Accommodation (eye)

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Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens.

Accommodation is the process by which the eye increases optical power to maintain a clear image (focus) on the retina.^[1] The principal focusing ability of the (terrestrial) eye is due to the difference in refractive index between air and the curved cornea, but the variable curvature of the lens allows for an additional adjustment. This varies from a maximum of over 15 diopters in an infant to only about 1.5 diopters in a person 70 years old, as the lens becomes less flexible with age.^{[citation needed]}

1 Theories of mechanism
2 Accommodative dysfunction
3 References
4 See also
- 4.1 Disorders of accommodation
- 4.2 Other
5 External links

[edit] Theories of mechanism

Helmholtz - The most widely held theory of accommodation is that proposed by Hermann von Helmholtz in the middle of the 19th century. When viewing a far object, the circularly arranged ciliary muscle relaxes causing the lens zonules 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. 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.

Bates - In 1920, William Bates stated in Perfect Sight Without Glasses that the contraction of two extraocular muscles, the superior and inferior obliques, were responsible for accommodation.^[2]

Schachar - Ronald Schachar argues that, in opposition to Helmholtz's theory, the ciliary muscle actually pulls on the crystalline lens' equator in an outwardly radial manner, and the consequent changes in lens shape increase the lens power.^[3] Counterintuitive indeed, this pulling reportedly steepens central curvature and flattens peripheral curvature. He goes further to explain that it is not sclerosing or stiffening of the crystalline lens that linearly reduces accommodative amplitude, rather that age-related increase in lens diameter reduces the space between ciliary body and lens, linearly reducing the effectivity of the ciliary body's effect on lens dynamics. The "Scleral Expansion Band" technique for the surgical reversal of presbyopia is based on this theory, however, the efficacy of the procedure has been met with skepticism.^[4]

[edit] Accommodative dysfunction

Duke-Elder classified a number of accommodative dysfunctions:^[5]

Accommodative insufficiency
Ill-sustained accommodation
Accommodative infacility
Paralysis of accommodation
Spasm of accommodation

[edit] References

^ Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
^ http://www.iblindness.org/books/bates/ch4.html
^ Burd HJ, Judge SJ, Flavell MJ. "Mechanics of accommodation of the human eye." Vision Res. 1999 May;39(9):1591-5. PMID 10343853.
^ http://aglasser.opt.uh.edu/presbyopia/presbyopia.html
^ Duke-Elder, Sir Stewart (1969). The Practice of Refraction (8th ed.). St. Louis: The C.V. Mosby Company. ISBN 0-7000-1410-1.