Accommodation (eye)
Overview
 
Accommodation is the process by which the vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 changes optical power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

 to maintain a clear image (focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

) on an object as its distance changes.

Accommodation acts like a reflex, but can also be consciously controlled. Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body
Ciliary body
The ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes. It is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. This epithelium produces the aqueous humor. The inner layer is transparent and covers...

 (in humans up to 15 diopters). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles.

The young human eye can change focus from distance to 7 cm from the eye in 350 milliseconds.
Encyclopedia
Accommodation is the process by which the vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 changes optical power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

 to maintain a clear image (focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

) on an object as its distance changes.

Accommodation acts like a reflex, but can also be consciously controlled. Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body
Ciliary body
The ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes. It is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. This epithelium produces the aqueous humor. The inner layer is transparent and covers...

 (in humans up to 15 diopters). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles.

The young human eye can change focus from distance to 7 cm from the eye in 350 milliseconds. This dramatic change in focal power of the eye of approximately 12 diopters (a diopter is 1 divided by the focal length in meters) occurs as a consequence of a reduction in zonular
Zonule of Zinn
The zonule of Zinn is a ring of fibrous strands connecting the ciliary body with the crystalline lens of the eye....

 tension induced by ciliary muscle
Ciliary muscle
The ciliary muscle is a ring of striated smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye not the size of the pupil which is...

 contraction. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude has declined so the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

 occurs, those who are emmetropic (do not require optical correction for distance vision) will need an optical aid for near vision; those who are myopic (nearsighted and require an optical correction for distance vision), will find that they see better at near without their distance correction; and those who are hyperopic (farsighted) will find that they may need a correction for both distance and near vision. The age-related decline in accommodation occurs almost universally to less than 2 dioptres by the time a person reaches 45 to 50 years, by which time most of the population will have noticed a decrease in their ability to focus on close objects and hence require glasses for reading or bifocal lenses. Accommodation decreases to to essentially 0 dioptres at the age of 70 years.

It is normally accompanied by a convergence of the eyes to keep them directed at the same point, sometimes termed accommodation convergence reflex.

Theories of mechanism

  • Helmholtz - The most widely held theory of accommodation is that proposed by Hermann von Helmholtz
    Hermann von Helmholtz
    Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...

     in 1855. When viewing a far object, the circularly arranged ciliary muscle
    Ciliary muscle
    The ciliary muscle is a ring of striated smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye not the size of the pupil which is...

     relaxes causing the lens
    Lens (anatomy)
    The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

     zonules
    Zonule of Zinn
    The zonule of Zinn is a ring of fibrous strands connecting the ciliary body with the crystalline lens of the eye....

     and suspensory ligaments to pull on the lens, flattening it. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera
    Sclera
    The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...

    . When viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form.

  • Schachar - Ronald Schachar has proposed an alternative theory which indicates that focus by the human lens is associated with increased tension on the lens via the equatorial zonules; that when the ciliary muscle contracts, equatorial zonular tension is increased, causing the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing. As a consequence of the changes in lens shape during human in vivo accommodation, the central optical power of the lens increases and spherical aberration of the lens shifts in the negative direction. Because of the increased equatorial zonular tension on the lens during accommodation, the stress on the lens capsule is increased and the lens remains stable and unaffected by gravity. The same shape changes that occur to the crystalline lens during accommodation are observed when equatorial tension is applied to any encapsulated biconvex object that encloses a minimally compressible material (volume change less than approximately 3%) and has an elliptical profile with an aspect ratio ≤ 0.6 (minor axis/major axis ratio). Equatorial tension is very efficient when applied to biconvex objects that have a profile with an aspect ratio ≤ 0.6. Minimal equatorial tension and only a small increase in equatorial diameter causes a large increase in central curvature. This explains why the aspect ratio of a vertebrate crystalline lens can be used to predict the qualitative amplitude of accommodation of the vertebrate eye. Vertebrates that have lenses with aspect ratios ≤ 0.6 have high amplitudes of accommodation; e.g., primates and falcons, while those vertebrates with lenticular aspect ratios > 0.6 have low amplitudes of accommodation; e.g. owls and antelopes. The decline in the amplitude of accommodation eventually results in the clinical manifestation of presbyopia
    Presbyopia
    Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

     As the equatorial diameter of the lens continuously increases over life, baseline zonular tension simultaneously declines. This results in a reduction in baseline ciliary muscle length that is associated with both lens growth and increasing age. Since the ciliary muscle, like all muscles, has a length-tension relationship, the maximum force the ciliary muscle can apply decreases, as its length shortens with increasing age. This is the etiology of the age-related decline in accommodative amplitude that results in presbyopia.

  • Catenary - D. Jackson Coleman
    D. Jackson Coleman
    D. Jackson Coleman is the John Milton McLean Professor of Ophthalmology and Chairman Emeritus of the Department of Ophthalmology at New York Presbyterian Hospital-Weill Medical College of Cornell University.-Career:...

     proposes that the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye. Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape in the mechanically reproducible state of a steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens, i.e. the shape, in cross section, of a catenary
    Catenary
    In physics and geometry, the catenary is the curve that an idealised hanging chain or cable assumes when supported at its ends and acted on only by its own weight. The curve is the graph of the hyperbolic cosine function, and has a U-like shape, superficially similar in appearance to a parabola...

    . The anterior capsule and the zonule form a trampoline shape or hammock shaped surface that is totally reproducible depending on the circular dimensions, i.e. the diameter of the ciliary body (Müeller’s muscle). The ciliary body thus directs the shape like the pylons of a suspension bridge, but does not need to support an equatorial traction force to flatten the lens.

Induced effects of accommodation

When someone accommodates to a near object, they also converge
Convergence (eye)
In ophthalmology, convergence is the simultaneous inward movement of both eyes toward each other, usually in an effort to maintain single binocular vision when viewing an object. This action is mediated by the medial rectus muscle, which is innervated by Cranial nerve III...

 their eyes and constrict their pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...

s. The combination of these three movements (accommodation, convergence and miosis
Miosis
Miosis is the constriction of the pupil of the eye to two millimeters or less...

) is under the control of the Edinger-Westphal nucleus
Edinger-Westphal nucleus
The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...

 and is referred to as the near triad. Although, it is clear that convergence allows to focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

 the object's image on the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

, the functional role of the pupillary contraction remains less clear. Arguably, it may increase the depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

 by reducing the aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 of the eye, and thus reduce the amount of accommodation needed to bring the image in focus on the retina.

There is a measurable ratio between how much convergence takes place because of accommodation (AC/A ratio, CA/C ratio). Abnormalities with this can lead to many orthoptic problems.

Accommodative dysfunction

Duke-Elder classified a number of accommodative dysfunctions:
  • Accommodative insufficiency
    Accommodative insufficiency
    Accommodative insufficiency involves the inability of the eye to focus properly on an object. Approximately 80 percent of children diagnosed with CI also demonstrate AI....

  • Ill-sustained accommodation
  • Accommodative infacility
  • Paralysis of accommodation
  • Spasm of accommodation
    Spasm of accommodation
    A spasm of accommodation is a condition in which the ciliary muscle of the eye remains in a constant state of contraction. Normally, this contraction bends the lens to allow the eye to "accommodate" for near-vision. However in a state of perpetual contraction, the ciliary muscle cannot relax when...


Disorders of accommodation

  • Myopia
    Myopia
    Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

  • Accommodative esotropia
    Esotropia
    Esotropia is a form of strabismus, or "squint", in which one or both eyes turns inward. The condition can be constantly present, or occur intermittently, and can give the affected individual a "cross-eyed" appearance...

  • Accommodative excess
  • Accommodative spasm
  • Accommodative lag
  • Latent hyperopia
    Hyperopia
    Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

  • Presbyopia
    Presbyopia
    Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

  • Pseudomyopia
    Pseudomyopia
    Pseudomyopia refers to an intermittent and temporary shift in refraction of the eye towards myopia, in which the focusing of light in front of the retina is due to a transient spasm of the ciliary muscle causing an increase in the refractive power of the eye...


Other

  • Accommodation reflex
    Accommodation reflex
    The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object , comprising coordinated changes in vergence, lens shape and pupil size...

  • Amplitude of accommodation
    Amplitude of accommodation
    Amplitude of accommodation is a measurement of the eye’s ability to focus clearly on objects at near distances . This eye focusing range for a child is usually about 5–7.5 cm . For a young adult, it is 10–15 cm . The focus range for a 45-year-old adult is about 50 cm...

  • Cycloplegia
    Cycloplegia
    Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation.-Anatomy:The iris is the heavily pigmented colored part of the eye. It has a contractile diaphragm in front of the lens with a central opening called the pupil...

  • Cyclospasm
    Cyclospasm
    Cyclospasm is the contraction of the ciliary muscle in the eye, in the accommodation of focus for near vision. Cyclospasm may also exert tensions on the trabecular meshwork, opening the pores and facilitating outflow of the aqueous humour into the canal of Schlemm. The increase in aqueous humour...

  • Edinger-Westphal nucleus
    Edinger-Westphal nucleus
    The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...

  • Mandelbaum Effect
    Mandelbaum Effect
    The Mandelbaum Effect is the tendency for people to focus nearby in conditions of poor visibility. It was first codified by Mandelbaum in 1960. Because dirty glass can aggravate the effect, potentially causing a pilot or driver to miss seeing an obstacle or hazard, it is a strong safety argument...

  • Negative relative accommodation
    Negative relative accommodation
    Negative relative accommodation was proposed by Prof. Joseph Kearney of Oxford University in 1967, is a measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision. This measurement is typically obtained by an orthoptist, ophthalmologist or optometrist...

  • Positive relative accommodation
    Positive relative accommodation
    Positive relative accommodation is a measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision. This measurement is typically obtained by an orthoptist, ophthalmologist or optometrist during an eye examination using a phoropter...

  • Adaptation (eye)
    Adaptation (eye)
    In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.-Efficacy:The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the...


External links

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