Much later, Kandel was able to show that sea slugs experience Hebbian learning, a process
hypothesized by the Canadian neuroscientist Donald Hebb. This kind of learning is captured in a
slogan that summarizes how two neural connections are formed: what fires together wires together.
Consider two neurons with a weak synaptic connection that are both made to fire by the same
stimulus. According to Hebb, there should be a mechanism by which their firing at the same time
wires the neurons together by increasing strength of the synaptic connection between them. From the
work of Kandel and many other researchers, we now know that this kind of learning occurs in sea
slugs and also in animals with much more complex brains.