Hebb's postulate of activity dependent plasticity.
Hebbian theory: a theory in neuroscience that proposes an explanation for the adaptation of neurons in the brain during the learning process.
Let us assume that the persistence or repetition of reverberatory activity (or "trace") tends to induce lasting cellular 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.