Eye, Brain, and Vision

inhibitory input from another set, as in the diagrams on the two preceding
pages, or by convergent input from many end-stopped simple cells.
The optimal stimulus for an end-stopped cell is a line that extends for a
certain distance and no further. For a cell that responds to edges and is end
stopped at one end only, a corner is ideal; for a cell that responds to slits or
black bars and is stopped at both ends, the optimum stimulus is a short white
or black line or a line that curves so that it is appropriate in the activating
region and inappropriate—different by 20 to 30 degrees or more—in the
flanking regions, as shown in the diagram of the curved contour on this page.
We can thus view end-stopped cells as sensitive to corners, to curvature, or to
sudden breaks in lines.

                                     THE IMPLICATIONS
                              OF SINGLE-CELL PHYSIOLOGY
                                         FOR PERCEPTION
The fact that a cell in the brain responds to visual stimuli does not
guarantee that it plays a direct part in perception. For example, many struc-
tures in the brainstem that are primarily visual have to do only with eye move-
ments, pupillary constriction, or focusing by means of the lens. We can never-
theless be reasonably sure that the cells I described in this chapter have a lot to
do with perception. As I mentioned at the outset, destroying any small piece of
our striate cortex produces blindness in some small part of our visual world,
and damaging the striate cortex has the same result in the monkey. In the cat
things are not so simple: a cat with its striate cortex removed can see, though
less well. Other parts of the brain, such as the superior colliculus, may play a
relatively more important part in a cat's perception than they do in the pri-
mate's. Lower vertebrates, such as frogs and turtles, have nothing quite like
our cortex, yet no one would contend that they are blind.
We can now say with some confidence what any one of these cortical cells is
likely to be doing in response to a natural scene. The majority of cortical cells
respond badly to diffuse light but well to appropriately oriented lines. Thus for
the kidney shape shown in the illustration on the next page, such a cell will fire
if and only if its receptive field is cut in the right orientation by the borders.
Cells whose receptive fields are inside the borders will be unaffected; they will
continue to fire at their spontaneous rate, oblivious to the presence or absence
of the form.
   This is the case for orientation-specific cells in general. But to evoke a re-
sponse from a simple cell, a contour must do more than be oriented to match
the optimum orientation of the cell; it must also fall in the simple cell's recep-
tive field, almost exactly on a border between excitation and inhibition, be-
cause the excitatory part must be illuminated without encroachment on the
inhibitory part. If we move the contour even slightly, without rotating it, it
will no longer stimulate the cell; it will now activate an entirely new popula-
tion of simple cells. For complex cells, conditions are much less stringent
because whatever population of cells is activated by a stimulus at one instant
will remain unchanged if the form is moved a small distance in any direction
without rotation. To cause a marked change in the population of activated
complex cells, a movement has to be large enough for the border to pass
entirely out of the receptive fields of some complex cells and into the fields of
others. Thus compared to the population of simple cells, the population of
activated complex cells, as a whole, will not greatly change in response to
small translational movements of an object.
Finally, for end-stopped cells, we similarly find an increased freedom in the
exact placement of the stimulus, yet the population activated by any form will
be far more select. For end-stopped cells, the contour's orientation must fit the
cell's optimum orientation within the activating region but must differ enough
just beyond the activating region so as not to annul the excitation. In short, the
contour must be curved just enough to fit the cell's requirements, or it must
terminate abruptly, as shown in the diagram of the curve (page 21).
One result of these exacting requirements is to increase efficiency, in that an
object in the visual field stimulates only a tiny fraction of the cells on whose
receptive fields it falls. The increasing cell specialization underlying this effi-
ciency is likely to continue as we go further and deeper into the central nervous
system, beyond the striate cortex. Rods and cones are influenced by light as
such. Ganglion cells, geniculate cells, and center-surround cortical cells com-
pare a region with its surrounds and are therefore likely to be influenced by
any contours that cut their receptive fields but will not be influenced by overall
changes in light intensity. Orientation-specific cells care not only about the
presence of a contour but also about its orientation and even its rate of change
of orientation—its curvature. When such cells are complex, they are also sensi-
tive to movement. We can see from the discussion in the last section that
movement sensitivity can have two interpretations: it could help draw atten-
tion to moving objects, or it could work in conjunction with microsaccades to
keep complex cells firing in response to stationary objects.
I suspect light-dark contours are the most important component of our
perception, but they are surely not the only component. The coloring of ob-
jects certainly helps in defining their contours, although our recent work tends
to emphasize the limitations of color in defining forms. The shading of ob-
jects, consisting of gradual light-dark transitions, as well as their textures, can
give important clues concerning shape and depth. Although the cells we have
been discussing could conceivably contribute to the perception of shading and
texture, we would certainly not expect them to respond to either quality with
enthusiasm. How our brain handles textures is still not clear. One guess is that
complex cells do mediate shades and textures without the help of any other
specialized sets of cells. Such stimuli may not activate many cells very effi-
ciently, but the spatial extension that is an essential attribute of shading or
texture may make many cells respond, all in a moderate or weak way. Perhaps
lukewarm responses from many cells are enough to transmit the information
to higher levels.
Many people, including myself, still have trouble accepting the idea that the
interior of a form (such as the kidney bean on the facing page) does not itself
excite cells in our brain—that our awareness of the interior as black or white
(or colored, as we will see in Chapter 8) depends only on cells sensitive to the
borders. The intellectual argument is that the perception of an evenly lit inte-
rior depends on the activation of cells having fields at the borders and on the
absence of activation of cells whose fields are within the borders, since such
activation would indicate that the interior is not evenly lit. So our perception
of the interior as black, white, gray, or green has nothing to do with cells
whose fields are in the interior—hard as that may be to swallow. But if an
engineer were designing a machine to encode such a form, I think this is
exactly what he would do. What happens at the borders is the only informa-
tion you need to know: the interior is boring. Who could imagine that the
brain would not evolve in such a way as to handle the information with the
least number of cells?
After hearing about simple and complex cells, people often complain that
the analysis of every tiny part of our visual field—for all possible orientations
and for dark lines, light lines, and edges—must surely require an astronomic
number of cells. The answer is yes, certainly. But that fits perfectly, because an
astronomic number of cells is just what the cortex has. Today we can say what
the cells in this part of the brain are doing, at least in response to many simple,
everyday visual stimuli. I suspect that no two striate cortical cells do exactly
the same thing, because whenever a microelectrode tip succeeds in recording
from two cells at a time, the two show slight differences—in exact receptive
field position, directional selectivity, strength of response, or some other attri-
bute. In short, there seems to be little redundancy in this part of the brain.
How sure can we be that these cells are not wired up to respond to some
other stimulus besides straight line segments? It is not as though we and others
have not tried many other stimuli, including faces, Cosmopolitan covers, and
waving our hands. Experience shows that we would be foolish to think that
we had exhausted the list of possibilities. In the early 1960s, just when we felt