Lecture1-human Eye System.pptx

THE HUMAN VISUAL SYSTEM THE EYE

IN THIS SECTION . . . • ANATOMY OF HUMAN EYE • IMAGE FORMATION BY HUMAN EYE • METHOD OF LIGHT DETECTION • RETINAL PROCESSING

HUMAN VISUAL SYSTEM

Image formation •Cornea •Lens

Exposure Control •Iris/pupil •Photoreceptor sensitivity

Detection

Processing

•Retina •Rods •Cones

•Brain

HUMAN EYE Ciliary Muscle Sclera Iris

Pupil

Ear side (Temporal)

Vitreous Humor Eyelens

Fovea Retina Optic Nerve

Cornea

Nose side (Nasal)

Aqueous Humor Suspensory ligament

Choroid

• HUMAN EYE IS A COMPLETE IMAGING SYSTEM.

IMAGE FORMATION Object

• THE CURVED SURFACES OF THE EYE FOCUS THE IMAGE ONTO THE BACK SURFACE OF THE EYE.

Image

IMAGE FORMATION • DISTANCE BETWEEN CENTER OF LENS AND RETINA (FOCAL LENGTH) VARY BETWEEN 14-17 MM. • WHEN OBJECT IS 3 M OR MORE AWAY, F = 17MM WITH LOWEST REFRACTIVE POWER. • IMAGE LENGTH H = 17(MM) X (15/100)

CORNEA • THE OUTER WALL OF THE EYE IS FORMED BY THE HARD, WHITE SCLERA. Sclera Cornea

• CORNEA IS THE CLEAR PORTION OF THE SCLERA. • 2/3 OF THE REFRACTION TAKES PLACE AT THE CORNEA.

IRIS AND PUPIL Iris

• COLORED IRIS CONTROLS THE SIZE OF THE OPENING (PUPIL) WHERE THE LIGHT ENTERS.

Pupil

• PUPIL DETERMINES THE AMOUNT OF LIGHT, LIKE THE APERTURE OF A CAMERA.

Iris open Dilated pupil

Iris closed Constricted pupil

LENS Ciliary muscle Lens

• EYE LENS IS MADE OF TRANSPARENT FIBERS IN A CLEAR MEMBRANE. • SUSPENDED BY SUSPENSORY LIGAMENT.

Suspensory Ligament Transparent Fibers

Cross section of the eye lens

• USED AS A FINE FOCUSING MECHANISM BY THE EYE; PROVIDES 1/3 OF EYE’S TOTAL REFRACTING POWER. • NON-UNIFORM INDEX OF REFRACTION.

ACCOMMODATION • THE SUSPENSORY LIGAMENTS ATTACH THE LENS TO THE CILIARY MUSCLE.

Distant object

Near object

Relaxed muscle Taut ligaments

• WHEN THE MUSCLE CONTRACTS, THE LENS BULGES OUT IN THE BACK, DECREASING ITS FOCAL LENGTH. • THE PROCESS BY WHICH THE LENS CHANGES SHAPE TO FOCUS IS CALLED ACCOMMODATION.

Contracted muscle Slack ligaments

AQUEOUS HUMOR AND VITREOUS HUMOR Vitreous Humor

• TRANSPARENT GELATINOUS LIQUID FILLING THE EYE. • PROVIDES NUTRIENTS TO THE CORNEA AND EYE LENS.

Aqueous Humor

• ALSO HELPS MAINTAIN THE EYEBALL SHAPE WITH ITS PRESSURE.

RETINA • RETINA IS THE PHOTOSENSITIVE “DETECTOR” FOR THE EYE. Retina

• TWO TYPES OF RECEPTORS IN THE RETINA: RODS FOR LOW LIGHT LEVEL, AND CONES FOR COLOR.

• LOCATED AT THE CENTER OF THE RETINA, FOVEA CONTAINS A GREATER CONCENTRATION OF CONES.

Fovea

Optic Nerve

• SIGNALS FROM THE RECEPTORS LEAVE THROUGH THE OPTIC NERVE TO THE BRAIN.

PLEXIFORM LAYER • THE RETINA IS MADE OF THREE LAYERS: • PLEXIFORM LAYER IS A NETWORK OF NERVES WHICH CARRY THE SIGNALS FROM THE PHOTO RECEPTORS.

Fovea Photo receptors

• CHOROID PROVIDES NOURISHMENT TO THE RECEPTORS, AS WELL AS ABSORB ANY LIGHT THAT DIDN’T GET ABSORBED BY THE PHOTO RECEPTORS, LIKE A ANTI-HALATION BACKING IN FILM.

Light Plexiform Layer

Optic Nerve

• PHOTO RECEPTORS.

Choroid

RODS AND CONES Synaptic endings Cell nucleus Inner segments Outer segments

Rod

Cone

• HIGHLY SENSITIVE TO LOW LIGHT LEVEL OR SCOTOPIC CONDITIONS.



• BLACK AND WHITE.



• DISPERSED IN THE PERIPHERY OF THE RETINA.



Sensitive to high light level or photopic conditions. Three types of cones responsible for color vision. Concentrated in the fovea.

BRIGHTNESS ADAPTATION • HVS CAN VIEW LARGE INTENSITY RANGE (1010) • BUT SIMULTANEOUS PERCEIVED INTENSITY RANGE IS MUCH SMALLER. • IF ONE IS AT BA INTENSITY (OUTSIDE) AND WALK INTO A DARK THEATER, HE CAN ONLY DISTINGUISH UP TO BB. IT WILL TAKE MUCH LONGER FOR EYE TO ADAPT FOR THE SCOTOPIC VISION TO PICK UP.

ADAPTATION • WHY CAN’T YOU SEE IMMEDIATELY AFTER YOU ENTER A MOVIE THEATER FROM DAYLIGHT?

Threshold of detection (log scale)

Photopic (cones)

0

• THE THRESHOLD OF DETECTION CHANGES WITH OVERALL LIGHT LEVEL.

Scotopic (rods)

5

10

15

20

25

Time in dark (minutes)

30

• THE SWITCH IS QUITE GRADUAL, UNTIL THE SENSITIVITIES OF CONES AND RODS CROSS OVER: AT ABOUT 7 MINUTES IN THE DARK.

WEBER RATIO • HVS’S SENSITIVITY TO INTENSITY DIFFERENCE DIFFER AT DIFFERENT BACKGROUND INTENSITIES.

• WEBER RATIO: I/I: JUST NOTICEABLE INTENSITY DIFFERENCE VERSUS BACKGROUND INTENSITY. IT IS A FUNCTION OF LOG I.

I

I+ I

SIMULTANEOUS CONTRAST

• THE PERCEIVED BRIGHTNESS OF INNER CIRCLE ARE DIFFERENT DUE TO DIFFERENT BACKGROUND INTENSITY LEVELS EVEN IF THEY ARE IDENTICAL.

DISTRIBUTION OF PHOTORECEPTORS

Number of receptors per mm2

Temporal

Visual Axis

80 º 60 º 40 º 20 º 160 140 120 100 80 60 40 20

Nasal

0º

80º 60 º 40 º 20 º

• CONES ARE CONCENTRATED IN THE FOVEA.

Blind spot Rods

Cones 60 º 40 º 20 º 0 º 20 º 40 º 60 º 80 º Angle

• RODS PREDOMINATE THE PERIPHERY. • THERE IS A BLIND SPOT WHERE THERE ARE NO PHOTORECEPTORS, AT THE POINT WHERE THE NERVES EXIT THE EYE (OPTIC NERVE).

• THREE CONE TYPES (S,I,L) CORRESPOND TO B,G,R

S

I

L

Relative response

•

HUMAN VISION HUMAN CONE RESPONSE TO COLOR

400

460 490 500

530

600

650

Wavelength (nm) Blue

Cyan

Green

Red

700

RETINA Light

Cones

Rods To optic nerve

• THE RETINA IS MADE OF NETWORK OF NERVE CELLS. • THE NETWORK WORKS TOGETHER TO REDUCE THE AMOUNT OF INFORMATION IN A PROCESS CALLED LATERAL INHIBITION.

Bipolar cells Amicrine cells Ganglion cells Horizontal cells

HERMANN GRID

• ILLUSTRATES LATERAL INHIBITION.

HERMANN GRID

A

B

• POINT A LOOKS DARKER BECAUSE THERE ARE 4 INHIBITORY INPUTS • POINT B LOOKS LIGHTER BECAUSE THERE ARE ONLY 2 INHIBITORY INPUTS

MACH BANDS

Actual brightness

Perceived by you

EYE DEFECTS Object at infinity

• IMAGE FOCUSES ON THE RETINA FOR A NORMAL EYE. Normal

• DISTANT OBJECTS LOOK BLURRY FOR A MYOPIC (NEAR SIGHTED) EYE. Myopic

Hyperopic Eyes at relax state.

• NEAR OBJECTS LOOK BLURRY FOR A HYPEROPIC (FAR SIGHTED) EYE.

MYOPIA - NEAR SIGHTEDNESS Far object

Near object

• DISTANT OBJECTS LOOK BLURRY BECAUSE THE EYE Myopic eye relaxed CANNOT RELAX ANY FARTHER Blurry SO THAT THE IMAGE IS FOCUSED BEFORE THE RETINA. Myopic eye relaxed • In focus

Far object Myopia corrected with a negative lens

NEAR OBJECT IN FOCUS WITHOUT ACCOMMODATION.

• CORRECTED WITH A NEGATIVE LENS.

The virtual image from the diverging lens appears to be closer.

HYPEROPIA - FAR SIGHTEDNESS Far object

Near object

• NEAR OBJECTS LOOK BLURRY BECAUSE THE EYE Hyperopic eye Partially accommodated CANNOT In focus ACCOMMODATE ENOUGH FOR NEAR Hyperopic eye Fully accommodated OBJECTS. Blurry

Near object Hyperopia corrected with a positive lens

• FAR OBJECT IN FOCUS. • CORRECTED WITH A POSITIVE LENS.

Light from the converging lens looks as though it is coming from the distance.

CONTACT LENS Contact lens Cornea

Fluid

• CONTACT LENS IS AN ALTERNATIVE TO CORRECTIVE LENSES. • CHANGES THE CURVATURE OF THE CORNEA BY ADHERING TO THE SURFACE WITH SOME FLUID.

YOUR EYE CARE Go see a doctor if you think there is something wrong with your eye-

Early detection is essential to keeping damage low and preventing permanent loss of your vision.