Eye, Brain, and Vision

satisfied with the striate cortex and wanted to move on to the next area (in fact,
we had moved on), we happened to record from a sluggishly responding cell in
the striate cortex and, by making the slit shorter, found that this very cell was
anything but a sluggish responder. In this way we stumbled on end stopping.
And it took almost twenty years work with the monkey striate cortex before
we became aware of blobs—pockets of cells specialized for color, described m
Chapter 8. Having expressed these reservations, I should add that I have no
doubt at all that some of the findings, such as orientation selectivity, are genu-
ine properties of these cells. There is too much collateral evidence, such as the
functional anatomy, described in Chapter 5, to allow for much scepticism.

BINOCULAR CONVERGENCE
I have so far made little mention of the existence of two eyes.
Obviously, we need to ask whether any cortical cells receive input from both
eyes and, if so, whether the two inputs are generally equal, qualitatively or
quantitatively.
To get at these questions we have to backtrack for a moment to the lateral
geniculate body and ask if any of the cells there can be influenced from both
eyes. The lateral geniculate body represents the first opportunity for informa-
tion from the two eyes to come together at the level of the single cell. But it
seems that the opportunity there is missed: the two sets of input are consigned
to separate sets of layers, with little or no opportunity to combine. As we
would expect from this segregation, a geniculate cell responds to one eye and
not at all to the other. Some experiments have indicated that stimuli to the
otherwise ineffective eye can subtly influence responses from the first eye. But
for all practical purposes, each cell seems to be virtually monopolized by one
or the other eye.
Intuitively, it would seem that the paths from the two eyes must sooner or
later converge, because when we look at a scene we see one unified picture. It
is nevertheless everyone's experience that covering one eye makes no great
difference in what we see: things seem about as clear, as vivid, and as bright.
We see a bit farther to the side with both eyes, of course, because the retinas do
not extend around as far in an outward (temporal) direction as they extend
inwardly (nasally); still, the difference is only about 20 to 30 degrees. (Remem-
ber that the visual environment is inverted and reversed on the retina by the
optics of the eye.) The big difference between one-eyed and two-eyed vision is
in the sense of depth, a subject taken up in Chapter 7.
In the monkey cortex, the cells that receive the input from the geniculates,
those whose fields have circular symmetry, are also like geniculate cells in
being monocular. We find about an equal number of left-eye and right-eye
cells, at least in parts of the cortex subserving vision up to about 20 degrees
from the direction of gaze. Beyond this center-surround stage, however, we
find binocular cells, simple and complex. In the macaque monkey over half of
these higher-order cells can be influenced independently from the two eyes.
Once we have found a binocular cell we can compare in detail the receptive
fields in the two eyes. We first cover the right eye and map the cell's receptive
field in the left eye, noting its exact position on the screen or retina and its
complexity, orientation, and arrangement of excitatory and inhibitory re-
gions; we ask if the cell is simple or complex, and we look for end stopping
and directional selectivity. Now we block off the left eye and uncover the
right, repeating all the questions. In most binocular cells, we find that all the
properties found in the left eye hold also for the right-eye stimulation—the
same position in the visual field, the same directional selectivity, and so on. So
we can say that the connections or circuits between the left eye and the cell we
are studying are present as a duplicate copy between the right eye and that cell.
We need to make one qualification concerning this duplication of connec-
tions. If, having found the best stimulus—orientation, position, movement
direction, and so on—we then compare the responses evoked from one eye
with the responses evoked from the other, we find that the two responses are
not necessarily equally vigorous. Some cells do respond equally to the two
eyes, but others consistently give a more powerful discharge to one eye than
to the other. Overall, except for the part of the cortex subserving parts of the
visual field well away from the direction of gaze, we find no obvious favorit-
ism: in a given hemisphere, just as many cells favor the eye on the opposite
side (the contralateral eye) as the eye on the same side (the ipsilateral). All shades
of relative eye dominance are represented, from cells monopolized by the left
eye through cells equally affected to cells responding only to the right eye.
We can now do a population study. We group all the cells we have studied,
say 1000 of them, into seven arbitrary groups, according to the relative effec-
tiveness of the two eyes; we then compare their numbers, as shown in the two
bar graphs on the preceding page. At a glance the histograms tell us how the
distribution differs between cat and monkey: that in both species, binocular
cells are common, with each eye well represented (roughly equally, in the
monkey); that in cats, binocular cells are very abundant; that in macaques,
monocular and binocular cells are about equally common, but that binocular
cells often favor one eye strongly (groups 2 and 5).
We can go even further and ask if binocular cells respond better to both eyes
than to one. Many do: separate eyes may do little or nothing, but both to-
gether produce a strong discharge, especially when the two eyes are stimulated
simultaneously in exactly the same way. The figure on this page shows a
recording from three cells (1, 2, and 3), all of which show strong synergy. One

of the three did not respond at all to either eye alone, and thus its presence
would have gone undetected had we not stimulated the two eyes together.
Many cells show little or no synergistic effect; they respond about the same
way to both eyes together as to either eye alone.
A special class of binocular cells, wired up so as to respond specifically to
near or far objects, will be taken up separately when we come to discuss
stereopsis, in Chapter 7.
These hookups from single cells to the two eyes illustrate once more the
high degree of specificity of connections in the brain. As if it were not remark-
able enough that a cell can be so connected as to respond to only one line
orientation and one movement direction, we now learn that the connections
are laid down in duplicate copies, one from each eye. And as if that were not
remarkable enough, most of the connections, as we will see in Chapter 9, seem
to be wired up and ready to go at birth.