. In this experiment Gary Blasdel applied a voltage-sensitive dye to a monkey's striate cortex and stimulated the visual fields with stripes of one orientation after the next, while imaging the cortex with TV techniques. Using computers, the results are displayed by assigning a color to each set of regions lit up by each orientation. For any small region of cortex the orientation slabs are parallel stripes, so that a complete set of orientations appears as a tiny rainbow.
particular distance rather than another. One indirect confirmation that orientation is involved in the deoxyglucose work is the absence of any patchiness or periodicities in layer 4C, where cells lack orientation preference. Another comes from a study in which Michael Stryker, at the University of California at San Fransisco, made long microelectrode penetrations parallel to the surface in cat striate cortex, planted lesions every time some particular orientation was encountered, and finally stimulated with stripes of one orientation after injecting radioactive deoxyglucose. These experiments showed a clear correlation between the pattern and stimulus orientation.
The most dramatic demonstration of orientation columns comes from the use of voltage-sensitive dyes, developed over many years by Larry Cohen at Yale and applied to the cerebral cortex by Gary Blasdel at the University of Pittsburgh. In this technique, a voltage-sensitive dye that stains cell membranes is poured onto the cortex of an anesthetized animal and is taken up by the nerve cells. When an animal is stimulated, any responding cells show slight changes in color, and if enough cells are affected in a region close enough to the surface, we can record these changes with modern TV imaging techniques and computer-aided noise filtration. Blasdel stimulated with stripes in some particular orientation, made a photograph of the pattern of activity in a region of cortical surface a few centimeters in area, and repeated the procedure for many orientations. He then assigned a color to each orientation—red for vertical, orange for one o'clock, and so on—and superimposed the pictures. Because an iso-orientation line should be progressively displaced sideways as orientation changes, the result in any one small region should be a rainbowlike pattern. This is exactly what Blasdell found. It is too early, and the number of examples are still too few, to allow an interpretation of the patterns in terms of fractures and reversals, but the method is promising.
MAPS OF THE CORTEX
Now that we know something about the mapping of orientation and ocular-dominance parameters onto the cortex, we can begin to consider the relation between these maps and the projections of the visual fields. It used to be said that the retina mapped to the cortex in point-to-point fashion, but given what we know about the receptive fields of cortical cells, it is clear that this cannot be true in any strict sense: each cell receives input from thousands of rods and cones, and its receptive field is far from being a point. The map from retina to cortex is far more intricate than any simple point-to-point map.
I have tried in the figure on the next page to map the distribution of regions on the cortex that are activated by a simple stimulus (not to be confused with the receptive field of a single cell). The stimulus is a short line tilted at 60 degrees to the vertical, presented to the left eye only. We suppose that this part of the visual field projects to the area of cortex indicated by the rounded-corner rectangle. Within that area, only left-eye slabs will be activated, and of these, only 6o-degree slabs; these are filled in in black in the illustration. So a line in the visual field produces a bizarre distribution of cortical activity in the form, roughly, of an array of bars.
Now you can begin to see how silly it is to imagine a little green man sitting up in our head, contemplating such a pattern. The pattern that the cortex happens to display is about as relevant as the pattern of activity of a video camera's insides, wires and all, in response to an outside scene. The pattern of activity on the cortex is anything but a reproduction of the outside scene. If it were, that would mean only that nothing interesting had happened between eye and cortex, in which case we would indeed need a little green man.
We can hardly imagine that nature would have gone to the trouble of grouping cells so beautifully in these two independently coexisting sets of columns if it were not of some advantage to the animal. Until we work out the exact wiring responsible for the transformations that occur in the cortex, we are not likely to understand the groupings completely. At this point we can only make logical guesses. If we suppose the circuits proposed in Chapter 4 are at all close to reality, then what is required to build complex cells from simple ones, or to accomplish end-stopping or directional selectivity, is in each case a convergence of many cells onto a single cell, with all the interconnected cells having the same receptive-field orientation and roughly the same positions. So far, we have no compelling reasons to expect that a cell with some particular receptive-field orientation should receive inputs from cells with different orientations. (I am exaggerating a bit: suggestions have been made that cells of different orientation affiliations might be joined by inhibitory connections: the evidence for such connections is indirect and as yet, to my mind, not very strong, but it is not easily dismissed.) If this is so, why not group together the cells that are to be interconnected? The alternative is hardly attractive: imagine the problem of having to wire together the appropriate cells if they were scattered through the cortex without regard to common properties. By far the densest interconnections should be between cells having common orientations;
if cells were distributed at random, without regard to orientation, the tangle of axons necessary to interconnect the appropriate cells would be massive. As it is, they are, in fact, grouped together. The same argument applies to oculardominance domains.
If the idea is to pack cells with like properties together, why have sequences of small orientation steps? And why the cycles? Why go through all possible orientations and then come back to the first, and cycle around again, instead of packing together all cells with 30-degree orientation, all cells with 42-degree orientation, and indeed all left-eye cells and all right-eye cells? Given that we know how the cortex is constructed, we can suggest many answers. Here is one suggestion: perhaps cells of unlike orientation do indeed inhibit one another. We do not want a cell to respond to orientations other than its own, and we can easily imagine that inhibitory connections result in a sharpening of orientation tuning. The existing system is then just what is wanted: cells are physically closest to cells of like orientation but are not too far away from cells of almost the same orientation; the result is that the inhibitory connections do not have to be very long. A second suggestion: if we consider the connections necessary to build a simple cell with some particular opitimal orientation out of a group of center-surround layer-4 cells, more or less the same inputs will be required to build a nearby simple cell with a different, but not a very different, orientation. The correct result will be obtained if we add a few inputs and drop a few, as suggested in the illustration on this page. Something like that might well justify the proximity of cells with similar orientations.
The topic to be considered in the next chapter, the relationship between orientation, ocular dominance, and the projection of visual fields onto the cortex, may help us understand why so many columns should be desirable.
When we add topography into the equation, the intricacy of the system increases in a fascinating way.
The group of center-surround layer 4 cells that is needed to build a simple cell that responds to an oblique four o'clock—ten o'clock slit is likely to have cells in common with the group needed to build a 4:30-10:30 cell: a few inputs must be discarded and a few added.
A tilted line segment shining in the visual field of the left eye (shown to the right)
may cause this hypothetical pattern of activation of a small area of striate cortex (shown to the left). The activation is confined to a small cortical area, which is long and narrow to reflect the shape of the line;
within this area, it is confined to left ocular-dominance columns and to orientation columns representing a two o'clock-eight o'clock tilt. Cortical representation is not simple! When we consider that the orientation domains are not neat parallel lines, suggested here for simplicity, but far more complex, as shown in the upper, deoxyglucose figure on page 29 and Blasdel's figure on page 30, the representation becomes even more intricate.