Alvaro Pascual-Leone holds a figure-eight-shaped paddle to his head and flips a switch. His left arm begins to twitch. He turns off the device-quelling its pulsing magnetic field, which was inducing an electrical current inside his brain-and his arm relaxes. But Pascual-Leone, a neuroscientist at Boston's Beth Israel Deaconess Medical Center, is interested in more than muscle twitches; he believes that magnetic stimulation provides the last, best hope for treating patients with severe depression. This fall, researchers will begin large-scale human trials of the technology to see if he is right.

Some 20 million Americans suffer from severe depression, and many of them don't respond to conventional drugs. Based on several small studies, including Pascual-Leone's, regulators in Israel, Europe and Canada gave magnetic stimulation the green light last year as an alternative depression treatment. With a version of the technology licensed from Emory University, Atlanta-based Neuronetics plans to begin human trials, involving 240 patients at eight sites around the country, in September. If all goes well, Neuronetics' CEO Stan Miller hopes to see U.S. psychiatrists offering magnetic treatments for severe depression by 2004.

Mark George, a neuroscientist at the Medical University of South Carolina participating in the trials, sees magnetic stimulation as an alternative to shock therapy. "While shock therapy is very effective, it has lots of side effects such as memory loss, and you have to give the patient a seizure," says George. Magnetic stimulation has none of those side effects. The technique excites nerve cells in a small area of the brain that is underactive in depressed patients.

Still, some believe that the move to the clinic may be premature. "I think the therapy has gotten way ahead of the science," says National Institutes of Health neuroscientist Eric Wasserman. But the reality, say other researchers, is that many depressed patients are desperate for alternative treatments and would gladly try magnetic stimulation despite uncertainties.