Components of the Visual System

1. Components of the Visual System • Eyes • Visual pathways (eye to brain) • Visual centers of the brain

2. Compound & Simple Eyes

3. The Diversity of Eyes

5. What is light a valuable thing to sense? • It travels essentially instantaneously through air. • EM radiation propagates rectilinearly. • Light is pervasive on earth. • Light provides differential information about many terrestrial objects.

6. Placement of Eyes

7. Eye movements • Head movements • Saccades • Tremor

8. Eye Muscles

9. Protection of the eye • Encased in the optic orbit • Surrounded by fat • Eyelid covers eye • Blinking cleans and moistens the eye • Blink reflex

11. Layers of the eye • Fibrous tunic (sclera) • 1mm thick • Fibrous to contain internal pressure of eye • Vascular tunic • Dark choroid tissue 0.2 mm thick • Dark color prevents light scatter • Retina

12. Anterior chamber • Contains aqueous humor • Nourishes the cornea and lens • Under pressure • Glaucoma is excessively high pressure in eye

13. Iris • Two layers • Pigmented • Vascular • Pupil • Two sets of muscles: circular & radial • Varies in size: • 2-8 mm in young adults (16-fold variation in light) • 5-2 mm in elderly adults • Variations in size influence depth of field (cf. p. 44)

14. Crystalline lens • Capsule • Modulates flow of aqueous humor • Modulates shape of lens • Epithelial layer • Lens proper • Grows constantly, quadrupling in size by 90 years of age. • Subject to hardening and opacities (cataracts) • Brunescence (yellowing)

15. Vitreous chamber • Contains vitreous gel-like substance • Not renewed, so can contain floaters.

18. Human & Turtle Retinas

20. Macular degeneration • Leading cause of impaired vision in industrialized nations. • Can sometimes be arrested by laser surgery. • One of the few health risks where African Americans have reduced rates over other racial groups.

21. Diabetes • Causes cataracts • Growth of blood vessels in eyes.

22. Objects structure light • Objects absorb and reflect light. • Highly reflective surfaces appear light. • Poorly reflective surfaces appear dark. • Reflectance indicates • continuities and discontinuities • texture

24. Light structure is usable only if • Light must reach the retina (~50% passes through cornea) • The image cast on the retina must be focused and not blurred. • The structural relations among points of light must be preserved.

25. Image formation in the eye • Optical power of cornea and crystalline lens • Variable due to change in shape of cornea and crystalline lens • Shape of eye

30. Presbyopia – Inability to accommodate • Astigmatism – Irregularities in the surface of the cornea

31. Photoreceptors • Rods • ~100 million • Cones • ~5 million • No new cells are formed, but parts are.

35. Most fish, frogs, turtles and birds have 3-5 types of cones. • Most mammals have only two types of cones. • Primates have three types of cones.

39. Movie made by Carlos Rozas (CanalWeb, Chile).

43. Both rods and cones contain photopigments • Each photopigment has two parts • Opsin • Comes in three forms • Retinal (vitamin A derivative) • Isomerizes when it absorbs light • Isomerization slows spontaneous dark current

46. Phototransduction (1 msec) • Retinal isomerizes when it absorbs light • Isomerization releases all-trans retinal which eventually decreases cGMP concentration. • Lower cGMP concentration lets ion channels at the synapse close.

48. Rods have one type of opsin • Cones have three different types • 440 nanometers (aka S-cone, blue cone) • Wavelength of light looks violet • 530 nanometers (aka M-cone, green cone) • Wavelength of light looks green • 560 nanometers (aka L-cone, red cone) • Wavelength of light looks yellow

49. Mammalian color processing • Blue cones are found in the retinas of most species, hence appear to be oldest in evolutionary terms. • Two cone retinas generally have blue and green, indicating that green is next oldest in evolutionary terms. • Primates have three cone types, indicating that the red cones are the most recent in evolutionary terms.