  Animals can learn efficiently from a single experience and change their
future behavior in response. However, in other instances, animals learn very
slowly, requiring thousands of experiences. Here I survey tasks involving fast
and slow learning and consider some hypotheses for what differentiates the
underlying neural mechanisms. It has been proposed that fast learning relies on
neural representations that favor efficient Hebbian modification of synapses.
These efficient representations may be encoded in the genome, resulting in a
repertoire of fast learning that differs across species. Alternatively, the
required neural representations may be acquired from experience through a slow
process of unsupervised learning from the environment.
