Neural Basis of Number Sense in Larval Zebrafish

Significance

• Numerically responsive neurons are detected by 3 days post-fertilization (dpf) in zebrafish

  • Number-selective neurons preferring quantities greater than one increases after 3 dpf

• Neurons that are highly responsive to specific numbers of objects are mainly localized to the forebrain and midbrain

• Number stimulus can be decoded from Ca ²⁺ activity and show increased performance with age

• Administration of ethanol decreases the activity of number-selective neurons in the forebrain

Number sense, the ability to discriminate the quantity of objects, is crucial for survival. To understand how neurons work together and develop to mediate number sense, we used two-photon fluorescence light sheet microscopy to capture the activity of individual neurons throughout the brain of larval Danio rerio , while displaying a visual number stimulus to the animal. We identified number-selective neurons as early as 3 days post-fertilization and found a proportional increase of neurons tuned to larger quantities after 3 days. We used machine learning to predict the stimulus from the neuronal activity and observed that the prediction accuracy improves with age. We further tested ethanol’s effect on number sense and found a decrease in number-selective neurons in the forebrain, suggesting cognitive impairment. These findings are a significant step towards understanding neural circuits devoted to discrete magnitudes and our methodology to track single-neuron activity across the whole brain is broadly applicable to other fields in neuroscience.