Learning generates a brain reward:
This preference for knowledge about the future was intimately linked to the monkeys' desire for water. The same neurons in the middle of their brains signalled their expectations of both rewards – the watery prizes and knowledge about them.
All the neurons in question release the signalling chemical dopamine. While the monkeys were making their choices, Bromberg-Martin and Hikosaka recorded the activity of 47 dopamine neurons in their midbrains. These neurons became very excited when the monkeys saw a symbol that predicted a large amount of water, while the symbol that cued a smaller drink inhibited the neurons. The same dopamine neurons were excited during trials where the monkey only saw the symbol that heralded forthcoming information, and they were inhibited if they monkey only saw the other non-informative symbol.
(via Raph Koster)