  One of the key points addressed by Per Bak in his models of brain function
was that biological neural systems must be able not just to learn, but also to
adapt--to quickly change their behaviour in response to a changing environment.
I discuss this in the context of various simple learning rules and adaptive
problems, centred around the Chialvo-Bak `minibrain' model [Neurosci. 90 (1999)
1137--1148].
