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In the course of a long penetration parallel to the cortical surface in a cat, receptive fields drifted through the visual field. The electrode traveled over 3 millimeters and recorded over sixty cells, far too many to be shown in a figure like this. I show instead only the positions of four or five receptive fields mapped in the first tenth of each millimeter, ignoring the other ninetenths. For the parts of the penetration drawn with a thick pen in the lower half of the diagram (numbered o, 1, 2, and 3), the receptive fields of cells encountered are mapped in the upper part. Each group is detectably displaced to the right in the visual field relative to the previous group.
The fields in group 2 do not overlap with those in group o, and group-3 fields do not overlap with group-1 fields; in each case the cortical separation is 2 millimeters.

A single module of the type discussed in this chapter occupies roughly the area shown in this photograph of a Golgistained section through visual cortex. The Golgi method stains only a tiny fraction of the nerve cells in any region, but the cells that it does reveal are stained fully or almost so; thus one can see the cell body, dendrites, and axon.

6


        MAGNIFICATION AND MODULES

In the last chapter I emphasized the uniformity of the anatomy of the cortex, as it appears to the naked eye and even, with most ordinary staining methods, under the microscope. Now, on closer inspection, we have found anatomical uniformity prevailing in the topography of the ocular-dominance columns: the repeat distance, from left eye to right eye, stays remarkably constant as we go from the fovea to the far periphery of the binocular region. With the help of the deoxyglucose method and optical mapping techniques, we have found uniformity in the topography of the orientation columns as well.
This uniformity came at first as a surprise, because functionally the visual cortex is decidedly nonuniform, in two important respects. First, as described in Chapter 3, the receptive fields of retinal ganglion cells in or near the fovea are much smaller than those of cells many degrees out from the fovea. In the cortex, the receptive field of a typical complex upper-layer cell in the foveal representation is about one-quarter to one-half a degree in length and width. If we go out to 80 or 90 degrees, the comparable dimensions are more like 2 to 4 degrees—a ratio, in area, of about 10 to 30.
The second kind ofnonuniformity concerns magnification, defined in 1961 by P. M. Daniel and David Whitteridge as the distance in the cortex corresponding to a distance of i degree in the visual field. As we go out from the fovea, a given amount of visual field corresponds to a progressively smaller and smaller area of cortex: the magnification decreases. If, near the fovea, we move i degree in the visual field, we travel about 6 millimeters on the cortex; 90 degrees out from the fovea, i degree in the visual field corresponds to about 0.15 millimeter along the cortex. Thus magnification in'the fovea is roughly thirty-six times larger than in the periphery.
Both these nonuniformities make sense—and for the same reason—namely, that our vision gets progressively cruder with distance from the fovea. Just try looking at a letter at the extreme left of this page and guessing at any letter or word at the extreme right. Or look at the word progressively: if you fix your gaze on the p at the beginning, you may just barely be able to see the y at the end, and you will certainly have trouble with the e or the / before the y.
Achieving high resolution in the foveal part of our visual system requires many cortical cells per unit area of visual field, with each cell taking care of a very small domain of visual field.



                             THE SCATTER AND DRIFT                                     OF RECEPTIVE FIELDS
How, then, can the cortex get away with being so uniform anatomically? To understand this we need to take a more detailed look at what happens to receptive-field positions as an electrode moves through the cortex.
If the electrode is inserted into the striate cortex exactly perpendicular to the surface, the receptive fields of cells encountered as the tip moves forward are all located in almost the same place, but not exactly: from cell to cell we find variations in position, which seem to be random and are small enough that some overlap occurs between almost every field and the next one, as shown in the illustration on this page. The sizes of the fields remain fairly constant in any given layer but differ markedly from one layer to another, from very small, in layer 4C, to large, in layers 5 and 6. Within any one layer, the area of visual field occupied by ten or twenty successively recorded receptive fields is, because of this random scatter, about two to four times the area occupied by any single field. We call the area occupied by a large number of superimposed fields in some layer and under some point on the cortex the aggregate receptive field of that point in that layer. In any given layer, the aggregate field varies, for example in layer 3, from about 30 minutes of arc in the to veal region to about 7 or 8 degrees in the far periphery.
Now suppose we insert the electrode so that it moves horizontally along any one layer, say layer 3. Again, as cell after cell is recorded, we see in successive receptive fields a chaotic variation in position, but superimposed on this variation we now detect a steady drift in position. The direction of this drift in the visual field is, of course, predictable from the systematic map of visual fields onto cortex. What interests us here is the amount of drift we see after i millimeter of horizontal movement along the cortex. From what I have said about variation in magnification, it will be clear that the distance traversed in the visual field will depend on where in the cortex we have been recording—
whether we are studying a region of cortex that represents the foveal region, the far periphery of the visual field, or somewhere between. The rate of movement through the visual field will be far from constant. But the movement turns out to be very constant relative to the size of the receptive fields them

 

 

 

 

 

 

 

 

 

 

 

 



selves. One millimeter on the cortex everywhere produces a movement through the visual field that is equal to about half the territory occupied by the aggregate receptive field—the smear of fields that would be found under a single point in the region. Thus about 2 millimeters of movement is required to get entirely out of one part of the visual field and into the next, as shown in the illustration at the top of this page. This turns out to be the case wherever in area 17 we record. In the fovea, the receptive fields are tiny, and so is the movement in the visual field produced by a 2-millimeter movement along the cortex: in the periphery, both the receptive fields and the movements are much larger, as illustrated in the lower figure on this page.


   
 


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In a macaque monkey, the upper-layer receptive fields grow larger as eccentricity increases from the fovea (0 degrees). Also growing by an equal amount is the distance the receptive fields move in the visual field when an electrode moves 2 millimeters along the cortex parallel to the surface.


 
 
 
 
 

These nine receptive fields were mapped in a cat striate cortex in a single microelectrode penetration made perpendicular to the surface. As the electrode descends, we see random scatter in receptive-field position and some variation in size but see no overall tendency for the positions to change.



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