Out of sight into mind
05 Sep 98
They look perfectly normal, but a select group of people with
weird vision may hold the answer to that huge puzzle,
consciousness. David Concar talks to one of them
THERE were no witnesses to the accident that almost killed the man
known to science as GY. "I think I was just a kid who ran into the
road, as eight-year-olds do," he says, sandwich in one hand, strong
coffee in the other. "I just don't recall it."
The lack of emotion in his voice is striking, but easily explained. First,
the accident happened a long time ago, 34 years to be precise. And
then, truth be told, GY-think of him as "Graham"-is not entirely
unhappy about the near-fatal blow to the back of his neck that the
car delivered that day. Without it he suspects he'd "probably be
married off with ten kids somewhere" by now.
Instead, Graham is greatly in demand as the owner of a very special
brain-a brain that is a magnet to psychologists and the talk of
neuroscience conferences. A few weeks ago it was the turn of a team
at the University of Durham to put Graham's brain through its paces.
Next week, he is off to Oxford, thence to Bangor in Wales, and
following that, who knows: Munich, Amsterdam, San Francisco,
Prague? They all want him in their labs.
Such demand, in fact, that these days Graham usually spends no
more than half the working week pursuing his "real" career as a
psychiatric nurse. In one year alone, he took part in 29 separate
experiments, each lasting about three days. Smart researchers know
to book him several months in advance to avoid disappointment.
For his services to science, Graham is paid expenses and
compensated for loss of earnings. But that is not the attraction.
Graham does it because many of the scientists he has worked with
over the years have become friends, and because in the end he is as
interested as they are in discovering what makes his brain special. He
reads and keeps virtually every paper written about him. And that is a
lot of papers.
So, what is so special about Graham's brain? Many relatives and
friends simply know that he has "funny vision", Graham says. But
psychologists and neuroscientists call it blindsight. Courtesy of that
blow to the head, Graham is totally blind on the right side of his
visual field: by all conventional measures, he sees nothing to the
right of his focal point with either eye. And yet, when cued by
experimenters, he can still use that blind field to accomplish tasks
you'd think of as requiring sight. Reaching out to grab an object,
discriminating between lines at different angles to each other, locating
spots of light on a screen, you name it, Graham can do it-even if he
says he sees nothing at all. To him, it usually feels like guesswork. But
clearly, even if Graham himself is guessing, his nervous system
"knows" what's out there. In other words, Graham sees in his blind
field- but unconsciously.
Actually, it's more complicated than that. If a bright light is flashed
rapidly enough, Graham does become aware of something, usually "a
dark shadow". And when objects or lights move fast enough, Graham
experiences a strange sensation he can only describe as something
akin to pure movement-motion stripped of form, colour or depth,
although even this, he says, doesn't really come close to capturing it.
Not surprisingly, from neuroscientists armed with brain scanners to
philosophers and experts in artificial intelligence, many researchers
treat blindsight with great respect these days. Partly because it
challenges everyday notions about what "seeing" involves and partly
because blindsight seems to offer researchers a rare opportunity to
investigate where and how in the brain conscious perceptions are
born.
Most of us think of sight as a sense that automatically involves and
requires consciousness, often of a rich and subjective kind. But
blindsight researchers know different. Having studied people like
Graham for years, they know that not all visual skills lead to or
require consciousness. Quite the opposite. And more and more of
these skills are being uncovered all the time.
Graham is not the only blindsight subject courted by scientists, nor
was he the first. In fact, some of the most influential early research
involved a monkey called "Helen" who lacked a segment of brain tissue
thought to be vital to normal vision but who could nevertheless find
objects and reach out for food. Among human subjects, though,
Graham's willingness to be experimented on and the anatomical
precision of the wound in his brain makes him more highly prized than
most (although, as blindsight researchers are quick to point out,
nobody is keeping any league tables).
Just after the accident, doctors feared he would die or suffer massive
brain damage. Amazingly, however, the impact of the wound was
confined to nerves in the left half of a segment of tissue, called V1,
at the back of the head-the same segment that was missing in Helen.
V1 is one of the main reception sites in the brain for signals from the
retina. It also plays a key role in normal vision-hence Graham's
right-sided blindness.
Graham knew nothing of his blindsight until the late 1970s when
researchers at Imperial College in London, led by the late Keith
Ruddock, began testing him. Then as now, many scientists had a hard
time accepting that someone could respond, often with stunning
accuracy, to visual stimuli they denied being able to see. Nor were
the scientists alone in their doubt. "For several years," Graham recalls,
"I thought I must be cheating somehow."
But equipment that tracked the direction of his gaze proved he wasn't
sneaking a look with his good field. And today, careful experimenters
don't even let blindsight subjects say what they can and can't see-a
subjective account which could be open to bias. Instead, the
experimenters measure perception directly by monitoring changes in
the pupils of the subjects' eyes, which contract slightly in response to
visual stimuli.
But even if the authenticity of blindsight is now unassailable,
researchers are still divided over how to interpret Graham's funny
vision. What, in the end, is blindsight really telling us about the nature
of visual perception and consciousness?
Some researchers have suggested that blindsight is little more than a
weak version of normal sight, akin perhaps to peripheral vision.
According to this view, what the subjects' brains have lost is not so
much the ability to produce visual consciousness as the ability to
process basic visual information. Signals from the retina follow the
usual pathways through the brain: they just seldom gather the
requisite strength.
But this explanation is rejected by Larry Weiskrantz, an Oxford
psychologist who has probably done more than anyone to raise the
scientific profile of blindsight in the past 25 years. "Blindsight is not
just having weak eyesight," he insists. After all, in one visual field
Graham is aware and the other he isn't, yet in both fields he can
achieve stunning levels of performance. On some tasks, such as
detecting a pattern of light and dark stripes, he sometimes does even
better in his blind field than in his normal one. And in a detailed
analysis of Graham's blindsight skills, Paul Azzopardi and Alan Cowey,
also at Oxford, found it impossible to simulate his signal detection
abilities using a model based on degraded normal vision.
The distinction here might seem like hair-splitting-but it isn't. If
Graham's blindsight is just weakened normal vision, there is no need to
argue that what is specifically lacking in his brain is visual
consciousness: he simply lacks sight. End of story.
And if he and similar subjects just lack sight, it becomes less obviously
crucial for researchers to distinguish visual awareness from basic
visual perception in their theories of vision-and more reasonable for
them to lump consciousness in with basic perception instead and say
that they're both produced by the same brain mechanisms (which is
what a handful of philosophers committed to "explaining away" the
problem of consciousness would prefer to do).
Instead, says Weiskrantz, blindsight subjects do not lack the ability to
detect things such as wavelengths, but rather visual consciousness
itself-the redness of red, and so on. If Weiskrantz is right, anyone
who thinks simulating such basic perceptual skills alone on a computer
will eventually produce a conscious machine is being a tad optimistic.
Instead, something else is required. No one knows what this second
ingredient is, but Weiskrantz and others believe that brain scans of
subjects like Graham can at least provide clues.
The idea is simple. Put someone like Graham in an fMRI brain scanner-
designed to look for brain function rather than structure-and get him
to perform a visual task in his blind field. In fact, get him to do it
twice, first in his unconscious seeing mode and then in his conscious
seeing mode. Subtract the brain scans and, bingo, the result should
tell you whether-and how-brain activity differs between vision with
and vision without awareness.
Of course, there's more to it than that, but experiments like this are
now under way in several labs, and so far the results seem to support
the idea that aware vision is not just a "more intense" version of
unaware vision. In Graham, for example, "conscious seeing" seems to
produce a different pattern of brain activation compared with
"unconscious seeing". There is more activity at the front of the cortex
and less in the lower regions when Graham is aware of something in
his blindfield. There is activity deep down in a midbrain structure, the
superior colliculus.
And that second finding helps to confirm the answer to a different
question of how blindsight happens in the brain. If Graham's eyes
cannot get signals to the V1 area that is the main reception site for
right field vision, how does all that visual information guiding his
blindsight "guessing" get into the cortex? The answer is along other,
secondary routes that can bypass V1. And one of these goes via the
superior colliculus.
Like other researchers, Weiskrantz believes this secondary pathway is
to some extent operating in all of us, although its activity seems to be
more fully developed in people with cortical blindness. In other words,
we may not realise it, but we probably all have the pathways used in
blindsight. "It would be a waste of effort for the brain to spend time
making events conscious that don't really require it," Weiskrantz
points out. "There are lots of times in life where we carry out visual
discriminations without any awareness at all. It's when we're going on
automatic pilot."
Some aspects of blindsight suggest a role for it as a defence
mechanism or early-warning system. Sudden flashes and fast moving
objects trigger strange awarenesses in Graham's blindfield, and in his
unconscious seeing mode he responds better to red, a colour that is
often linked to danger. Nevertheless, the notion that blindsight is
nothing more than a primitive form of visual perception or
evolutionary throwback is beginning to look more suspect.
At the Medical Research Council's Cognition and Brain Sciences Unit in
Cambridge, Anthony Marcel discovered that blindsight subjects not
only perceive aspects of shape, curvature and form rather better than
was previously thought in their blindfields-they can even register
whole words.
In one cunning experiment involving Graham and another subject,
Marcel flashed a word with various meanings (say, "bank") into the
good field, just after a word relating to one of the meanings (such as
"money" or "river") had been flashed in the blindfield. Even though the
subjects reported no awareness of the word in their blindfields, it
biased their interpretation of the word they could see. For the first
time, comments Weiskrantz, we have an observation linking blindsight
with higher-level cognition. "This is a major departure."
Marcel, however, believes researchers will have to build up an even
fuller picture of what blindsight subjects can-and can't-perceive
before they can tackle the most important question of all: how the
nonconscious visual perceptions seen in blindsight relate to the
conscious visual perceptions of normal sight. Even though the
known repertoire of visual discrimination that subjects seem able to
achieve without consciousness continues to expand, there may be
some visual skills-such as the selective focusing of attention on one
of several objects- that really cannot be achieved without
consciousness because they are part of the very mechanism that
creates visual consciousness.
Other limitations of blindsight are already clear. It cannot be used to
identify targets for purposive action, Marcel notes. No matter how
thirsty Graham becomes, he can't use blindsight to grasp a glass of
water unless someone else tells him the glass is there. Nor can he
create thoughts and imaginings out of what he perceives in that field.
"I can't replicate the sensation of an event in my imagination,"
Graham says. "I know that little about it. I can't actually go home and
think that's what it looked like to me."
Even so, Graham's blindsight skills have been improving of late. In the
past few years, the levels of brightness and speed he requires in a
visual stimulus before he can consciously sense it have dropped quite
sharply, possibly because of all the experiments he does. Graham
jokes that one day he would like to use his blind field to read a novel,
but he knows that is unlikely. Indeed, right now he'd just like a
straight answer to a straight question. "If I'm so good at
discriminating wavelengths, orientations, shapes and movement in my
blind field," he asks, "why can't I see in it? For 20 years, I've been
doing these experiments, but still no one can tell me that." Perhaps
one day someone will. But first they'll have to answer another
question. What is the nature of consciousness?
David Concar
From New Scientist magazine, vol 159 issue 2150, 05/09/1998, page
38
© Copyright New Scientist, RBI Limited 2001