When we see 2 to 4 apples, we can quickly identify the number. However, when seeing five or more apples, our recognition time increases and we often make miscalculations. In fact, the brain recognizes smaller quantities of things differently than it recognizes larger quantities of things. New research from the Universities of Tübingen, Bonn and Bonn University Hospital shows that the brain processes smaller quantities differently than larger quantities. The findings were recently published in the journal Nature Human Behavior.
Imagine that someone shows us a picture of a string quartet and asks us to tell how many people are in the picture. There wasn't enough time to count, but we all blurted out, "Four!" The next photo showed a septet, again giving us just enough time to take a quick look. We hesitated, not so confident this time: the correct number for "eight" was actually seven, but we were very close.
We humans seem to have two unique ways of processing quantities of things: We are generally able to identify small amounts of things quickly and correctly. This is also known as "subization" in the research world. However, when there are five or more elements, this approach suddenly changes: we need more and more time to answer, and the answers become less and less precise.
Using ultrathin electrodes implanted in the temporal lobes of epilepsy patients, researchers can observe the activity of individual neurons in different brain regions. Photo credit: Christian Burkert/Volkswagen Stiftung/University of Bonn
Some researchers have therefore speculated that there are two different processing methods in the brain - a precise method for dealing with small numbers, and an estimating mechanism for dealing with large numbers. Professor Florian Mormann from the Department of Epilepsy at the University Hospital Bonn explains: "However, this idea is still controversial. It is also possible that our brains are always making estimates, but the error rate for smaller quantities is so low that it is not even noticed."
Neurons are more selective for smaller amounts of things
However, recent research actually shows that we do process small and large quantities of things differently. The research team involved in the project had shown several years ago that nerve cells in the brain are responsible for processing each quantity. For example, some neurons are mainly responsible for two elements, some are responsible for four elements, and some are responsible for seven elements. Professor Andreas Nieder from the University of Tübingen explains: "Nevertheless, neurons also respond to subtle changes in numbers. Therefore, brain cells for the element 'seven' also respond to elements 'six' and 'eight', but more weakly. The same cells are still activated, but even more weakly for elements five or nine."
Study participants saw a set of dots on a screen for half a second. After a short pause, they had to indicate whether the number was even or odd. If the number of points is less than 5, they usually give the correct answer without hesitation. Beyond this number, reaction times and error rates gradually increase. Image credit: AGMormann/University of Bonn
Needle has been able to demonstrate this "numeric distance effect" in experiments on monkeys. This effect seems to only occur in larger populations of humans. "For numbers with fewer than five elements, there seems to be an additional mechanism that makes these neurons more precise," the neurobiologist said.
"When a brain cell representing three numbers reacts, it simultaneously inhibits brain cells representing two and four numbers," said the neurobiologist. "This reduces the risk that these cells will also falsely trigger the number three. However, this mechanism does not apply to neurons that fire for the numbers five, six or eight. This is why the error rate is higher for these numbers."
Observe the work of individual brain cells
One feature of the University Hospital Bonn has greatly benefited the researchers in their research: the hospital's epilepsy department specializes in brain surgery. Doctors there try to treat epilepsy by surgically removing diseased nerve tissue. To determine the location of the epileptogenic zone, they sometimes first insert electrodes into the patient's brain.
Seventeen patients participated in the latest study. In preparation for surgery, they inserted hair-thin microelectrodes into the temporal lobes. "We can use them to measure the response of individual nerve cells to visual stimuli," explains Esther Kutter.
Subjects sat in front of a computer screen and a varying number of dots appeared on the screen for half a second. The subjects were then asked to tell whether they saw an even or odd number of dots. They responded very quickly and made few mistakes until four points. Later, as the number of points increased, so did the number of errors, as did the thinking time participants needed to complete the task.
This work will provide new insights into how the human brain processes numbers. In the long term, these findings may lead to a better understanding of dyscalculia, a developmental disorder associated with poor understanding of numbers.