Numbers are not just abstract symbols—they also have a “home” in our brain’s spatial map. And yes, the “mental number line” really does exist, shaped in part by exposure to light. New research from Italy now provides direct evidence of this.

Most of us imagine numbers arranged along a mental line running from left to right: smaller numbers on the left, larger ones on the right. It has long been thought that this association is primarily cultural, tied to habits of reading and writing. However, studies with children and animals challenge this view, suggesting that the link between numbers and space may have biological roots.

At the heart of this phenomenon lies brain lateralization—also known as hemispheric specialization—the idea that the two hemispheres of the brain have different roles in cognitive processes.

A study published in eLife and coordinated by the Department of General Psychology at the University of Padua offers direct proof of this hypothesis through experiments with newly hatched chicks. The results show that cerebral lateralization—the specialization of the left and right hemispheres—is essential for the development of a left-to-right mental number line and that light exposure influences it.

“Light exposure during embryonic development induces cerebral lateralization in domestic chicks, enhancing their spatial-numerical skills and their tendency to ‘count’ from left to right,” explains Rosa Rugani, lead author of the study and professor at the University of Padua. “Various theoretical models have proposed that the mental number line originates in brain lateralization, but until now we lacked direct experimental proof. Our study provides it.”