Download Coding of Visual Information in the Retina Coding of Light d D k and

Coding of Visual Information in the Retina
Coding of Light
and
d Dark
D k
Coding of Color
Le Parade, by Seurat
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Copyright © Allyn & Bacon 2007
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A quick comparison:
Characteristic
Foveal Vision
Peripheral Vision
Receptors
Cones in the fovea
itself; cones and rods
mix in the surrounding
area.
Proportion of rods
increases toward the
periphery; the extreme
periphery has only rods.
Convergence
of receptors
One or a few receptors
send their input to each
postsynaptic cell
Increasing numbers of
receptors send input to
each postsynaptic cell.
Brightness
sensitivity
iti it
Useful for
distinguishing
g
g among
g
bright lights; responds
poorly to faint lights.
Responds well to faint
lights;
g
; less useful for
making distinctions in
bright light.
Color vision
Good (many cones)
Poor (few cones)
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Color discrimination problem…
Problem: how does one cell code for
yp of information?
two types
A neuron can only vary its frequency of
action potentials.
If the cone’s response indicates
brightness then it cannot signal for
brightness,
color.
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The solution…
No single neuron can simultaneously
indicate brightness
g
and color
Our perceptions must depend on
patterns of responses by a number of
different neurons.
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Trichromatic Theory
AKA: Young-Helmholtz Theory
g (1773
(
– 1829))
Thomas Young
among the first to recognize that color
required a biological explanation.
He proposed that we perceive color by
comparing the responses of some small
number of receptors, each of which is
sensitive to a different part of the range
of visible wavelengths.
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Hermann von Helmholtz
Modified Young’s theory
“We perceive color through the
relative rates of response by
three kinds of cones, each kind
maximally sensitive to a different
set of wavelengths.”
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How did he know that?
Psychophysical observations
He found that people could match any
color by mixing appropriate amounts of
just three colors.
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Trichomatic theory:
We discriminate
among wavelengths
l
th by
b
the ratio of activity
across the tree types of
cones
cones.
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Cone distribution…
Long and medium cones are more
abundant
Easier to see tiny red, yellow, and green
dots.
Really difficult to see tiny blue dots you
perceive them to be black.
Cones are distributed randomly within the
retina.
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The Opponent-Process Theory
Theory proposed by Ewald Hering (19 C)
Negative color afterimage
Replaces
green and red with each other;
yellow and blue with each other; &
white and black with each other.
We perceive color in terms of paired opposites
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Copyright © Allyn & Bacon 2007
Copyright © Allyn & Bacon 2007
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Copyright © Allyn & Bacon 2007
The Stimulus
sensory receptors
A specialized neuron that detects a particular
category of physical events.
sensory transduction
The process by which sensory stimuli are
transduced into slow, graded receptor potentials.
receptor potential
A slow, graded electrical potential produced by a
receptor cell in response to a physical stimulus.
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The Stimulus
hue
One of the perceptual dimensions of color; the
dominant wavelength.
brightness
One of the perceptual dimensions of color;
intensity.
saturation
One of the perceptual dimension of color; purity.
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Anatomy of the Visual System
vergence movement
The cooperative movement of the eyes, which
ensures that the image of an object falls on
identical portions of both retinas.
saccadic movement
The rapid, jerky movements of the eyes used in
scanning a visual scene.
pursuit movement
The movement that the eyes make to maintain an
image of a moving object on the fovea.
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9
Anatomy of the Visual System
accommodation
Changes in the thickness of the lens of the eye, accomplished
by the ciliary muscles, that focus images of near or far
objects
j
on the retina.
retina
The neural tissue and photoreceptive cells located on the
inner surface of the posterior portion of the eye.
rod
One of the receptor cells of the retina; sensitive to light of low
intensity.
cone
One of the receptor cells of the retina; maximally sensitive to
one of three different wavelengths of light and hence
encodes color vision.
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Anatomy of the Visual System
photoreceptor
One of the receptor cells of the retina; transduces photic
energy into electrical potentials.
fovea
The region of the retina that mediates the most acute vision
of birds and higher mammals. Color-sensitive cones
constitute the only type of photoreceptor found in the
fovea.
optic
p disk
The location of the exit point from the retina of the fibers of
the ganglion cells that form the optic nerve; responsible
for the blind spot.
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10
Anatomy of the Visual System
bipolar cell
A bipolar neuron located in the middle layer of the retina,
conveying information from the photoreceptors to the
ganglion cells.
ganglion cell
A neuron located in the retina that receives visual information
from bipolar cells; its axon give rise to the optic nerve.
horizontal cell
A neuron in the retina that interconnects adjacent
photoreceptors
p
p
and the outer p
processes of the bipolar
p
cells.
amacrine cell
A neuron in the retina that interconnects adjacent ganglion
cells and the inner processes of the bipolar cells.
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Anatomy of the Visual System
lamella
A layer of membrane containing photopigments; found in the rods and
cones of the retina.
photopigment
A protein dye bonded to retinal, substance derived from vitamin A;
responsible for transduction of visual information.
opsin
A class of protein that, together with retinal, constitutes the
photopigments.
retinal
A chemical synthesized from vitamin A; join with opsin to form a
photopigment.
rhodopsin
A particular opsin found in rods.
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Anatomy of the Visual System
dorsal lateral geniculate nucleus
A group of cell bodies within the lateral geniculate
body of the thalamus; receives inputs from the
retina and projects to the primary visual cortex.
magnocellular layer
One of the inner two layers of neurons in the
dorsal lateral geniculate nucleus; transmits
information necessary for the perception of form,
movement, depth, and small differences in
brightness to the primary visual cortex.
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Anatomy of the Visual System
parvocellular layer
One of the four outer layers of neurons in the
dorsal lateral geniculate nucleus; transmits
information necessary for perception of color and
fine details to the primary visual cortex.
koniocellular layer
One of the sublayers of neurons in the dorsal
lateral geniculate nucleus bound to each of the
magnocellular and parvocelllar layers; transmits
information from short-wavelength (“blue”) cones
to the primary visual cortex.
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Anatomy of the Visual System
calcarine fissure
A horizontal fissure on the inner surface of the
posterior cerebral cortex; the location of the
primary visual cortex.
striate cortex
The primary visual cortex.
optic chiasm
A cross-shaped connection between the optic
nerves, located below the base of the brain, just
anterior to the pituitary gland.
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Coding of Visual Information in the
Retina
receptive field
The portion of the visual field in which the presentation of
visual stimuli will produce an alteration in the firing rate
of a particular neuron.
protanopia
An inherited form of defective color vision in which red
and green hues are confused; “red” cones are filled
with “green” cone opsin.
deuteranopia
An inherited form of defective color vision in which red
and green hues are confused; “green” cones are filled
with “red” cone opsin.
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Coding of Visual Information in the
Retina
tritanopia
An inherited form of defective color vision in which hues
with short wavelengths are confused; “blue” cones are
either lacking or faulty.
negative afterimage
The image seen after a portion of the retina is exposed
to an intense visual stimulus; consists of colors
complementary
l
t
t those
to
th
off the
th physical
h i l stimulus.
ti l
complimentary colors
Colors that make white or gray when mixed together
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Charles Angrand. Hay Ricks in Normandy
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