Eye, Brain, and Vision
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In the scheme in which one plate projects to the next, an important complication arises in the transition from retina to geniculate; here the two eyes join up, with the two separate plates of retinal ganglion cells projecting to the sextuple geniculate plate. A single cell in the lateral geniculate body does not receive convergent input from the two eyes: a cell is a right-eye cell or a left-eye cell. These two sets of cells are segregated into separate layers, so that all the cells in any one layer get input from one eye only. The layers are stacked in such a way that the eyes alternate. In the left lateral geniculate body, the sequence in going from layer to layer, from above downwards, is right, left, right, left, left, right. It is not at all clear why the sequence reverses between the fourth and fifth layers—sometimes I think it is just to make it harder to remember. We really have no good idea why there is a sequence at all.
As a whole, the sextuple-plate structure has just one topography. Thus the two left half-retinal surfaces project to one sextuple plate, the left lateral geniculate (see the figure on the facing page). Similarly, the right half-retinas project to the right geniculate. Any single point in one layer corresponds to a point in the animal's field of vision (via one eye or the other), and movement along the layer implies movement in the visual field along some path dictated by the visual-field-to-geniculate map. If we move instead in a direction perpendicular to the layers—for example, along the radial line in the figure on page 65—as the electrode passes from one layer to the next, the receptive fields stay in the same part of the visual field but the eyes switch—except, of course, where the sequence reverses. The half visual field maps onto each geniculate six times, three for each eye, with the maps in precise register.
The lateral geniculate body seems to be two organs in one. With some justification we can consider the ventral, or bottom, two layers {ventral means "belly") as an entity because the cells they contain are different from the cells in the other four layers: they are bigger and respond differently to visual stimuli. We should also consider the four dorsal, or upper, layers {dorsal means "back" as opposed to "belly") as a separate structure because they are histologically and physiologically so similar to each other. Because of the different sizes of their cells, these two sets of layers are called magnocellular (ventral) and parvocellular (dorsal).
Fibers from the six layers combine in a broad band called the optic radiations, which ascends to the primary visual cortex (see the illustration on page 14). There, the fibers fan out in a regular way and distribute themselves so as to make a single orderly map, just as the optic nerve did on reaching the geniculate. This brings us, finally, to the cortex.



                               RESPONSES OF CELLS                                        IN THE CORTEX
The main subject of this chapter is how the cells in the primary visual cortex respond to visual stimuli. The receptive fields of lateral geniculate cells have the same center-surround organization as the retinal ganglion cells that feed into them. Like retinal ganglion cells, they are distinguishable from one another chiefly by whether they have on centers or off centers, by their positions in the visual field, and by their detailed color properties. The question we now ask is whether cortical cells have the same properties as the geniculate cells that feed them, or whether they do something new. The answer, as you must already suspect, is that they indeed do something new, something so original that prior to 1958, when cortical cells were first studied with patterned light stimulation, no one had remotely predicted it.
The primary visual, or striate, cortex is a plate of cells 2 millimeters thick, with a surface area of a few square inches. Numbers may help to convey an impression of the vastness of this structure: compared with the geniculate, which has 1.5 million cells, the striate cortex contains something like 200 million cells. Its structure is intricate and fascinating, but we don't need to know the details to appreciate how this part of the brain transforms the incoming visual information. We will look at the anatomy more closely when I discuss functional architecture in the next chapter.
I have already mentioned that the flow of information in the cortex takes place over several loosely defined stages. At the first stage, most cells respond like geniculate cells. Their receptive fields have circular symmetry, which means that a line or edge produces the same response regardless of how it is oriented. The tiny, closely packed cells at this stage are not easy to record from, and it is still unclear whether their responses differ at all from the responses of geniculate cells, just as it is unclear whether the responses of retinal ganglion cells and geniculate cells differ. The complexity of the histology (the microscopic anatomy) of both geniculate and cortex certainly leads you to expect differences if you compare the right things, but it can be hard to know just what the "right things" are.
This point is even more important when it comes to the responses of the cells at the next stage in the cortex, which presumably get their input from the center-surround cortical cells in the first stage. At first, it was not at all easy to know what these second-stage cells responded to. By the late 1950s very few scientists had attempted to record from single cells in the visual cortex, and those who did had come up with disappointing results. They found that cells in the visual cortex seemed to work very much like cells in the retina: they found on cells and off cells, plus an additional class that did not seem to respond to light at all. In the face of the obviously fiendish complexity of the cortex's anatomy, it was puzzling to find the physiology so boring.
The explanation, in retrospect, is very clear. First, the stimulus was inadequate: to activate cells in the cortex, the usual custom was simply to flood the retina with diffuse light, a stimulus that is far from optimal even in the retina, as Kuffler had shown ten years previously. For most cortical cells, flooding the retina in this way is not only not optimal—it is completely without effect.
Whereas many geniculate cells respond to diffuse white light, even if weakly, cortical cells, even those first-stage cells that resemble geniculate cells, give virtually no responses. One's first intuition, that the best way to activate a visual cell is to activate all the receptors in the retina, was evidently seriously off the mark. Second, and still more ironic, it turned out that the cortical cells that did give on or off responses were in fact not cells at all but merely axons coming in from the lateral geniculate body. The cortical cells were not responding at all! They were much too choosy to pay attention to anything as crude as diffuse light.
This was the situation in 1958, when Torsten Wiesel and I made one of our first technically successful recordings from the cortex of a cat. The position of microelectrode tip, relative to the cortex, was unusually stable, so much so that we were able to listen in on one cell for a period of about nine hours. We tried everything short of standing on our heads to get it to fire. (It did fire spontaneously from time to time, as most cortical cells do, but we had a hard time convincing ourselves that our stimuli had caused any of that activity.)
After some hours we began to have a vague feeling that shining light in one particular part of the retina was evoking some response, so we tried concentrating our efforts there. To stimulate, we were using mostly white circular spots and black spots. For black spots, we would take a 1-by-2-inch glass microscope slide, onto which we had glued an opaque black dot, and shove it into a slot in the optical instrument Samuel Talbot had designed to project images on the retina. For white spots, we used a slide of the same size made of brass with a small hole drilled through it. (Research was cheaper in those days.) After about five hours of struggle, we suddenly had the impression that the glass with the dot was occasionally producing a response, but the response seemed to have little to do with the dot. Eventually we caught on: it was the sharp but faint shadow cast by the edge of the glass as we slid it into the slot that was doing the trick. We soon convinced ourselves that the edge worked only when its shadow was swept across one small part of the retina and that

   
 
 
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The stacked-plate organization is preserved in going from retina to geniculate, except that the fibers from the retinas are bundled into a cable and splayed out again, in an orderly way, at their geniculate destination.


This Golgi-stained section of the primary visual cortex shows over a dozen pyramidal cells—still just a tiny fraction of the total number in such a section. The height of the section is about 1 millimeter. (The long trunk near the right edge is a blood vessel.)

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