Mathematics is a strange beast. It uses our language, but it isn’t quite the same – our brains hear it completely differently from normal speech. For example: when we hear a sentence like “cats like warm milk,” our brains process that information mostly in the left hemisphere. Something like “eight plus one is nine,” though, will fire neurons in both.
A new study, published this week in the journal Current Biology, has dug even deeper into Your Brain On Math – and it turns out that as far as your neurons are concerned, not all equations are created equal.
“We found that different neurons fired during additions than during subtractions,” explained study co-lead and Professor of Cognitive and Clinical Neurophysiology, Florian Mormann. “This study marks an important step towards a better understanding of one of our most important symbolic abilities, namely calculating with numbers.”
“Even when we replaced the mathematical symbols with words, the effect remained the same,” agreed co-author Esther Kutter. “For example, when subjects were asked to calculate '5 and 3', their addition neurons sprang back into action; whereas for '7 less 4,' their subtraction neurons did.”
When you come across an equation – whether it’s written out using symbols like + and - or spoken aloud – it’s your temporal lobe that fires up. To measure what was going on, therefore, the researchers recorded neural activity in nine volunteers by implanting electrodes into this area of their brains. That may sound like a procedure straight out of sci-fi, but it’s quite familiar to a certain section of the population – for some people with epilepsy, intracerebral electroencephalography, as this procedure is technically known, is a valuable diagnostic tool.
In this experiment, it revealed some pretty amazing insights. Throughout the temporal lobe, different neurons fired depending on whether test subjects were performing addition or subtraction – in fact, the phenomenon was so predictable that a computer program was able to accurately identify which operation was being carried out based only on what neural activity had been recorded.
“We know from experiments with monkeys that neurons specific to certain computational rules also exist in their brains,” said Professor of Animal Physiology Andreas Nieder.
However, so far, data in humans has been lacking, he explained. With the new study, the team confirmed that human brains have these same kinds of “math neurons”: specific cells which encode a particular mathematical instruction for action.
But that wasn’t all.
The team also stumbled onto a surprising discovery in an area of the brain called the parahippocampal cortex, located at the junction of our brains’ memory formation and high-level visual processing centers. While the same distinction was found between the neurons used for addition and subtraction, the researchers noticed that different nerve cells would alternate activity across tasks – a phenomenon known as “dynamic coding,” which they likened to a calculator where the plus or minus symbol keeps jumping about to different locations.
This has some really interesting implications for our understanding of how the brain processes arithmetic – an ability the researchers describe as a “cornerstone of scientifically and technologically advanced human culture.”
The medial temporal lobe sits right at the junction of several very important areas of the brain, the study points out, and it’s becoming increasingly recognized as a hub of mathematics inside our heads.
For instance, the study explains, while “a dynamic code seems to suffice for short maintenance of more implicit information in memory, the intense mental manipulation of the attended working memory contents may require a static code.” That may be what’s going on in the parahippocampal cortex, the authors write: it may be that activity in this area “may represent a short-term memory of the arithmetic rule, whereas downstream [the] hippocampus may ‘do the math’ and process numbers according to the arithmetic rule at hand.”