Dr. Nicolelis and his colleagues began by implanting two sets of electrodes in the brains of four rats. One set delivered a signal into one part of each brain, while the other eavesdropped on a different brain patch.

The four rats received the same signal, and then a computer monitored how their brains responded. If all four rats produced synchronized signals in their brains, they were rewarded with a sip of water.

Through trial and error, the rats learned how to consistently synchronize their brains, making it possible for the rats to act like a simple computer. In one experiment, the animals learned how to produce different brain responses to two different signals: a single burst of electric pulses, or four bursts.

The rats learned how to produce synchronized brain activity in response to one of the signals, and unsynchronized activity in the other. Their collective response was correct as often as 87 percent of the time — substantially better than an individual rat learning on its own.

The scientists also found that the brains of three rats could be linked into an information-processing chain. First, they trained one rat to produce the correct kind of brain activity to two different electrical bursts in the brain. Then they linked the first rat’s brain signals to the brain of a second rat.

The second rat learned to produce the same response as the first rat, the scientists found, and a third rat could reliably interpret the second rat’s brain responses. And when they delivered the third rat’s brain signals back to the first rat, it also responded correctly much of the time.

Dr. Nicolelis and his colleagues then turned from rats to monkeys, with a new twist on earlier experiments in which individual monkeys learned to control a robot arm. This time the scientists implanted electrodes into two monkeys instead of one.