Transcripts/01_28 2

Neuroscience: 2:00 - 3:00
Scribe: Teresa Kilborn
Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
Page 1 of 8
He present at picture at the beginning of the lecture that was not in the powerpoint.
a. Look at one detail? Why is it that you can see that detail? What allows you to be able to pick it out and not
get lost in the background? Contrast. The edges.
b. You can see the house compared to the snow because of the edges. The difference in the photon count
following off the screen and back into your eye.
c. But look at specific places, like the yellow dot, the burch tree, the left side which is in the snow is bright like
the snow. So you distinguish it by its right side which has a shadow.
d. Follow the same tree up into the forest and you can still see it. The same tree in the dark forest is now visible
because of the bright edge. The edges are important. Light to dark and dark to light
e. That is encoded in the retina in the on/ off system. The retina is very interesting in light coming on and going
off. You have parallel processing of the signals in the retina from the same location.
f. While the yellow dot is on your retina you have several neural circuits look at the image. Some are focused
on the details and allow you to pick out the branches. Others have lower spatial resolution and are looking at
larger portions of the object at the same time.
g. You have ganglion cells with receptive fields that are tiny and are good for transmitting information in fine
h. Others in the same location and looking at the same object, but pooling them to give you information about
lower spatial resolution for ex. the mountain side.
i. You have ganglion cells look at the tiny dot and the entire mountain side. If you were to focus intently with one
eye on the branch point, while you are focusing on one branch the mountain side is going to look uniformly
j. You can suppress the image of this white patch of snow if you are not looking at it. This happens because
you have a very large receptive field ganglion cell that can’t pick out the detail in there, so your brain fills it in
and tells you it is a very large black area.
k. The point is that every spot in visual space you are sampling many things. If you could color this, then that
would be another thing that would be processed by the retina and the visual system with different circuits.
The same things is true of motion.
Retinal Physiology Vocabulary [S1]
a. Remember: receptive fields and the idea of spatial summation.
b. Large receptive field then large area of spatial summation and very poor spatial discrimination. Same thing is
true in time.
c. Many of the things he will present will be related to space. Because it is easy to represent space on images,
not flashing lights. Just be flexible in your mind.
d. You can have lateral inhibition in time. You can have different spatial summation over time.
e. Convergent and divergent wiring, that is pooling of signals or taking it from one place and sending it to
multiple different circuits.
f. We’ll talk about lateral inhibition and center/surround organization-key in retina physiology.
Receptive Fields of Sensory Neurons [S2]
a. The receptive field is just the physical area over which the appropriate modality of energy is absorbed by a
receptor and converted to an electrical impulse.
b. In the skin, you might have receptive fields that are tiny on the tips of your fingers, lips, tongue where you
have a high density of neurons.
c. The receptive field for the same exact sensation, light touch, would be much larger like your forearms or your
back. That gives you an idea of what receptive field sizes are.
d. But for different modalities, cold, heat, pain you have receptive fields for those same skin senses, and they
may be different sizes.
e. A particular part of skin encodes many sense modalities in terms of touch.
f. In the same way in the retina, a particular set of photons going to a set of rods or cones can be fed out to a
large set of circuits.
Fine Touch Discrimination [S3]
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Scribe: Teresa Kilborn
Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
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a. Fine touch discrimination we use two point discrimination, if the two points are put on skin that has a high
density of receptors then you can detect the fact that you feel two pin pricks and not just one.
b. The same exactly stimulus if presented somewhere else where there is only a single receptor with a large
receptive field, you will not be able to tell if it is two or one large pencil.
c. However, you might have more sensitivity because you have more physical indentation of the skin when there
are two pencils compared to one with a small receptive field.
d. Used as an example of lateral inhibition because lateral inhibition helps to define the edge of the stimulus.
Lateral Inhibition: What does it do for you? [S4]
a. This is a mathematical example. If you go though the numbers it will help you understand how it works.
b. The importance is that if you have an array of sensors: 5 different touch sensors, and a pencil point on B and
c. That leads to an excitatory signal over those immediate sensors, and because there is a slight overlap, in the
receptive field you get a little bit of indentation and excitation in the neighboring cells.
d. The important thing here is that 50 is the largest signal.
e. What do you think your brain is more interest in? trick question, neither. It is interested in the ratio of
excitation of those two. It is interested in differences.
f. If you got an 82 on your last midterm, would you be happy? It would depend on what the class average is. If
it was 97, then probably not. It is relative to what is going on in the background.
g. The brain is interested in encoding this signals, no so much interested in the 50, but the ratio between the 50
and the 25. The relative difference in adjacent sensory neurons is what is important.
h. The brain is going to say that you can see two different stimuli if the ratio of neighboring stimuli is at least 5:1.
That means that you have an edge because the ratio is 5:1 between A and B. You do not have an edge here
because ratio is 2:1 likewise from C to D. Single large object as far as the brain is concerned with edge
between A/ B and D/E. We do not have fine 2 point discrimination, it is only 1 point.
i. But, throw the trick of lateral inhibition in. It’s like in the example of the test but this time everyone around you
gets deducted 10 points making you look better. Interneurons do the same thing.
j. The signal from B is passed on to second quarter neuron and then through an interneuron an inhibitory signal
is passed on to the neighboring neurons in proportion to the main stimulus. In this case it is minus 20%.
Minus 20% gives us a minus 10 in either direction, but that same circuit is occurring everywhere in this array
so the signal from A is going to send an inhibitory signal to B, etc. etc. Work that out ourselves.
k. Important thing where is that in the secondary neurons, you see two things. The white bars are the excitation
of the primary sensory neurons- 10, 50, 25, 50, and 10 and the gray bars are the excitation of the secondary
neurons after lateral inhibition. Notice two things, everyone has gone down. (the GPA for the whole class
has gone down) but those getting the direct stimulus. Now, you know have a ratio between B and C of 43:5
or 8.5:1. So the ratio of excitation between these neighboring cells has been enhanced by lateral inhibition
and if the critical threshold is 5:1, you now have that between A/B, B/C, C/D, and D/E.
l. Now with the same raw stimulus, you have 2 point discrimination. The nervous system has created an
enhanced edge effect through the circuitry of lateral inhibition.
m. The same thing occurs in vision and auditory space. You’re visual system is so interested in differences that
it can be tricked. On the left is a numerical example. 9’s are big, 0 is small, and 1 is intermediate. What does
lateral inhibition do for you? It enhances edges.
Mach bands [S5]
a. This is a picture of Mach bands- on the right you see a white stimulus and on the left you see and black
stimulus. 10 shades of gray between.
b. Is the gray uniform in one hue or does it changes slightly? That is the perception people have.
c. If he took a gray encoder it would show that the step function of the intensity of pixel one to pixel two is
exactly a square function.
d. The step function is exactly like (draws a stair step picture) that is the pixels/ photons are doing out there. But
because you have lateral inhibition in your retinas and they are interested in change they super imposes a
neural code that does this (draws dotted line over the stair step).
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Scribe: Teresa Kilborn
Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
Page 3 of 8
e. You have the perception of a dark edge when its next to a litter bar. You can find a lot of visual illusions that
are dependent on this.
Retinal Physiology [S6]
a. Building a light sensor.
b. Take our light sensor and put it on top of something that allows it to see.
c. We have something called a stalk upon which the eye sits.
d. We are really interested in the eye and the layer of nervous tissue at the back of the eye called the retina.
e. It is part of the CNS.
f. The layers of the retina have been known for many years.
Tartuferi [S7]
a. Drawing from Italian anatomist. In this picture you can see the 5 basic neural types identified.
b. Rods and cones in the outer retina
c. The outer retina is close to the sclera.
d. Choroid RPE and then the photoreceptor outer segments
e. The photoreceptors synapse in the outer plexiform layer on bipolar cells and horizontal cells
f. The horizontal cells provide the lateral feedback in the outer plexiform layer
g. bipolar cells bring the signal from the photoreceptor toward the outer layer plexiform layer where they
synapse with ganglion cells
h. in the outer layer, amacrine cells provide horizontal interact
i. there are 5 types of neurons
j. dual photoreceptor system
Key points to remember: [S8-9]
a. Dual photoreceptor system- Huge range of sensitivity which the retina has to function that is 10 orders of
magnitude. The way we can do that is by having two photoreceptor classes. One is highly tuned to low
levels of light, high gain and the other that is sensitive to bright light and highly adaptable and that is the rods
and cone system.
b. The miniumal direct pathway for a signal to get out of your eye is from a photoreceptor to bipoloar cell to
ganglion cell. The ganglion is the first cell in the retina that generates an axons. The axons coalesce to form
the optic nerve and go back to innervate many structures for vision, primarily the lateral geniculate nucleus
c. Horizontal and amacrine cells are there for lateral interacts. At the outer plexiform for the horizontal cells and
the amacrine cells. They are responsible for the creation of the center- surround organization and lateral
d. Retina is there for contrast and differences, not so much in total photon flux. If he change the light level in the
room by a factor of ten slowly, you wouldn’t noticed. But if he changed it by a factor of 2.5 quickly you would
notice. The temporal change is important, but the magnitude is not.
Review of the anatomy of the retina [S10]
a. Retinal pigment epithelium is the outermost
a. Rod and cone layer
b. The external limiting membrane is made up of muller cell processes which are making tight junctions
with cone intersegments
c. Outer plexiform layer
d. Inner nuclear layer which house the cell bodies of the bipolar, horizontal, amarcine, and muller cells.
e. Larger plexiform layer called the inner plexiform layer which is divided into the on and off sublamina
f. Ganglion cells which give rise to nerve fibers that are in the nerve fiber layer
g. The internal limiting membrane which is important to ophthalmologist as surgical place where they
are pulling about the internal limiting membrane in their ventral retinal surgery that is made up of
muller cells and the tight junctions between end feet of muller cells.
Slide missing (upside picture of previous slide)
a. This is an anatomically correct picture of the retina. This is the clinical view of the retina in which the
photoreceptors are down. You can see the outer nuclear layer, the inner nuclear layer, the very large inner
Neuroscience: 2:00 - 3:00
Scribe: Teresa Kilborn
Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
Page 4 of 8
plexiform layer, ganglion cell layer and nerve fiber layer. You can even see individual cone and rod receptors.
This is the RPE.
b. If you look at a physiologist or anatomist’s picture, you will see it upside-down.
c. We’ll talk about circuits in this orientation.
d. This is an example of divergent wiring. Where a single cone photoreceptor is synapsing with two bipolar
neurons giving rise to two circuits. In this case it is the on and off pathways.
e. Light finds it way into this outer segment. If the light increases, it will generate an excitation in this on
ganglion cell and if this light decreases, it will generate an excitation in this neighboring off ganglion
f. Cones make synapses with dozens of bipolar neurons. Some are for very narrow field others wide field.
g. Convergent wiring is what you have for summation. When you were looking at that tiny dot you need a
ganglion like this. When you are looking at the forest you need a circuit that pulls information from a large
number of receptors to a smaller number of biopolar neurons and to smaller number of ganglion cells
h. Convergent wiring is necessary because the retina contains 100 million photoreceptors but only 1 million
ganglion cells. There has to be convergence 100:1 to get information out. Otherwise your optic nerve would
have to be larger than your eye which would negate the entire point of ocular motor activities.
Ganglion cell receptive Fields [S11]
a. Cells around receptive fields and lateral inhibition.
b. at the ganglion cell layer, we have midget and non midget bipolar and ganglion systems
c. described based on the fact that their dendritic arm is either very broad, a diffuse cell or very small and small
are midget
d. midget can have only a single cone photoreceptor
e. midget biopolar cell connected to midget ganglion cell that receives excitation from this sensor pathway
bipolar and cone and through lateral inhibition that goes through neighboring bipolar cell onto amarcrine cell
that then feeds a negative signal onto the ganglion cell and what you get is a inhibitory surround. The cells in
the surrounding area are providing a negative or hyperpolarizing signal, whereas the center is providing a
depolarizing or excitatory signal.
f. This is an example of lateral inhibition generating the center/surround of a ganglion cell
Ganglion cell receptive Fields [S12]
a. When you rotate them 90 degrees and plot them, you would say this ganglion cell has a excitatory center (+)
when the light is on and inhibitory in the black area. Photoreceptors would send their signal through their
bipolar and lateral interactions either horizontal cells or amacrine. They would feed into the ganglion cell with
the signal of the opposite sign. The center is from excitatory on bipolar and the surround comes from either
off biopolars or interneurons like amacrine
b. We are interested in light going on and off in every location on the retina. The off signals are encoded and
diagramed just the opposite: dark in the middle and white ring around. The dark means that when the light
goes off that there is an excitatory burst that is an increase in AP firing for that off cell.
Ganglion cell receptive fields [S13]
a. All summarized in slide- study it.
b. On center field and an off center field
c. We’ll talk about the on and you figure out the off.
d. In all of these cases, a recording from the ganglion cell.
e. The yellow line indicates the light coming on which is for ½ s.
f. If the small dot is in the center and it is an on ganglion cell you have a background rate of firing and when light
comes on you have a depolarization of that cell and an increase in firing frequency.
g. If a small dot is presented in the periphery or the surround then you get an inhibitory signal, a
hyperpolarization of the cell, and a decrease in the firing rate.
h. If you could get the spot of light to exactly fill the center and complete avoid the surround then you would have
the optimal excitation.
i. On the other hand if you were staring at a bullseye, then you would have optimal stimulation of the surround
with no stimulus of the center, you will have optimal hyperpolarization and you could probably evoke complete
cessation of light response.
Neuroscience: 2:00 - 3:00
Scribe: Teresa Kilborn
Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
Page 5 of 8
j. The important thing is that diffuse illumination, illumination of the entire field, is boring. The retina in this case
is not interested in uniform light.
k. This also shows the timing. When the light goes off from the surround, produces an excitatory burst. If you
get that then good, if not don’t worry about it.
Photoreceptors [S14]
a. Where do these signals come from and how do we get the responses?
b. The visual system can respond over 10 orders of magnitude.
c. It can do so because you have two special photoreceptors for low light levels and high light levels.
d. Rod like cell that has disks completely incased in the cell; about 2,000 in every rod, they are packed with
e. Rhodopsin is easy to harvest, if you take a retina out and you shake it then the outer segments will fall out
and each rod outer segment contains 100 million molecules of rhodopsin.
f. The cone is physical different- smaller with conically shaped outer segment. Physiological different also.
g. Both transduce light information into an electrical signal.
h. The cone has low sensitivity and the responses are over quickly. High temporal resolution. If you want to see
something fast, and know if the light is going on and off quickly you need the cone system. Try turning off the
light in a batting cage.
Rod Photoreceptor (system) [S15]
a. The rod photoreceptor system take up 95% of the photoreceptor in the human eye. We are nocturnal
b. We have sensitivity in individual rod cells to generate an electrical response
c. Cones can also generate electrical response and can respond to individual photon, but don’t generate a big
d. Rods have single photon sensitivity – can generate a single that can detected over the noise; signal to noise
e. In order for a cone to generate a signal you have to have at least 7-10 photo-isomerizations inside that cell for
there to be a detectable change in the membrane potential to be passed to the bipolar cell
f. There is a high degree of spatial summation. Rods pool onto rod bipolars. Rod bipolar pool onto a2
amacrine cells. A2 amacrine cells pool onto ganglion cells.
g. But for that you pay a price- spatial resolution. Rod system has very low spatial resolution
h. If you ever try to walk around in the living room in the dark you can’t see the edges of the coffee table
i. They also have a very slow timed peak
j. Low temporal summation- trade off due to biochemistry of photo transduction. If you want to amplify the
signal then it takes time. That time generates a large response which is good for sensitivity, but you sacrifice
temporal resolution. Rods have low sensitivity to flicker. At a high rate of flicker it looks steady. Then if you
increase the light level, it would appear flickering again, because you make a transition from rods to cone
k. The rods cover about 2.5 log units of sensitivity. The rods summation occurs another 2 log units. The rods
system covers about 4.5 log units of sensitivity.
l. One of them is an anatomical trick and that is that if you take the rod bipolar and connect it to 500 rod cells
and connect 10 of those rod bipolars to an output then you have 5,000s rods that are pool. You can stimulate
them at a rate of 1 out of 1000 rods and still get 5 photo-isomerizations in that pathway and that is enough to
be passed on to the ganglion cell for you to be able to see it.
XVII. Cone (system) function [S16]
a. The cone system, in phobic animals like we are, the there is tiny area of retina that is specialized for high
acuity. In that area there are only cone receptors, no rods
b. The layers of the retina are all different
c. There are 3 flavors of cones- red, green and blue sensitive. The blue are kicked out and you only have red
and green in this area of the fovea.
d. High acuity, spatial acuity due to this tiny patch that is .5 mm in diameter. You have 200,000 cones per sq
mm here, each connected to an off and an on bipolar.
Neuroscience: 2:00 - 3:00
Scribe: Teresa Kilborn
Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
Page 6 of 8
e. Single cone in the fovea is connected to the an on and off bipolar connected to a ganglion cell because you
need pixel resolution of very high fidelity to have high acuity and that information has to get back to the brain
without summation
f. Summation occurs in the periphery.
g. What you don’t realize is that when you are looking straight ahead you don’t realize fine details in your
periphery. If you wanted to see them, then you would look over there. Move your eyes to put the image of
interest on the fovea.
h. Fast response time, cone are sensitive to a high rate of flicker. Very wide range of adaptation. You can
produce a burst of light that will saturate the cone, but in about 30 seconds it would adapt. The bright burst of
light of rod would make it insensitivity for 1-2 minutes.
XVIII. Picture [S17]
a. This all happens because of the g coupled receptor called rhodopsin.
b. The ligand for rhodopsin is a photon of light that is absorbed by retinal.
c. Retinal is a chemical that comes from vitamin A.
d. Vitamin A you can’t manufacture, you have to get it from your diet- carrots and sweet potatoes.
e. People in Africa go blind because low retinal.
f. Absorption of a photon of light can rearrange the subunits of the rhodopsin molecule and create an activate
enzyme. Active rhodopsin that interacts with up to 500 transducing molecules in a rod cell, which then
activates phosphodieseterase which chews up cGMP which keeps the channels open.
g. You go to photon of light, to second messenger system, to closed channels. From light to electrical signal.
Biochemistry of Phototransduction [S18]
a. The important thing to remember is that rods have high gain.
b. The photoreceptor cascade goes on for a long time and all of the numbers are very large.
c. Cones have smaller responses, due to the fact that the shut off mechanism in cones is more robust.
d. The activation of rhodopsin, activation of transducin, activation of PDE all have to be shut off. The destruction
of cGMP that causes the channels close has to be recreated. You don’t get the end of the light response until
all of those things have happened.
e. If the cone cells do those biochemical events faster then the signal will be smaller, faster, shorter.
f. The big difference between cones and rods is their photo-transduction systems.
Picture [S19]
a. Rod outer segment with a lot of open channels with inward, depolarizing current carried by Na and Ca.
b. When a photon of light is absorb it leads to a local depletion of cGMP, the channels close, and the normally
flowing current is eliminated leading to a local hyperpolarization which spreads very quickly down the cell and
modulates the membrane potential at the synaptic ending.
c. The photoreceptor, like other neurons, when it is depolarized then NT is released and when it is not then NT
release goes down
d. When the light comes on, the cell is hyperpolarized, and the NT release of the rod and cone photoreceptors
goes down.
Current and Photovoltages[S20]
a. Words that say the same thing I just said to you
XXII. Rod vs. cone Kinetic and sensitivity [S21]
a. The NT releases glutamate
b. Photoreceptors produce graded responses
c. Pico amps of current that are normally flowing into the outer segment in the darkness. In time you have a
stimulus of light that opens and closes. When photons are absorbed it causes a transit closure of channels in
the rod cells
d. 4x as bright = 4x the signal, all linear ranges
e. If you produce a large range of photoisomerizations, you can close all of the channels. Once you’ve closed
all of the channels by the stimulus, when you make the stimulus brighter, all of the cGMP has already been
Neuroscience: 2:00 - 3:00
Scribe: Teresa Kilborn
Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
Page 7 of 8
f. You can’t close any more of the channels, but you can create photo bi-products that the channels stay closed
longer. Longer periods of saturation.
g. Plotted to the right is the intensity response diagram. Intensity in log units and the number of photons.
Amplitude is carried over. For a dim stimulus, you get a linear changes then the saturation period. Cone
photoreceptors respond similarly to rod, they have transient channels, transient hyperpolarization. As you
make the light brighter and brighter, you close more of the channels; however, the shut off functions are more
robust. The inactivation of PDE in cones is much faster. Cones carry more Ca. The Ca feeds back and
changes things more quickly in the cones and you have an interruption in the signal much faster. More light
needed to generate a signal in cone than in a rod. If you choose a stimulus intensity of 200 photons. That
stimulus will produce a response in the rods outlined by the red line. A very bright saturating response of the
rod cell. You are at the upward end of the response range. That same stimulus is producing a threshold or
minimal response in the cone cell. We call this area of vision the scotopic area of vision or rod-dominated
vision. The area where both of them are function are called mesoptic and the area to the right where only the
cones are functioning is called photopic vision.
XXIII. Themes of retinal circuitry [S22]
a. Divergent wiring is the fact that a single cone photoreceptor can go out to multiple bipolar cells such as the on
and the off pathway.
b. Convergent wiring is the area where we have the greatest summation
XXIV. Picture [S23] Same pictures as seen before
XXV. Themes of retinal circuitry [S24]
a. The highest acuity in the visual system is carried by the midget system diagramed here.
b. A single cone photoreceptor is connected to a bipolar cell which is connected to a ganglion cell.
c. The ganglion cell has no direct influence from other cells and that information goes back to the brain, if you
have a large number of ganglion cells each with its own private line to the brain then you will have high spatial
acuity and high representation of that area of the retina in the brain.
d. Horizontal cells and amarcine cell provide lateral feedback for any inner plexiform and outer plexiform.
e. Horizontal cells in the outer plexiform create a center surround organization
f. The center surround organization that you hear about is present in ganglion cells and bipolar cells.
g. In the bipolar cells, the center surround organization comes from horizontal feedback and lateral interaction to
the center cell.
h. Amacrine cells provide much great complexity at the inner plexiform layer to give you more complex center
surround receptive fields for ganglion cells
i. For example, there are some that are in the center, red on, green off, and the surround is green on and red
off. You have double opponent color cells in the ganglion cells but you don’t see that in the bipolar cells
j. There are feedback synapses that occur, synapses that occur to provide additional complexity. For example
the creation of the transient versus sustained. Some of it comes from the individual cells, some of it comes
from synapses from surrounding cells. You have directionally sensitive. They are ganglion cells sensitive to
motion in a certain direction. It occurs primarily in the periphery. It’s a complex system that involves
amacrine cells, complex t4 and t? at the inner plexiform layer
k. Spatial filtering that provide lateral inhibition
l. Directional selectivity
m. When you dark or light adapt your retina under lighting conditions, some of gain of function is coming from the
photoreceptors themselves. Some of it is an adjustment of the photoreceptor cascade and biochemistry.
Also a network gain control. You can have gain control of the photoreceptor to bipolar cell synapse so that
you have less effective communication across that synapse. That would extend the range of function for the
cone circuit, that may not be present in the cone cell.
XXVI. On and Off pathways [S25-27]
a. You can read the words
b. Off pathway you have a stimulus that goes on and off, hyperpolarization of the photoreceptor which through a
sign conserving synapse hyperpolarizes the bipolar cell which through a sign conserving synapse
hyperpolarizes the ganglion cell decreasing the spikes.
Neuroscience: 2:00 - 3:00
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Wednesday, January 28, 2009
Proof: Ashley Holladay
Dr. Kraft
Early visual processing
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c. Ionotropic channels which you’ve heard about in the off system and metabotropic glutamate receptors for the
on system so when you have hyperpolarization of the photoreceptor, you have a depolarization of the bipolar
cell which depolarizes the ganglion cell which leads to an increase in spike frequency.
XXVII. Picture [S28]
a. The on and off system can come from a single cone photoreceptor.
b. Can synapse with 25-55 bipolar cells, some of which are on, some off, some midget, some diffuse bipolar
cells feeding into fine spatial acuity versus low spatial acuity circuits in the retina
XXVIII. Building receptive Fields [S29]
a. The center surround organization begins at the bipolar cells and the synaptic interactions are occurring
through the horizontal cells. It is much more active at the ganglion cell layer due to the fact that there are 35
different amacrine cells (we only know what 12 do)
XXIX. Picture [S30]
a. horizontal cell, cone photoreceptor, if it was sending a signal then it could send a lot of inhibitory signals back
to a lot of surrounding cones through the horizontal cell.
XXX. Diagram [S31]
a. Center surround receptive field for an on and off center receptive field
XXXI. Biopolar Cell [S32-33]
a. There are different types of bipolar cells:
b. connected to the rods and cones, midgets that are only connected to cones, and midget system to cones to
provide for high spatial acuity, and diffuse bipolar that connect to cones to provide spatial filtering.
c. We have on and off subtypes because spatial filtering the view of the world needs to tell the light coming on or
the light going off
d. Bipolar cells have a high contrast- high gain. They encoding a high sensitivity for the image you saw with
high gain.
XXXII. The rod Piggyback-pathway [S34-35]
a. Rod system only has an on bipolar. The rod signal goes out through the cone ganglion. No ganglion devoted
to the cones. In the retina in the dark, your cones aren’t active. Most of your ganglion cells are devoted to
those cone receptors so they are going to be silent. In the dark, those cones send a signal by an amacrine
cell and feed into the cone pathway. Dual use of the ganglion cell in dark and light condition. The rod goes to
the rod bipolar to a A2 amacrine, feeds to a off and on cone bipolar and out of the retina through the off and
on ganglion cells.
XXXIII. Picture [S36]
a. High gain picture of A2 amacrine cells with all of the synapses there are.
[end 47:08]