photo chemistry of vision

PHOTOCHEMISTRY OF VISION

Photochemistry of vision

Photopic vision:Day light vision due to conesColor visionBrightness above 1mA Scotopic vision:Dim light vision due to rodsBelow 0.001 mA Mesopic vision:Full moonlight visionboth rods & cones

Visible light: 400-750 nmPurkinje shift: shifting of sensitivity of eye

from photopic to scotopic vision

Electromagnetic Spectrum

Photons are classified according to their wavelength

Longest wavelength: radio and television waves

Shortest wavelength: gamma rays Middle of the spectrum: visible light

Rods and Cones

Retinal photoreceptors contain pigments that preferentially absorb photons with wavelengths 400-700 nm

Shortest wavelength: blue and green Longer wavelengths: yellow, orange,

Visual Pigments

Four visual pigments: Rhodopsin: present in rods 3 cone pigments

Erythrolabe (R cones): red, 570 nm Cyanolabe (B cones): blue, 440 nm Chlorolabe (G cones): green, 540 nm

Visual cycle

Rhodopsin: visual purpleScotopsin- protein partRetinene1- 11-cis retinal- derivative of

(Metarhodopsin II)- Activated rhodopsin- brings about electrical changes in rods

Structure of Rhodopsin

Rhodopsin-Retinal visual cycle

Phototransduction

Rhodopsin Kinase inactivates metarhodopsin II within seconds

Ca2+ activates adenylyl cyclase which in turn increases cGMP & inhibits phosphodiesterase

Activation of rods by light In dark, Na+ ions are continually

pumped out from inner segment Outer segment is very leaky to Na+

ions In dark, rods are less negative (-40

mv) On activation there will be closure of

leaky Na+ channels leading to hyperpolarization (upto -70 mv)

Receptor potential peaks in 0.3 secs

It is 4 times faster in cones

Receptor potential is directly proportional to logarithm of light intensity

Regulation of retinal sensitivity

Dark adaptation: person exposed to light for many hours is suddenly exposed to darkness.

Difficulty in visualizing for long timeLight adaptation: reverse of the above Sensitivity of eye can change by 1

million times Registration of image requires both

light & dark spots

Dark adaptation

Mechanisms of dark & light adaptations

1. Availability of light sensitive pigments

2. Changes In pupillary size

3. Neural adaptation

Night Blindness

Impaired vision at night or in dim light situations

Rhodopsin deficiency affecting rods Most common cause - prolonged

Vitamin A deficiency Rods degenerate

Color Vision

Complementary colorsPrimary colors: Red (647-723nm), Green

(492-575) & Blue (450-492)

RedGreenBlue

Primary Colors

3 Attributes of Color Hue

“color” color perception denoted by blue, red, purple, etc Depends largely on what the eye and brain

perceive to be the predominant wavelength present in the incoming light

yellowgreenblue

Wavelength

Mean Hue

3 Attributes of Color Saturation

purity or richness of a color When all the light seen by the eye is the same

wavelength, the color is fully saturated e.g. pink is a desaturated red

Wavelength

medium

low# P

Variance Saturation

3 Attributes of Color Brightness

Quantity of light coming from an object (the number of photons striking the eye)

Wavelength

B. Area Lightness

bright

Area Brightness

Young-Helmholtz theory: Three types of cones with sensitivity to three primary colors

S, M & L pigments

S pigment gene- Chromosome 7

M & L pigment genes on X Chromosome

Color perception depends on the percentage stimulation of all 3 cones

Visual Pigments

Four visual pigments: Rhodopsin: present in rods 3 cone pigments

Erythrolabe (R cones): red, 570 nm Cyanolabe (B cones): blue, 440 nm Chlorolabe (G cones): green, 540 nm

Color Blindness Congenital lack of one or more cone

types Deficit or absence of red or green

cones most common Sex-linked trait Most common in males

What numbers can you see in each of these?

Tests for color vision Pseudo-isochromatic chart test

(Ishihara’s plates) Elridge Green lantern Holmgren’s wool test

Color blindness

-anomaly: weakness-anopia: absence or loss-prot: red color-deter: green color-trit: blue color• Monochromat• Dichromat• Trichromat

Tests for color blindness:(i) Ishihara’s chart(ii) Edridge Green Lantern(iii) Holmgren’s Wool testColor Blindness:Trichromats- Protanomaly, DeutranomalyDichromats- Protanopia, Deutranopia,

TritanopiaMonochromats Red-Green color Blindness: difficulty in

distinguishing red, orange, green & yellow; X-linked inheritance

Trichromats

92% of the population who have “normal” color vision

Have all 3 different kinds of cones, normal concentration of cone pigments, normal retinal wiring

Congenital Dichromatism

Cones themselves are normal, but one of the 3 contains the wrong pigment

Deutranopes: Lack green pigment

Protanopes Lack red pigment

Tritanopes Lack blue pigment

Congenital Dichromatism

Mode of inheritance: sex-linked recessive Men almost exclusively manifest the

disorder Women are carriers

Red-Green color blindness

• Seen in 8% of males and 0.4% of females• X-linked recessive disorder• Females are carriers• Defect of red or green cones

Protanomaly Deutranomaly Tritanomaly Protanopia Deutranopia

Electrical activity of retinal cells

Only ganglion cells produce action potentials

Receptors- hyperpolarization

Bipolar cells- depolarization/hyperpolarization

Receptive field structure in bipolar cells

Receptors

Bipolar Cell

A. WIRING DIAGRAM

HorizontalCells

Direct excitatory component (D)

B. RECEPTIVE FIELD PROFILES

Direct Path

Indirect Path

Indirectinhibitory

component (I)

Retinal Receptive Fields

Processing of visual image in retina

Formation of three images

First image: Photoreceptors

Second image: Bipolar cells

Third image: Ganglion cells

Processing of Visual Information in the RetinaIn a sense, the processing of visual information in the retina involves the formation of three images. The first image, formed by the action of light on the photoreceptors, is changed to a second image in the bipolar cells, and this in turn is converted to a third image in the ganglion cells. In the formation of the second image, the signal is altered by the horizontal cells, and in the formation of the third, it is altered by the amacrine cells. There is little change in the impulse pattern in the lateral geniculate bodies, so the third image reaches the occipital cortex.

Receptive field structure in ganglion cells:On-center Off-surround

Stimulus condition Electrical response

Response

RF of On-center Off-surround cells

Receptive FieldNeural Response

Center

Surround

On Off

Response Profile

on-center

off-surround

Horizontal Position

FiringRate

RF of Off-center On-surround cells

Receptive Field

Horizontal Position

on-surround

off-center

Response Profile

FiringRate

Center

Surround

On Off

Neural Response

Surround

Center

Cortical Receptive Fields

Three classes of cells in V1

Simple cells

Complex cells

Hypercomplex cells

Visual cortex processing

Most fibers from LGB end in layer 4 Fibers from intralaminar portion end

in blobs present in layer 2 & 3 Simple cells: respond to bars of light,

lines & edges if in particular orientation

Complex cells: fire when lines are moved laterally

Simple Cells: “Line Detectors”

A. Light Line Detector

Horizontal Position

FiringRate

B. Dark Line Detector

Horizontal Position

FiringRate

Simple Cells: “Edge Detectors”

C. Dark-to-light Edge Detector

Horizontal Position

FiringRate

D. Light-to-dark Edge Detector

Horizontal Position

FiringRate

Constructing a line detector

Receptive Fields

Retina LGN

Center-Surround Cells

Simple Cell

CorticalArea V1

Complex Cells

STIMULUS NEURAL RESPONSE

Complex Cells

Constructing a Complex Cell

Simple Cells

Cortical Area V1

Complex CellReceptive Fields

Retina

Hypercomplex Cells

Constructing a Hypercomplex Cell

Receptive Fields

RETINA CORTICAL AREA V1

Complex Cell End-stopped Cell

This is the so-called "Ice Cube" model of the visual cortex illustrating cortical architecture. This 1mm by 1mm region of cortex contains all orientations, columns for both the left and right eyes, and blobs.

Orientation columns: vertical columns of 1mm diameter

Ocular dominance columns: layer 4 cells alternate with inputs from two eyes

Color pathways project to ‘blobs’ & layer IV c of area 17 and from there on to V8

Neurons in many cortical areas are arranged into functional columnar structures spanning from the pial surface to the white matter tracts. A cortical column is defined by a group of neurons arranged vertically that share a similar receptive field. For example, as an electrode oriented perpendicular to the surface of the primary visual cortex is penetrated deeper into the cortex, all neurons encountered will respond to a bar of light angled at 45 degrees from the horizon (Figure 1, left)1. However, neurons recorded from an electrode inserted parallel to the cortical surface will show gradually changing orientation selectivity (Figure 1, right).

photo chemistry of vision

red color

cones color vision brightness

normal color vision

green color trit

cones visual pigments

dim light vision

red visual pigments

day light vision

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