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Neuro: 1:00 - 2:00
Scribe: Marjorie Hannon
Wednesday, February 04, 2009
Proof: Caitlin Cox
Gawne
Higher Visual Processing
Page 1 of 7
I. Introduction [S1]: Higher Visual Processing
a. He will be happy to answer any questions. Reach him at the email and phone provided on the slide.
II. So what is the visual system anyhow? [S2]
a. Optical System: cornea, lens, etc.
b. Phototransduction (rods and cones). If the light energy is not transduced into some form the nervous system
can handle, then there is no vision.
c. Computation (*Largest part by far*)
i. By far the biggest part of our visual system is computation.
ii. Something like 1/3 or 1/4 of the human brain is required for our full visual capability. That is an enormous
amount of tissue, which is more than for language.
iii. Why is this? Why does visual processing take so much of the brain?
d. The problem here is something called the “inverse problem”. Imagine that you have objects in the world; there
are lights, different surfaces and shadows, and geometry. They are focused to make a two dimensional image
on the back of the retina.
III. The forward Problem [S3]
a. This is basic optics, basic physics. You can write down a formula or equation to solve this. In fact, if you go and
see a movie with 3D graphics in it, someone has generated an abstract model of reality and calculated what the
2D image should be, given the 3D image scene. That is easy, but that is not vision it is just focusing an image.
b. Vision is the process of: “here is an image on the retina, what is out there in the world? That is much harder
because information is destroyed when you go from here to here.
c. For every 3D scene, there is one unique retinal image that will result (it is a mathematical certainty).
d. For every 2D retinal image, there are an infinite number of possible scenes that could give rise to that.
i. For example in this image, the objects occlude each other. Maybe this is a complete circle (but parts are
missing), or maybe it is some other shape and you just can’t see it.
ii. The problem with the visual system is it has to take this and guess what’s out there.
e. The visual system has to reconstruct it using incomplete data, using knowledge of the likelihood of things being
present in the world. The visual system is very complex and in fact, we don’t know how to do this.
IV. Picture [S4]:
a. This is just to orient us. This is a horizontal section through the visual system. You have the two eyes, the optic
nerves (ganglion cell axons), things cross over at the optic chiasm, and synapse in these retinal nuclei which are
more or less in the middle of the head. Stuff goes on here, but mostly it goes back to the occipital lobe to the
primary visual cortex.
b. Maybe a quarter or more of your cortex is involved in the visual system.
V. How does the visual system work?[S5]
a. We don’t really know! But learning fast.
b. It sounds simple, it sounds like vision should be primitive and language and reasoning should be hard. But
when you actually think about it computationally, they are really of equal difficulty. We are not just studying
vision; we are studying consciousness and the mind because it is really tied up there. One of the most
fundamental and exciting problems in modern science
c. But for now, robots that ‘see’ are all still very much science fiction. Remember this crucial distinction: A video
camera can RECORD and image (and transmit it from place to place), but it doesn’t know what it is seeing.
Vision is interpreting the image, and that is what we don’t know how to do.
VI. Picture [S6]
a. This robot was in the movie Red Planet.
b. One of the reasons we couldn’t build this is that we don’t know how to give a robot sight, not yet.
c. The full visual capability that even a 3 year old child has is beyond the reach of the most sophisticated computer
or robot in existence.
VII. The major targets of the retinal ganglion cells [S7]
a. Here is a slide you should know. *KNOW THIS SLIDE “that is a hint for the test”!
b. Retina  LGN  Cortex (vision)
i. You have the retina and the ganglion cells. The ganglion cell axons go places, most of them go to the
lateral geniculate nucleus (LGN) which is part of the thalamus. From there they are relayed up to cortex.
c. By far, 90% or more of the ganglion cell axons go here (the LGN) and this is the part that is involved in vision.
d. The retina  LGN  Cortex pathway is known as the geniculostriate or thalamocortical system; this is
conscious vision.
e. A smaller fraction goes to a place called the superior colliculus and that is involved in saccadic eye movement.
i. For example, when you look from somewhere to somewhere else, the circuitry that helps you look different
places rapidly and accurately, the superior colliculus is involved in that.
Neuro: 1:00 - 2:00
Scribe: Marjorie Hannon
Wednesday, February 04, 2009
Proof: Caitlin Cox
Gawne
Higher Visual Processing
Page 2 of 7
f. Relatively smaller numbers of axons (out of maybe a million ganglion cell axons, maybe only a few thousand go
to the suprachiasmatic nucleus in the hypothalamus. This is involved with making sure your circadian rhythms
are in phase with the light dark cycle.
g. Some of them go to the pretectum for the pupillary light reflex.
h. Some go to the accessory optic system which is involved in helping stabilize gaze. (So that your eyes don’t
bounce around when you are walking or moving). There are a lot of systems in the brain that help to keep the
gaze stable and this is part of that system.
i. He stressed again at the end of the slide that you should certainly know this slide. (Where do the ganglion cell
axons go, what’s the name, and what they do).
VIII. Picture [S8]
a. Here is another picture to get oriented. The eyes are on the frontal lobe, you go back to the LGN in the middle of
the brain, and from here most of the information goes back to the occipital lobe into the primary visual cortex.
b. In the box is a blowup of this region. You have the LGN (the layered structure) and then the pretectum and
superior colliculus in the midbrain.
IX. Picture [S9] Reflex loop for the pupillary light reflex.
a. Light comes in  pretectal olivary nuclei  edinger westphal  cilliary ganglion  constrictor muscle.
b. That would be a useful thing to know (hint for test).
X. Blindsight [S10]
a. There are a lot of circuits going in, there are these 5 major retinal nuclei.
b. If you take out the thalamocortical system, you lose conscious vision. The other circuits (superior colliculus,
accessory optic system) can mediate some reflexive behaviors. Sometimes if you do it right, people can do
some very simple visually guided tasks like say when they think a light is flashing, but it is in the absence of
conscious perception.
c. The bottom line is: if you take out the LGN-cortical system then you are for all practical purposes consciously
blind, you have no perception of vision. This is important because, if you get a lesion anywhere in
retina/LGN/primary visual cortex and you get complete loss of conscious vision in specific parts of the visual
field from one or both eyes.
d. When you get away from primary visual cortex, you can get more subtle deficits that don’t involve visual field
defects; they don’t involve loss of visual perception they result in subtle inability to analyze some aspects of an
image and not others.
XI. The LGN… [S11]
a. The LGN is part of the thalamus, which itself is part of the diencephalon.
b. The thalamus contains sensory relay nuclei for all the senses: sometimes called “the gateway to the cortex”.
i. You have primary sensory organs, they go to the thalamus, and then to the cortex. (Olfactory is a partial
exception, but basically they all work that way).
ii. Sometimes the LGN is referred to as the LGNd because it is a part of the dorsal LGN.
XII. Picture [S12]
a. *Know this slide! This one “I might put on the test”.
b. This is a top down view. The eyes are facing forward, so they mostly see the same thing- give or take- but we
want to bring the information together.
c. It is very simple: imagine this is a room and there is this little porthole or window. If you are on this side of the
room, you can only see out to the left. If you are on this part of the room, then you can only see out to the other
side. This hemi-retina and this hemi-retina see the same thing. The visual system wants to bring that
information together so that it can operate on it as a unit. This is done at the chiasm.
d. The optic nerve, chiasm, and tract all have the same nerves, same axons.
i. Optic nerve: all the fibers from one eye.
ii. Optic chiasm: this is like the highway crossing, things change over. There are no synapses here, the fibers
are just sorting themselves out.
iii. Optic tract: it is the same ganglion cell axons, but now they come from both eyes.
e. After the chiasm, we have all the fibers from the right or left hemi-retina from both eyes.
f. So once you get past the chiasm, it involves everything from both eyes to the left of what you are looking at, and
the same thing for the other side. After the chiasm, all of the fibers involve everything from both eyes equally to
the right/left of what you see. It is basically just inverting: everything on the left ends up on the right and
everything from the right ends up on the left
i. SQ: I thought you just said everything from both eyes, from point 2 to LGN, ends up after the chiasm?
ii. A: If you lesion out here at 1, you affect one eye (the eye who’s optic nerve has been hit). After the chiasm
the deficit will be roughly equal in both eyes and on the opposite side from the lesion. That is standard for
everything past the chiasm.
iii. SQ: if you have a lesion where 1 is, would you still be able to see?
Neuro: 1:00 - 2:00
Scribe: Marjorie Hannon
Wednesday, February 04, 2009
Proof: Caitlin Cox
Gawne
Higher Visual Processing
Page 3 of 7
iv. A: If you lesion at 1, you completely lose vision in the right eye and the left eye is unaffected and retains
normal vision.
v. Q: What about if there is a lesion just on the left hemi-retina on the right eye?
vi. A: Then you just lose half a retina.
vii. Q: Wouldn’t you still be able to see the image because the left hemi-retina on the left eye is seeing the
same thing?
viii. A: Yes. This is like the physiological blindsight.
g. If you cut where 3 is, you end up being blind to the left visual field in both eyes. You have a left homonymous
hemianopia. Left because it is a visual field defect (when you talk about defects it is always about the visual
field, not the retina). You can’t see to the left, half of the visual field, and it is homonymous (same in both eyes).
You would have the same thing if you took out the entire primary visual cortex on the right.
h. When you go from LGN to primary visual cortex, the axons are diffuse and go out through other tissues without
stopping (as compared to the tightly bound bundles).
i. Some of them sweep forward into the temporal lobe in what is called Meyer’s loops. These tend to represent
more of the superior visual field. So if you have a problem with the temporal lobe (for example a resection for
temporal lobe epilepsy), you would get a left superior quandrantanopia (you would lose a quadrant of your upper
contralateral visual field).
j. He states again that you should know this slide.
XIII. Chiasm crosses fibers … [S13]
a. Imagine that you are looking out at the visual world and this is the point you are looking at. Here is the fovea,
here is the periphery. You also have these monocular crescents.
b. The eyes mostly see the same thing, but not quite. Ex: The left eye gets blocked looking to the right because
the nose is in the way. Likewise, the right eye gets blocked looking to the left because the nose is in the way.
So out here on the side is a region that one eye can see but the other can’t; a monocular zone. But as you go
up there is no monocular zone, it is equal in both eyes. The monocular zones are crescent shaped and is
therefore called the monocular crescent of the visual field.
c. Looking at the medial surface, this would be the left cortical hemisphere. The primary visual cortex is in the
calcarine sulcus. The fovea representation is right near the surface. The more peripheral representation is
deeper and the most peripheral (the monocular crescent) is deeper into the brain.
d. There is an inversion of up and down as well. The superior visual field is mapped onto the inferior bank of the
calcarine sulcus, and the inferior is mapped onto the superior. Since this is the left hemisphere, it would
represent everything to the right from both eyes equally (except for the monocular crescent).
e. SQ: We have learned that everything is inverted on the retina. Is this why we see it upright?
f. A: That is an extremely subtle point and we could spend all day arguing philosophy. The point is it doesn’t
matter. You are making the assumption that the point of the visual system is to get the visual system back into
the head for someone to look at it. The point of the visual system is to take the image and analyze it. If you
could somehow take the neurons in the back of someone’s head and rotate them all 90 degrees clockwise but
keep all of the connections the same, you wouldn’t notice any difference because all of the computations are the
same. It is irrelevant whether it is upside down, reversed left/right, as longs as the connections are correct to
make the right computations, the relative orientation of it in the brain has no bearing. For example, in the
somatosensory system, there is a map of your skin surface on your brain. Touching one part of the skin
stimulates a specific part of the brain. It is irrelevant whether that is oriented right side up or upside down.
Good question though, and an interesting point.
XIV.
Migraine Visual Aura [S14]
a. Some people who get migraine headaches get a visual aura. We are still not quite sure what it is, the theory is
that it is a disturbance not on the retina or thalamus, but in primary visual cortex. It is a direct correlation of
something irritating the primary visual cortex.
b. People will see these sorts of jagged psychadellic things in both eyes.
c. How can you tell it is on the visual cortex? What kinds of characteristics would it have?
i. Since it is post chiasmatic, it will be the same in both eyes. If it were a retinal lesion, it would be present in
one eye, but not in the other.
XV.
Parvocellular and Magnocellualr Systems [S15]
a. There are two major pathways for analyzing an image in our visual system.
b. Parvocellular System: Originates with the midget ganglion cells in the retina, connects to parvo cells in LGN,
and is most strongly associated with extrastriate visual areas in the inferior temporal lobe. Neurons respond
well to color and fine detail, not so strongly to rapid motion or low contrast.
c. Magnocellular System: originates with the parasol ganglion cells in the retina, connects to magno cells in LGN,
more strongly associated with extrastriate visual areas in the posterior parietal lobe. Neurons respond well to
rapid motion and low contrasts, not so well to color or fine detail.
Neuro: 1:00 - 2:00
Scribe: Marjorie Hannon
Wednesday, February 04, 2009
Proof: Caitlin Cox
Gawne
Higher Visual Processing
Page 4 of 7
d. This is a division of labor. It starts in the retina and is maintained deep into cortex.
XVI.
Picture [S16]
a. You have two LGN’s (one on the left and one on the right). They are a layered structure and the layers are
labeled 1-6 (with 1 on the ventral and 6 is dorsal). Layers 1 and 2 are magnocellular, layers 3,4,5, and 6 are
parvocellular.
b. They get this interesting alternating patterning: 1,4,6 are from the contralateral eye. 2,3,5 are from the ipsilateral
eye.
XVII. Figure 4.5 [S17]
a. The LGN is more or less in the middle and you can see the six laminations.
XVIII. Figure 7.8 [S18]
a. We don’t know what the LGN does. We do know that it is relaying information from the eye to primary visual
cortex, and clinically that is all you need to know. But there is a lot of circuitry that is there that we don’t know
what is doing.
b. When talking about the LGN neurons, we are mostly talking about the relay neurons. We have ganglion cells
coming in from the retina, they synapse on the relay cell, and they send outputs to the visual cortex.
c. There are other nuclei around it, there are interneurons, there is feedback from visual cortex, there’s input from
brainstem; most of the synapses on these relay cells actually come from other sources other than retina. The
input from retina is only a few percent of the total synaptic drive to this relay cell. Yet the relay cell has receptive
field properties that seem to be identical to the receptive field properties of the ganglion cells that contact them.
d. We really don’t know why we have all of this stuff.
XIX.
LGN relay neurons… [S19]
a. This is all we do know for sure:
b. LGN relay neurons project (I.e., send axons) to primary visual cortex.
c. LGN relay neurons have response properties that are very similar to the ganglion cells that contact them.
d. LGN interneurons make only local connections.
e. There are more interneurons than relay neurons!
f. LGN neurons get feedback connections from cortex. (The one-way connection from retina to rest of brain is
unique in the visual system—ganglion axons talk to the brain but the brain doesn’t really get to talk back).
g. We presume that all of this circuitry is helping to modulate or regulate alertness or awareness of visual space.
But it doesn’t seem to have very big effects; it must be the case that small subtle shadings of the response
patterns of these cells must have behaviorally big consequences.
XX.
In primates… [S20]
a. In primates (nearly) all visual information that gets to cortex must first go through primary visual cortex. Four
main synonyms (which are used almost interchangeably) for this part of cortex:
i. Primary Visual Cortex
ii. Area V1 (V for visual, 1 for first)
iii. Brodmann’s area 17 (When Brodmann was naming areas, it was the 17th area that he came to).
iv. Striate cortex
1. It’s called striate cortex because of a heavy band of myelinated axons in layer 4, the stria of Gennari,
about the only landmark you can see in cortex without special stains (every other cortex to the naked
eye looks pretty much the same)
XXI.
Points… [S21]
a. All cortex is basically the same with a few little tweaks and specializations. It is a 6 layered structure.
b. There is the white matter which is packed axons, connections (relatively acellular). There is the grey matter, the
surface of the brain near layer 1, the grey/white matter boarder near layer 6.
c. Layer 4 is traditionally the input layer of the cortex, no matter where you go. Because primary visual cortex is a
primary sensory area, it is big on input. Layer 4 is more expanded relative to other cortical regions. 4 is
subdivided into different spots (4Calpha, 4Cbeta, etc). Most notably, the magnos come into 4Calpha and
parvos to 4Cbeta.
d. The bottom line is: the inputs are coming into layer 4. Layers 2 and 3 are projections to other cortical regions.
Layers 5 and 6 are projections to non-cortical regions. You should know the basic physiology.
e. Pyramidal cells are the ones whose cell bodies are pyramid shaped, and have an apical dendrite pointing to the
surface. These are the ones that send axons that actually project elsewhere into the brain. Their output is
always excitatory.
f. There are interneurons, smooth and spiny which make local inter connections.
XXII. V1 Simple Cells … [S22]
a. What do the cells do? We have already talked about retinal ganglion cell receptive field, the center surround
structure. It is basically exactly the same in LGN. If you know retinal receptive field, you know LGN receptive
fields. But when you get to primary visual cortex, things get a little trickier.
Neuro: 1:00 - 2:00
Scribe: Marjorie Hannon
Wednesday, February 04, 2009
Proof: Caitlin Cox
Gawne
Higher Visual Processing
Page 5 of 7
b. Most neurons in primary visual cortex don’t just respond to spots of light, they respond to oriented lines or
edges. If you are doing a study in the retina recording ganglion cells, you could just flash spots of light and the
cells would be stimulated. That doesn’t work in primary visual cortex. The stimulus of choice for these cells is
edges, lines, contours, and things like that.
c. SQ: Are you talking about the area in the calcarine sulcus when you say primary visual cortex? A: Yes.
d. They are cells that are selected for orientation. There are two main types of cells: simple and complex.
e. Remember from the retinal ganglion cell lecture that you have an on and off part of the receptive field. Simple
cells are basically like that. If you consider the receptive field as sort of the window onto the world of that
particular neuron, there are regions where light excites the cell and regions where light inhibits the cell. It is just
like we saw in retina, but with one twist. Instead of being in donuts or targets, the zones are in elongated strips.
There will be a strip that is plus and a strip that is minus.
f. What that means is that the stimulus is no longer just a spot of a certain diameter; the optimal stimulus of this
cell is a strip of light going to the plus region flanked by black going through the minus region. That would give
you the highest firing rate, or the most numbers of spikes per second.
g. These cells are orientation selective (unlike cells in the retinal ganglia) because something that goes the wrong
orientation will get equal plus and minuses and it will cancel out. Some cells like the orientation horizontal,
vertical, 45 degrees, so on and so forth.
h. The simple cells are sensitive to polarity and exact contrast.
i. If you have white on a black, you will get a good response. If you have a black in the plus region and a
white in the minus, it doesn’t work.
XXIII. V1 Complex Cells … [S23]
a. There is another class called complex cells.
b. The receptive fields of these cells do not have plus or minus sub regions anymore. They don’t have specific
zones where light always excites, or specific zones where light always inhibits.
c. They are like simple cells in that they like a certain orientation. This particular cell likes vertical: it is firing lots of
spikes and action potentials for vertical things and not many for horizontal things.
d. However, these cells are more abstract and a little more generalized. For example: a little to the left, a little to
the right, black on white, or white on black, two halves; there is more of an abstract sense of orientation.
e. SQ: When you are talking about lines and edges, is this the projected image that is in 2D? A: Yes. If you were
doing an experiment of an animal looking at a TV monitor and you could find the signal of a single neuron with a
microelectrode, somewhere in space you would be able to move an edge and the cell would fire. It would fire if
it was in one orientation, but it wouldn’t fire if it was another. It is responding to the orientation of the projected
image on the retina.
XXIV. Orientation-selective … [S24] skipped
XXV. Grossly simplified block diagram… [S25]
a. “Don’t memorize this”. This is just to give you a sense of the rest of the brain.
b. Retina to LGN to primary visual cortex (or V1) is pretty much obligate for conscious vision. If you wipe them out
you are blind, except for some reflexive behaviors. If you wipe out part of them, you are blind in specific ways
depending on exactly where the damage occurs. But all of this doesn’t really let you see.
c. Initially, people thought vision was an easy thing to do because anyone can do it, and that vision comes in to
V1, but the rest of the brain is not involved. Then they realized that there was this little strip of cortex around V1
that also does vision, and they called this extrastriate visual cortex. As time went on, people realized that
extrastriate visual cortex is enormous and is about 1/3 of the brain.
d. The bottom line is this: when you get to V1 that is only the start. Analyzing the image is a very complex
process; it occurs at many levels and involves a lot of knowledge of the world, your expectations, and behaviors.
It is a very complex computation process. It takes lots of modules and systems.
e. This is actually a rhesus monkey brain. The basic pattern seems to be true in humans. And there are several
points to take home:
i. It is very complicated. There is a lot of stuff in the extrastriate visual cortex system.
ii. It is no longer strict hierarchy (like we had retina  LGN  V1). Everything is connected to everything
else. There is sort of a hierarchy, from V1 V2V3V4TFTE, but there are short cuts and
roundabouts and cross talk. The system is no longer a strict hierarchy and it is also very spread out.
iii. If you lesion in here or here because you are in a more abstract area of processing, you won’t get specific
blindness or specific visual field defects. You would get something like normal acuity, normal visual fields,
but they might have trouble telling faces apart. It is a more subtle loss of the ability to analyze some aspect
of a visual scene. Remember that the reason the visual system is so complex is not just so that it can get
the image in there, but it is to analyze it. That is why so much of the brain is involved. Therefore, if you
damage or lesion one of these areas, you don’t lose raw acuity, you lose your ability to make sense out of
whatever aspect of a visual scene this area was involved in helping you analyze.
Neuro: 1:00 - 2:00
Scribe: Marjorie Hannon
Wednesday, February 04, 2009
Proof: Caitlin Cox
Gawne
Higher Visual Processing
Page 6 of 7
XXVI. Even more grossly simplified… [S26]
a. It is never drawn the same way twice because it is so complicated. Here is an even more primitive version of it.
b. You have retina, LGN, V1, and some of the extrastriate visual cortex area (this is by no means inclusive).
c. V2 is closely associates with V1.
d. V4 and inferior temporal cortex are now in the ventral stream of processing. This is dominated by the
parvocellular system. It goes down into the bottom part of the temporal lobe and it seems to be more involved in
telling you what it is you are looking at.
i. For example, telling fine patterns, telling faces apart, recognizing objects.
e. MT and the back of the parietal lobe are now in the dorsal stream of processing (it is a stream because there
are a lot of areas involved). It is dominated by magno inputs (although not exclusively – these two systems do
have some cross talk) and is more involved with navigating in space, hand-eye coordination, telling where
objects are.
f. The reality is very complex, but at the crudest possible level, just remember this:
i. Ventral stream: in cortex. Dominated by parvo. More involved in what things are.
ii. Dorsal stream: relatively dominated by magnocellular inputs and where things are in space.
iii. This ventral and dorsal has nothing to do with what is ventral and dorsal in the LGN. The fact that the
magnos are in the ventral LGN has nothing to do with this at all. When people talk about ventral and dorsal
streams of visual processing, they are referring exclusively to the extrastriate cortical visual system.
XXVII. Very crudely: [S27]
a. This has been done in monkeys, but similar things have been found with human strokes or injuries.
b. You can train the monkey to grab the striped triangle in the left image. If he grabs it he gets a treat, but if he
grabs the rectangle, he doesn’t. If you have a lesion in the bottom of the temporal lobe, they have trouble
performing this task.
c. In the right image is a different task: landmark discrimination. If he pushes the button near the post then he gets
a treat and pushes the button away from the post and he doesn’t get a treat. If there is a lesion in the back of
the parietal lobe, they have trouble with this.
d. The temporal lesion monkeys don’t have so much trouble with the landmark discrimination and the parietal
lesion monkeys don’t have trouble with the triangle task.
e. This is basically saying that the dorsal cortical pathway is dominated by magno, is concerned with relative
location of objects in space, and related things like hand eye coordination. Ventral pathway is parvo and is
more of the ‘what’ like what are the details or pattern.
XXVIII. V2 [S28]
a. Closely associated with V1
b. Has BOTH magno and parvo sections
c. Neurons in V2 have, at first glance, roughly similar receptive field properties to those of neurons in V1: they are
selective for orientation.
XXIX. Neurons in V2, Fig 1 [S29]
a. Neurons in V2 seem to be able to fill in the gaps of an image. If you look at this image for the differences just
between light and dark, the edges defined only by light and dark are incomplete. This is because in some spots
you have shadows, part of an object may be the same brightness of the background, and there are missing
gaps.
b. Neurons in V1 act like they see this (I believe he was pointing to image B), some neurons in V2 act like they see
this (I believe he was pointing to image C). The bottom line is: it looks like V2 is starting to fill in the gaps.
XXX. V4 [S30]
a. One of the larger and more important of the post-V1/V2 visual cortical areas. It has been identified in Rhesus
monkeys, but almost certainly has a homolog in humans. Some people call it human-V4, or H-V4.
b. More dominated by ‘parvo’. In the ‘Ventral Stream’.
c. It has complex color properties but it doesn’t just respond to color, it responds to color and form.
d. Lesions cause more subtle problems than just visual field defects. If you lesion V4 in a monkey, they have
normal visual fields, normal acuity. However, they have trouble telling objects apart.
XXXI. Figure 1 (A) [S31]
a. The point of this slide is that some neurons in V4 like oriented lines, but some seem to respond to bulls eye
patterns, curves, or spirals. We are not sure exactly what they are responding to but the point is it is more
complex and more abstract aspects of form than just orientation.
XXXII. IT [S32]
a. Inferior temporal cortex (has names such as: IT, TE, TEO, etc.), down in the bottom of the temporal lobe.
b. Neurons can have very large receptive fields but their specificity for visual stimuli can be VERY high
c. A neuron might respond to something within +/- your whole field of view, but only to one specific object or one
specific face, as far as we can tell.
Neuro: 1:00 - 2:00
Scribe: Marjorie Hannon
Wednesday, February 04, 2009
Proof: Caitlin Cox
Gawne
Higher Visual Processing
Page 7 of 7
d. Lesions of inferior cortex can have devastating consequences for the ability to recognize specific objects (the
classic one is faces: PROSOPAGNOSIA) with no corresponding loss of acuity or visual field deficits.
i. Prosopagnosia: greek for face blindness. They have normal vision and they can read an eye acuity chart
perfectly well, and their visual fields are fine with no unusual blind spots, but they can’t tell people apart.
e. Lesions of temporal cortex can cause visual field deficits by interrupting the passing fibers of Myer’s loops.
f. Remember in the real world lesions and strokes are complicated. They can wipe out the primary area and they
can cause secondary effects by wiping out axons that go through that area without stopping.
XXXIII. Fig 9 [S33]
a. If you record from neurons in the inferior temporal cortex of a rhesus monkey, you find things very fuzzy.
b. For example, this represents the amount of firing of a neuron in the inferior temporal cortex. This represents an
image flashed in front of the animal on a computer screen. There are not many firings when you flash the
frontal view of the face, but there is much firing when you show the profile view. It is not completely specific for
shape, it can generalize a little. It is hard to know exactly what aspect of the form it is responding to.
c. The bottom line is: these cells seem to respond in very narrow ways to specific objects and sometimes specific
views of specific objects.
d. If you wipe out these areas, the animal is not blind it just has trouble telling them apart. It seems reasonable to
assume that somehow the specific firing of these cells helps us tell objects apart.
XXXIV. MT/V5 [S34]
a. MT and V5: these are synonyms.
b. MT stands for Middle Temporal cortex. V5 is for the putative fifth visual area.
c. When you think MT, think motion. Cells in MT are in the dorsal pathway and like motion.
XXXV. Fig 28-9 [S35] Skipped
XXXVI. Important things to know [S36] “I think I’ll end here with what I want you to know”:
a. Know the targets of the retinal ganglion cells:
i. Know their names and their functions. (ex: circadian rhythms or saccadic eye movement)
b. Know the laminations of the LGN (this is a common board question)
c. Know where the magnos are and where the parvos are.
d. Know the pupillary light reflex
e. Know the basic visual field defects resulting from lesions in the retina-LGN-striate pathway
i. Ex: When you cut the right optic tract, you get a left homonymous hemianopia.
f. Know the receptive field properties of neurons in:
i. LGN (they are just like retina)
ii. Striate cortex (orientation selective, simple and complex)
1. Simple is orientation selective but depends on exact position and contrast.
a. If white on black is a good stimulus, black on white will not be.
2. Complex is also orientation selective, but is a little more abstract
a. If white on black is a good stimulus at 13 degrees to the left, black on white will be a good
stimulus as long as it is 13 degrees to the left.
iii. IT cortex: respond very specifically to specific views of objects.
g. Know what V1 is (primary visual cortex), know where it is in the calcarine sulcus, and know its four synonyms:
i. V2: kind of like V1, it’s associated with it; it is not really parvo or magno, dorsal or ventral. It seems to be
involved in extending V1 further and letting it fill in gaps based on partial information.
ii. V4: more of a ventral stream, more parvo dominated, the cells respond in more complex ways to color and
form than cells earlier in the system.
iii. IT: we’ve talked about this.
iv. MT (V5): cells like motion
[End 50 min]