Those old Doublemint chewing gum ads got it right: Two is better than one. That's the idea behind a new study, published today, that showed off a system that hooked up monkeys' brains directly to a computer. On the computer screen, the monkeys saw two avatar arms. Motivated by a few sips of juice if they got it right, the monkeys were able to move those arms to targets just by thinking about it. After training, they could produce the movements while keeping their real arms still. It was like playing a first-person computer game, but instead of tapping out commands on a keyboard, the monkeys controlled their avatar arms with their thoughts. 

Previous experimental systems have allowed both people and monkeys to control one arm, whether on a computer screen or a physical, robotic arm. This is the first time researchers have been able to add in a second arm.