Eye, Brain, and Vision

tion or inhibition to the cell. Because methods for obtaining this kind of
knowledge don't yet exist, we are forced to use less direct approaches, with
correspondingly higher chances of being wrong. The mechanism summarized
in the diagram on this page seems to me the most likely because it is the most
simple.

                                       COMPLEX CELLS
Complex cells represent the next step or steps in the analysis. They
are the commonest cells in the striate cortex—a guess would be that they make
up three-quarters of the population. The first oriented cell Wiesel and I re-
corded—the one that responded to the edge of the glass slide—was in retro-
spect almost certainly a complex cell.
Complex cells share with simple cells the quality of responding only to
specifically oriented lines. Like simple cells, they respond over a limited region
of the visual field; unlike simple cells, their behavior cannot be explained by a
neat subdivision of the receptive field into excitatory and inhibitory regions.
Turning a small stationary spot on or off seldom produces a response, and even
an appropriately oriented stationary slit or edge tends to give no response or
only weak, unsustained responses of the same type everywhere—at the onset
or turning off of the stimulus or both. But if the properly oriented line is swept
across the receptive field, the result is a well-sustained barrage of impulses,
from the instant the line enters the field until it leaves (see the cell-response
diagram on page 17). By contrast, to evoke sustained responses from a simple
cell, a stationary line must be critically oriented and critically positioned; a
moving line evokes only a brief response at the moment it crosses a boundary
from an inhibitory to an excitatory region or during the brief time it covers the
excitatory region. Complex cells that do react to stationary slits, bars, or edges
fire regardless of where the line is placed in the receptive field, as long as the
orientation is appropriate. But over the same region, an inappropriately ori-
ented line is ineffective, as shown in the illustration on this page.
The diagram on this page for the complex cell and the one on page 73 for
the simple cell illustrate the essential difference between the two types: for a
simple cell, the extremely narrow range of positions over which an optimally
oriented line evokes a response; for a complex cell, the responses to a properly
oriented line wherever it is placed in the receptive field. This behavior is re-
lated to the explicit on and off regions of a simple cell and to the lack of such
regions in a complex cell. The complex cell generalizes the responsiveness to a
line over a wider territory.
Complex cells tend to have larger receptive fields than simple cells, but not
very much larger. A typical complex receptive field in the fovea of the
macaque monkey would be about one-half degree by one-half degree. The
optimum stimulus width is about the same for simple cells and complex cells—
in the fovea, about 2 minutes of arc. The complex cell's resolving power, or
acuity, is thus the same as the simple cell's.
As in the case of the simple cell, we do not know exactly how complex cells
are built up. But, again, it is easy to propose plausible schemes, and the sim-
plest one is to imagine that the complex cell receives input from many simple
cells, all of whose fields have the same orientation but are spread out in over-
lapping fashion over the entire field of the complex cell, as shown in the
illustration on this page. If the connections from simple to complex cells are
excitatory, then wherever a line falls in the field, some simple cells are activated;
the complex cell will therefore be activated. If, on the other hand, a stimulus
fills the entire receptive field, none of the simple cells will be activated, and the
complex cell won't be activated.

    The burst of impulses from a complex cell to turning on a stationary line and
not moving it is generally brief even if the light is kept on: we say that the
response adapts. When we move the line through the complex cell's receptive
field, the sustained response may be the result of overcoming the adaptation,
by bringing in new simple cells one after the next.
   You will have noticed that the schemes for building simple cells from
center-surround ones, as in the illustration on page 18, and for building com-
plex cells out of simple ones, as in the illustration on this page, both involve
excitatory processes. In the two cases, however, the processes must be very
different. The first scheme requires simultaneous summed inputs from center-
surround cells whose field centers lie along a line. In the second scheme, acti-
vation of the complex cell by a moving stimulus requires successive activation
of many simple cells. It would be interesting to know what, if any, morpho-
logical differences underlie this difference in addition properties.

DIRECTIONAL SELECTIVITY
Many complex cells respond better to one direction of movement
than to the diametrically opposite direction. The difference in response is often
so marked that one direction of movement will produce a lively response and
the other direction no response at all, as shown in the diagram on this page. It
turns out that about 10 to 20 percent of cells in the upper layers of the striate
cortex show marked directional selectivity. The rest seem not to care: we have
to pay close attention or use a computer to see any difference in the responses
to the two opposite directions. There seem to be two distinct classes of cells,
one strongly direction-selective, the other not selective.
Listening to a strongly direction-selective cell respond, the feeling you get is
that the line moving in one direction grabs the cell and pulls it along and that
the line moving in the other direction fails utterly to engage it—something like
the feeling you get with a ratchet, in winding a watch.
We don't know how such directionally selective cells are wired up. One
possibility is that they are built up from simple cells whose responses to oppo-
site directions of movement are asymmetric. Such simple cells have asymmet-
ric fields, such as the one shown in the third diagram of the illustration on page
17. A second mechanism was proposed in 1965 by Horace Barlow and William
Levick to explain the directional selectivity of certain cells in the rabbit retina—
cells that apparently are not present in the monkey retina. If we apply their
scheme to complex cells, we would suppose that interposed between simple
and complex cells are intermediate cells, colored white in the diagram on the
next page. We imagine that an intermediate cell receives excitation from one
simple cell and inhibition from another (green) cell, whose receptive field is
immediately adjacent and always located to one side and not the other. We