©1999 Melissa Kaplan
Snakes have rods and cones in their eyes, as do we, though in different numbers. They do not have the diversity colored oil droplets (presumed to have been lost when snakes when nocturnal and subterranean) in their photoreceptors that mammals and birds do, so, while they do have color vision, it isn't as broad ranged as ours is. They do have a yellow filter which, filling the lens, absorbs ultraviolet light, protecting the eye.
Snakes use a combination of infrared vision (developed in the trigeminal nerve), variable (by species) visual acuity and color detection, limited eye mobility, and chemosensation to find prey and recognize features in their environment (including their keepers).
Lizard (including geckos) and turtle retinas contain multicolored oil droplets in their photoreceptors, so they can perceive color. The opsin proteins in the cones in the eye are "calibrated" to detect different wavelengths. In many species, this enables them to see into the higher wavelengths beyond the scope of unaided human vision: into the UV range.
Nocturnal reptiles usually have smaller eyes than diurnal ones, but relatively large pupillary and lens aperture and cornea. This improves their light-gathering ability, but at the same time reduces visual acuity.
Lizards can focus on near and far by squeezing or stretching their lenses, using the ciliary muscles and annular pads. Pupils dilate and contract in response to light. Nocturnal geckos like the tokay have a stenopaic pupil: contracts into a vertical slit composed of a linear array of dots. Some nocturnal lizards have slit pupils, others are round. Lizards, unlike other reptiles, have a choroid body, called the conus papillaris. Projecting out into the vitrious humor, it nourishes the cornea.
Sinclair, Sandra. 1985. How Animals See: Other Visions of Our World. Croom Helm, London.
Grace, Michael S. 1997, The visual system and non-visual photoreception. In: The Biology Husbandry, and Health Care of Reptiles. Lowell Ackerman, DVM, ed. Vol. I, pp. 325-341. TFH Publishing, Neptune City, NJ.
about binocular coordination of eye movements: is there a chameleon in
the primate family tree?
Many animals with laterally placed eyes, such as chameleons, move their eyes independently of one another. In contrast, primates with frontally placed eyes and binocular vision must move them together so that both eyes are aimed at the same point in visual space. Is binocular coordination an innate feature of how our brains are wired, or have we simply learned to move our eyes together? This question sparked a controversy in the 19(th) century between two eminent German scientists, Ewald Hering and Hermann von Helmholtz. Hering took the position that binocular coordination was innate and vigorously challenged von Helmholtz's view that it was learned. Hering won the argument and his hypothesis, known as Hering's Law of Equal Innervation, became generally accepted. New evidence suggests, however, that similar to chameleons, primates may program movements of each eye independently. Binocular coordination is achieved by a neural network at the motor periphery comprised of motoneurons and specialized interneurons located near or in the cranial nerve nuclei that innervate the extraocular muscles. It is assumed that this network must be trained and calibrated during infancy and probably throughout life in order to maintain the precise binocular coordination characteristic of primate eye movements despite growth, aging effects, and injuries to the eye movement neuromuscular system. Malfunction of this network or its ability to adaptively learn may be a contributing cause of strabismus.
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