Slide 1

Vision is Remarkable!

•Extremely complicated and costly process•As a reward over the course of evolution, vision provided

•New ways of communication•Ability to predict the trajectory of objects and events in time and space•New froms of mental imagery and abstraction•World of visual art

•Eyes like a camera •can adjust to differences in illumination and focus itself on objects of interest•Additional functions such as the ability to track moving objects and self-cleaning system

•Retina like film (but much more than that) •Photoreceptors: Converts light energy into neural activity•Output does not faithfully reproduce the intensity of the light falling on it

•Detects differences in intensity of light falling on different parts of it•Image processing on the retina

Slide 2

Properties of LightLight Electromagnetic radiation is all around us!

• radio, wireless phones, x-ray machines, sun Light is the electromagnetic radiation that is visible to our eyes! (400-700 nm)

Energy content is proportional to frequency • Hot colors: Orange, red : lower energy

• Cool colors: blue, violet: higher energy

• Colors are themselves “colored” by the brain

Slide 3

Properties of LightOptics Light rays travel in straight lines until they interact with atoms and molecules in the medium• Reflection

Bouncing of light rays off a surface Most of what we see is reflected light

• Absorption Transfer of light energy to a particle or surface

Basis for color perception - reflected light off a surface is absorbed in the retina

• Refraction Bending of light rays from one medium to another - toward a line that is perpendicular to the surface

Due to the speed (of light travel) difference - the greater the difference, the greater the angle of refraction

Basis for image forming on the retina

Slide 4

The Structure of the EyeGross Anatomy of the Eye Pupil: Opening where light enters the eye

Iris: Control the amount of light coming into eye (aperture)• The sphincter muscle lies around the very edge of the pupil. In bright light, the sphincter contracts, causing the pupil to constrict. The dilator muscle runs radially through the iris, like spokes on a wheel. This muscle dilates the eye in dim lighting.

• Of which pigmentation determines the eye color

Sclera: White of the eye• provides tough wall of eyeball• Extraocular muscles (3pairs) are embedded and control the movement of eyeball

Slide 5

The Structure of the EyeGross Anatomy of the Eye

Eye’s orbit: bony socket of skull

Conjunctiva: membrane connecting sclera with eyelids

Cornea: • Glassy transparent external surface of the eye (lens-like regractive power)

• Innervated by unmyelinated nerve endings : very sensitive to pressure (touch)

Optic nerve: Bundle of axons from the retina

Slide 6

The Structure of the EyeOphthalmoscopic Appearance of the Eye Blood vessels on the surface of Retina Optic disk :

• A pale circular region• Gate for entering blood vessels andExiting optic nerve fibers • Blind spot

No photoreceptors present Brain is deceiving you!

Macula (spot):• Central vision• Relative absence of blood vessels - improves the quality of central vision

Fovea (pit):• A thinner center of macula• The center of retina - serves as a anatomical reference point Nasal vs temporal Superior vs inferior

Slide 7

The Structure of the EyeOphthalmoscopic Appearance of the Eye Blood vessels on the surface of Retina Optic disk :

• A pale circular region• Gate for entering blood vessels andExiting optic nerve fibers • Blind spot

No photoreceptors present Brain is deceiving you!

Macula (spot):• Central vision• Relative absence of blood vessels - improves the quality of central vision

Fovea (pit):• A thinner center of macula• The center of retina - serves as a anatomical reference point

Nasal vs temporal Superior vs inferior

Slide 8

The Structure of the EyeCross-Sectional Anatomy of the Eye Aqueous Humor

• fluid filling space between corneaand lens• supply nourishment

Ciliary muscles• Ligaments (zonule fibers) that suspend lens are attached• Connect to sclera

Lens: Change shape to adjust focus• Aqueous humor in anterior chamber• Jelly-like vitreous humor in posterior chamber

its pressure serves to keep the spherical shape of eyeball

Slide 9

Image Formation by the EyeEye collects light, focuses on retina, forms images Refraction of light by the cornea Parallel lights from far distance must be bent by refraction

Air to aqueous humor changemakes refraction on the surfaceof cornea Focal distance depends onthe curvature of cornea Refractive power - reverse ofFocal distance (diopter)

• Cornea has 42 diopters• Depends on the slowing of light (Blurry vision underwater)

Slide 10

Image Formation by the EyeAccommodation by the Lens Changing shape of lens allows for extra focusing power (~12 diopters)

Important for focusing images of objects within 9m ranges - requires greater refraction for diverging (not parallel) rays

Accommodations by ciliary muscle contraction

• tension of zonule fubers Decrease• lens becomes rounder • increased curvature• lose this function with age (presbyopia)

Slide 11

Image Formation by the Eye

The Pupillary Light Reflex Connections between retina and brain stem neurons that control muscle around pupil

Continuously adjusting to different ambient light levels

Consensual : bilateral reflex Pupil similar to the aperture of a camera • increase the depth of focus by the constriction of pupil

• same as increasing the f-stop

Slide 12

Image Formation by the Eye

The Visual Field Amount of space viewed by the retina when the eye is fixated straight ahead

Visual Acuity Ability to distinguish two nearby points : depends on several factors including the spacing of photoreceptors in the retina and the precision of eye’s refraction

Visual Angle: Distances across the retina described in degrees

Slide 13

Microscopic Anatomy of the Retina

Photoreceptors: Cells that convert light energy into neural activity

Direct (vertical) pathway:

Horizontal cells, Amacrine cells : modify the responses of bipolar cells and ganglion cells via lateral connections

Ganglion cells Output from the retina

Photoreceptors bipolar cells

ganglion cells

Slide 14

Microscopic Anatomy of the RetinaThe Laminar

Organization of the Retina Cells organized in layers

Inside-out :• Upper cells are relatively transparent

• Pigmented epithelium is critical to maintain photoreceptors and photopigments

Slide 15

Microscopic Anatomy of the RetinaPhotoreceptor Structure

Electromagnetic radiation to neural signals

Four main regions• Outer segment

Stack of membraneous disks that contain photopigments

Lights are absorbed by photopigments and lead to changes in membrane potential

• Inner segment• Cell body• Synaptic terminal

Types of photoreceptors• Rods

have more disks and higher photopigments concentration - 1000 times more sensitive to light than cones

Scotopic retina• Cones detect colors

Photopic retina

Slide 16

Microscopic Anatomy of the RetinaRegional Differences in Retinal Structure Varies from fovea to retinal periphery

Peripheral retina• Higher ratio of rods to cones

• Higher ratio of photoreceptors to ganglion cells - lower acuity

• More sensitive to light (rods are specialized for low light)

Rods outnumber cones in the human retina (20 to 1)

Slide 18

Microscopic Anatomy of the RetinaRegional Differences in Retinal Structure Cross-section of fovea:

• Pit in retina due to lateral displacement of the cells above the photoreceptors

• Maximizes visual acuity by allowing light to strike photoreceptors directly (no scattering)

Central fovea: All cones (no rods)• 1:1 ratio with ganglion cells• Area of high visual acuity

Slide 19

Phototransduction Phototransduction in Rods Depolarization in the dark: “Dark current” -30 mV• Due to steady influx of Na+ through cGMP gated sodium channel

• Constant production of cGMP by guanylyl cyclase

Hyperpolarization in the light • Light reduces cGMP to close Na+ channel - hyperpolarize

transducin

Slide 20

Phototransduction Phototransduction in Rods Depolarization in the dark: “Dark current” to -30 mV• Due to steady influx of Na+ through cGMP gated sodium channel

• Constant production of cGMP by guanylyl cyclase

Hyperpolarization in the light • Light reduces cGMP to close Na+ channel - hyperpolarize

PDE

Slide 21

Rhodopsin•Photopigment that absorb electromagnetic radiation•Receptor protein that is embedded in the membrane of the stacked disks in the rod outer segments•Receptor protein with a prebound chemical agonist [Opsin (GPCR) + retinal (vitamin A derivative)]•Bleaching : change in conformation of retinal by light absorption leading to the activation of opsin (by dissociation)

Phototransduction

11-cis-retinal all-trans-retinal

Slide 22

Light - retinal - opsin - transducin - PDE - cGMP - cGMP gated Na+ channelSignal amplification : very low number of photons can be detected

Phototransduction

Slide 23

Phototransduction Phototransduction in Cones Similar to rod phototransduction

• Rods are hyperpolarized constantly - saturated

• Daytime vision depends on cones, whose photopigments require more energy to become bleached

Different opsins : major difference• Red, green, blue cones

Color detection• Contributions of blue, green, and red cones to retinal signal

• Young-Helmholtz trichromacy theory of color vision Brain assigns colors based on a comparison of the readout of the three color types

Color blindness - significant spectral abnormality (beyond normal variation), mostly due to genetic errors

Abnormal red-green vision is the most common abnormality that is more frequently found in men

Peak sensitivity of rods is to a wavelength of 500 nm (blue-green)

Slide 24

Phototransduction

Dark and Light Adaptation

Dark adaptation—Increasing the sensitivity to light (106 fold)• Dilation of pupils - 2-8 mm diameter; 16 fold • Regeneration of unbleached rhodopsin• Adjustment of functional circuitry - signals from more rods are available to each ganglion cells

All-cone daytime vision All-rod nighttime vision20–25 minutes

Slide 25

Phototransduction

Dark and Light Adaptation Calcium’s Role in Light Adaptation

• Blinding sensation reflects saturation of both rods and cones (hyperpolarization)

• Cones gradually adapt their membrane potential to - 35 mV

• cGMP-gated Na+ channel also allow Ca++ that inhibit guanylyl cyclase (balancing GC in the dark)

• Under the light, low Ca++ concentration in the hyperpolarized cones gradually activates GC, recovering cGMP - open the Na+ channel again

• Ca++ also affect photopigments and PDE to reset their responses to light

• What we see is the relative difference in light level, not the absolute level!

Slide 26

Retinal Processing

Only ganglion cells fire APs! All other cells respond with graded changes in membrane potential : difficult to detect!

Transformations in the Outer Plexiform Layer Photoreceptors form synapses with bipolar cells and horizontal cells

Output of photoreceptors is generated by dark rather than light

Dark is the preferred stimulus• When a shadow passes across a photoreceptor, it responds by depolarizing and releasing neurotransmitter glutamate

Slide 27

Retinal Processing

Bipolar Cell Receptive Fields Two classes of bipolar cells

• OFF bipolar cells : depolarized through glutamate gated Na+ channel - active when photoreceptors are depolarized (dark)

• ON bipolar cells : GPCR respond to glutamate to hyperpolarize - active (depolarized) when there’s less glutamate (light)

Receptive Field• Area of retina that changes the cell’s mem-brane potential upon Light stimulation• Consists of centerand surround• Antagonistic center-surround receptive fields; complex interaction ofhorizontal cells, photoreceptors, and bipolar cells

Slide 28

Light on surround

On center becomes off

Slide 29

Retinal OutputGanglion Cell Receptive Fields On-Center and Off-Center cells

(corresponding connections with bipolar cells) Center-surround cancellation

Responsive to differences in illumination within their receptive fields

Slide 30

Retinal Output

Ganglion Cell Receptive Fields The center-surround organization of the receptive fields leads to a neural response that emphasizes the contrast at light-dark edges

Illusion of the perception of light level can be explained by the level of inhibition by surround

Slide 31

Retinal Output

Types of Ganglion Cells Two types of ganglion cells in monkey and human retina• M-type (Magno, 5%), P-type (Parvo, 90%), nonM-nonP (5%)

Slide 32

Retinal OutputColor-Opponent Ganglion Cells Some P cells and nonM-nonP cells Response to one wavelength in the receptive field center is canceled by another wavelength in the receptive field surround

Red versus green and blue versus yellow

• White light will equally activate both center and surround to cancel each other

Ganglion cells output A stream of information concerning three different comparisons : Light vs dark, red vs green, blue vs yellow

Slide 33

Retinal Output

Parallel Processing - independent but simultaneous information processing Simultaneous input from two eyes

• Information from two streams is compared in the central visual system to give the perception of ‘Depth’

Information about light and dark• ON-center and OFF-center ganglion cells provide independent

streams of information Different receptive fields and response properties

of retinal ganglion cells• M cells : sensitive to subtle contrasts over large receptive

field and contribute to low-res vision• P- cells : small receptive field contribute to high-res

vision (detail)• nonM-nonP cells : color opponency