[P]hilippens and her colleague Raymond Vanwersch attached electrodes to the brains of marmoset monkeys to pick up electroencephalogram (EEG) signals from the brain. Rather than showing the monkeys the EEG signal, as might be done in humans, Philippens and Vanwersch simply gave them a marshmallow reward every time they tuned their brain activity to a certain frequency range – in this case, 12 to 16 hertz.

http://www.newscientist.com/article/dn20989