Vision
The eyes are a very interesting sensory system and allow us (as well as reptiles) to detect shapes, movement, colour and detail within our environment. Vision is made possible through the existance of two types of photoreceptors in the eye:- rods and cones, which are distinguished from eachother by their shapes (from which their names are derived). These photoreceptors are found within the retina, located at the back of the eye and are responsible for converting light signals into electronic signals allowing us to visually perceive the world around us. Each are specialised for a different aspect of vision, as described below.
Rods are sensitive to the detection of shapes and movement and are sensitive to light so are responsible for vision in low light situations such as at night. However, rods have very low spatial acuity, meaning vision from these photoreceptors lacks sharpness. Furthermore, they do not allow for colour vision. Rods are in their greatest numbers around the 'periphery' of the retina, meaning they're most active in our peripheral vision, the corner of our vision. Conversely, cones work best in brighter conditions as they are relatively insensitive to light. They have very high spatial acuity meaning vision is sharp and focused, colour vision is also made possible with the presence of cones. The central part of the eye (the fovea) is populated exclusively by cones, processing the information of what you are directly looking at as opposed to what's perceived in the corner of your eye.
Retinas of all vertebrates possess rods but not all vertebrates have cones. Some amphibians possess colour vision, many reptiles do but comparatively few mammals see in colour with the exception of humans and some other higher primates. Although reptiles do possess colour vision, they do not have the same level of acuity as humans. The proportion of rods and cones varies considerably between different species. Animals active in bright conditions (during the day), possess both rods and cones. Photoreceptor cells contain a pigment with high light-absorbing capabilities. The pigments are composed of a protein called opsin, which is combined with the light absorbing derivative of vitamin A. Human eyes possess three types of cones sensitive to three different spectra, these are Red, Green and Blue receptors which allow us to perceive a number of colours within the visible light spectrum. Reptiles have an additional receptor, totalling four in number, which allows herptiles (and their cousins, the birds) to see within the Ultraviolet spectrum. This allows the animal to see wavelengths beyond those that humans can perceive and may allow the animal to distinguish colours that appear identical to humans.
The position of the eyes represents a trade-off between visual field and depth perception. Eyes positioned either side of the head allows each eye to scan separate portions of the environment and the total field of view at any moment is extensive. This is termed 'monocular vision' and is a common feature among prey species as it allows threat detection from most directions. Strictly monocular vision where the field of view between both eyes does not overlap is relatively rare but does occur in salamanders. Binocular vision occurs when the visual field from each eye overlaps. Reptiles demonstrate a 40° overlap of binocular vision, in comparison humans have as much as 140° of binocular vision.
Within the overlap two single fields of vision merge into one image producing stereoscopic vision. This form of vision gives the animal a sense of depth perception.
Rods are sensitive to the detection of shapes and movement and are sensitive to light so are responsible for vision in low light situations such as at night. However, rods have very low spatial acuity, meaning vision from these photoreceptors lacks sharpness. Furthermore, they do not allow for colour vision. Rods are in their greatest numbers around the 'periphery' of the retina, meaning they're most active in our peripheral vision, the corner of our vision. Conversely, cones work best in brighter conditions as they are relatively insensitive to light. They have very high spatial acuity meaning vision is sharp and focused, colour vision is also made possible with the presence of cones. The central part of the eye (the fovea) is populated exclusively by cones, processing the information of what you are directly looking at as opposed to what's perceived in the corner of your eye.
Retinas of all vertebrates possess rods but not all vertebrates have cones. Some amphibians possess colour vision, many reptiles do but comparatively few mammals see in colour with the exception of humans and some other higher primates. Although reptiles do possess colour vision, they do not have the same level of acuity as humans. The proportion of rods and cones varies considerably between different species. Animals active in bright conditions (during the day), possess both rods and cones. Photoreceptor cells contain a pigment with high light-absorbing capabilities. The pigments are composed of a protein called opsin, which is combined with the light absorbing derivative of vitamin A. Human eyes possess three types of cones sensitive to three different spectra, these are Red, Green and Blue receptors which allow us to perceive a number of colours within the visible light spectrum. Reptiles have an additional receptor, totalling four in number, which allows herptiles (and their cousins, the birds) to see within the Ultraviolet spectrum. This allows the animal to see wavelengths beyond those that humans can perceive and may allow the animal to distinguish colours that appear identical to humans.
The position of the eyes represents a trade-off between visual field and depth perception. Eyes positioned either side of the head allows each eye to scan separate portions of the environment and the total field of view at any moment is extensive. This is termed 'monocular vision' and is a common feature among prey species as it allows threat detection from most directions. Strictly monocular vision where the field of view between both eyes does not overlap is relatively rare but does occur in salamanders. Binocular vision occurs when the visual field from each eye overlaps. Reptiles demonstrate a 40° overlap of binocular vision, in comparison humans have as much as 140° of binocular vision.
Within the overlap two single fields of vision merge into one image producing stereoscopic vision. This form of vision gives the animal a sense of depth perception.
©2014 Cat Read