Neural Basis of Number Sense in Larval Zebrafish

Background

Numerical cognition is a fundamental cognitive ability that allows animals to assess quantities in their environment, crucial for survival-related tasks like foraging, predator avoidance, and social interactions. This ability is not limited to humans; it is observed in a range of species, from insects to primates. Most animals make use of an ‘Approximate Number System,’ enabling them to estimate quantities without relying on language or symbols, a skill that appears early in development¹. Understanding how the brain processes and represents numerical information has significant implications for neuroscience, as it sheds light on both basic cognitive mechanisms and their evolutionary roots.

There is growing evidence that the neural circuits for numerical processing may not be exclusive to complex species such as primates. Studying organisms like fish or birds can offer new insights into the universality and evolutionary conservation of these cognitive abilities. Zebrafish larvae, for instance, provide a powerful model for such research due to their transparency, well-mapped genetic structure, and ease of brain-wide imaging during development.

Until recently, research on numerical cognition in animals has focused mainly on behavioral observations, while the neural underpinnings remained elusive. With advances in neuroimaging technologies, it has become possible to explore how individual neurons across the brain respond to specific stimuli. This preprint uncovers the neural basis of number sense in Danio rerio larvae using cutting-edge microscopy (Figure 1) and computational techniques to track brain-wide neuronal activity. The work by Luu et al., the authors of this preprint, not only addresses a gap in the literature on how different vertebrates process numbers but also establishes zebrafish as a viable model for studying cognitive function at the neural circuit level.

 

Figure 1. Fluorescence light sheet microscopy captures neuronal activity related to number perception in larval zebrafish. (c) Maximum intensity projections (MIPs) of a 7 dpf zebrafish brain: dorsal (top), frontal (middle), and lateral (bottom) views, averaged over 60 seconds, with white boxes highlighting areas shown in (d) and (e). (Preprint Figure 1)

Main Findings

Numerically-responsive neurons emerge early:

By using two-photon fluorescence light-sheet microscopy, the researchers discovered that numerically-tuned neurons appeared as early as 3 days post-fertilization (dpf). These neurons responded to visual stimuli with varying dot numbers, projected onto a diffuser 19 mm from the larvae’s right eye for 1-second intervals, indicating a basic numerical processing capacity. The early emergence of this ability underscores the fundamental role of numerical cognition for survival, suggesting that the neural circuitry required for processing numbers develops significantly before observable behaviors manifest.

Age-dependent changes:

As zebrafish age, there is a notable increase in the proportion of neurons that can recognize more numbers of objects. Specifically, this study found an increase in the number of neurons preferring three or more objects from 3 dpf onward, indicating that the capacity for number discrimination improves with age. This development raises important questions about whether the increase in number-selective neurons is due to the generation of new neurons or the re-tuning of existing ones, warranting further investigation into the dynamics of neural plasticity during early development.

Regional specificity:

Number-selective neurons were predominantly observed in the forebrain and midbrain regions of the zebrafish brain. At 3 dpf, significantly fewer number-selective neurons were located in the forebrain compared to the midbrain (Figure 2). However, by 5 and 7 dpf, the proportions of number-selective neurons in both regions became more comparable, suggesting a developmental trajectory that enhances the forebrain’s role in numerical processing. This spatial distribution aligns with findings in other vertebrates, suggesting an evolutionarily conserved network for numerical cognition.

 

Figure 2. 3D brain map showing number-selective neurons in larval zebrafish, primarily located in the forebrain and midbrain. Point maps illustrate neuron locations across three individuals at different developmental stages, with white circles marking the centers of identified neurons. (Preprint Figure 4)

Why I highlight this preprint

As a prospective PhD student interested in systems neuroscience, I am also working in zebrafish, studying the decision-making process and cognitive switching mechanism. This study stands out for its use of zebrafish as a model organism, which allows whole-brain imaging and detailed neural mapping at early developmental stages. The discovery that number-selective neurons are present before any observable numerically-driven behaviors (such as hunting) is especially exciting. It underscores the importance of early neural development in shaping future behavioral capabilities. This work not only deepens our understanding of how number sense evolves in zebrafish but also provides a comparative framework for studying this cognitive function across species. By identifying number-selective neurons early in zebrafish development, the study offers insights into how neural circuits supporting cognitive functions emerge and mature. It also contributes to a broader understanding of number sense across species and offers exciting possibilities for future research into neural plasticity and cognitive function. 

Question for the authors

1. Could the observed improvements in numerical sensitivity with age also be linked to developmental changes in visual acuity?
2. What mechanisms underlie the increase in neurons tuned to larger numerosities? Are new neurons generated, or do existing neurons become re-tuned?
3. How do you see the role of the optic tectum in number sense differ from its established functions in visual mapping?

Reference

1. Brannon, E. M., & Merritt, D. J. (2011). Evolutionary foundations of the approximate number system. In Space, time and number in the brain: Searching for the foundations of mathematical thought (pp. 207–224). Elsevier Academic Press. https://doi.org/10.1016/B978-0-12-385948-8.00014-1

Tags: behavior, cognition, systems neuroscience, zebrafish

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