Chameleon vision

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The chameleon is among the most highly visually-oriented lizards, using this sense in prey capture, mating behavior, and predator avoidance. [1] Unique features of chameleon vision include a negative lens, a positive cornea, and monocular focusing. The development of the chameleon visual system could have evolved to aid in prey capture and/or in predator avoidance.

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Chameleon eye Chamaleon2.jpg
Chameleon eye

The angle, or amplitude, of eye movement in chameleons is very large for a vertebrate [1] and the eyes move independently of each other. [2] This allows a chameleon to watch an approaching object while simultaneously scanning the rest of its environment. [1] Chameleon eyes protrude laterally from the head, giving the lizard panoramic sight. [2] An eyelid fused to the pupil protects the eyes, leaving only a small part exposed. [2] With a negative (nearsighted or concave) lens and a positive (farsighted or convex) cornea, chameleons use a method of monocular focusing to judge distance called corneal accommodation. Each eye focuses independently, which is achieved by the chameleon eye’s unique anatomy of separated nodal and center points of the eye. [3] Finally, “striated rather than smooth ciliary muscle in sauropsids” allows for rapid focusing. [3]

Negative lens

Chameleon eyes feature a negative lens, meaning that the lens is concave. This increases retinal image size, allowing more precise focusing. [3] [4] In fact, image magnification in chameleons is higher in a scaled comparison to all other vertebrates eyes. [4]

Positive cornea

While the lens are negative, the cornea of chameleon eyes are positive, meaning that it is convex. The increased power of the cornea also contributes to more precise focusing than in other vertebrates. [3] The cornea improves sight resolution in a narrower field of vision. [3]

Monocular focusing and corneal accommodation

The combination of a negative lens and a positive cornea in the chameleon eye allow for accurate focusing by corneal accommodation. [4] Using corneal accommodation for depth perception [5] makes the chameleon the only vertebrate to focus monocularly. [1] While sight is primarily independent in the two chameleon eyes, the eye that first detects prey will guide accommodation in the other eye. [3] Contrary to the previous belief that chameleons used stereopsis (both eyes) for depth perception, research has shown monocular focusing to be more likely. [6] Depending on the chameleon's step in the predation sequence, corneal accommodation can be coupled, meaning the eyes independently focus on the same object. [3] When scanning the environment and in judging distance to prey, vision and accommodation are uncoupled: the eyes are focusing on different objects, such as the environment and the newly-sighted prey. Immediately before the chameleon's characteristic tongue is extended, accommodation in both eyes is coupled: both eyes focus independently on the prey. [3] Imprecise alignment of the images from each eye, as demonstrated by measuring various angles from eye to target, shows that stereopsis is unlikely for depth perception the chameleon. [3]

Nodal point separation

The nodal point in the eye is the point at which "lines connecting points in the scene and corresponding points in the image intersect." [5] In chameleons, the nodal point is located a significant distance before the center of rotation, the point around which the eye rotates in the eye socket. As a result of this nodal point separation, images of objects move more or less on the retina based on their distance from the chameleon. The position of an image on the retina is the "primary means by which chameleons judge distance." [3] Therefore, the rotation of one eye informs the chameleon of the "relative distances of different objects." [5] An important effect of the ability to judge distance with one eye is that the head does not have to be turned to allow stereoptic viewing of the object. [7]

Evolution

Chameleons as an evolutionary transition to stereopsis

A suggested theory for the evolution of squamate vision is that corneal accommodation and monocular depth perception are "primitive" mechanisms in comparison to binocular vision and stereopsis. [3] Chameleons use an alternative strategy to stereopsis in functional coupling of the eyes immediately before the tongue shot. This differs from stereopsis in that the images from both eyes are not reconciled into one. However, it is possible that this was first used for neural static reduction. [3] This suggests that chameleons could be seen as a transition between independent and coupled eye use. [8] However, it is also possible that the chameleon vision system is an alternative, equally successful mode of prey capture and predator avoidance, and perhaps more appropriate for the chameleon's niche as a camouflaged, arboreal hunter than other vision systems.

Prey/predator causes of chameleon eye development

The chameleon, a camouflaged, slow-moving lizard, is an arboreal hunter that hides and ambushes prey. [2] Prey and predators alike can be sighted and monitored using monocular depth perception. Also, nodal point separation allows distance to be judged with one eye, so minimal head movement is needed by the chameleon in watching its surroundings, reinforcing the chameleon strategy of inconspicuousness. [5]

Prey capture

The specialized strategy by which chameleons capture prey is reflected in their sensory anatomy and use, in particular that of vision. [2] First, prey is sighted and distance assessed using one eye. [6] To avoid detection by prey, a chameleon uses minimal head movement, made possible by nodal point separation. [5] The chameleon then slowly turns its head toward the prey. Both eyes focus independently on the prey before the tongue shot. [3]

Predator avoidance

The chameleon predator avoidance response is vision-mediated. [1] In predator avoidance, chameleons use minimal head movement and a unique method to monitor potential threats. Due to nodal point separation, a chameleon can judge distance to a potential threat with minimal head movement needed. When confronted with a potential threat, chameleons rotate their slender bodies to the opposite side of their perch to avoid detection. [1] They will keep moving around the branch to keep the branch between themselves and the threat and to keep the threat in their line of sight. [1] If the branch is narrow, a chameleon can observe a threat binocularly around the branch. While a wide branch might present a difficulty in depth perception to another lizard as it is forced to view the threat monocularly, a chameleon due to corneal accommodation and nodal point separation can judge distance between itself and a potential threat with only one eye viewing the threat. [1]

Comparison to the sandlance fish

While the chameleon eye is unique in lizards, parallels exist in other animals. In particular, the barred sandburrower fish shares key vision features with the chameleon. This is because the environmental circumstances such as the need for camouflaged quick prey capture that led to the development of the chameleon eye seem to have acted on the sandburrower fish as well. [7] Rapid predatory attacks are made possible through the chameleon and the sandlances' striated corneal muscles allowing for corneal accommodation, a reduced power lens, and increased corneal power. [3] A nearly complete eyelid covering and lack of head movement due to nodal separation reduce conspicuousness to prey and predators. [7]

Related Research Articles

Eye Organ that detects light and converts it into electro-chemical impulses in neurons

Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.

Binocular vision Ability to perceive a single three-dimensional image of surroundings with two eyes

In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Neurological researcher Manfred Fahle has stated six specific advantages of having two eyes rather than just one:

  1. It gives a creature a "spare eye" in case one is damaged.
  2. It gives a wider field of view. For example, humans have a maximum horizontal field of view of approximately 190 degrees with two eyes, approximately 120 degrees of which makes up the binocular field of view flanked by two uniocular fields of approximately 40 degrees.
  3. It can give stereopsis in which binocular disparity provided by the two eyes' different positions on the head gives precise depth perception. This also allows a creature to break the camouflage of another creature.
  4. It allows the angles of the eyes' lines of sight, relative to each other (vergence), and those lines relative to a particular object to be determined from the images in the two eyes. These properties are necessary for the third advantage.
  5. It allows a creature to see more of, or all of, an object behind an obstacle. This advantage was pointed out by Leonardo da Vinci, who noted that a vertical column closer to the eyes than an object at which a creature is looking might block some of the object from the left eye but that part of the object might be visible to the right eye.
  6. It gives binocular summation in which the ability to detect faint objects is enhanced.
Stereoscopy Technique for creating or enhancing the illusion of depth in an image

Stereoscopy is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word stereoscopy derives from Greek στερεός (stereos) 'firm, solid', and σκοπέω (skopeō) 'to look, to see'. Any stereoscopic image is called a stereogram. Originally, stereogram referred to a pair of stereo images which could be viewed using a stereoscope.

Depth perception Visual ability to perceive the world in 3D

Depth perception is the visual ability to perceive the world in three dimensions (3D) and the distance of an object. Depth sensation is the corresponding term for non-human animals, since although it is known that they can sense the distance of an object, it is not known whether they perceive it in the same subjective way that humans do.

Far-sightedness Eye condition in which light is focused behind instead of on the retina

Far-sightedness, also known as long-sightedness, hypermetropia, or hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blurred effect is due to incoming light being focused behind, instead of on, the retina wall due to insufficient accommodation by the lens. Minor hypermetropia in young patients is usually corrected by their accommodation, without any defects in vision. But, due to this accommodative effort for distant vision, people may complain of asthenopic symptoms during prolonged reading. Some hypermetropes can see clear at distance, but near vision may be blurred due to insufficient accommodation. For this reason, this defect is referred as far-sightedness. If the hypermetropia is high, there will be defective vision for both distance and near. People may also experience accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus. Newborns are almost invariably hypermetropic, but it gradually decreases as the newborn gets older.

Presbyopia Medical condition associated with aging of the eye

Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Often referred to as 'reading blur' that affects many adults over the age of 40, symptoms include difficulty reading small print which results in having to hold reading material farther away. Other symptoms associated can be headaches, and eyestrain. Different people will have different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia. This condition is similar to hypermetropia or far-sightedness which starts in childhood and exhibits similar symptoms of blur in the vision for close objects.

Autostereogram Visual illusion of 3D scene achieved by unfocusing eyes when viewing specific 2D images

An autostereogram is a single-image stereogram (SIS), designed to create the visual illusion of a three-dimensional (3D) scene from a two-dimensional image. In order to perceive 3D shapes in these autostereograms, one must overcome the normally automatic coordination between accommodation (focus) and horizontal vergence. The illusion is one of depth perception and involves stereopsis: depth perception arising from the different perspective each eye has of a three-dimensional scene, called binocular parallax.

Intraocular lens Lens implanted in the eye to treat cataracts or myopia

Intraocular lens (IOL) is a lens implanted in the eye as part of a treatment for cataracts or myopia. If the natural lens is left in the eye, the IOL is known as phakic, otherwise it is a pseudophakic, or false lens. Such a lens is typically implanted during cataract surgery, after the eye's cloudy natural lens has been removed. The pseudophakic IOL provides the same light-focusing function as the natural crystalline lens. The phakic type of IOL is placed over the existing natural lens and is used in refractive surgery to change the eye's optical power as a treatment for myopia (nearsightedness). This is an alternative to LASIK.

Eye examination A series of tests assessing vision and pertaining to the eyes

An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, ophthalmologist, orthoptist, or an optician. Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.

Stereopsis is a term that is most often used to refer to the perception of depth and three-dimensional structure obtained on the basis of visual information deriving from two eyes by individuals with normally developed binocular vision. Because the eyes of humans, and many animals, are located at different lateral positions on the head, binocular vision results in two slightly different images projected to the retinas of the eyes. The differences are mainly in the relative horizontal position of objects in the two images. These positional differences are referred to as "horizontal disparities" or, more generally, "binocular disparities". Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are naturally present when viewing a real three-dimensional scene with two eyes, they can also be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy. The perception of depth in such cases is also referred to as "stereoscopic depth".

Monocular vision is vision in which both eyes are used separately in animals and monocular vision in human species is vision when only one eye is used. By using the eyes in this way the field of view is increased, while depth perception is limited. The eyes of an animal with monocular vision are positioned on opposite sides of the animal's head, giving it the ability to see two objects at once. This is usually most commonly seen with prey animals, as the reason why their eyes are placed on either side of their head is to make it easier for them to look out for predators, which usually have forward-facing eyes to make it easier to find prey. However, there are some exceptions to this rule, usually if the predator is an animal that is often preyed upon by a greater predator or sport an anatomy that makes it very difficult for it to see straight, such as a short, stiff neck that would limit its head movement, and therefore would require its eyes to be on either side. Bimonocular vision also named two-eyed monocular vision or seeing in a monocular way over the entire field of view without visual field loss and without fusion is vision in which both eyes are used separately in human species and was discovered in 2018 by John Post a Belgian inventor in optics. The word monocular comes from the Greek root, mono for single, and the Latin root, oculus for eye.

Emmetropia State of vision

Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfect vision. A human eye in a state of emmetropia requires no corrective lenses; the vision scores well on a visual acuity test.

Equine vision

The equine eye is one of the largest of any land mammal. Its visual abilities are directly related to the animal's behavior; for example, it is active during both day and night, and it is a prey animal. Both the strengths and weaknesses of the horse's visual abilities should be taken into consideration when training the animal, as an understanding of the horse's eye can help to discover why the animal behaves the way it does in various situations.

Vision in toads

The neural basis of prey detection, recognition, and orientation was studied in depth by Jörg-Peter Ewert in a series of experiments that made the toad visual system a model system in neuroethology. He began by observing the natural prey catching behavior of the common European toad.

Chromostereopsis Visual illusion whereby the impression of depth is conveyed in two-dimensional color images

Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red-blue or red-green colors, but can also be perceived with red-grey or blue-grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration.

Vision of humans and other organisms depends on several organs such as the lens of the eye, and any vision correcting devices, which use optics to focus the image.

Vision in fish

Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish retinas generally have both rod cells and cone cells, and most species have colour vision. Some fish can see ultraviolet and some are sensitive to polarised light.

The eye, like any other optical system, suffers from a number of specific optical aberrations. The optical quality of the eye is limited by optical aberrations, diffraction and scatter. Correction of spherocylindrical refractive errors has been possible for nearly two centuries following Airy's development of methods to measure and correct ocular astigmatism. It has only recently become possible to measure the aberrations of the eye and with the advent of refractive surgery it might be possible to correct certain types of irregular astigmatism.

Eagle eye

The eagle eye is among the sharpest in the animal kingdom, with an eyesight estimated at 4 to 8 times stronger than that of the average human. Although an eagle may only weigh 10 pounds (4.5 kg), its eyes are roughly the same size as those of a human. Eagle weight varies: a small eagle could weigh 700 grams (1.5 lb), while a larger one could weigh 6.5 kilograms (14 lb); an eagle of about 10 kilograms (22 lb) weight could have eyes as big as that of a human being who weighs 200 pounds (91 kg). Although the size of the eagle eye is about the same as that of a human being, the back side shape of the eagle eye is flatter. Their eyes are stated to be larger in size than their brain, by weight. Color vision with resolution and clarity are the most prominent features of eagles' eyes, hence sharp-sighted people are sometimes referred to as "eagle-eyed". Eagles can identify five distinctly colored squirrels and locate their prey even if hidden.

<i>Limnichthys fasciatus</i> Species of fish

Limnichthys fasciatus, the barred sand burrower, is a species of sandburrower. It is noted for its highly developed eyes, with a structure similar to the eyes of a chameleon, which has led it to be described as marine chameleon. Its fully grown length measures between 20 millimetres (0.79 in) and 40 millimetres (1.6 in). The species is native to reefs in the Indo-Pacific. The fish preys on plankton prey by surprise attacking it from a hiding in loose sand, with only the eyes protruding from the sand.

References

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  2. 1 2 3 4 5 Sandor, Peter S.; Maarten A. Frens; Volker Henn (2001). "Chameleon eye position obeys Listing's law". Vision Research. 41: 2245–2251. doi: 10.1016/s0042-6989(01)00111-0 .
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Ott, M.; F. Schaeffel; W. Kirmse (1998). "Binocular vision and accommodation in prey-catching chameleons". Journal of Comparative Physiology A. 182: 319–330. doi:10.1007/s003590050182.
  4. 1 2 3 Ott, Matthias; Frank Schaeffel (1995). "A negatively powered lens in the chameleon". Nature. 373: 692–694. doi:10.1038/373692a0.
  5. 1 2 3 4 5 Srinivasan, Mandyam V. (1999). "Ecology: When one eye is better than two". Nature. 399: 305–307. doi:10.1038/20550.
  6. 1 2 Harkness, Lindesay (1977). "Chameleons use accommodation clues to judge distance". Nature. 267: 346–349. doi:10.1038/267346a0.
  7. 1 2 3 Pettigrew, John D.; Shaun P. Collin; Matthias Ott (1999). "Convergence of specialised behaviour, eye movements and visual optics in the sandlance (Teleostei) and the chameleon (Reptilia)". Current Biology. 9: 421–424. doi: 10.1016/s0960-9822(99)80189-4 .
  8. ↑ Ott, Matthias (2001). "Chameleons have independent eye movements but synchronise both eyes during saccadic prey tracking". Experimental Brain Research. 139: 173–179. doi:10.1007/s002210100774.
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