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.
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.
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.