Transcript RETINA

RETINA
BLIND SPOT
close left eye and stare at plus
move paper back and forth
• Rhodopsin= retinal (lipid) + opsin
• Different opsins=different
photoreceptors (rods vs. cones, etc.)
• Vitamin A is a source of retinal
• Why are photoreceptors located where
they are? Show animation.
RODS VERSUS CONES: approx 130,000 in eye
•RODS: monochromatic, most sensitive to low
light, on periphery, poor acuity (detail)
•CONES: color, sensitive to mod/high light,
concentrated in fovea, good acuity (why? We
will discuss later). Make up only 3-5 % of the
photoreceptors
SENSORY TRANSDUCTION
Sensory transduction
• Review: Amplifies (100k, 5 photon sensitive)
• Note that light closes the sodium channels
which causes a hyperpolarization.
• This hyperpolarization causes a decrease in
glutamate neurotransmitter release to a
bipolar cell (more on this later)
• Unique to the nervous system. So in
darkness, the photoreceptors are
depolarized. Evolution significance?
DARK/LIGHT ADAPTATION
• To prepare for another photon, all trans
retinal leaves the cone/rod, is
metabolized back to 11-cis form and
returned to the rod/cone.
• How long does this take?
• Rods:20-30 minutes. Respond to slow
changes
• Cones:2-3 minutes. Respond to more
rapid changes.
• What happens when we enter a dark movie
theatre from outside in the sun?
• 99% of the retinal had been converted to the
all trans and fallen off of the opsin
• There would be no signal until enough 11-cis
production has occurred.
• Cones first (2-3 minutes) then rods(20-30
minutes)
• Pupil dilation
• What happens when we walk out again from
the movie theatre?
• Since we have been in the dark, we have
regenerated all of the retinal into the 11-cis
form.
• We walk out into the sun and every cell is
ready to fire (blinding) until a short time later
when the 11-cis is mostly converted to all
trans and is used up (adjustment)
review
• Light actually tells a photoreceptor to stop
releasing glutamate. The glutamate then sends
a signal to a bipolar nerve cell, which in turn, can
also release glutamate if activated. This bipolar
nerve cell can be an “on cell” which fires when
light is shown. In order to do this, it therefore
has to have a glutamate receptor that inhibits an
action potential. An “off” bipolar nerve ganglion
has a glutamate receptor that opens sodium
channels and allows an action potential.
Receptive fields
• Think of the retina as a “screen”
• Center surround organization
• Our vision depends on viewing contrasts
between objects and backgrounds
• Rod receptive field larger than cones
• “on”/”off” center surround organization
• Concept of lateral inhibition, and rebound
effect. Draw picture on board and show
animation. Can explain after images and
optical illusions.
Let us assume an “on” center and “off” surround field.
In other words, the center of the receptive field has “on”
ganglion cells (fire when photoreceptors receive light),
while the surrounding ganglion cells are “off” when light
is shown on them. Note that there are other fields where
the opposite is occuring.
Field A is more activated than field D. Field B is
more activated than field C. When assuming an
on center off surround field.
The surrounding “off” surround ganglion cells are
inhibiting the “on” center ganglion cells in A more
than B, creating a gray (less contrast image) spot
When you look directly inbetween the
grids, the spot goes away because
receptive fields get smaller when you look
directly at them.
Light contrast: to understand you
must know the difference between
intensity and lightness. Both are
indicators of light reflecting off an
object but….
intensity=physical and constant level
lightness=subjective and perceptual.
Can change based on differences of
intensity between images.
Each strip differs in intensity, but
notice within each strip, lightness
differs.
In B the second light will inhibit the
strength of the “on” ganglion in the center.
Field A detects the least amount of light,
while field B detects the most amount of
light. Fields C and D receive light from
both the darker and lighter stripes, which
creates antagonism. With field C,
although its center detects the same
intensity of light as field A (because they
are located within the same stripe), part
of its surround detects brighter light from
the lighter stripe. As a result, the
surround of field C creates more
antagonism than the surround of field A,
which decreases the activity of field C
making the area around it appear darker
in lightness. With field D, its center
detects the same intensity of light as
field B, but part of the surround is
detecting the reduced light from the
darker stripe. This creates less
antagonism resulting in a lighter field.
Color vision
• Do not confuse mixing colors of light and colors
of pigment like paint
• Pigments: yellow + blue = green. Mix all colors
and you get black (subtractively). The more
pigments you add the more wavelengths will be
absorbed and the less reflected wavelengths
that will reach the eye.
• Light: yellow + blue = white (NOT GREEN) Red
+ green = yellow. The more lights you add, the
closer you get to white light (additive). Newton
and prisms.
• The uniqueness of yellow.
Chemistry review
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Visible light is a part of the electromagnetic energy spectrum
Photons of light are energy, and they travel in waves
Wavelengths are measured in nanometers (nm)
Visible light is only between 400nm and 750nm
Visible light spectrum ranges from Purple, Blue Green, Yellow,
Orange, to Red
• Below 400 is called Ultraviolet (Can’t See)
• Above 750 is called Infrared (Can’t See)
• Newton: “The Rays to speak properly are not coloured. In
them there is nothing else than a certain Power and Disposition
to stir up a Sensation of this or that Colour”. Wavelengths are
not colored, color is created by our perceptual system in
response to these wavelengths
Young & Helmholtz’s Trichromatic
Theory of Color
• You can combine three primary colors of light to
reproduce all colors of the spectrum. Two colors are
too few.
• Therefore we have only 3 kinds of receptors to
detect an infinite # of colors
• Blue cones: have a unique opsin that maximally
responds to 420 nm wavelength
• Green cones: 530 nm
• Red cones: 559 nm
• No yellow cones!
• Any single wavelength of visible light will stimulate
a unique ratio of these 3 cones—allowing for an
infinite # of colors (shades of colors)
Color wheels, spectrums etc.
• We will review demonstrations
• Again, remember that our perception of color depends
on the ratio of excitation in the three different cone
classes. Two different wavelength lights can evoke the
same response (color perception) as long as the ratio of
excitation is the same. These are called metamers
• Max absorbency of photoreceptors is 555nm for us and
old world apes. Evolution significance?
• People can distinguish about 2 million different colors.
200 colors X 500 steps in intensity X 20 steps in
saturation. Saturation=amount of whiteness in a color.
Red is more saturated than pink
Opponency theory
• Name a color that is yellowish red. How
about reddish blue. How about bluish
green. How about reddish green? How
about bluish yellow?
• In lateral geniculate nucleus (lgn) there are
color-opponent neuron cells. This is
similar to on/off center surround receptive
fields.
• Demonstration will explain
Color blindness
• Most common types are protanopia and
deuteranopia (95% of all color blindness)
• See the world in shades of yellow and
blue. Both red and green look yellowish.
• Acuity is normal.
• 7% of men are affected but only 0.4% of
women affected.
• What does this imply? What could be
happening here?
• Sex linked. The gene that codes for red cones and
green cones must reside on the X chromosome.
• Since normal acuity, the red or green cones are actually
being formed, but are not responding properly to the
different wavelengths. (remember that color vision is
dependent on “ratio” of excitation)
• Therefore, protanopes have their red cones filled up with
green cone opsin. Deuteranopes have their green
cones filled up with red opsin.
• If you don’t have different opsin molecules to respond to
red and green wavelengths, then you cannot distinguish
between red and green. Remember that yellow is
perceived when red and green cones respond equally,
which they would if they had the same opsin molecule.
More things to consider….
• The primates of South America, which broke from the continent of
Africa about 40 million years ago, possess only a single functional
copy of a red or green gene, much like color-blind men. Old World
primates—the monkeys and apes of Africa and the ancestors of
humans have two different opsin receptors for red and green like
normal men. Researchers have sequenced human red and green
opsin genes and discovered that they differ by only two percent.
• Can you suggest an explanation for this? What might have
happened to our ancestral genome in old world apes that led us
to normally see red and green? What is the evolutionary
significance of this?
• A primordial red-green gene must have duplicated and then
diverged slightly in sequence, leading to separate receptors of the
red and green type. Being able to distinguish red berries in a green
canopy might be advantageous.
• Curiously, researchers have also found that when they sequenced
DNA in this region from some men who have normal color vision,
they found that lying head to tail along their X chromosome were not
just the two genes for the red and green receptors, but also an extra
copy of the green receptor gene.
• This explains the why color blindness is a relatively common genetic
disorder. What could be happening here? Hint, recall our
studies with sry gene and recombination.
• One X chromosome may receive an extra
green receptor gene. This does no harm.
But then the other chromosome with which
it is exchanging bits of genetic information
is left with only a red receptor gene. The
man who inherits this slightly truncated
chromosome will be color-blind, without
the genetic information needed to make a
green receptor.
• Remember that more than 95 percent of all
variations in human color vision involve the red
and green receptors in men's eyes. It is very
rare for anyone—male or female—to be "blind"
to the blue end of the spectrum.
• Why do you think that blue end blindness is
so rare? Do you think that it affects more
men, or women?
• The gene coding for the blue receptor lies
on chromosome 7, which is shared equally
by men and women, and this gene does
not have any neighbor whose DNA
sequence is similar. Blue color blindness is
caused by a simple mutation in this gene.
Binocular vision
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Sacrifices peripheral vision(panorama)
Total cross over gives panorama vision only
We have approx. 50% crossover. For instance the left visual cortex
receives optic nerve input from the left temporal region and the right nasal
region.
Parallax: Two slightly different images from each eye come together
(fusion) in the brain creating a 3-D sensation (depth perception). This
fusion process is a learned response and develops during infancy. If this
didn’t happen we would have “double” vision.
Convergence of eyes occur to help overcome double images.(one finger
test). Lens focus is also important.
Stereopsis is strongest with near objects. Why?(prove it to yourself)
3D vision depends on saccadic eye movements. It is optimal with images
on the fovea.
You can still have monocular depth perception. How? (movement stereo,
etc.)
3D movies