A paralyzed man with a small sensor implanted in his brain was able to control a computer, a television and a robot using only his thoughts, scientists reported today.

The development offers hope that in the future, people with spinal cord injuries, Lou Gehrig’s disease or other ailments that impair movement might be able to better communicate with or control their world.

“If your brain can do it, we can tap into it,’’ said John P. Donoghue, a professor at Brown University who led the development of the system and was the senior author of a report published today in the journal Nature.

In separate experiments, the first person to receive the implant, Matthew Nagle, was able to move a cursor, open e-mail, play a simple video game called Pong and draw a crude circle on the screen. He could change the channel or volume of a television set, move a robot arm somewhat, and open and close a prosthetic hand.

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Although his cursor control was sometimes wobbly, the basic movements were not hard to learn. “I pretty much had that mastered in four days,’’ Mr. Nagle, now 26, said in a telephone interview from the New England Sinai Hospital and Rehabilitation Center in Stoughton, Mass., where he lives. He said the implant did not cause any pain.

A former high school football star in Weymouth, Mass., Mr. Nagle was paralyzed below the shoulders after being stabbed in the neck during a melee at a beach in July 2001. He said he was not involved in starting the brawl and didn’t even know what sparked it. The man who stabbed him is now serving ten years in prison, he said.

There have been some tests of a simpler sensor implant in people, as well as tests of systems using electrodes outside the scalp. And Mr. Nagle has spoken about his experiences before.

But the paper in Nature is the first peer-reviewed publication of an experiment using a more sophisticated implant in a human.

The paper helps “shift the notion of such ‘implantable neuromotor prosthetics’ from science fiction towards reality,’’ Stephen H. Scott of Queen’s University in Canada wrote in a commentary in the journal.

The implant system, known as the BrainGate, is being developed by Cyberkinetics Neurotechnology Systems of Foxborough, Mass. The company is now testing the system in three other people whose names have not been released — one with a spinal cord injury, one who had a brain-stem stroke and one with amyotrophic lateral sclerosis, known as Lou Gehrig’s disease.

Timothy R. Surgenor, the president of the company, said Cyberkinetics hoped to have an implant approved for use as early as 2008 or 2009. Mr. Donoghue of Brown is a cofounder of the company and its chief scientist. Some of the authors of the research paper work for the company, while others work at Massachusetts General Hospital and other medical or academic institutions.

The sensor measures 4 millimeters — about one sixth of an inch — on a side and contains 100 tiny electrodes. The device was implanted in the area of Mr. Nagle’s motor cortex that is responsible for arm movement, and was connected to a pedestal that protruded from the top of his skull.

When the device was to be used, technicians connected the pedestal to a computer with a cable. So Mr. Nagle was directly wired to a computer, somewhat like a character in the “Matrix” movies.

Mr. Nagle would then imagine moving his arm to hit various targets, as technicians calibrated the machine, a process that took about half an hour each time. The implanted sensor eavesdropped on the electrical signals emitted by nearby neurons as they controlled the imaginary arm movement.

Scientists said the study was important because it showed that the neurons in Mr. Nagle’s motor cortex were still active, years after they had any role to play in physically moving his arms.

Cursor control was not very smooth. In a task where the goal was to guide the cursor from the center of the screen to a target on the perimeter, Mr. Nagle hit the target about 73 to 95 percent of the time. When he did, it took an average of 2.5 seconds, though sometimes much longer. The second patient tested with the implant had worse control than Mr. Nagle, the paper said.

By contrast, healthy people moving the cursor by hand can hit the target almost every time and in only one second.

Dr. Jonathan R. Wolpaw, a researcher at the New York State Department of Health in Albany, said the BrainGate performance did not appear to be substantially better than a non-invasive system he is developing using electroencephalography, in which electrodes are placed outside the scalp.

“If you are going to have something implanted into your brain, you’d probably want it to be a lot better,’’ he said.

Dr. Donoghue and other proponents of the implants say they have the potential to be a lot better, because they are much closer to the relevant neurons. The scalp electrodes get signals from millions of neurons all over the brain.

One way to improve implant performance was suggested by another paper in the same edition of Nature. In a study involving monkeys, Krishna V. Shenoy and colleagues at Stanford University eavesdropped not on the neurons controlling arm movement but on those expressing the intention to move.

“Instead of sliding the cursor out to the target, we can just predict which target would be hit, and the cursor simply leaps there,’’ said Mr. Shenoy, an assistant professor of electrical engineering and neurosciences.

He said a patient using the system could do the equivalent of typing 15 words a minute, about four times the speed of the other devices.

Other obstacles must be overcome before brain implants become practical. The ability of the electrodes to detect brain signals begins to deteriorate after several months, for reasons that are not fully understood. Also, ideally, the implant would transmit signals out of the brain wirelessly, doing away with the permanent hole in the head and the accompanying risk of infection.

Mr. Nagle, meanwhile, had his implant removed after a bit more than a year, so he could undergo another operation that allowed him to breathe without a ventilator. He can control a computer with voice commands, so he does not really need the brain implant. But he said he was happy he volunteered for the experiment.

“It gave a lot of people hope,’’ he said.

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