Tom Jonard's Perceptual Filling In Page
Our
senses are the fundamental mechanism by which we are connected to our
world.
They sit between the interior world of our minds and the exterior world
through which we move. In the history of the Philosophy of the
Mind
much attention has been paid to this interface and rightfully so.
Our experience is so intimately mediated by our senses that we easily
take
them for granted and seldom consider their workings. We see with
the eyes without perceiving the very act of seeing. So too we
hear,
taste, smell and touch without perceiving any of the mechanisms of the
senses. Yet how we perceive is likely to effect our relationship
with the world and also with the body which is our physical presence in
that world. Just ask any blind person.
Filling
in is one of those perceptual phenomenon that is just out of view most
of the time. Most of us are familiar with it in the form of the
visual
blind spot. Hopefully in a grade school science class you were
told
about the fact that the retina in each of your eyes has a blind spot
where
there are no light receptors. This occurs at the point where the
optic nerve exits the eye. It is to one side of the central line
of sight in each eye so that when using both eyes the entire visual
field
is covered with receptors in at least one eye. Yet when you close
an eye the blind spot in that eye is not readily visible. You
need
to know where
to look but you can discover it.
The
odd thing about the blind spot is that no one sees it even though at 6o
it is 36 times the size of the full moon! Even if you do one of
the
exercises to discover it you still don't actually see it. What
you
see is that a spot or mark that enters the blind spot disappears.
As an astronomer people ask me what a black hole is like. Well
the
blind spot is sort of like a black hole in the visual field. You
can only see it indirectly when something disappears into it.
There
is no corresponding black spot in your visual field. As a matter
of fact your visual field appears to be continuous right across the
blind
spot. There is no sense of anything missing in the blind spot
because
there is nothing there -- no visual receptors and therefore no visual
field.
We don't just ignore these two holes in our vision -- they simply
aren't
there.
The
retina of the eye is not uniformly populated with light
receptors.
A spot in the center of the line of sight called the fovea has the
densest
aggregation of light sensitive cells. Moving away from that point
the number of these cells per unit area falls off. The type of
cell
also varies over the surface of the retina. In the fovea the
color
sensitive cone cells predominate. White light sensitive rod cells
predominate in the periphery surrounding the fovea. All of these
variations in the visual field are ignored by the visual system.
We think that we see the periphery of our vision with the same acuity
and
color content as the central portion. This is not true. In
fact much of what we "see" there is filled in from our visual memory of
what we have seen or what we
expect to see there based on what
we can see.
Sometimes
what we see in our peripheral vision is filled in after the fact.
This is shown by experiments
in which subjects read text from a computer screen while the movements
of their eyes across the text were tracked by the same computer.
When the computer detected the start of a move from one word to another
it erased the second word and replaced it with another of the same
length.
Subjects did not notice this change. Other observers saw the text
flicking as the subject read. For subjects the text appeared as
stable
as a printed page. For other observers it was hard to believe
that
this could be so. Remarkably subjects who observe the experiment
first and therefore know what is going on are still unable to detect
that
the experiment is working.
This
experiment touches on another phenomenon of vision that is totally
transparent
to us -- the saccade (from the French for a sudden movement or
jerk).
It turns out that when we think we are gazing steadily at anything in
our
field of view we are not. The eye exhibits continuous jerky
motions
even during the steadiest gaze. The reason maybe that because the
area of the fovea where highest visual acuity is possible is rather
small
constant shifts in the position of the retina allow the visual system
to
scan the visual field for detail and build up a visual memory which
allows
us to fill in areas at which we are not directly looking.
Another
reason for saccades may be that our visual system primarily detects
motion.
If an image is fixed on the retina for a few seconds it
disappears.
This was originally demonstrated by using a very small projector
attached
to a contact lens so that shapes it projected into the eye moved with
the
eye. Subjects wearing this apparatus reported that the shapes
disappeared
over several seconds. You can come close to the same result (as I
have done) by picking a point and staring at it. Almost
everything
in your field of vision surrounding the point will shortly disappear
into
a white fog -- at least until an involuntary eye movement brings it all
back.
Apparently
saccades also serve to prevent the visual system from losing sight of
the
stationary world. In a system that developed through evolution
this
functionality makes sense. Vision that brings to our attention
any
motion in our visual field would be adaptive for sensing environmental
threats. If that system had the side effect that anything that
didn't
move would disappear from our attention then saccades would be an
adaptive
modification that corrects for it. The result is a visual system
that is adaptively tuned to detect movement and yet also not lose sight
of the fixed background.
From
our perspective the most interesting thing about saccades is that we
don't
notice them. If we did our vision might be something like those
handheld
camera shots you see in movies or on TV. But vision is not like
that.
The jerky motions are ignored by the visual system. In fact as
eye
tracking experiments like the one described above show something even
more
unexpected happens. During a saccade visual perception is
inhibited.
This phenomenon is known as saccade blindness. The
subjects
in the eye tracking reading experiments do not see the word change
before
reading it. In fact in their visual field it either does not
change
or some sort of fill in editing occurs so that they perceive that the
word
has always been the one they see at last even though other observers
know
this is not true.
Visual
perception has been shown to be inhibited over the whole visual field
during
a saccade. In testing general vision instead of just reading
other
experiments show that any disruption in the visual field (blinking,
flashing
or a sceen cut in a move) can produce a similar change
blindness (see also here
and some
examples).
The size of a background building in a scene or the number of people in
a crowd can change under these circumstances without the viewer
noticing.
Instead of being like a handheld video vision is more like a series of
still photos each taken from a slightly different angle. As these
still photos are presented to the mind the previous one is hidden from
view. With no other evidence we believe that the last view was
substantially
like the current one and if we notice some detail in the current view
we
believe it to have been in the previous one as well as all its
predecessors.
Even if this is not true.
We
say, "Seeing is believing", and that would appear to be true but not in
the way commonly meant. For what is commonly meant by this saying
is that seeing is a way of verifying the way things are. In fact
experiment shows that seeing is quite literally believing -- we do
believe
what we see is the way things are but what we see may not actually be
the
way things are. We believe that what we see here and now is what
we saw then and there or what we will see when and where.
But
it may be that what we believe we see is not really anywhere. We
simply do not see the whole picture the way that we believe that we do.
Though
I and the researchers in cognitive science have concentrated on vision
do not suppose that these phenomenon are unique among all the
senses.
In hearing for instance we all perceive stereo sound as three
dimensional
when we listen to recorded music even though the true source is one or
more pairs of speakers. Many other examples exist in this and the
other senses that we won't go into here. The evidence from vision
is enough to make a point.
We
have avoided referring to these phenomenon as illusions because that
might
be misleading. The word illusion suggests that we sense something
that is not there or are otherwise deceived. It does not seem
right
to call any normal process of perception an illusion. It might be
better to say that our perceptions are constructions that allow us to
make
the most out of our sensory input. This input is often less than
we think it is. In everyday life where clever experimentalists
are
not trying to trick our perceptual apparatus into revealing its secrets
this may not be a problem. The construction is a normally
reliable
representation of the external world as long as that world continues to
meet our expectations -- expectations that are the result of
evolutionary
adaptations.
There
is not a direct connection between our perceptions and the external
world.
When the world ceases to conform to our evolutionarily formed
expectations
the perceptions constructed by our minds can be seriously in
error.
We use the term eyewitness to emphasize the veracity of some
report
about the world. But the phenomenon we have discussed show that
what
we see cannot be universally naively trusted. Whatever we
otherwise
perceive may likewise be questioned. Our sensory connection to
the
external world has built-in tricks, shortcuts and edits that most of
the
time facilitate our perception of reality. But they may also
inhibit
it in ways that are hidden from us because that is one of the
tricks.
What we witness with our eyes and other senses may in fact not be what
is.
