it is so childishly simple that anyone who hasn't should! You start by closing one eye, say the left; keeping it closed, you fix your gaze with the other eye on a small object across the room. Now hold the Q-tip at arm's length directly in front of the object and slowly move it out to the right exactly horizontally (a dark background helps). The white cotton will vanish when it is about 18 degrees out. Now, if you place the stick so that it runs through the blind spot, it will still appear as a single stick, without any gap. The region of blindness constituting the blind spot is like any scotoma; you are not aware of it and cannot be, unless you test for it. You don't see black or white or anything there, you see nothing.
In an analogous way, if looking at a big patch of white paper activates only cells whose fields are cut by the paper's borders (since a cortical cell tends to ignore diffuse change in light), then the death of cells whose fields are within the patch of paper should make no difference. The island of blindness should not be seen—and it isn't. We don't see our blind spot as a black hole when we look at a big patch of white. The completion phenomenon, plus looking at a big white screen and verifying that there is no black hole where the optic disc is, should convince anyone that the brain works in ways that we cannot easily predict using intuition alone.
ARCHITECTURE OF THE CORTEX
Now we can return to our initial question: How are the physiological properties of cortical cells related to their structural organization? We can sharpen the question by restating it: Knowing that cells in the cortex can differ in receptive-field position, complexity, orientation preference, eye dominance, optimal movement direction, and best line length, should we expect neighboring cells to be similar in any or all of these, or could cells with different properties simply be peppered throughout the cortex at random, without regard to their physiological attributes?
Just looking at the anatomy with the unaided eye or under the microscope is of little help. We see clear variations in a cross section through the cortex from one layer to the next, but if we run our eye along any one layer or examine the cortex under a microscope in a section cut parallel to the layers, we see only a gray uniformity. Although that uniformity might seem to argue for randomness, we already know that for at least one variable, cells are distributed with a high degree of order. The fact that visual fields are mapped systematically onto the striate cortex tells us at once that neighboring cells in the cortex will have receptive fields close to each other in the visual fields. Experimentally that is exactly what we find. Two cells sitting side by side in the cortex invariably have their fields close together, and usually they overlap over most of their extent. They are nevertheless hardly ever precisely superimposed. As the electrode moves along the cortex from cell to cell, the receptive-field positions gradually change in a direction predicted from the known topographic map.
No one would have doubted this result even fifty years ago, given what was known about geniculo-cordcal connections and about the localized blindness resulting from strokes. But what about eye dominance, complexity, orientation, and all the other variables?
It took a few years to learn how to stimulate and record from cortical cells reliably enough to permit questions not just about individual cells but about large groups of cells. A start came when, by chance, we occasionally recorded from two or more cells at the same time. You already saw an example of this on page 22. To record from two neighboring cells is not difficult. In experiments where we ask about the stimulus preferences of cells, we almost always employ extracellular recording, placing the electrode tip just outside the cell and sampling currents associated with impulses rather than the voltage across the membrane. We frequently find ourselves recording from more than one cell at a time, say by having the electrode tip halfway between two cell bodies.
Impulses from any single cell in such a record are all almost identical, but the size and shape of the spikes is affected by distance and geometry, so that impulses from two cells recorded at the same time are usually different and hence easily distinguished. With such a two-cell recording we can vividly demonstrate both how neighboring cells differ and what they can have in common.
One of the first two-unit recordings made from visual cortex showed two cells responding to opposite directions of movement of a hand waving back and forth in front of the animal. In that case, two cells positioned side by side in the cortex had different, in fact opposite, behaviors with respect to movement. In other respects, however, they almost certainly had similar properties.
Had I known enough to examine their orientation preferences in 1956, I would very likely have found that both orientation preferences were close to vertical, since the cells responded so well to horizontal movements. The fact that they both responded when the moving hand crossed back and forth over the same region in space means that their receptive-field positions were about the same.
Had I tested for eye dominance, I would likely have found it also to be the same for the two cells.
Even in the earliest cortical recordings, we were struck by how often the two cells in a two-unit recording had the same ocular dominance, the same complexity, and most striking of all, exactly the same orientation preference.
Such occurrences, which could hardly be by chance, immediately suggested that cells with common properties were aggregated together. The possibility of such groupings was intriguing, and once we had established them as a reality, we began a search to learn more about their size and shape.
EXPLORATION OF THE CORTEX
Microelectrodes are one-dimensional tools. To explore a threedimensional structure in the brain, we push an electrode slowly forward, stop at intervals to record from and examine a cell, or perhaps two or three cells, note the depth reading of the advancer, and then go on. Sooner or later the electrode tip penetrates all the way through the cortex. We can then pull the electrode out and reinsert it somewhere else. After the experiment, we slice, stain, and examine the tissue to determine the position of every cell that was recorded. In a single experiment, lasting about 24 hours, it is usual to make two or three electrode penetrations through the cortex, each about 4 to 5 millimeters long, and from each of which some 200 cells can be observed.
The electrodes are slender, and we do well if we can even find their tracks under a microscope; we consequently have no reason to think that in a long penetration enough cells are injured to impair measurably the responses of nearby cells. Originally it was hard to find the electrode track histologically, to say nothing of estimating the final position of the electrode tip, and it was consequently hard to estimate the positions of the cells that had been recorded.
The problem was solved when it was discovered that by passing a tiny current through the electrode we could destroy cells in a small sphere centered on the electrode tip and could easily see this region of destruction histologically.
Luckily, passing the current did no damage to the electrode, so that by making three or four such lesions along a single penetration and noting their depth readings and the depth readings of the recorded cells, we could estimate the position of each cell. The lesions, of course, kill a few cells near the electrode tip, but not enough to impair responses of cells a short distance away. For cells beyond the electrode tip, we can avoid losing information by going ahead a bit and recording before pulling back to make the lesion.