09/01/97
By Tom Siegfried / The Dallas
Morning News
DALLAS - For centuries, philosophers
have
strained their brains trying to
figure out
consciousness. It was purely a
philosophical
question, they reasoned, since it
was impossible to
look inside a brain.
But that was before scientists
figured out how to
monitor blood flow in various brain
regions. Now
science has something to say about
consciousness,
and the philosophers ought to be
listening.
Of course, even with their images of
what goes on
inside the brain, scientists haven't
exactly been able
to explain what consciousness is all
about. But they
have found significant clues,
including a new study
published last month linking
conscious awareness
to specific parts of the brain.
The finding comes from studies of
G.Y., a man in
his early 40s. He suffered a head
injury at age eight
that damaged the vision area at the
back of his
brain, leaving him blind on the
right side of his field
of view.
G.Y. is ordinarily unaware, or not
conscious, of
light or color in his blind field.
But something in his
brain can still get the message.
Experiments have
shown that even when G.Y. cannot see
a spot of
light, he can correctly guess which
direction it is
moving.
Some other patients have shown
similar ability,
called "blindsight" by Lawrence
Weiskrantz, a
neuropsychologist at Oxford
University in England.
Experiments over the last two
decades by Dr.
Weiskrantz and others have revealed
intriguing
nuances about blindsight. G.Y., for
example,
sometimes expresses a "feeling" that
something is
present when light moves in his
blind field of view,
although he denies explicitly seeing
anything. In
other instances, though, he can
correctly "guess"
the direction of the light's motion
even without the
vaguest sense of any awareness at
all.
Comparing brain activity in these
two
circumstances - aware and unaware -
should
provide clues to what part of the
brain is at work
when G.Y. is aware, or "conscious,"
of the moving
light.
In the experiment reported last
month, Dr.
Weiskrantz and colleagues beamed a
laser on a
screen to display a moving dot of
light in G.Y.'s
blind area. An audio beep signaled
G.Y. to press a
button indicating which direction
the dot moved.
He signaled by another key whether
he was aware
of the dot's presence.
When the dot moved more rapidly,
G.Y. was likely
to indicate some awareness of it,
and he could
guess its direction nearly every
time. When the dot
moved slowly, he rarely sensed it,
yet he could
guess its direction more than 80
percent of the
time, anyway.
Then G.Y. was hooked up to an MRI
brain
scanner and asked to tell when he
was aware or
unaware of the moving light. He
again reported
awareness of fast-moving dots but
not of the slow
ones. The brain scans were then
analyzed to see if
the "aware" brain differed from the
unaware.
Sure enough, parts of the frontal
cortex were more
active when G.Y. was conscious of
the light. When
he was unaware, a deeper brain
structure called the
superior colliculus was active.
These findings support the view
advocated by
neursocientists Francis Crick and
Christof Koch
that frontal parts of the brain play
a critical role in
visual awareness. But other
interpretations are
possible.
"An activation found in an imaging
study may reveal
only the tip of a very large
iceberg," Dr. Weiskrantz
and colleagues from the Institute of
Psychiatry and
City University in London wrote in
"Proceedings of
the National Academy of Sciences."
"The results
must be considered provisional,
given the limited
number of observations possible in
this single
subject. Further studies are
needed."
The point is not that the frontal
part of the brain is
the scene of consciousness, the
researchers say,
but that the brain's pattern of
activity changes
depending on whether G.Y. is aware
of the light or
not. Presumably he can guess its
movement
direction even while unaware of it
because different
brain circuits are engaged from
those at work in
conscious vision.
The new MRI study provides clues to
how this
circuitry might work. A further clue
is the apparent
importance for visual awareness of
brain regions
traditionally not considered part of
the vision
system. The superior colliculus, for
example, may
be part of a parallel visual pathway
activated in
case of damage to other pathways.
Intriguing as these clues are, the
nature of
consciousness remains an enormously
complex
question. Visual awareness is only
one element of
the whole constellation of conscious
activity. But it
is certainly relevant to realize
that some people
have abilities, like blindsight,
that involve perceiving
certain things without conscious
unawareness. And
as Dr. Weiskrantz points out in his
new book,
"Consciousness Lost and Found"
(Oxford
University Press), similar abilities
without
awareness have been reported for
senses other
than vision.
Studies of such patients show that
explaining the
brain is not a philosophical
question after all. Most
of what's been written about
consciousness in
centuries past is now pretty much
meaningless, and
progress in understanding
consciousness is likely to
come not from philosophy, but from
experiments.