Brain Lateralization Shapes Chick Number Line

A recent study published in eLife sheds new light on the relationship between brain asymmetry and the development of a mental number line in young domestic chicks. This fascinating research posits that the lateralization of brain functions—where each hemisphere specializes in different cognitive abilities—plays a crucial role in shaping how numbers are mentally organized from left to right.

The Concept of the Mental Number Line

The mental number line is a cognitive concept where individuals visualize numbers arranged along a continuum. Typically, smaller numbers are conceptualized on the left, while larger numbers occupy the right side. This representation is often attributed to cultural influences, particularly through processes such as reading and writing.

However, emerging evidence suggests that the origins of the left-to-right mental number line may have biological, rather than solely cultural, roots. This notion has significant implications for understanding numerical cognition across different species, including humans.

Study Overview

Lead author Rosa Rugani, a Professor in the Department of General Psychology at the University of Padua, Italy, and her colleagues conducted an experiment involving 100 domestic chick embryos. Half of these embryos were exposed to light during the final days of incubation, promoting brain lateralization, while the other half were kept in the dark, resulting in weakly lateralized chicks.

The experimental design aimed to assess the spatial-numerical abilities of these chicks through a series of food-locating tasks. Essential features of the study included:

• Training chicks to find food hidden behind the fourth of ten identical caps arranged vertically.
• Testing their directional preferences after rotating the cap arrangement horizontally, allowing researchers to monitor left vs. right selection.

Findings and Implications

The study produced compelling results. Strongly lateralized chicks exhibited a clear preference for the fourth cap from the left, corroborating the existence of a left-to-right spatial mapping for numbers. In contrast, weakly lateralized chicks showed no consistent preference, indicating randomness in their selections.

The experiment further employed monocular vision tests, where the chicks' left or right eye was covered to determine which hemisphere was processing spatial and numerical information. Results demonstrated that:

This experiment highlights the essential role of brain lateralization in integrating spatial and numerical cues for cognitive tasks.

Exploration of Spatial Cues

In a follow-up experiment, the research team tested the chicks' performance under conditions where spatial cues were unreliable. The spacing between the caps was varied so that the fourth cap's physical position was inconsistent. Under these conditions, neither group showed a directional preference, further confirming the reliance on lateralized brain function for spatial-numerical associations.

“For the first time, we show that brain lateralization is not just related to the mental number line—it's necessary for it,” says Professor Rugani.

Evolutionary Perspectives

The researchers postulate that a natural left-to-right scanning pattern may provide evolutionary advantages for chicks in their natural environments. Efficiently processing and quantifying food resources may enable these animals to forage more effectively, a skill crucial for survival.

Furthermore, understanding brain lateralization and its influence on numerical cognition could shed light on the development of cognitive abilities in humans. Insights from this research may ultimately help address atypical brain organization and its effects on numerical reasoning.

Conclusion and Future Directions

This study underscores the intricate relationship between brain structure and cognitive function, particularly in the context of numerical thinking. As suggested by Professor Lucia Regolin, senior author of the study:

“Understanding the biological basis of numerical thinking may help us identify why certain cognitive abilities emerge when they do in development.”

Future research endeavors could further explore the implications of prenatal sensory experiences on cognitive outcomes and continue to examine the developmental trajectories of numerical reasoning across species.

References

Rugani, R., et al. (2025). Prenatal light exposure affects number sense and the mental number line in young domestic chicks, eLife, DOI: 10.7554/eLife.106356.3.

For further reading, visit the article on [Medical Xpress](https://medicalxpress.com/news/2025-09-brain-asymmetry-mental-line-early.html).