The origin of the mental number line may be biological, not cultural, according to a new study

A new study has found that a chick’s ability to mentally organize numbers along a line from left to right is not learned but is instead a direct result of brain specialization that occurs before hatching. Researchers found that exposing chick eggs to light in the final days of incubation causes the two hemispheres of the brain to develop distinct functions, which in turn establishes an innate tendency for the chicks to count from left to right.

For many years, scientists and philosophers have debated the origins of the “mental number line,” a common intuition where people visualize smaller numbers on the left and larger numbers on the right. The prevailing theory suggested this was a cultural artifact, learned through years of reading and writing in a left-to-right direction.

However, this idea has been challenged by findings that pre-verbal infants and even some animals exhibit a similar spatial bias for numbers, suggesting a deeper, biological foundation. Researchers have hypothesized that this foundation lies in brain lateralization, the process where the left and right hemispheres of the brain become specialized for different cognitive tasks. While this connection seemed plausible, there was little direct experimental evidence to confirm that brain specialization actually causes this numerical mapping.

To investigate this connection, a team of researchers turned to the domestic chick, a model animal that allows for direct manipulation of brain development. In birds, brain lateralization is heavily influenced by a simple environmental factor: light. During the last few days of incubation, a chick embryo is positioned inside the egg so that its right eye is turned toward the shell, while its left eye is obscured by its own body. If the egg is exposed to light, the right eye receives more stimulation, which promotes the development of specialized pathways in the opposite (left) hemisphere of the brain.

This simple asymmetry in light exposure produces chicks with strongly lateralized brains. Conversely, incubating eggs in complete darkness prevents this asymmetric stimulation, resulting in chicks with weakly lateralized brains. This unique developmental feature provided the scientists with a perfect opportunity to compare the numerical abilities of strongly and weakly specialized brains.

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The research team, led by Professor Rosa Rugani at the University of Padua, designed an experiment to test for a directional bias in counting. They incubated one batch of one hundred eggs in the presence of light and another batch in total darkness. After the chicks hatched, they were individually trained to perform a specific task.

Each chick was placed in an arena with a row of ten identical red bottle caps arranged in a straight line, pointing directly away from the chick. Their task was to learn that a food reward, a mealworm, was consistently hidden under the fourth cap in the sequence. Once a chick reliably located the food at the fourth position, the testing phase began.

In the first test, the researchers rotated the line of ten caps by 90 degrees, so it was now arranged horizontally in front of the chick. This created a new challenge. From the chick’s perspective, there were now two correct options: the fourth cap from the left end of the line and the fourth cap from the right end. The researchers observed which cap the chicks chose to investigate first.

The strongly lateralized chicks, those that had been exposed to light as embryos, showed a distinct preference for searching the fourth cap from the left. This behavior indicates an inherent left-to-right mapping of numbers onto space. In contrast, the weakly lateralized chicks, which had developed in darkness, showed no directional preference at all. They chose the fourth cap from the left and the fourth from the right at random, suggesting they lacked an internal mental number line.

To gain a deeper understanding of which brain hemisphere was driving this behavior, the scientists conducted a second, more specific test. They temporarily placed a small, harmless patch over one of the chick’s eyes. Because the visual systems of chicks are almost completely crossed, covering the left eye forces the chick to rely on its right brain hemisphere, while covering the right eye engages the left hemisphere. When the strongly lateralized chicks used only their left eye (and right hemisphere), they once again consistently chose the fourth cap from the left.

This confirmed that the right hemisphere plays a key role in integrating spatial and numerical information to create the left-to-right bias. When these same chicks used only their right eye (and left hemisphere), their preference flipped, and they tended to select the fourth cap from the right. The weakly lateralized chicks were unable to solve the task at all when one of their eyes was covered, further demonstrating that brain specialization enhances overall cognitive performance.

In a final experiment, the researchers explored the importance of spatial consistency. They repeated the horizontal test, but this time they varied the spacing between the bottle caps in each trial. This meant that the physical location of the fourth cap changed from one trial to the next, making spatial information an unreliable guide. To find the food, the chicks had to rely purely on their ability to count to the fourth item, regardless of its position.

Under these conditions, the left-to-right preference vanished. Even the strongly lateralized chicks no longer favored the fourth cap from the left, instead choosing randomly between the left and right options. This result shows that the mental number line is not an abstract concept alone; it is tightly linked to the spatial arrangement of objects in the environment. The brain’s left-to-right bias is activated when there are consistent spatial cues to process.

The study’s authors conclude that these findings provide the first direct experimental proof that brain lateralization is not merely associated with the mental number line but is a necessary condition for its development. The work strongly supports the idea that our sense of number is biologically grounded.

The researchers propose that this left-to-right scanning bias may have evolutionary roots. For a chick foraging for food, having a default scanning direction could be an efficient strategy, ensuring that no potential food sources are missed. The right hemisphere, known for its specialization in spatial awareness, appears to establish the left side of space as a natural starting point, or anchor, for processing the environment.

This research, while conducted in chicks, has significant implications for understanding human cognition. “Understanding the biological basis of numerical thinking may help us identify why certain cognitive abilities emerge when they do in development, and why they might be altered in individuals with atypical brain organisation,” commented senior author Lucia Regolin. The study opens new avenues for research into how early sensory experiences, such as light exposure, can have lasting effects on the organization of the brain and subsequent cognitive functions.

One limitation of the study is that it focuses on a single species, and further work is needed to see how these principles apply to other animals, including primates and humans. Future research could explore how different types of early environmental stimulation might influence the development of numerical and spatial reasoning. By continuing to examine the biological origins of fundamental cognitive abilities, scientists can build a more complete picture of how the brain learns to make sense of the world.

The study, “Prenatal light exposure affects number sense and the mental number line in young domestic chicks,” was published in eLife and authored by Rosa Rugani, Matteo Macchinizzi, Yujia Zhang, and Lucia Regolin.