Researchers at the Universities of Bonn and Tübingen are now one step closer to answering this question. They were able to demonstrate that some brain cells fire mainly for quantities of three, others for quantities of four and others for other quantities.
A similar effect can be observed for digits: In humans, the neurons activated in response to a "2" are for instance not the same as the neurons activated for a "5". The results also demonstrate how we learn to handle number symbols in comparison to quantities. The study is published online in the journal Neuron.
We are born with the ability to count: Shortly after birth, babies can estimate the number of events and even perform simple calculations. Researchers from the Department of Epileptology at the University of Bonn and neurobiologists from the University of Tübingen have investigated these two questions.
They benefited from a special feature of Bonn University Hospital: The epileptology clinic located there specializes in brain surgery. With this, doctors try to cure epilepsy patients using an operation in which they remove the diseased nerve tissue.
In some cases, they first have to insert electrodes into the affected person's brain to ascertain the location of the epileptogenic focus. As a side effect, researchers can use this to watch patients think.
Algorithm recognizes how many points test subjects see
This is also the case in the current study: The nine participants were epilepsy patients who had microelectrodes as fine as a single hair inserted into their temporal lobes. "This enabled us to measure the reaction of individual nerve cells to visual stimuli.
The scientists now showed their subjects a different number of points on a computer screen – sometimes only one, sometimes four or even five. We were able to demonstrate that certain nerve cells fired primarily in response to very specific quantities.
Each quantity, therefore, creates a specific activity pattern in the human brain. They have written a classification algorithm that evaluates this pattern. How we process digits, i.e., symbols that represent quantities can hardly be answered with the help of animals.
The scientists have now been able to show for the first time that this works in principle in a similar way as with quantities: When we see a certain digit, certain brain cells fire. However, the digit neurons and the quantity neurons are not identical: The digit "3" excites completely different nerve cells than a quantity of three points.
The digit neurons also have a numerical distance effect. They are also stimulated not only by the exact digit but also by its neighbors – but only very weakly. Nevertheless, this phenomenon shows that we learn digits differently from simple characters.
In a sense, the neurons have learned that the value of a 3 is only slightly different from a 2 or a 4, otherwise, they would not also respond to these two digits. Digits, therefore, seem to be firmly interwoven with a certain idea of quantity.
The researchers hope that their results will also contribute to a better understanding of dyscalculia, a developmental disorder accompanied, among other things, by a poorer understanding of quantity.