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 genuine properties of these cells. There is too much collateral evidence,
such as the functional anatomy, described in Chapter 5, to allow for much scepticism.
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
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
information 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
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.
(Remember 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
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
regions; 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
We need to make one qualification concerning this duplication of connections. 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
favoritism: 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
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
effectiveness 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
together 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.
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 remarkable 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.