Fish are helping researchers track down the origins of how brains compute math, according to a review (“Quantity as a Fish Views It: Behavior and Neurobiology”) in Frontiers in Neuroanatomy.

An international team has reviewed more than 200 publications, which together show that fish perceive quantities using similar parts of their brains as mammals and birds. Research is still underway to find the specific brain circuitry that makes number processing possible, but these findings could eventually help treat human diseases that impair the ability to do math, noted the scientists.

“An ability to estimate quantities, such as the number of conspecifics or the size of a predator, has been reported in vertebrates. Fish, in particular zebrafish, may be instrumental in advancing the understanding of magnitude cognition. We review here the behavioral studies that have described the ecological relevance of quantity estimation in fish and the current status of the research aimed at investigating the neurobiological bases of these abilities,” wrote the investigators.

“By combining behavioral methods with molecular genetics and calcium imaging, the involvement of the retina and the optic tectum has been documented for the estimation of continuous quantities in the larval and adult zebrafish brain, and the contributions of the thalamus and the dorsal-central pallium for discrete magnitude estimation in the adult zebrafish brain.

“Evidence for basic circuitry can now be complemented and extended to research that make use of transgenic lines to deepen our understanding of quantity cognition at genetic and molecular levels.”

The video shows experiments described in the lab of Giorgio Vallortigara, PhD, of the University of Trento, and reported in Messina et al. (2022). On the left side, an example is shown of the procedure of habituation of a zebrafish to a certain numerosity; on the right side, one can notice that during the dishabituation test when the numerosity is abruptly changed the zebrafish exhibit increased stay close to the stimulus to explore the novelty. Controls showed that zebrafish responded to numerosity as such and not just to change in area or spatial positions of the dots. [Giorgio Vallortigara/University of Trento]

“Fish are on par with other animals in possessing a sense of quantity,” said corresponding author Giorgio Vallortigara, PhD, of the University of Trento in Italy. “There are species, most notably the zebrafish, that are ideal models for studying the molecular and genetic bases of the sense of quantity. This could have important implications for neurodevelopmental diseases affecting number cognition, such as developmental dyscalculia, which impairs maths skills in up to 6% of children.”

Lessons from fish

Estimating quantity is essential to a fish’s survival. Vallortigara and his collaborators began with a collection of behavioral studies showing fish rival mammals and birds in recognizing the difference between larger and smaller quantities—of food or other fish, for example.

Many investigations have also used behavior to try to understand how fish gauge quantity, but Vallortigara’s team found this required looking more closely at the cellular and genetic level. To answer these questions, researchers have used brain imaging to show fish use the equivalent parts of their brains as many other vertebrates.

“Another open issue is whether numerical quantities are really computed as an abstract property or whether animals always think about numbers on the basis of other cues from their surroundings (such as surface area, contour length, or density),” said Vallortigara. “However, experiments are described in this review  show pure numerousness is indeed used by fish.”

The evolution of math

On an even more detailed level, other studies have moved closer to finding the specific neurons that form the circuits that process quantity, including those specific for discrete quantities. Genetic analyses are also revealing exactly how similar these strategies are between different species.

“A big ongoing question is whether the mechanisms for quantity cognition in the different parts of the animal kingdom evolved from a common ancestor or separately as a result of convergent evolution under similar selective pressures,” added Vallortigara.

On a genetic level, model systems like zebrafish are surprisingly close to humans and many researchers have been using zebrafish to better understand learning disabilities in humans.