Seeing and the Retina

1. Seeing and the Retina Tiffany Lai

2. Outline • Anatomy of the eye • Refractive errors of the eye • 20/20 • Astigmatism • Hyperopia(farsightedness) • Myopia (nearsightedness) • Human and experimental studies • Contact Lenses • What all the numbers mean • Hard lenses • Soft lenses

3. Anatomy of the eye • Cornea- thin transparent protective shield on the front of the eye. The corneal also functions as a lens and begins focusing the rays of light by bending them (refracting) these as they enter. • Pupil- black hole in the center of the eye, and is a door way which, along with the iris, regulates how much light comes through. • Lens- focuses the light rays . Controlled by ciliary body located above and below the lens. • Optic Nerve- sends information to the brain • Fovea- most concentrated cones. Therefore, high acuity. • Para Fovea

4. Anatomy of the eye

5. What does 20/20 mean? • normal visual acuity • measured by the ability to read charts at a distance of 20 feet • 20/40 vision means that the test subject see at 20 feet what a normal person sees at 40 feet

6. Astigmatism • With astigmatism, the cornea is abnormally curved, causing vision to be out of focus • Light rays not focused on a single point on the retina • Usually concurrent with myopia/hyperopia • Unknown cause

7. Hyperopia • Close objects = blurry • Far objects = clear • Also known as farsightedness • Cornea is flatter than normal • Visual image from environment focused not on the retina but behind it. • Cause is unknown but could be either hereditary or due to visual stress

8. Myopia • Close objects = clear • Far objects = blurry • Also known as nearsightedness • Eyeball is too long or the cornea has too much curvature • Light entering eye isn’t focused right at the retina (falls short) • Cause is unknown but could be either hereditary or due to visual stress

10. Emmotropization • Process by which young eyes regulate the growth of their eyes to eliminate refractive errors • Animals compensate for imposed defocus • Chicks • Monkeys • Shrews

11. Wildsoet’s study on chicks at U. of Berkeley Optometry School (2003) • Purpose: to assess whether a retina-brain link and/or an intact ciliary nerve are required for this emmetropizing response. • Lesions at optic nerve section or ciliary nerve section or both to interrupt communication between eye and brain. • After a recovery of 4 days, lesioned eyes were fitted either diffusers, -5 D lenses, or -15 D lenses • All three lesions produced hyperopic shifts in refraction • Conclusions : • data imply that an intact retina-brain link is not required for compensation to hyperopic defocus and thus emmetropization • However there are interactions between higher centers and the eye • ciliary nerve as an important conduit for signals that exercise a restraining influence on eye growth

12. ONS & compensation to delayed imposed defocus:Compensation with accommodation?

14. All the Numbers • +/– sphere +/– cylinder x axis. • Sphere - the amount of nearsightedness (myopia) or farsightedness (hyperopia). If the sphere number is negative, the patient is nearsighted. If it is positive, he or she is farsighted • Cylinder- the amount of astigmatism. If no astigmatism is present, then the prescription will only have a number for sphere. Cylinder can be expressed as either a positive or negative number. • Axis- the direction on the clock hour of the astigmatism. • BC- base curve (size of the lense) • DIA- (size of the lense)

15. Contacts • Hard Contacts • Ortho K (Orthokeratology) – used for individuals with myopia or astigmatism. Best for individuals with prescriptions no more than -4.00 • Gas permeable contact lenses are rigid lenses made of durable plastic that transmits oxygen • Usually includes silicone • Usually worn at night • While asleep, the shape of the lense is structured in such a way that in turn reshapes the cornea • good for children not eligible for LASIK or for athletes • Daytime Use- • Stabilizes prescription

16. Contacts • Soft Contacts • gel-like, water-containing plastics • Larger in diameter and greater in thickness than hard contacts. • Does not correct vision; still vulnerable to worsening prescription (contrast with hard contacts used in the day time)

17. Taking off Contacts • Accomodation • the ability of the eye to adjust in order to see clearly objects at different distances. • Due to the naturally elastic lens of the eye changes in shape • Ciliary muscles adjust focusing power.

18. How glasses work. • The glasses offer a concave lens that bends light rays outward, which normalizes the eyeball. In farsightedness, the eyeball is too short to focus upon objects that are near. Glasses use a convex lens that bends the light inward before it reaches the eye's lens, thereby correcting vision.

19. References • http://www.aoa.org/myopia.xml • http://www.eyesandeyesight.com/2009/02/anatomy-of-the-eye/ • http://blog.eyebuydirect.com/?tag=myopia • http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002010/ • http://health.howstuffworks.com/wellness/cosmetic-treatments/contact-lens2.htm • http://www.allaboutvision.com/contacts/contact_lenses.htm • http://www.ehow.com/how-does_4564464_glasses-work.html • http://www.lasik.md/learnaboutlasik/understandingprescriptions.php • http://www.ncbi.nlm.nih.gov/pubmed/14704921 • Seeing: The Computational Approach to Biological Vision by Frisby and Stone • Sensation & Perception by Jeremy Wolfe