Neuron 63, 119–126 (2009)

Monkeys like to know the size of rewards coming their way, and, in the brain, this desire is signalled by the same dopamine neurons that signal primitive rewards like sex and food.

Ethan Bromberg-Martin and Okihide Hikosaka of the National Eye Institute in Bethesda, Maryland, trained two monkeys to glance at one of two targets on a computer screen in order to receive a drink reward, which was randomly large or small. When one target included information about reward size the monkeys preferred to go for that target, rather than be surprised by a randomly sized reward.

The scientists recorded from single neurons in the brain's 'reward' circuitry and found that they fired when the monkeys learned information about the future. This suggests that the act of prediction may be intrinsically rewarding.