About science About vision
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This is about science produced by the California Institute of Technology
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and originally broadcast by station KPCC Pasadena California. The
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programs are made available to the station by national educational radio.
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This program is about vision with host Dr. Peter less a man and his
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guest Dr. Derrick Fandor.
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Here now is Dr. listen.
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Have you ever taken the time to think about that old phrase the mind's eye.
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Of course what it means is the ability to remember something in picture form to see
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something as ones who saw it. But that phrase comes close to telling us how things
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really are. The eye without the brain's ability to translate what it sees
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is useless while the mind without the eyes ability to recognize and tell
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about light is limited. So we see that this business of seeing is a
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cooperative venture that works so well for most of us that we seldom realize what's
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going on. Scientists have known for years of course that the eye is a device for
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detecting light changing the light it receives into coded nerve impulses
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and sending them along the optic nerve to the brain. But what are these impulses.
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Are they tiny pictures long messages or maybe bits and pieces of
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numbers similar to what would work in a computer. That is what some of the latest work
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is being aimed about specifically at how the brain gets what it
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gets from the eye what form the information is in where it goes in
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the brain and what the brain does with the information across CVS.
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Here to discuss these problems with us now is Dr. Derek Fender professor of
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biology and Applied Science accounting. Dr. fender and his colleagues have been
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working with a very small electric signals generated by the optic centers of the brain
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when the eyes see flashing lights. Dr. fender a native of Heath
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England obtained his bachelor's degree at Reading University in 1939 and his
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Ph.D. in physics at the same university in 1956.
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He was a senior lecturer at the university before joining the Kel-Tec faculty in
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1961. Derek friend has done research on the function of eye
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movements in the visual process and is now engaged in biological systems
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analysis. Derek What exactly does and how does the
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brain analyze visual information and recognize what the eyes see.
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Well before before I get into that problem itself I would like to
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give due credit to a colleague of mine who works with me in this field.
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This is Dietrich Lehmann who is a neuro
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psychiatrist who actually works at the Institute for visual Sciences in
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San Francisco. We run this as a cooperative research project.
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So you are really speaking just as much for himself or for here as for yours you must blame
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him for at least half of what I thought you were being very noble in sharing the credit
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for to have already taken the way out.
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As for the problem itself this is really a very big problem.
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Just how the human brain analyzes the visual information and
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enables us to recognize specific things in the visual field. There are
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many people in fact working many research teams working on this all over America
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and in fact all over the rest of the world and they're attacking this problem at
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various levels and you'll see what I mean by levels in a few moments I
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hope we ourselves are interested in one
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very small level of this information chain and that is what
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actually happens. Do we believe visual information as it
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makes its way from the eye to the visual cortex of the
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brain. Now that in fact is a long path.
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If if I can just trace this the
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passage of the information over this path with you it goes something like this. Jerry could I
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interrupt for a second to ask as a very simple person what is the cortex
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to which this message is transmitted and the cortex is specifically
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the surface layers of the brain or the surface layers
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of nerves. And we usually refer to reserve the term
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cortex for these layers of nerve cells which
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respond to primary sensory input. That is if you like goes into
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some visual cortex like an active sound in the
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ear then some cells in the auditory cortex become active and so on.
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But these are all surface layer cells not the deep seeded cells
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which perform more fundamental processes of the
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thinking and knowing and making
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abstractions proving theorems. And so on.
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So this is this is sort of the entry point it's like the mailbox of
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the brain that's where the messages come in first the cortex is really the first
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point at which the brain receives information from the outside world.
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Yes this is quite true. Now if I can get back to my
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to my long or long route the like from the
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outside world goes in through the eye through the pupil
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and through the lens of the eye in his image on the retina or the simple textbooks
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have pictures of cameras at this point and compare the eye with a camera. And in fact it
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behaves much the same as a camera up to that point. That is the image is upside down and
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backwards and so on in the psyche.
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The retina images formed on the retina of the retina is at the
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back of the eye inside the eyeball. And it is a mosaic of
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retinal receptors. There are the hundred million or so of these in each
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eye. They have visual pigments in them. The pigment
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absorbs the light and in the act of absorbing the light it
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changes the light signal into an electrical signal.
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A very small electrical signal which flows off as a
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nerve impulse and this is the first point at which a change is being made.
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But from there on the light and the picture the visual scene has been lost.
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The brain only has nerve impulses little pulses of electricity on which to
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do its work.
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The amount of information encoded by the retina such as a small only
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twelve brightness and color probably the code brightness and color
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into nerve impulses and usually the coding is made by the
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rapidity at which these pulses follow one after the other.
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The nerve impulses then flow off from the retina to the brain
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through a big bundle of nerves about a million there was in the bundle called the
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optic nerve. This nerve makes its way
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from the two are used with nerves make their way in place from the two
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ears through the quiet ASM which is just behind the eyes underneath the brain which
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some of the nerves cross over but some of the nerves do not
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on their way to the brain. You remember of course of the brain is split into
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two hemispheres left and right hemisphere. The crossing of the
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nerves in the Kiowas is such that if you were to look
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exactly straighter and or move the visual information which is out to your left
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hand side finishes in the right hand hemisphere
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of the brain and all that which is up to the right hand side finishes in the left hemisphere of
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the brain. This is of course quite typical of a
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human but the control the sensory input is
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crossed and the control from the brain to the limbs is crossed. Left side of
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the brain controls the right hand and so on.
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Do we have any idea why that is so.
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Why you have a crossing is necessary. You know that's a very difficult problem to answer.
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If you follow up through the development of the various animals you
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find that that the insect level there is no crossing left
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his left and right is right. But they don't get mixed up as you go up to
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higher levels. Certainly by the time you get to things
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like frogs then there is a crossing of some information but
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other information is not crossed. And finally by the time you get to things as advanced as cats
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say then this crossing has occurred. But at what stage in the life or in the
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evolution of the animal kingdom. The decision was made to
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cross the hemispheres completely like this is really very difficult to say and the reason for which
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the crossing occurred so anyhow the information passed through the chi or somewhere it's
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crossed and.
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Essentially left goes in to right and right goes to the left.
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And this information finishes in the cortex. We've been talking about
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before but specifically in the visual cortex and the
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cortex itself is quite large. It is as many subdivisions. They all
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have silly Latin names but also fortunately have numbers and we are
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talking specifically today about areas 17 18 and 19 of the visual
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cortex. I was 17 18 and 19 are a lot of the back of your head if you know so
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interlaced your fingers and put your hands behind your head in a comfortable relaxed
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gesture the palms of your hands would be neatly fitting over
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the area 17 in each hemisphere of your heart of your brain.
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OK well now that is the first relay station in the brain as it were. These are the primary
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projection in the areas where sensory stimuli
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first become available to the brain itself and the point at which the brain can
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first start working on the little electrical impulses which
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are representing the visual scene from the outside
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world fact that the brain still has to make something of this message. Yeah that is
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true the perception has to go on perception that is
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converting it into a picture which we see in the mind's eye you that you talked about earlier on
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perception goes on in other areas of the brain which I could just call the higher centers the
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higher levels of the brain. And my work specifically
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today stops at areas 17 18 19. And
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perception in higher levels is another problem not only another problem
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for me but another problem for other people who are not wonder what about the coding
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mechanism that that is used.
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Well.
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This of course brings us really to the next big problem. Just what sort of information
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is transmitted from the outside world to
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the cortex. I have said that the retinal intercept is really only sample
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brightness and color in the visual world
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and it would be possible just to transmit this information and
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no more to the cortex and then to use the rest of the brain as a
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big computer like make an ism to figure it all out and figure it all out. Yes
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there are other ways of doing it however and the
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one of the other way is to start. Making
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abstractions of the visual field as far forward in the visual
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system as you can I mean as close to the to the eyeball as you
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can by abstractions I mean things
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like straight lines edges movement. It would be
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perfectly possible to detect the use parameters of the
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scene. And rather than transmit all of this hash about brightness
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to transmit the information which say it is there is a straight line
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in this part of the visual field. This is a
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bank tree produces a reduction of information and many
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conserved. Capacity in the nerve
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fibers and in the brain when the brain has to perform its final
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analysis of the picture.
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So in the characteristically scientific way you strive to use the
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simplest of model in the simplest of our impasse to find out how the
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system works.
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Yes that is true although it's very difficult to
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say what is simplest in this case. You see this coding principle
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has been examined on animals and animals you
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actually open up the brain of the animal you stick an electrode into a
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cell in area 17 in the
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cortex and then you run around in front of the animal and wave a straight line in
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his visual field until eventually if you have chosen the right
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cell in the cortex you find that that cell fires
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when the straight line is in the right position. No straight lines seem
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to be pretty simple but are really pretty simple to one animal. In fact the work which has
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