Neurons in the dorso-central pallium of zebrafish respond to change in visual numerosity.

It has been hypothesized that our ability to accurately represent the number of objects in a set (numerosity), and to carry out numerical comparisons and arithmetic, developed from an evolutionarily conserved system for approximating numerical magnitude, the so-called Approximate Number System. Non-symbolic number cognition based on an approximate sense of magnitude has been documented in a variety of vertebrate species. However, we know little of its neural and molecular bases, in particular in those species, such as zebrafish, that in recent years became established as developmental and behavioral genetic model systems. In our experiments, we first examined the ontogeny of numerosity discrimination using a group size preference assay. Fish showed group size preference from 26 days post fertilization (dpf) and from 27 dpf fish reliably chose the larger group when presented with discrimination ratios from 1:8 to 2:3. When the ratio between the number of conspecifics in each group was maintained at 1:2, fish could discriminate between 1 vs. 2 individuals and 3 vs. 6, but not when given a choice between 2 vs. 4 individuals. These findings suggest numerosity processing is the result of an interplay between attentional, cognitive and memory-related mechanisms that orchestrate numerical competence both in humans and animals. To explore the neural circuits of numerosity cognition in fish, adult zebrafish were habituated to sets of small dots that changed in individual size, position and density while maintaining their numerousness and overall surface. During dishabituation tests, zebrafish faced a change in number (with the same overall surface), in shape (with the same overall surface and number), or in size (with the same shape and number) of the dots, whereas in a control group zebrafish faced the same stimuli as during the habituation. Using a combination of early gene expression and in situ hybridization experiments, we identified for the first time a small region in the caudal part of the dorso-central division of the zebrafish pallium that shows selective activation upon change in numerosity of visual stimuli. As pallial regions are implicated in number cognition in mammals and birds, these findings support the existence of an evolutionarily conserved ANS and provide an avenue for exploring the underlying molecular correlates.