TIL https://en.wikipedia.org/wiki/Hebbian_theory
/Let us assume that the persistence or repetition of a reverberatory activity (or "trace") tends to induce lasting cellular changes that add to its stability.… When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A‍ '​s efficiency, as one of the cells firing B, is increased./