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Whether non-symbolic encoding of quantity is predisposed at birth with dedicated hard-wired neural circuits is debated. Here we presented newly-hatched visually naive chicks with stimuli (flashing dots) of either identical or different numerousness (with a ratio 1:3) with their continuous physical appearance (size, contour length, density, convex hull) randomly changing. Chicks spontaneously tell apart the stimuli on the basis of the number of elements. Upon presentation of either fixed or changing numerousness chicks showed different expression of early gene c-fos in the visual Wulst, the hippocampal formation, the intermediate medial mesopallium, and the caudal part of the nidopallium caudolaterale. The results support the hypothesis that the ability to discriminate quantities does not require any specific instructive experience and involves a neural network with several populations of number-selective neurons. Evidence for innateness of non-symbolic numerical cognition have implications for both neurobiology and philosophy of mathematics.

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. 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 ability to represent, discriminate, and perform arithmetic operations on discrete quantities (numerosities) has been documented in a variety of species of different taxonomic groups, both vertebrates and invertebrates. We do not know, however, to what extent similarity in behavioral data corresponds to basic similarity in underlying neural mechanisms. Here, we review evidence for magnitude representation, both discrete (countable) and continuous, following the sensory input path from primary sensory systems to associative pallial territories in the vertebrate brains. We also speculate on possible underlying mechanisms in invertebrate brains and on the role played by modeling with artificial neural networks. This may provide a general overview on the nervous system involvement in approximating quantity in different animal species, and a general theoretical framework to future comparative studies on the neurobiology of number cognition.