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For this "tuned excitatory" cell, it makes a lot of difference whether the stimulus is at the distance the animal is looking, or is nearer or farther away. The cell fires only if the slit is roughly at the distance the ani- mal is looking. In these experiments, the direction of gaze of one eye is varied hori- zontally with a prism, but bodily moving the screen nearer or farther away would amount to the same thing.

The amount of horizontal malpositioning, or disparity, that can be tolerated before the response disappears is a fraction of the width of the receptive field. It therefore fires if and only if the object is roughly as far away as the distance on which the eyes are fixed. The second cell (top of this page) fires only when the object is farther away than that distance. Still other cells respond only when the stimulus is nearer. As we vary disparity, these two cell types, called near cells and far cells, both show very rapid changes in responsiveness at or near zero disparity. All three kinds of cells, called disparity-tuned cells, have been seen in area 17 of monkeys. It is not yet clear just how common they are or whether they occur in any special layers or in any special relation to oculardominance columns. Such cells care very much about the distance of the object from the animal, which translates into the relative positions of the stimulus in the two eyes. Another characteristic feature of these cells is that they fail to respond to either eye alone, or give only weak responses. All these cells have the common characteristic of orientation specificity; in fact, as far as we know, they are like any ordinary upper-layer complex cell, except for their additional fussiness about depth. They respond very well to moving stimuli and are sometimes end stopped.
Gian Poggio at Johns Hopkins Medical School has recorded such cells in area 17 of alert implanted monkeys trained to keep their eyes fixed on a target.
In anesthetized monkeys, such cells, although certainly present, seem to be rare in area 17 but are very common in area 18. I would be surprised if an animal or human could assess and compare the distances of objects in a scene stereoscopically if the only cells involved were the three types—tuned excitatory, near, and far—that I have just described. I would have guessed that we would find a whole array of cells for all possible depths. In alert monkeys, Poggio has also seen tuned excitatory cells with peak responses not at zero but slightly away from zero, and it thus seems that the cortex may contain cells with all degrees of disparity. Although we still do not know how the brain reconstructs a scene full of objects at various distances (whatever "reconstructs" means), cells such as these seem to represent an early stage in the process.



                             SOME DIFFICULTIES                                POSED BY STEREOPSIS
For some years, psychophysicists have recognized the difficult problems that stereopsis poses. Our visual system handles some binocular stimuli in unexpected ways. I could give many examples but will confine myself to two.
We saw in the stereo diagrams on page 37 that displacing two identical images (circles, in that example) inward leads to the sensation "near"; outward, to "far". Now suppose we do both things at once, putting the two circles side by side in each picture. Clearly that could give us two circles, one nearer and one farther than the fixation plane. But we could also imagine that the result might simply be two circles lying side by side in the plane of fixation: either situation leads to the same retinal stimuli. In fact, such a pair of stimuli can be seen only as two circles side by side, as you will see by fusing the two squares on this page by any of the methods described. Similarly, we can imagine that looking at two diagrams, each consisting of a row of x's, say six of each, side by side, might result in any of a large number of sensations, depending on which x in one eye you pair with which x in the other. In fact, what you get when you view two such diagrams is always six fused x's in the plane of fixation. We don't yet know how the brain resolves the ambiguities and reaches the simplest of all the possible combinations. It is hard to imagine, given the opportunities for ambiguity, how we ever make sense of a scene consisting of a bunch of twigs and branches all at different depths. The physiology, at any rate, tells us that the problem may not be so difficult, since different twigs at different depths are likely to be in different orientations, and as far as we know, stereoscopically tuned cells are always orientation tuned.
The second example of the unpredictability of binocular effects has direct bearing on stereopsis but involves retinal rivalry, which we allude to in our discussion of strabismus in Chapter 9. If two very different images are made to fall on the two retinas, very often one will be, as it were, turned off. If you look at the left black-and-white square on the facing page with the left eye and the right one with the right eye, by crossing or uncrossing your eyes or with a stereoscope, you might expect to see a grid, or mesh, like a window screen.
Actually, it is virtually impossible to see both sets of orthogonal stripes together. You may see all vertical or all horizontal, with one set coming into view for a few seconds as the other fades, or you may see a kind of patchwork mosaic of the two, in which the patches move and blend in and out from one orientation to the other, as shown by the figure on this page. For some reason the nervous system will not put up with so different simultaneous stimuli in any one part of the visual field—it suppresses one of them. But here we use the word suppress as a short way ofredescribing the phenomenon: we don't really know how the suppression is accomplished or at what level in the central nervous system it takes place. To me, the patchy quality of the outcome of the battle between the two eyes suggests that the decision takes place rather early in visual processing, conceivably in area 17 or 18. (I am glad I do not have to defend such a guess.)
That we experience retinal rivalry implies that in cases in which the visual system cannot get a sensible result out of the combination of the two sets of inputs from the two eyes—either a single fused flat scene if the images are identical or a scene with depth if the images differ only in small horizontal disparities—it gives up and simply rejects one of the two, either outright, as when you look through a monocular microscope, keeping the other eye open, or in patchy or alternating fashion, as in the example described here. In the case of the microscope, attention surely plays a role, and the neural mechanisms of that role are likewise unknown.

   
 
You cannot fuse this pair in the way you can fuse other pairs, such as those on page the previous. Instead, you get "retinal rivalry" a patchwork quilt of vertical and horizontal areas whose borders fade in and out and change position.


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If the figure on page 37 gave you the usual depth effects of a circle standing out in front or floating back behind, the one which combines the two should give both a circle in front and one behind. It gives neither, just a pair of circles at the same depth as the frame.




When both eyes are stimulated together by a vertical slit of light moving leftward, an ordinary binocular cell in area 17 will have similar responses to three different relative alignments of the two eyes. Zero disparity means that the two eyes are lined up as they would be if the monkey were looking at the screen onto which the stimuli are being projected. The exact alignment makes little difference in the response of the cell.
 
 
 
 
 



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For this cell, a "far" cell, objects closer than the screen evoke little or no response;
at about zero disparity (screen distance), a small shift of the screen has a very large influence on the effectiveness of the stimulus. The response rises sharply to a plateau for distances farther away than where the animal is looking. Beyond a certain point the two receptive fields no longer overlap;
in effect, the eyes are being stimulated separately. Response then falls to zero.