For eighteen years, Jan Scheuermann has been paralyzed from the neck down. She is six feet tall, and she spends all day and all night in a sophisticated, battery-powered wheelchair that cradles her—half sitting, half reclining—from head to toe. In effect, the chair has become an extension of her body. To navigate the world in it, Scheuermann manipulates a cork-tipped joystick with her chin. She can move in this way with remarkable agility, but her height, combined with the bulk of the chair and the unrelenting nature of gravity and matter, can limit her. Over the phone, though, it is possible to not ever think of her paralysis. She has a soft voice, a wry sense of humor, and a warm, gentle manner. Sometimes when she speaks she pauses to inhale; the deliberate breaths are necessary because her lungs do not automatically pull in enough air, but a listener tends not to notice them. Across a fibre-optic network, her words are converted into weightless digital information. She floats to you.

When I first met Scheuermann, it was by phone. I had called her at home, in Pittsburgh, after learning that she had participated in a neuroscience experiment that allowed her to partially escape the confines of her paralyzed body. Scheuermann is one of a very few Americans to have experienced a direct brain-computer interface, a complex assemblage of technology—transistor-like cortical implants, wires, algorithmic decoders, robotics, all in their early stages of development—designed to fuse minds with machines. For decades, the idea of plugging a brain into a computer has been a mainstay of cyberpunk fiction, not biotechnology. (“I jack in and I’m not here,” a character explains in William Gibson’s 1984 novel, “Neuromancer.”) The human brain is the most complicated object in the known universe. A single brain contains more electrical connections than there are galaxies in space. Understanding the behavior of its eighty-six billion neurons is as formidable a scientific challenge as interstellar travel.

Scheuermann was not always paralyzed. The second of nine siblings, she grew up in Pittsburgh, in the nineteen-sixties and seventies. Her childhood took place in a self-contained, analog world: family, school, church, all within a few city blocks. Her father was a baker, and on Saturdays she worked at his doughnut shop, near her school. She loved reading mysteries; later, at the University of Pittsburgh, she studied nonfiction writing, and after graduating she founded a company, Deadly Affairs, that staged murder mysteries in clients’ homes. Guests wore outlandish costumes and acted out parts that she wrote, while she played Inspector Clueless, a befuddled detective who helped guide the narrative. She met her husband through the business, and moved to California to live with him in 1987. While developing Deadly Affairs, she became a contestant on “Wheel of Fortune” and other game shows. By the early nineties, she had two children. She was writing new scripts. Her life was as she wanted it.

In 1996, Scheuermann was in a client’s living room, orchestrating a production, when suddenly her legs felt heavy and numb. She thought of her father, who had died at the age of fifty-seven, from multiple sclerosis, a disease that tends to run in families. That night, Scheuermann tried not to become alarmed, assuring herself that the heaviness was only fatigue. With rest, the feeling dissipated, but soon it returned and began to spread. Her doctors ruled out M.S. but could offer no diagnosis, and the medical uncertainty inspired its own worries. Scheuermann began to use a cane, then a wheelchair. In 1998, fearing death, she moved with her family back to Pittsburgh so that her relatives could help care for her children.

In her home town, Scheuermann’s life began to change. Doctors there settled on a diagnosis, spinocerebellar degeneration, a rare ailment that ruins lines of communication between the brain and the spine; although there was no cure for it, the concreteness of the diagnosis offered a kind of relief. She was prescribed Prozac, which alleviated a gathering depression. After her children left for college, Scheuermann felt a deep emptiness, but, buoyed by the drug and by support from friends, she regained her bearings and wrote a humorous whodunnit, “Sharp as a Cucumber: A Brenda LaVoom Mystery.”

Then, in October, 2011, a friend sent her a YouTube clip of a young man, Tim Hemmes, who was paralyzed after a motorcycle accident. The video documented his work with researchers at the University of Pittsburgh, which had joined with the U.S. military in an unprecedented scientific effort—a program, with a budget of more than a hundred million dollars, to develop sophisticated prosthetics that could be controlled directly by the human brain. Microsensors embedded on a wafer were surgically placed between Hemmes’s skull and his brain, allowing him to manipulate a robotic arm—even to hold his girlfriend’s hand, for the first time in years. At the end of the clip, he told the camera, “I believe in my heart that this is the future. Anybody out there who has the courage, and the want, to try to do this—you gotta go for it!” The work with Hemmes was only a pilot study. The researchers, contemplating a genuine trial, needed a new subject. As soon as the video ended, Scheuermann vowed to sign up. “I was just so eager for this,” she told me. “I had one goal: to move that robotic arm with my mind!”

II.

The human animal is a creature of movement. For each of us, the gift of consciousness resides in a cellular vehicle, made from bone and blood, skin and fat, and driven by muscles—a body, as Walt Whitman put it, “cunning in tendon and nerve.” One cardiac muscle and countless smooth visceral muscles operate automatically within—the unseen engines of life. They are joined by hundreds of skeletal muscles, which can be commanded to run marathons, to perform music, to write, to speak.

How the mind instructs the body to move is a mystery that has preoccupied Andrew Schwartz, a neuroscientist at the University of Pittsburgh, for more than three decades. It might seem reasonable to expect that a relationship so fundamental would by now be known, but the chain of events that connect the firing of neurons to, say, a punch to the nose remains the subject of pitched scientific controversy.

Schwartz, whose research was central to helping Hemmes move the robotic arm, was a partisan in that controversy, and also a pioneer in the field of neural prosthetics. Since the nineteen-nineties, he had been competing with a small cadre of scientists to develop a system that could circumvent the body and translate raw mental activity into robotic movements. In a rarefied, often showy, sometimes bitter scientific milieu, he seldom sought attention. But his incremental approach had produced remarkable results, earning him a reputation as a rigorous researcher who was as comfortable with bioengineering as he was with neuroscience. The video of Hemmes was one artifact of an intellectual quest, combining Schwartz’s personal journey through the science of the brain with an effort to build the world’s most advanced anthropomorphic robotic arm, underwritten by the most heterodox part of the federal bureaucracy: the Defense Advanced Research Projects Agency.

Schwartz grew up outside Minneapolis. He has the compact physique, and the ruddy complexion, of a cycling enthusiast. He tends to maintain a quiet presence, but he holds strong scientific views, and they often surface beyond the veneer. “Movements are beautiful, and I want to restore that beauty,” he once told me. “When engineers say, ‘Well, heck, you could do this with a claw and a magnet, to lift something up and transport it,’ I’m, like, ‘Yeah, but that’s not what I want to do!’ When we move, there is a very efficient, almost simplistic, elegant way to do it.”