components of the visual system

Components of the Visual System

• Eyes

• Visual pathways (eye to brain)

• Visual centers of the brain

Compound & Simple Eyes

The Diversity of Eyes

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.

Placement of Eyes

Eye movements

• Head movements

• Saccades

• Tremor

Eye Muscles

Protection of the eye

• Encased in the optic orbit

• Surrounded by fat

• Eyelid covers eye

• Blinking cleans and moistens the eye

• Blink reflex

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

Anterior chamber

• Contains aqueous humor– Nourishes the cornea and lens– Under pressure– Glaucoma is excessively high pressure in eye

• 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)

• 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)

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

Human & Turtle Retinas

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.

Diabetes

• Causes cataracts

• Growth of blood vessels in eyes.

Objects structure light

• Objects absorb and reflect light.– Highly reflective surfaces appear light.– Poorly reflective surfaces appear dark.

• Reflectance indicates – continuities and discontinuities– texture

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.

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

• Presbyopia – Inability to accommodate

• Astigmatism – Irregularities in the surface of the cornea

Photoreceptors

• Rods– ~100 million

• Cones– ~5 million

• No new cells are formed, but parts are.

• 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.

Movie made by Carlos Rozas (CanalWeb, Chile).

• 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

Phototransduction (1 msec)

1. Retinal isomerizes when it absorbs light

2. Isomerization releases all-trans retinal which eventually decreases cGMP concentration.

3. Lower cGMP concentration lets ion channels at the synapse close.

• 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

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.