Eye, Brain, and Vision

ocular-dominance columns. You see the result in the photographs above.
In a very pretty extension of the same idea, Roger Tootell, in Russel De
Valois's laboratory at Berkeley, had an animal look with one eye at a large
pattern of concentric circles and rays, shown in the top image of the figure on
the next page. The resulting pattern on the cortex contains the circles and rays,
distorted just as expected by the variations in magnification (the distance on
the cortex corresponding to i degree of visual field), a phenomenon related to
the change in visual acuity between the fovea and periphery of the eye. Over
and above that, each circle or ray is broken up by the fine ocular-dominance
stripes. Stimulating both eyes would have resulted in continuous bands. Sel-
dom can we illustrate so many separate facts so neatly, all in a single experi-
ment.
Cats, several kinds of monkeys, chimpanzees, and man all possess ocular-
dominance columns. The columns are absent in rodents and tree shrews; and
although hints of their presence can be detected physiologically in the squirrel
monkey, a new world monkey, present anatomical methods do not reveal the
columns. At present we don't know what purpose this highly patterned segre-
gation of eye influence serves, but one guess is that it has something to do with
stereopsis (see Chapter 7).
Subdivisions of the cortex by specialization in cell function have been found
in many regions besides the striate cortex. They were first seen in the somato-
sensory cortex by Vernon Mountcastle in the mid-1950s, in what was surely
the most important set of observations on cortex since localization of function
was first discovered. The somatosensory is to touch, pressure, and joint posi-
tion what the striate cortex is to vision. Mountcastle showed that this cortex is
similarly subdivided vertically into regions in which cells are sensitive to touch
and regions in which cells respond to bending of joints or applying deep pres-
sure to a limb. Like ocular-dominance columns, the regions are about half a
millimeter across, but whether they form stripes, a checkerboard, or an ocean-
and-islands pattern is still not clear. The term column was coined by Mountcas-
tie, so one can probably assume that he had a pillarlike structure in mind. We
now know that the word slab would be more suitable for the visual cortex.
Terminology is hard to change, however, and it seems best to stick to the
well-known term, despite its shortcomings. Today we speak of columnar subdi-
visions when some cell attribute remains constant from surface to white matter
and varies in records taken parallel to the surface. For reasons that will become
clear in the next chapter, we usually restrict the concept to exclude the topo-
graphic representation, that is, position of receptive fields on the retina or
position on the body.

ORIENTATION COLUMNS
In the earliest recordings from the striate cortex, it was noticed that
whenever two cells were recorded together, they agreed not only in their eye
preference, but also in their preferred orientation. You might reasonably ask at
this point whether next-door neighboring cells agree in all their properties: the
answer is clearly no. As I have mentioned, receptive-field positions are usually
not quite the same, although they usually overlap; directional preferences are
often opposite, or one cell may show a marked directional preference and the
other show none. In layers 2 and 3, where end-stopping is found, one cell may
show no stopping when its neighbor is completely stopped. In contrast, it is
very rare for two cells recorded together to have opposite eye preference or
any obvious difference in orientation.
Orientation, like eye preference, remains constant in vertical penetrations
through the full cortical thickness. In layer 4C Bata, as described earlier, cells
show no orientation preference at all, but as soon as we reach layer 5, the cells
show strong orientation preference and the preferred orientation is the same as
it was above layer 4C. If we pull out the electrode and reinsert it somewhere
else, the whole sequence of events is seen again, but a different orientation very
likely will prevail. The cortex is thus subdivided into slender regions of con-
stant orientation, extending from surface to white matter but interrupted by
layer 4, where cells have no orientation preference.
If, on the other hand, the electrode is pushed through the cortex in a direc-
tion parallel to the surface, an amazingly regular sequence of changes in orien-
tation occurs: every time the electrode advances 0.05 millimeter (50 microme-
ters), on the average the preferred orientation shifts about 10 degrees clockwise
or counterclockwise. Consequently a traverse of i millimeter typically records
a total shift of 180 degrees. Fifty micrometers and 10 degrees are close to the
present limits of the precision of measurements, so that it is impossible to say
whether orientation varies in any sense continuously with electrode position,
or shifts in discrete steps.

In the two figures on this page, a typical experiment is illustrated for part of a close-to-horizontal penetration through area 17, in which
23 cells were recorded. The eyes were not perfectly aligned on the screen
(because of the anesthetic and a muscle-relaxing agent), so that the projections
of the foveas of the two eyes were separated by about 2 degrees. The color
circles in the figure above represent roughly the sizes of the receptive fields,
about a degree in diameter, positioned 4 degrees below and to the left of the
foveal projections—the records were from the right hemisphere. The first cell,
96, was binocular, but the next 14 were dominated strongly by the right eye.
From then on, for cells in to 118, the left eye took over. You can see how
regularly the orientations were shifting during this sequence, in this case al-
ways counterclockwise. When the shift in orientation is plotted against track
distance (in the graph on the next page), the points form an almost perfect
straight line. The change from one eye to the other was not accompanied by
any obvious change either in the tendency to shift counterclockwise or in the
slope of the line. We interpret this to mean that the two systems of groupings,
by eye dominance and by orientation, are not closely related. It is as though
the cortex were diced up in two completely different ways.
In such penetrations, the direction of orientation shifts may be clockwise or
counterclockwise, and most penetrations, if long enough, sooner or later show
shifts in the direction of rotation; these occur at unpredictable intervals of a