Even snails and slugs can learn. The American neuroscientist Eric Kandel won a Nobel Prize for his
research on how learning works in the sea slug, Aplysia. These slugs have only a few thousand large
neurons, so Kandel was able to map out the connections among them and investigate the chemical
mechanisms responsible for the formations of these connections. Sea slugs don't learn much, but they
are able to modify such behaviors as eating and withdrawing from noxious stimuli. Kandel showed in
the 1960s how these behavioral changes result from changes in synapses, the connections between
neurons. For example, when a sea slug is exposed to a new substance and given an electric shock, its
neurons undergo chemical changes that alter its behavior, enabling it to avoid the substance.
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.
Cognitive neuroscience is still far from having a full explanation of all the different kinds of human
learning right up to the most creative leaps made by human scientists, inventors, and artists. But
thanks to research by Kandel and others, there is no doubt that many kinds of learning are the result of
identifiable brain processes. Conditioning and Hebbian learning occur in humans as well as lower
animals. The psychiatrist Norman Doidge has written an accessible book about neuroplasticity, the
enormous adaptability of the human brain that is the result of its learning mechanisms. We do not need
to have a fully worked-out account of every kind of human learning to note the substantial ongoing
explanatory successes of the hypothesis that learning is a brain process.