photo chemistry of vision
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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,
red
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
Vit A
(Metarhodopsin II)- Activated rhodopsin- brings about electrical changes in rods
Structure of Rhodopsin
Rhodopsin-Retinal visual cycle
Phototransduction
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
# P
hoto
ns
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
high
medium
low
hi.
med.
low# P
hoto
ns
Variance Saturation
3 Attributes of Color Brightness
Quantity of light coming from an object (the number of photons striking the eye)
# P
hoto
ns
Wavelength
B. Area Lightness
bright
dark
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
Light
© Stephen E. Palmer, 2002
Receptive field structure in bipolar cells
Receptors
Bipolar Cell
A. WIRING DIAGRAM
HorizontalCells
Direct excitatory component (D)
B. RECEPTIVE FIELD PROFILES
LIGHT
Direct Path
Indirect Path
Indirectinhibitory
component (I)
D + I
Retinal Receptive Fields
© Stephen E. Palmer, 2002
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
Time
Response
© Stephen E. Palmer, 2002
Receptive field structure in ganglion cells:On-center Off-surround
Stimulus condition Electrical response
Time
Response
Retinal Receptive Fields
© Stephen E. Palmer, 2002
Receptive field structure in ganglion cells:On-center Off-surround
Stimulus condition Electrical response
Time
Response
Retinal Receptive Fields
© Stephen E. Palmer, 2002
Receptive field structure in ganglion cells:On-center Off-surround
Stimulus condition Electrical response
Time
Response
Retinal Receptive Fields
© Stephen E. Palmer, 2002
Receptive field structure in ganglion cells:On-center Off-surround
Stimulus condition Electrical response
Time
Response
Retinal Receptive Fields
© Stephen E. Palmer, 2002
Receptive field structure in ganglion cells:On-center Off-surround
Stimulus condition Electrical response
Time
Response
Retinal Receptive Fields
© Stephen E. Palmer, 2002
RF of On-center Off-surround cells
Receptive FieldNeural Response
Center
Surround
On Off
Response Profile
on-center
off-surround
Horizontal Position
FiringRate
Retinal Receptive Fields
© Stephen E. Palmer, 2002
RF of Off-center On-surround cells
Receptive Field
Horizontal Position
on-surround
off-center
Response Profile
FiringRate
Retinal Receptive Fields
© Stephen E. Palmer, 2002
Center
Surround
On Off
Neural Response
Surround
Center
Cortical Receptive Fields
Three classes of cells in V1
Simple cells
Complex cells
Hypercomplex cells
© Stephen E. Palmer, 2002
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
Cortical Receptive Fields
Simple Cells: “Line Detectors”
A. Light Line Detector
Horizontal Position
FiringRate
B. Dark Line Detector
Horizontal Position
FiringRate
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Simple Cells: “Edge Detectors”
C. Dark-to-light Edge Detector
Horizontal Position
FiringRate
D. Light-to-dark Edge Detector
Horizontal Position
FiringRate
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Constructing a line detector
Receptive Fields
Retina LGN
Center-Surround Cells
Simple Cell
CorticalArea V1
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Complex Cells
STIMULUS NEURAL RESPONSE
Time
00o
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Complex Cells
STIMULUS NEURAL RESPONSE
Time
060o
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Complex Cells
STIMULUS NEURAL RESPONSE
Time
090o
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Complex Cells
STIMULUS NEURAL RESPONSE
Time
0120o
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Constructing a Complex Cell
Simple Cells
Cortical Area V1
Complex CellReceptive Fields
Retina
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Hypercomplex Cells
Time
STIMULUS NEURAL RESPONSE
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Hypercomplex Cells
Time
STIMULUS NEURAL RESPONSE
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Hypercomplex Cells
Time
STIMULUS NEURAL RESPONSE
© Stephen E. Palmer, 2002
Cortical Receptive Fields
Hypercomplex Cells
Time
STIMULUS NEURAL RESPONSE
“End-stopped” Cells© Stephen E. Palmer, 2002
Cortical Receptive Fields
Constructing a Hypercomplex Cell
Receptive Fields
RETINA CORTICAL AREA V1
Complex Cell End-stopped Cell
© Stephen E. Palmer, 2002
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).
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