Eye, Brain, and Vision

further suppose that the inhibitory path involves a delay, perhaps produced by
still another intermediate cell. Then, if the stimulus moves in one direction,
say, right to left, as in the illustration of Barlow and Levick's model, the
intermediate cell is excited by one of its inputs just as the inhibition arrives
from the other, whose field has just been crossed. The two effects cancel, and
the cell does not fire. For left-to-right movement, the inhibition arrives too
late to prevent firing. If many such intermediate cells converge on a third cell,
that cell will have the properties of a directionally selective complex cell.
We have little direct evidence for any schemes that try to explain the behav-
ior of cells in terms of a hierarchy of complexity, in which cells at each succes-
sive level are constructed of building blocks from the previous level. Never-
theless, we have strong reasons for believing that the nervous system is
organized in a hierarchical series. The strongest evidence is anatomical: for
example, in the cat, simple cells are aggregated in the fourth layer of the striate
cortex, the same layer that receives geniculate input, whereas the complex cells
are located in the layers above and below, one or two synapses further along.
Thus although we may not know the exact circuit diagram at each stage, we
have good reasons to suppose the existence of some circuit.
The main reason for thinking that complex cells may be built up from
center-surround cells, with a step in between, is the seeming necessity of doing
the job in two logical steps. I should emphasize the word logical because the
whole transformation presumably could be accomplished in one physical step
by having center-surround inputs sum on separate dendritic branches of com-
plex cells, with each branch doing the job of a simple cell, signaling electroton-
ically (by passive electrical spread) to the cell body, and hence to the axon,
whenever a line falls in some particular part of the receptive field. The cell itself
would then be complex. But the very existence of simple cells suggests that we
do not have to imagine anything as complicated as this.

                                  THE SIGNIFICANCE
                      OF MOVEMENT-SENSITIVE CELLS,
                         INCLUDING SOME COMMENTS
                                       ON HOW WE SEE
Why are movement-sensitive cells so common? An obvious first
guess is that they tell us if the visual landscape contains a moving object. To
animals, ourselves included, changes in the outside world are far more impor-
tant than static conditions, for the survival of predator and prey alike. It is
therefore no wonder that most cortical cells respond better to a moving object
than to a stationary one. Having carried the logic this far, you may now begin
to wonder how we analyze a stationary landscape at all if, in the interests of
having high movement sensitivity, so many oriented cells are insensitive to
stationary contours. The answer requires a short digression, which takes us to
some basic, seemingly counterintuitive facts about how we see.
First, you might expect that in exploring our visual surroundings, we let our
eyes freely rove around in smooth, continuous movement. What our two eyes
in fact do is fixate on an object: we first adjust the positions of our eyes so that
the images of the object fall on the two foveas; then we hold that position for a
brief period, say, half a second; then our eyes suddenly jump to a new position
by fixating on a new target whose presence somewhere out in the visual field
has asserted itself, either by moving slightly, by contrasting with the back-
ground, or by presenting an interesting shape. During the jump, or saccade,
which is French for "jolt", or "jerk" (the verb), the eyes move so rapidly that
our visual system does not even respond to the resulting movement of the
scene across the retina; we are altogether unaware of the violent change. (Vi-
sion may also in some sense be turned off during saccades by a complex circuit linking eye-movement centers with the visual path.) Exploring a visual scene, in reading or just looking around, is thus a process of having our eyes jump in rapid succession from one place to another.
Detailed monitoring of eye movements shows vividly how unaware we are
of any of this. To monitor eye movements we first attach a tiny mirror to a
contact lens, at the side, where it does not block vision; we then reflect a spot
of light off the mirror onto a screen. Or, using a more modern version in-
vented by David Robinson at the Wilmer Institute at Johns Hopkins, we can
mount a tiny coil of wire around the rim of a contact lens, with the subject
seated between two orthogonal pairs of bicycle-wheel size hoops containing
coils of wire; currents in these coils induce currents in the contact-lens coil,
which can be calibrated to give precise monitoring of eye movements. Neither
method is what you would call a picnic for the poor subject.
In 1957, Russian psychophysicist A. L. Yarbus recorded eye movements of
subjects as they explored various images, such as a woods or female faces (see the illustrations below), by showing the stopping places of a sub-
ject's gaze as dots joined by lines indicating the eyes' trajectory during the
jumps. A glance at these amazing pictures gives us a world of information
about our vision—even about the objects and details that interest us in our
environment.
So the first counterintuitive fact is that in visual exploration our eyes jump
around from one point of interest to another: we cannot explore a stationary
scene by swinging our eyes past it in continuous movements. The visual sys-
tem seems intent instead on keeping the image of a scene anchored on our
retinas, on preventing it from sliding around. If the whole scene moves by, as

occurs when we look out a train window, we follow it by fixating on an object
and maintaining fixation by moving our eyes until the object gets out of range,
whereupon we make a saccade to a new object. This whole sequence—
following with smooth pursuit, say, to the right, then making a saccade to the
left—is called nystagmus. You can observe the sequence—perhaps next time
you are in a moving train or streetcar—by looking at your neighbor's eyes as
he or she looks out a window at the passing scene—taking care not to have