It's a well known "fact" that we use just 10 percent of our
brains. This is why creativity gurus are always urging us to learn
to tap the other silent 90 percent. It's also been a staple point
for those who want to argue that consciousness has little to do with
brain circuitry and more to do with some intangible
soul-stuff.
So where did this
particular old wives' tale spring from? Well, there are at least
three famous bits of neuroscientific research that have fed the
myth. And here are the modern countering arguments.
In the 1920s, the
behaviourist psychologist Karl Lashley carried out an experiment in
which he trained rats to run a maze and then chopped away increasing
amounts of their cortex to find out
which grey matter bump might house the memory trace for the route.
Lashley was surprised to find that what counted was not which part
he cut out, just how much. So the memory seemed evenly spread over
the brain tissue
Today we can answer
that memories are indeed distributed across the cortex – though not
evenly, but in a hierarchically organised fashion. The visual
aspects of a memory will find their way to visual areas of the
cortex, olfactory cues to the olfactory regions. And while parts of
the brain like the hippocampus are specialist memory organs, they
play a role mostly at specific stages like the fixing and recalling
of memories. Furthermore, Lashley's study did not even cut into
lower brain centres like the basal ganglia which would have carried
the most habitual or over-learnt aspects of the response (McCrone
1999). We now know that any kind of mental activity is the result of
a team effort by the brain's hierarchy and so damage to one area
normally degrades rather than eradicates the ability to
perform.
Of course, Lashley was
no fool and rightly concluded that his experiment merely showed that
brain organisation was far more dynamic and distributed than
existing theories recognised (Lashley 1930). But the indelible image
of almost brainless rats still running mazes encouraged others to
wonder if the cortex really did anything at all?
Then in the 1930s, the
pioneering Canadian neurosurgeon Wilder Penfield probed the brains
of his patients with an electrode while operating on them for
epilepsy. Such surgery is carried out while the patients are
conscious and able to talk about what they are
experiencing. The probing is done to ensure that surgery is not
cutting into any vital areas like the language centres. Famously,
Penfield found that jolts to some regions sparked vivid imaginary
scenes or surges of emotion. But equally he was puzzled that there
were large areas of "silent" cortex where he got no reaction.
Penfield later came to argue that this uncertain connection between
physical stimulation and mental response meant that there must be
more to being a mind than just a set of brain circuits (Penfield,
1975).
The modern view on
these silent regions is that Penfield was simply using too crude a
stimulus to stir the more delicate integrative parts of the cortex.
Again, the brain being a distributed hierarchy, it seems that while
the lower processing areas, such as the primary sensory cortices,
will respond quite readily to an electrode, trying to interpret it
as a real sensory event, the higher areas need to be hearing from a
wider range of inputs to start to find any concrete meaning in
them.
Nevertheless, vivid
newspaper accounts of Penfield's and Lashley's work helped foster
the myth that much of the cortex, our wrinkled grey hemispheres,
appeared mysteriously unused, or at least not completely necessary
for everyday mental function. Einstein even jokingly declared that
these untapped regions of the brain must be the secret of his own
success, showing just how quickly this factoid entered into popular
folklore. However today when anti-abortionists argue before
Parliamentary committees about foetal sentience and cortical
development, or psychologists rail against research linking IQ to
brain volume, it is the research of an English neurologist, John
Lorber, that still really gets them going.
In the 1970s, Lorber
was part of a world-leading spinal surgery team at Sheffield
Children's Hospital treating kids with spina bifida. A frequent
complication of this complaint is hydrocephalus where the
fluid-filled ventricles in the middle of the brain expand, causing
the cortex to be squashed against the bone of the skull. This can
leave sufferers severely mentally handicapped or even kill them.
Lorber was inserting shunts – plastic valves – to drain the cerebral
fluid and so relieve the pressure.
What surprised Lorber
was that a few of his patients showed no outward sign of mental
deterioration and yet X-rays revealed "wall to wall" ventricles. The
chambers had ballooned to such an extent that there was barely any
cortex visible inside the skull. The most celebrated case was that
of a 26-year-old student at the University of Sheffield who had an
IQ of 126 anda
first-class honours degree in mathematics. This was despite a
cortical mantle apparently crushed to paper thinness, the usual four
or five centimetres having been reduced to a bare millimetre or so.
Lorber estimated that the man's whole brain weighed only about 100
grams compared to the adult average of about 1500 grams.
So an honours student
with a brain mass not much more than that of a dog or monkey! Little
wonder that Lorber was moved to ask: "Is your brain really
necessary?" when talking up his findings at medical conferences. Or
that the journal Science headlined with the very same question when
it picked up on the story (Lewin 1980). The X-rays did make many
people wonder what was the point of millions of years of careful
evolutionary tuning to develop the very large and complex human
brain if it still worked just as well when reduced to no more than a
slick of neural tissue.
Lorber's claims were
never publicly refuted. And Lorber – who died in 1996 – stuck firmly
to his story, claiming that in 500 CT scans he had found many
hydrocephalics with hardly any brain left above the level of the
brainstem and yet living ordinary lives (Lorber, 1981). So a little
detective work was needed to get to the bottom of this
one.
Talking to colleagues
and contemporaries of Lorber, it was revealed he was probably
greatly exaggerating the extent of brain loss in his cases. Said one
source: "If the cortical mantle actually had been compressed to a
couple of millimetres, it wouldn't even have shown up on his
X-rays." Another agreed, adding that brain scans with modern
techniques such as MRI (magnetic resonance imaging) show stretching,
but not much real loss of brain weight with slow-onset
hydrocephalus. He says the brain structure adapts to the space it is
allowed: "The cortex and its connections are still there, even if
grossly distorted."
Sufferers with
hydrocephalus also report many subtle symptoms that don't show up in
standard tests of cognition. They do well on basic reading and
arithmetic or IQ-type questions, but struggle with focused
attention, spatial imagination, general motor co-ordination, and
other skills that rely on longer-range integrative links across the
brain. This fits a picture of a brain in which all the cortical
processing regions are in place but where the white matter - the
wealth of insulated connections that actually occupies much of the
centre of the cerebral hemispheres - has been pulled out of
shape.
So Lorber's results
were striking but overplayed. And certainly the rise of neuroimaging
over the past decade ought finally to have put paid to this
long-running myth about the 10 percent brain. One of the most
important lessons from the first scanning studies of brains actually
caught in the act of thinking - with areas lighting up with
increased metabolic activity – was just how widespread were the
patterns of activation for the most minor mental responses. No areas
were silent, just relatively active or inactive in forming the
reaction to the moment.
As Lashley came to
realise, the brain is not a simple device but a complex organ whose
supple logic we are only beginning to be able to appreciate. New
kinds of causal thinking are needed to model systems in which there
is a localisation of function yet also global cohesion (McCrone
2004). Nevertheless you can be pretty sure that without any special
effort on your part, you are indeed using the whole of your brain
the whole of the time.