Quantities in the brain: Behavioral and neurobiological evidence in zebrafish

The ability to deal with quantity developed from an evolutionarily conserved system for approximating non-symbolic numerical magnitude. Neuronal systems selectively tuned to the number of items of a set have been described in the parietal and prefrontal cortex of human and non-human primates and in equivalent areas in crows and young chicks. However, little is known about the neural correlates of this ability in fishes. By combining a habituation/dishabituation paradigm with molecular biology assays, we have recently identified part of the neural network associated with quantity discrimination in adult zebrafish brain. Briefly, fish were habituated to sets of 3 or 9 red dots for four consecutive days. During dishabituation, zebrafish faced a change (i) in number (from 3 to 9 dots or vice versa), or (ii) in shape (3 or 9 red squares), or (iii) in size. A control group was tested with the same stimuli as during the habituation. After thirty minutes, their brains were dissected to quantify immediate early genes expression. Results showed an involvement of the retina and optic tectum in the encoding of continuous magnitude. We also found a role of the habenula, the optic tectum, the preglomerular complex, and of the caudal regions of the dorso-lateral and dorso-central pallium, in the encoding of discrete magnitude (e.g., change in numerosity). In conclusion, our results suggest an early involvement of thalamic and tectal areas for encoding of continuous quantity, and of more pallial (via tectal and thalamic nuclei) regions for discrete quantity.