Hebb actually talked about causation, not synchrony (firing together): "When an axon of cell A is near enough to excite cell B and repeatedly or persistently t

Hebb actually talked about causation, not synchrony (firing together): "When an axon of cell A is near enough to excite 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” Synchrony is extremely important, particularly for the formation of cortical columns and neural pruning. But in spike timing dependent plasticity, where growth is potentiated if the presynaptic fires just before post synaptic firing, the connection is actually depressed if up and downstream neurons fire exactly synchronously. (there is a huge amount of variation in this across the brain, though)