The number 5 is to the left of the number 10. The number 15 is to the right of 10, but to the left of 20. Most people would have no problem agreeing with these statements, because most people have spatial associations for numbers. This is called the “mental number line”: smaller numbers go to the left, increasing to the right.
What is not clear is where this number line comes from. It is possible that it is a product of mathematical education, or even that it is a product of human culture and language, found universally regardless of educational level. On the other hand, it’s possible that it’s a built-in feature of our brain, one that may be shared with other species.
A recent study in Science tested three-day-old chicks to see if they have number-space mapping like humans. By using such young animals, they could ensure that the chicks were not overtrained in the use of numbers, which could lead to atypical behavior.
Each chick was placed in front of a five-dot panel. There was food behind the panel, and when the chick figured this out enough to walk around the panel looking for food 20 times in a row, it moved past the training phase into the testing phase. The test phase included two panels equidistant from the chick, both with the same number of dots: both panels showed either two dots or eight dots. The test was whether the chick would choose to walk around the left panel or the right panel.
If the panels had two dots, the chicks walked around the left panel about 70 percent of the time. If the panels had eight dots, the chicks walked around the right panel about 70 percent of the time. This suggests that they associated the smaller number with ‘left’ and the larger number with ‘right’ compared to the number they were trained on.
The next group of chicks were trained on a panel with 20 dots. This time the panels had 8 or 32 dots in the test phase. Again, when the panels had 8 dots, more chicks went around the left panel; when there were 32 dots, they moved to the right.
The results “provide further evidence that language and culture are not necessary for the development of mathematical cognition,” said lead researcher Rosa Rugani. They also show that chicks process the size of numbers in relation to other numbers, not absolutes. When the chicks were trained on five dots, panels with 8 dots were larger and so on the right side; when trained with 20 dots, the eight dot panels were smaller. Therefore, the processing of a given number on the left or right side will vary depending on which number it is compared to, Rugani explains.
It’s important not to think that the chicks really count, she adds. Being able to tell the difference between amounts is not the same as being able to count to 32. But while animals clearly don’t have the same advanced number skills as humans, we can still draw important conclusions from studying them. “The research on animal cognition could help us understand the cognitive skills that can be mastered without language,” she explains.
Previous research on mental number lines has led to some contradictions. One study found that people who grow up reading text and numbers from right to left in Arabic have an inverted number line, from right to left, suggesting that cultural factors influence the spatial mapping. On the other hand, babies as young as seven months seem to prefer seeing dots in a 1-2-3 left-to-right pattern, compared to a 3-2-1 left-to-right pattern, suggesting that the ability arises before much cultural conditioning or language acquisition can take place.
Showing that other animals had a number line could influence this debate. However, it’s not so clear whether the conclusions of this paper are supported by the evidence, says Dr Andrew Wilson, a researcher in psychology and cognitive sciences at Leeds Beckett University.
There are two main problems with the study, Wilson argues. “The mental number line effect in humans manifests as a nudge, a tendency,” he says. “But the behavior of the chicks was surprisingly reliable: only two of the 64 chicks in the study had an effect in the ‘wrong’ direction. That suggests that something much more stable and reliable was the driving force behind their search.” While the researchers were careful to control the visual stimuli as carefully as possible, there may have been an uncontrolled factor in the mix, he explains.
More importantly, he adds, it’s not clear that a mental number line concept would come into play in how a chick forages for food. Either the chicks saw some sort of similarity between the training card and the one they chose to look behind, or they were somehow forced to forage for food according to their mental number lines. The latter explanation is not convincing, argues Wilson.
“The authors worked hard to address some potential confusions in the study,” he says. “But there are many reasons why the chicks may be inclined to go in one direction, and jumping to a ‘mental number line’-like explanation so quickly is premature at best.”
Science2015. DOI: 10.1126/science.aaa1379 (About DOIs).