# Down for the Count

Imagine a child at age 10 needing to count on fingers to tell you whether 6 is bigger than 5. That's not as rare as you might think. Affecting some 2.5-7.5% of the population, researchers say, the learning disability dyscalculia prevents people from comprehending or manipulating numbers, even small ones, easily. Though you may have never heard of it, the condition is much more than being bad at math. "You need to hear people suffering from dyscalculia, how hard it is for them to do everyday things, just going to the shop, counting change," says Roi Cohen Kadosh, a research fellow at University College London (UCL). Other practical impossibilities for dyscalculics: balancing a checkbook, planning for retirement, being a baseball fan. The list goes on.

Cohen Kadosh may not have the solution, but it turns out he does have a pretty good grasp of the problem. For the first time ever, he and others at UCL have figured out how to induce dyscalculia — temporarily — in people with normal mathematical ability. That may not sound very useful. But in doing so, the UCL researchers have pinpointed the part of the brain they believe is responsible for humans' intuitive sense of magnitude — or what makes a number big.

Cohen Kadosh took five volunteers with normal math abilities through more than 500 trials. In an experiment published this month in Current Biology, he targeted different regions of the test subjects' brains with magnetic pulses while they performed number-recognition problems. A normal subject, when asked to identify whether a 2 or a 4 is written in larger text, will be a split-second faster on those occasions that the 4 is printed bigger. Normal subjects process that 4 is a bigger quantity than 2, and that information aids their pick — just as number recognition slows them down slightly when a larger 2 sits next to a smaller 4. But when Cohen Kadosh stimulated a specific part of each subject's right parietal lobe, they performed exactly the same as a group of dyscalculics, whose ability to select which numeral was printed the largest was unaffected by the numbers' underlying values. No other region of the brain responded the same way.

This means that scientists can, in effect, switch off a person's grasp of numbers. It's fascinating, both because it reduces a serious learning disability to the mere flick of a neural switch and because, by doing so, it holds out a tantalizing possibility that one day a cure may be as simple as flicking that switch in reverse. Cohen Kadosh hopes the result will allow scientists to develop a diagnostic tool for dyscalculia based on neuroimaging. Identifying children with developmental dyscalculia would let parents intervene earlier to teach important math concepts, just as they can intervene today to help dyslexic children read better.

Until then, the most significant outcome might be to remind teachers and parents — even those math-minded scientists — that dyscalculia is a neurological condition, quite separate from not paying attention in class or just being a bit slow. "Dyscalculia is where dyslexia was 30, 40 or 50 years ago," says Mahesh Sharma, a professor of mathematics education at Cambridge College in Massachusetts. Indeed, even the definition is a bit fuzzy. Some researchers count disabilities in spatial perception or arithmetic operations as dyscalculia, while others restrict it to difficulty recognizing numbers normally. Cohen Kadosh's tests hold out the possibility that different math dysfunctions could well be processed elsewhere in the brain. "I won't say this study provides all the answers," says Sharma. Definitely not, but at least it helps show why, for some, two plus two equals trouble.