Electricity ups knack for numbers?

Electrically stimulating the brain makes it easier for people to perform particular numerical tests, suggesting a similar technique may one day help people with math deficiencies, according to a small study published today in Current Biology.

One of the study subjects
with the TDCS machine
Image: Tudor Popescu

The researchers used a noninvasive technique called transcranial direct current stimulation (TDCS), in which scalp electrodes emit current that modulates the activity of populations of neurons that lie underneath them. Volunteers who received current over their right parietal lobe, a brain region at the back of the head known to contribute to spatial and math aptitude, became more proficient at tricky numerical tasks as they learned them over the course of about a week. "This is the first time I've seen that TDCS can modulate performance in basic number [tests]," said cognitive neuroscientist linkurl:Daniel Ansari;http://psychology.uwo.ca/faculty/ansari_res.htm of the University of Western Ontario, who was not involved in the research. While the results are exciting, the study included only 15 subjects, and the data are not particularly strong, Ansari added. "I'm unconvinced that there is clear-cut evidence that excitatory stimulation over the right hemisphere actually enhances performance," he said. TDCS increases or decreases the threshold of neural excitability, depending on the direction of current flow between electrodes, causing neurons to become more or less likely to respond to the same stimuli. Current flow in one direction reduces the activity of inhibitory neurons, causing overall excitation, whereas current flow in the other direction reduces activity in excitatory neurons, causing overall inhibition. Past studies indicate the tool can augment cognitive skills, such as memory, attention, language and decision-making. So far, scientists have used the method to treat depression, chronic pain, and stroke-related motor and cognitive deficits. The technique is not associated with major side effects, but it can cause skin irritation.In the study, one group of subjects received excitatory current over the right parietal lobe and inhibitory current over the left parietal lobe, and a second group experienced the opposite pattern of stimulation. Over the span of six days, the investigators applied current for 20 minutes at the beginning of training sessions in which they taught volunteers to associate numbers with arbitrary symbols, such as triangles or cylinders.After practicing, volunteers participated in two tasks that simulate children's experiences as they learn new digits. In one task, subjects were presented with two differently sized symbols they'd already associated with numbers, and they had to choose which one was physically larger. In some trials, the physically larger item was associated with a lower number than the smaller item. Adults who are highly familiar with digits and process them automatically find these trials challenging because they're tempted to choose the number of greater magnitude rather than the physically larger item, and this conflict slows their reaction times. On the other hand, young children who are less familiar with the numbers do not show slower responses for conflicting trials compared to non-conflicting trials.By the fourth day, subjects who received excitatory current over the right hemisphere became slower for incongruent trials compared to congruent trials, similar to adults responding to real digits. But those whose right hemispheres were inhibited showed no difference between these trials. The results suggest that right-hemisphere excitation causes subjects to learn to process the symbols automatically and therefore perform more like adults with everyday digits, whereas inhibition keeps performance in a child-like state. But the data connecting excitatory stimulation to performance were not particularly strong, Ansari cautioned. The performance differences between groups were small, and the effects were probably highly variable because of the small number of subjects, he said. "The strongest suggestion is that inhibitory stimulation over the right hemisphere disrupts performance, while the other groups perform normally."The team then gave the same tasks using everyday digits and found no differences between groups, suggesting that stimulation specifically affects the learned material, linkurl:Roi Cohen Kadosh,;http://cohenkadosh.psy.ox.ac.uk/ a cognitive neuroscientist at the University of Oxford and first author of the paper, told The Scientist. But given that the parietal lobe plays a critical role in attention, spatial, visual, and motor processes, the researchers should have added control conditions to confirm that stimulation specifically enhances numerical ability, said psychologist linkurl:Wim Fias;http://www.ugent.be/en/@@people?ugentid=801001323301 of Ghent University, who was not affiliated with the study. Six months after training, volunteers who previously underwent right-hemisphere excitation repeated the tasks and performed similarly. Still, there were no control groups to prove that the precise pattern of stimulation caused the long-lasting effects, Fias said. In the future, the authors plan to investigate how the technique induces changes in the brain and try it in people with developmental dyscalculia, a learning disability that affects math skills. But mental representations of numbers may be organized differently in those people, so the tool may unexpectedly impair proficiency if it is used inappropriately, Fias warned. And it's not clear that the approach will help kids with math disabilities solve arithmetic problems in the classroom, Ansari said. "It's too early for rehabilitative use. I would be very scared to see this being whole-heartedly embraced by a community of people doing an intervention for dyscalculia." R. Cohen Kadosh, et al., "Modulating neural activity produces specific and long-lasting changes in numerical competence," Current Biology, 20: 1-5, 2010.
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