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.

Photon capture → all-trans-retinal release → G protein activation → cGMP phosphodiesterase activation → cGMP decrease → cGMP released from ion channels  ion channel closure → cone hyperpolarization → decreased glutamate release.

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.

Computing visual Angle Objects project an image on the retina. The size of the projection is proportional to the distance of the object from the retina. Visual angle (degrees) = 57.3 * object size/viewing distance 360 degrees for a full circle. 60 minutes/degree.