-
- Turn to the small advertisements in many a Sunday magazine,
says John McCrone, and you'll find something rather like this: "Are
you good at logic, great at analysis and working out maths problems? Then
you're probably a left-brainer . . . Want to get in touch with your intuitive,
creative right brain and find a whole other you . . . "
-
-
- Many a myth has grown up around the brain's asymmetry.
The left cerebral hemisphere is supposed to be the coldly logical, verbal
and dominant half of the brain, while the right developed a reputation
as the imaginative side, emotional, spatially aware but suppressed. Two
personalities in one head, Yin and Yang, hero and villain.
-
- To most neuroscientists, of course, these notions are
seen as simplistic at best and nonsense at worst. So there was general
satisfaction when, a couple of years ago, a simple brain scanner test appeared
to reveal the true story about one of neurology's greatest puzzles: exactly
what is the difference between the two sides of the human brain? Fortunately,
or unfortunately, depending on how you like your theories, the big picture
revealed by that work is proving far less romantic than the logical-creative
split, intriguingly complex and tough to prove.
-
- The people behind the scanner test, clinical neurologists
Gereon Fink of the University of Düsseldorf in Germany and John Marshall
from the Radcliffe Infirmary in Oxford, had been pursuing the idea that
the difference between the two hemispheres lay in their style of working.
The left brain, they reckoned, focused on detail. This would make it the
natural home for all those mental skills that need us to act in a series
of discrete steps or fix on a particular fragment of what we perceive--skills
such as recognising a friend's face in a crowd or "lining up"
words to make a sentence.
-
- By contrast, the right brain concentrated on the broad,
background picture. The researchers believed it had a panoramic focus that
made it good at seeing general connections; this hemisphere was best able
to represent the relative position of objects in space and to handle the
emotional and metaphorical aspects of speech. So, in a neat and complementary
division of labour, one side of the brain thought and saw in wide-angle
while the other zoomed in on the detail.
-
- Good response
-
- To test this idea, the pair teamed up with the imaging
laboratory at London's Institute of Neurology and scanned the brains of
people who were looking at a series of images called letter navons. These
are pictures in which a single large letter such as an S is made up of
many smaller letters--perhaps a series of Fs (see Diagram, p 29). The researchers
asked their subjects to report whether they saw the global image (the big
S) or the local elements (the Fs) while a radioactive chemical injected
into their bloodstream revealed which side of the brain worked hardest
to make each report.
-
- The results seemed beautifully clear. When the subjects
concentrated on the small letters, areas on the left side of the brain
fired; when they mentally stepped back to take in the overall shape, the
right side fired. So wham, bam, and a few months later in August 1996,
Fink, Marshall and their colleagues published a neat, tidy paper in Nature
(vol 382, p 626). "The study was in the textbooks within a year,"
says Marshall with a smile. "The only other work that I've done that's
got into the textbooks took about twenty years to get there."
-
- Other work appeared to be converging on the same conclusion,
which no doubt helped the speedy acceptance of the paper's findings. The
popular myth about the hemispheres grew largely from "split-brain"
research in the 1960s, such as that which later won Roger Sperry of Caltech
a Nobel prize. In a drastic treatment for epilepsy, surgeons had operated
on a number of patients by cutting the corpus callosum--the thick bundle
of nerve fibres that forms the main connection between the cerebral hemispheres.
The surgery revealed what Sperry described as "two spheres of consciousness"
locked in the one head, the left-hand side having speech and a rational,
intellectual style, while the right was inarticulate, but blessed with
special spatial abilities.
-
- Fork or hat?
-
- For example, in a test in which split- brain patients
had to match a series of household objects, the left brain would match
by function while the right would match by appearance. So, when seeing
a cake on a plate, the left brain would connect to a picture of a fork
and spoon while the right brain would select a picture of a broad-brimmed
hat. This evidence appeared to support the idea of a highly modular brain
in which, for example, thinking in logical categories was a strictly left
hemisphere function while mental imagery and spatial awareness were handled
on the right.
-
- But, says Joseph Hellige, a psychologist at the University
of Southern California, this picture changed dramatically as soon as brain-scanning
experiments began to show that both sides of the brain played an active
role in such processes. Rather, it seemed to be processing styles that
distinguished the two halves. Under the scanner, language turned out to
be represented on both sides of the brain, in matching areas of the cortex.
Areas on the left dealt with the core aspects of speech such as grammar
and word production, while aspects such as intonation and emphasis lit
up the right side. In the same way, the right brain proved to be good at
working with a general sense of space, while equivalent areas in the left
brain fired when someone thought about objects at particular locations.
-
- Tests of reaction times also seemed to back up the notion
that the two hemispheres differed in their processing styles.
-
- A trick researchers use to ensure that an image goes
to one hemisphere first, before crossing over via the corpus callosum,
is to flash up a navon in either the extreme left or right of the visual
field, from where it passes first to the opposite side of the brain. If
the nature of the stimulus and the preference of the hemisphere match up,
then the person can respond slightly more quickly and accurately in identifying
the local or global letter.
-
- Still more startling, researchers found that the same
appears to hold for the brains of chimps and perhaps other primates. The
assumption has always been that handedness and brain asymmetry are strictly
human traits--part of the great brain reorganisation that allowed our ancestors
to use tools, speak and perhaps even think rationally. But handedness is
now widely claimed for primates and even birds, amphibians and whales.
And in the past few years, psychologists such as William Hopkins of Berry
College in Georgia have tested chimps and baboons with navon-type stimuli
and suggested their two hemispheres also differ in processing style.
-
- Hopkins carried out reaction tests similar to those performed
on humans, but used patterns of small geometric shapes, such as squares,
making up a single large shape such as a circle--a local versus global
distinction that chimps could recognise. The chimps then had to pick out
either the local or global shape from a selection of symbols presented
after each trial. He admits that his results are not as clear-cut as those
from humans--he could only demonstrate a left brain advantage for local
stimuli--but there was some degree of lateralisation.
-
- Hopkins also tested chimps that had been taught to communicate
in a picture-based language. He flashed word symbols to one side of the
visual field or the other and again measured reaction times. Even though
the chimps' language ability was rudimentary, it looked to be the left
brain that took on responsibility for handling the "local" task
of interpreting meaningful from non-meaningful symbols, says Hopkins (Journal
of Experimental Psychology: General, vol 120, p 45). The intriguing conclusion
from this work is that a division of labour between the two sides of the
brain seems to have been a good thing long before humans came along.
-
- With all this evidence, Hellige says that researchers
have come to see the distinction between the two hemispheres as a subtle
one of processing style, with every mental faculty shared across the brain,
and each side contributing in a complementary, not exclusive, fashion.
A smart brain became one that simultaneously grasped both the foreground
and the background of the moment.
-
- The next problem was to work out exactly how the brain
manages to produce these two contrasting styles. According to Hellige,
he and many other researchers originally looked for the explanation in
a simple wiring difference within the brain. This theory held that neurons
in the left cortex might make sparser, short-range connections with their
neighbours, while cells on the other side would be more richly and widely
connected. The result would be that the representation of sensations, memories
and even motor plans would be confined to smallish, discrete areas in the
left hemisphere, while exactly the same input to a corresponding area of
the right side would form a sprawling, even impressionistic, pattern of
activity.
-
- Supporters of this idea argued that these structural
differences would explain why left-brain language areas are so good at
precise representation of words and word sequences while the right brain
seems to supply a wider sense of context and meaning. A striking finding
from some people who suffer right-brain strokes is that they can understand
the literal meaning of sentences--their left brain can still decode the
words--but they can no longer get jokes or allusions. Asked to explain
even a common proverb, such as "a stitch in time saves nine",
they can only say it must have something to do with sewing. An intact right
brain is needed to make the more playful connections.
-
- Even though this theory has no anatomical backing (just
try counting neural connections under a microscope), computer simulations
made it seem a decent enough hypothesis. For example, researchers including
Robert Jacobs at the University of Rochester, New York, showed that varying
the richness and distance of interconnections between neurons in an artificial
neural network changes the network's performance. It can be made good at
recognising either specific shapes or at grouping shapes generally.
-
- But wiring differences are not the only contender to
account for the origin of the brain's hemispheric bias. One of the main
reasons why Fink and Marshall's Nature paper attracted so much attention
is that it was seen to support a quite different theory: that the bias
is orchestrated by "higher" cortex areas.
-
- Visual perception seems to emerge in the brain through
a hierarchical process in which "low" areas of the brain send
out signals when they detect simple aspects of the image falling on the
retina--such as vertical or horizontal lines, or movement in different
directions. These signals are then turned into meaningful scenes by "higher"
areas. But this is not a passive process. High-level attentional areas
can tell low-level sensory areas what they should be concentrating on ("<ns/971213/features.htmlWild
Minds", New Scientist, 13 December 1997, p 26). This feedback system
can suppress the activity of some cells and increase the sensitivity of
others. In effect, the brain can highlight what it wants to see.
-
- Fink and Marshall's experiment appeared to show exactly
this. Fink says that areas around high-level regions known to be crucial
for directing the brain's attention--the inferior parietal cortex and its
junction with the temporal cortex--fired every time attention switched
between local and global features.
-
- But they also found flurries of activity at lower levels
of the visual cortex--areas known as V2 and V3. These areas on the right
side glowed with the effort of seeing the global picture, and the left-
side equivalents fired when the demand was to concentrate on local shapes.
While brain-scanning images show only that these areas are active, not
why, the results fit well with the idea that the brain can direct its attention
locally or globally, says Fink.
-
- He now plans to explore this idea using a magnetoencephalography
(MEG) system at the University of Düsseldorf that can record the tiny
magnetic fields generated by active neurons, and so will be able to follow
the exact time-course of events. This should show whether the high-level
areas drive the low-level areas into a fleeting state of bias.
-
- But very little about the brain is ever straightforward.
And just as a clear picture of locally and globally directed attention
was emerging, Marshall had to go and spoil it. He could not resist pushing
for a replication--and with a twist. This time, the team used an object
navon--an image in which a large shape such as an anchor is made up of
smaller shapes such as cups. Naturally, the team expected to get exactly
the same result as before.
-
- Marshall remembers the day the results came back and
Fink silently handed them over, waiting to see how quickly he would spot
that something was very wrong. "Everything went pear-shaped,"
says Marshall. The pattern of activity was utterly reversed. The scans
showed left-brain activation for processing the global picture and right-brain
activation for the local elements.
-
- The hot spots were so precisely switched that at first
Marshall joked that the subjects must have gone into the scanner lying
on their bellies instead of their backs. Then more seriously, he wondered
if the image-analysis software had somehow turned positive readings into
negatives. But there had been no mistake. The crestfallen team was forced
to publish a paper concluding exactly the opposite of their own now famous
Nature paper (Proceedings of the Royal Society B, vol 264, p 487, 1997).
Then they had to try to find what had gone wrong--the hypothesis or the
method. Why should using an object navon reverse the side of the brain
that is spurred into activity?
-
- Fink and Marshall have yet to find an answer. They have
run a number of further experiments and several more are planned this year.
In one unpublished study, they have ruled out differential eye movements
as a possible explanation. For a while they thought that subjects might
habitually look to one side when picking out smaller shapes and so cause
excessive activity on one side. But controlling for that failed to make
a difference.
-
- Fink has a strong feeling that the wayward result is
something to do with the fact that in the object navon, the local elements
are very small--much smaller than the letters making up the letter navons.
It could be that the difficulty of discerning such small shapes changes
the nature of the task. Instead of the brain increasing the sensitivity
of the local pathway, it may be busy inhibiting awareness of the global
shape, so apparently creating a metabolic hot spot in the "wrong"
hemisphere. This, of course, is speculation and the team plans to run more
tests when they find how to match the ease of switching attention between
the local and global views of their object and letter navons. This may
mean altering the relative sizes of the elements and perhaps using more
geometrical shapes.
-
- For some the situation is a mess. The expected result
was achieved only to be reversed the following year, leaving the big story
about brain lateralisation as far away as ever. But Fink believes the message
is quite different. Overall, the bulk of the evidence still suggests that
the left brain is orchestrated to a state of local bias, while the right-side
processing is tilted towards the global. But just how these attention effects
express themselves in terms of the activity of individual brain areas such
as V2 and V3 depends rather on the nature of the task, he argues.
-
- Even if attention does shape how the brain chooses to
process a signal, it does not mean that the neural wiring theory is necessarily
dead, argues Fink. There could still be a wiring bias, formed as the brain
develops, that does some coarse initial sorting of the information coming
into the brain. Attention would then exaggerate the effect when the call
came to focus in a particular way.
-
- Such brain-aching complexities mean that this new line
in hemispheric research is still in its early days. But at least there
seems no prospect of a return to the old left-right caricatures that inspired
so many self-help books exhorting people to liberate their right brains
and avoid too much sterile left-brain thinking. As Fink says, whatever
the story about lateralisation, simple dichotomies are out. It is how the
two sides of the brain complement and combine that counts.
-
-
-
-
- MainPage
http://www.rense.com
-
-
-
- This
Site Served by TheHostPros
|