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 1

Properties of Light Light

   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 2

Properties of Light

Optics  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 3

The Structure of the Eye Gross 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 4

The Structure of the Eye

Gross 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 5

The Structure of the Eye

Ophthalmoscopic Appearance of the Eye  Blood vessels on the surface of Retina  Optic disk : • • A pale circular region • Gate for entering blood vessels and Exiting 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 6

The Structure of the Eye

Ophthalmoscopic Appearance of the Eye  Retina  Blood vessels on the surface of Optic disk : • • A pale circular region Gate for entering blood vessels and • Exiting 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 Eye Cross-Sectional Anatomy of the Eye

   Aqueous Humor • • fluid filling space between cornea and 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 8

Image Formation by the Eye Eye 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 change makes refraction on the surface of cornea  Focal distance depends on the curvature of cornea  Refractive power - reverse of Focal distance (diopter) • • Cornea has 42 diopters Depends on the slowing of light (Blurry vision underwater) Slide 9

Image Formation by the Eye

Accommodation 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 10

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 11

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 12

Microscopic Anatomy of the Retina Photoreceptors:

 Cells that convert light energy into neural activity

Direct (vertical) pathway:

Photoreceptors bipolar cells ganglion cells

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

Ganglion cells

 Output from the retina Slide 13

Microscopic Anatomy of the Retina

The 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 14

Microscopic Anatomy of the Retina

Photoreceptor 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 15

Microscopic Anatomy of the Retina Regional 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 16

Microscopic Anatomy of the Retina Regional 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 18

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 19

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 20

Phototransduction

 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) 11-

cis-

retinal all-

trans-

retinal Slide 21

Phototransduction

 Light - retinal - opsin - transducin - PDE - cGMP - cGMP gated Na + channel  Signal amplification : very low number of photons can be detected Slide 22

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  Peak sensitivity of rods is to a wavelength of 500 nm (blue green)  Abnormal red-green vision is the most common abnormality that is more frequently found in men Slide 23

Phototransduction Dark and Light Adaptation



All-cone daytime vision All-rod nighttime vision

20 –25 minutes Dark adaptation— Increasing the sensitivity to light (10 6 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 Slide 24

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 25

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 26

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 center and surround • Antagonistic center surround receptive fields ; complex interaction of horizontal cells, photoreceptors, and bipolar cells Slide 27

On center becomes off Light on surround Slide 28

Retinal Output

Ganglion 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 29

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 30

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 31

Retinal Output

Color-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 32

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 Slide 33