Exploding the 10 percent myth

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 and a 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.