Vergence movement, Accommodation & Diseases, disorders, and age-related changes

Vergence movement








The two eyes converge to point to the same object

When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate 'towards each other' (convergence), while for an object farther away they rotate 'away from each other' (divergence). Exaggerated convergence is called cross eyed viewing (focussing on the nose for example) . When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge.

Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.


Accommodation

Accommodation is the process by which the eye increases optical power to maintain a clear image (focus) on the retina 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 ageto regain its thicker form.

Diseases, disorders, and age-related changes









The stye is a common irritating inflammation of the eyelid.
There are many diseases, disorders, and age-related changes that may affect the eyes and surrounding structures.

As the eye ages certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of aging eye diseases. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses in bright light and extremely dark glasses in dim light. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting.

With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis, and ptosis. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities — visible as floaters — gradually increase in number.Various eye care professionals, including ophthalmologists, optometrists, and opticians, are involved in the treatment and management of ocular and vision disorders. A Snellen chart is one type of eye chart used to measure visual acuity. At the conclusion of an eye examination, an eye doctor may provide the patient with an eyeglass prescription for corrective lenses.

External links

1. DJO Digital Journal of Ophthalmology (http://www.djo.harvard.edu/)
2. Glossary of Eye Conditions (http://www.afb.org/Section.asp?DocumentID=2139)
3. Evolution of the Eye (http://www.pbs.org/)
4. Diagram of the eye (http://webvision.med.utah.edu/anatomy.html)
5. Webvision. The organisation of the retina and visual system. (http://webvision.med.utah.edu/)
6. VisionSimulations.com Images and vision simulators of various diseases and conditions of the eye (http://www.visionsimulations.com/)
7. How the eye works and common vision problems (http://vision101.com/)
8. Asian Eyes This website discusses the differences in Asian eyes. 9 http://kennethomura.tripod.com/asian_eyes/)
9. Computer Vision Syndrome (eyestrain, eye fatigue, dry eyes, light sensitivity, etc.) Who Is Affected by Computer Vision Syndrome? What Can I Do About It?
   (easysoft.2mcl.com/computer-vision-sydrome.html)