Illusion - Is Seeing Really Believing?/Illusion - Is Seeing Really Believing (1998)(Marshall Media)[Mac-PC].iso/pc/illusion/hub_fie.cxt/00097_Field_frep15b1.txt

Text File | 1996-12-30 | 4KB | 138 lines

    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.