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Special Senses
Published in Pritam S. Sahota, James A. Popp, Jerry F. Hardisty, Chirukandath Gopinath, Page R. Bouchard, Toxicologic Pathology, 2018
Kenneth A. Schafer, Oliver C. Turner, Richard A. Altschuler
The tapetum lucidum is a specialized layer in the inner choroid of certain laboratory animals, but not pigs, rodents, rabbits, and nonhuman primates (Samuelson 2007). The tapetum lucidum is mostly in the superior half of the ocular fundus and, in the dog, is composed of specialized epithelium (tapetum cellulosum) with the purpose of reflecting light to aid vision in low ambient light. The epithelial cells have intracytoplasmic, reflective, crystalline rods (tapetal rods) rich in zinc. The cells are arranged in multiple layers, centrally tapering to a single cell thickness at the periphery, and the RPE overlying the tapetum lucidum is nonpigmented. Funduscopically, the tapetum lucidum is hyperreflective when the overlying sensory retina is thinned (usually the outer retina) and may be hyporeflective in appearance when it is a target of toxicity (e.g., β-adrenergic blocking agents) (Schiavo et al. 1984).
Contrast adaptation
Published in Pablo Artal, Handbook of Visual Optics, 2017
Because the light sensitivity of humans rods is similar to the one of nocturnal mammals (also our rods can respond to a single photon), the only way for nocturnal animals to increase their CS at night is to increase retinal illuminance. This is achieved by lowering the aperture stop, that is, the ratio of focal length to pupil size (Figure 21.6). For instance, a barn owl has an aperture stop of less than 1, and a diurnal animal like a chameleon has an aperture stop of about 5. The ratio of retinal illuminance in both types of eyes is determined by the ratio of the squares of the two aperture stops, 1/25. Accordingly, the retinal image is 25 times brighter in the owl compared to the chameleon. Young children may have an aperture stop of around 2 (anterior focal length 16.7 mm, pupil size 8 mm), which means that their retinal image is only about four times darker than in an owl. Since CS rises with the square root of luminance in dim light, retinal illuminance can explain a difference in CS of a factor of 2. However, it was found that nocturnal mammals, like cat and owl, have about a six times higher CS at low light levels (Pasternak and Merigan 1981; Orlowski et al. 2012) than human subjects, and the difference cannot be fully explained only by optics. Cats and dogs have developed highly reflective layers behind the photoreceptors, the tapetum lucidum, to increase the chance that photons can be absorbed in a second pass. It has been calculated that the tapetum increases light sensitivity (and thereby CS) by further 29%. Because reflected photons are more scattered, this may be at the cost of visual acuity.
Scene through the eyes of an apex predator: a comparative analysis of the shark visual system
Published in Clinical and Experimental Optometry, 2018
There are two types of choroidal tapeta – a choroidal tapetum fibrosum consisting of closely packed, regularly arranged, collagenous fibrils running parallel to the retinal surface; and a choroidal tapetum cellulosum, which is formed by arrays of specialised endothelial cells containing reflective guanine or riboflavin crystals or zinc cysteine rodlets. Sharks possess a choroidal tapetum cellulosum or tapetum lucidum, which lies behind the retinal photoreceptors in the choroidal coat,1973 and reflects light, not initially absorbed by the photoreceptors, back onto the photoreceptor outer segments to enhance retinal sensitivity1973 (Figure 8).