Analysis of neural plasticity genes' expression in fish brain reveals the basis of individual differences in learning

Individual differences in cognition have been historically recognized in humans, and recent evidence suggests that such variation is widespread across the animal kingdom. Despite its importance for individuals' behaviour and fitness, the biological roots of cognitive variation remain poorly understood. We hypothesize that variation in brain gene expression is important in determining individual cognitive differences. To test this, we focused on 6 neural plasticity genes and examined fish, which exhibit the largest cognitive variation reported for vertebrates. Zebrafish (Danio rerio) exposed to visual discrimination tasks showed substantial variation in their performance, with some learning over 7 times faster than others. Expression of two genes positively predicted learning performance. However, expression levels of most genes were related at the individual level, suggesting that multi-gene expression patterns may be more relevant than single gene variation. Principal component analysis identified two axes of multi-gene expression variation: the first loaded by all genes except neurotrophin bdnf, the second mainly loaded by bdnf and neurod1 expression. Only the latter component significantly predicted learning performance in a visual discrimination task, indicating that individual variation in bdnf expression and with lesser extend neurod1 are critical for learning. Our study bridges the gap between cognitive differences and molecular mechanisms underlying brain function, providing foundation for new understanding what makes individual unique.