of data is a gargantuan task, akin to
understanding the entire U.S. economy
by mapping how all of its money flows.
But rather than looking at the whole
financial picture, insights can be gained
from tracking a single dollar by its serial
number as it jumps from wallet to cash
register to bank.
Similarly, scientists are attempting to
clarify the path that leads
to consciousness by following a single, bite-sized
piece of information — the
redness of an apple, for
instance — as it moves into
a person’s inner mind.
Recent research into
the visual system suggests that a sight simply
passing through the requisite vision channels in the
brain isn’t enough for an
experience to form. Studies that delicately divorce awareness
from the related, but distinct, process
of attention call into question the role
of one of the key stops on the vision pipeline in creating conscious experience.
Other experiments that create the sensation of touch or hearing through sight
alone hint at the way in which different
kinds of inputs come together. So far,
scientists haven’t followed enough individual paths to get a full picture. But they
are hot on the trail, finding clues to how
the brain builds conscious experience.
After planting a vision in a person’s
retina, scientists can then watch how
one image moves through the brain. By
asking viewers when they become aware
of the vision, researchers may pinpoint
where along the pipeline it pops into
consciousness.
One of the most fiercely debated stops
for visual information is a small patch
of wrinkles at the very
back of the brain called
the primary visual cortex, or V1. By virtue of
its prestigious locale on
the cortex — the sophisticated outer shell of the
brain where thoughts
are formed — V1 seems to
many like a reasonable
place for visual consciousness to arise.
But some researchers
argue that V1 is too simple:
Instead of the final authority, V1 may be
a relay station that conveys the message
to higher-ups, more specialized brain
regions that have their say in what’s conscious and what’s not. “V1 is kind of the
battlefield,” says Masataka Watanabe of
the Max Planck Institute for Biological
Cybernetics in Tübingen, Germany.
A key way to study V1 is to make an
object visible to the retina while keeping it outside the mind’s awareness. The
eyes may see the object just fine, but the
brain will completely miss it.
Scientists can do this by showing one
picture to one eye and a different picture
to the other. Because it’s impossible to
integrate the two images into a single
Demystifying
the Mind
this feature is the
second installment
in a three-part series
on the scientific
struggle to explain
consciousness.
to read the previous
installment and
see what’s in the
next issue, visit
sciencenews.org/mind
The mind’s eye
C.-Y. Kim and r. blaKe/ TRENDS IN COGNITIVE SCIENCES 2005
One of the best-understood systems
in the brain is the complex network of
nerve cells and structures that allow a
person to see. Imprints on cells in the
eye’s retina get shuttled to the thalamus, to the back of the brain and then
up the ranks to increasingly specialized
cells where color, motion, location and
identity of objects are discerned.
After decades of research, today’s map
of the vision system looks like a bowl of
spaghetti thrown on the floor, with long,
elegant lines connected by knotty tangles. But there’s an underlying method
in this ocular madness: Information
appears to flow in a prescribed direction.
vision, people toggle back and forth.
Input to each eye’s retina holds steady,
while perception — whether an image
pops into awareness — flips back and
forth. Scientists can measure brain
activity to track this perceptual switch.
As expected, cells in each retina
react to the information the same way,
regardless of which image the person
perceives. “This eyeball doesn’t really
care if the brain behind it is conscious
or not,” says neuroscientist Christof
Koch of Caltech and the Allen Institute
for Brain Science in Seattle.
After a quick stop-off at the thalamus, the info heads to V1, where the
story gets complicated. Functional MRI
scans, which measure big changes in
blood flow, have found that activity in
V1 tracks with toggling perceptions. But
other data call V1’s gatekeeper role into
question. For instance, a technique that
uses electrodes to measure the behavior
of nerve cells found that V1 behavior did
not change as perception switched.
Such conflicting findings stymied scientists until two new studies carefully
separated out the confounding effects
of a contaminator: attention. Attention
is the focusing of the mind on a particular subject, often described as the mind’s
spotlight.
“If you really want to understand consciousness, then you need to separate
the effects of attention,” says neuroscientist Naotsugu Tsuchiya of Monash
University in Australia.
Separating the two effects wasn’t
an issue until recently, because people
believed them to be the same thing. After
Flip-flop perception Visual illusions offer a good way to study awareness. though input into
the retina remains constant, the mind toggles between perceiving a vase versus faces (left) or a
box with a red back versus red front (middle). When each eye is shown a different image, a condition called binocular rivalry, the brain perceives one or the other rather than melding the two (right).
www.sciencenews.org
February 25, 2012 | SCIENCE NEWS | 19
