Eye Brain Vision Eye Every organism is light

Eye + Brain = Vision

Eye ● Every organism is light sensitive – Plants – Insects – ● Complex eye ● Simple brain Vertebrates ● Simple eye ● Complex brain

Eyes ● ● Sensory without contact Optical Instrument – ● ● Described by Descartes in 1637 Some parts isolated from blood flow Image is inverted, just like a camera!

Eye Anatomy ● Eyelid ● Eye Muscles ● Cornea ● Iris ● Pupil ● Lens ● Retina ● Optic Nerve

Eyelid ● ● Provides protection from objects Keeps eyes moist – Covers eye when sleeping – Blinking ● ● keeps eyes moist during activity ● occurs without stimulus ● Increases with stress ● Decreases with concentration Semi-transparent

Muscles ● 3 pairs move and stabilize eyes – ● Always in tension Provide stimulus to brain – Search or saccades ● – ● Blind during saccade motion Follow You cannot see your eyes move in a mirror!

Saccade Motion ● Hard to detect subtle differences in pictures From www. evanovich. com

Cornea ● ● ● Provides most of the refraction and focussing of light No bloodflow Shape can be changed – With eyeglasses – With surgery

Cornea Shape ● Can be corrected with eyeglasses – 1286 convex – 15 th century concave – Often worn in secret

Iris ● ● Colored part of the eye Greek for rainbow Restricts incoming light Color not related to function – Albinos have no pigment and cannot block light

Pupil ● ● ● Window into eye We can't see into our own eyes Closes for better acuity 16: 1 Change in size Can oscillate if a laser or bright light is shown in the edge

Lens ● ● Changes to allow us to focus objects near and far Normally transparent – ● Cataracts are cloudy regions of the lens Looses elasticity over time

Retina ● ● ● Contains the photoreceptors Nerves form an extension of the brain Kepler – 1684: Saw it as a light sensitive screen – 1625: Verified by examining ox eye

Retina ● ● Fovea – Central region – high accuity – Color vision Outside fovea – ● Monochrome Periphery – Sensitive only to movement – Stimulates eyes to

Retinal Structure and Function ● Photoreceptors form a network, a cobweb tunic

Rods ● ● Found throughout retina None in the fovea Smallest is about the size of two wavelengths of red light Take over in low lighting conditions – Removes color

Rods ● ● Rhodopsin Most sensitive at 498 nm

Cones ● ● The 420 curve is for the short wavelength cones, and the 534 and 564 curves are for the middle and long wavelength sensitive cones respectively. About 3 million in each eye

Distribution of Photoreceptors

Color Vision ● ● Varies among species – Most mammals don't – Birds do – Insects do – Fishes do – Reptiles do Why this narrow EMR band?

Color Vision ● ● ● Depends on wavelength Depends on relative stimulation of photoreceptors Not all colors are unique

Optic Nerve ● ● ● Sends signals from retina to brain Results in a blind spot in the visual field Brain fills in missing image!

Nerves ● Brain contains billions of neurons ● Send signals at a finite rate ● Electrical signals sent via Na-K conduction

Chiasm ● ● Optic nerve sections cross at chiasm Eye is split into L/R sides, which go to R/L side of brain