in retinae from cold- and war m-blooded vertebrates have demonstrated the ability of the photorecept ors to respond in a graded manner to light increments and decrements e ven if these are applied during a background illumination that is expe cted to saturate the cells. In all photoreceptors regardless of type a nd species, three cellular mechanisms have been identified that contri bute to background desensitization and light adaptation. These gain co ntrolling mechanisms include. response-compression due to the non-line arity of the intensity-response function, biochemical modulation of th e phototransduction process and pigment bleaching. The overall ability of a photoreceptor to adapt to background lights reflects the relativ e contribution of each of these mechanisms and the light intensity ran ge over which they operate. In rods of most species, response-compress ion tends to dominate these mechanisms at light levels too weak to cau se significant pigment bleaching and therefore, rods exhibit saturatio n. In contrast, cones are characterized by powerful background-induced modulation of the phototransduction process at moderate to bright bac kground intensities where pigment bleaching becomes significant. There fore, cones do not exhibit saturation even when the level of ambient i llumination is raised by 6-7 log units. (C) 1998 Elsevier Science Ltd. All rights reserved.