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 to the left and 
those shown previously), by 
showing the stopping places of a 
subject'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 system 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 your attentions 
misunderstood! The process of 
making visual saccades to items 
of interest, in order to get their 
images on the fovea, is carried 
out largely by the superior 
colliculus, as Peter Schiller at 
MIT showed in an impressive 
series of papers in the 1970s.

    The second set of facts about 
how we see is even more 
counterintuitive. When we look 
at a stationary scene by fixating 
on some point of interest, our 
eyes lock onto that point, as 
just described, but the locking 
is not absolute. Despite any 
efforts we may make, the eyes 
do not hold perfectly still but 
make constant tiny movements 
called microsaccades; these 
occur several times per second 
and are more or less random in 
direction and about 1 to 2 
minutes of arc in amplitude. In 
1952 Lorrin Riggs and Floyd 
Ratliff, at Brown University, 
and R. W. Ditchburn and B. L. 
Ginsborg, at Reading 
University, simultaneously and 
independently found that if an 
image is optically artificially 
stabilized on the retina, 
eliminating any movement 
relative to the retina, vision 
fades away after about a second 
and the scene becomes quite 
blank! (The simplest way of 
stabilizing is to attach a tiny 
spotlight to a contact lens; as 
the eye moves, the spot moves 
too, and quickly fades.) 
Artificially moving the image 
on the retina, even by a tiny 
amount, causes the spot to 
reappear at once. Evidently, 
microsaccades are necessary 
for us to continue to see 
stationary objects. It is as if the 
visual system, after going to the 
trouble to make movement a 
powerful stimulus--wiring up 
cells so as to be insensitive to 
stationary objects--had then to 
invent microsaccades to make 
stationary objects visible.

    We can guess that cortical 
complex cells, with their very 
high sensitivity to movement, 
are involved in this process. 
Directional selectivity is 
probably not involved, because 
microsaccadic movements are 
apparently random in 
direction. On the other hand, 
directional selectivity would 
seem very useful for detecting 
movements of objects against a 
stationary background, by 
telling us that a movement is 
taking place and in what 
direction. To follow a moving 
object against a stationary 
background, we have to lock 
onto the object and track it 
with our eyes; the rest of the 
scene then slips across the 
retina, an event that otherwise 
occurs only rarely. Such 
slippage, with every contour in 
the scene moving across the 
retina, must produce a 
tremendous storm of activity in 
our cortex.