According to one scientist, “You don’t see with the eyes. You see with the brain.” Illustration by Chad Hagen

The climbers at Earth Treks gym, in Golden, Colorado, were warming up: stretching, strapping themselves into harnesses, and chalking their hands as they prepared to scale walls stippled with multicolored plastic holds. Seated off to one side, with a slim gray plastic band wrapped around his brow, Erik Weihenmayer was warming up, too—by reading flash cards. “I see an ‘E’ at the end,” he said, sweeping his head over the top card, from side to side and up and down. “It’s definitely popping—is it ‘please’?” he asked me. It was. Weihenmayer moved triumphantly on to the next card.

Erik Weihenmayer is the only blind person to have climbed Mt. Everest. He was born with juvenile retinoschisis, an inherited condition that caused his retinas to disintegrate completely by his freshman year of high school. Unable to play the ball games at which his father and his brothers excelled, he took to climbing after being introduced to it at a summer camp for the blind. He learned to pat the rock face with his hands or tap it with an ice axe to find his next hold, following the sound of a small bell worn by a guide, who also described the terrain ahead. With this technique, he has summited the tallest peaks on all seven continents.

A decade ago, Weihenmayer began using the BrainPort, a device that enables him to “see” the rock face using his tongue. The BrainPort consists of two parts: the band on his brow supports a tiny video camera; connected to this by a cable is a postage-stamp-size white plastic lollipop, which he holds in his mouth. The camera feed is reduced in resolution to a grid of four hundred gray-scale pixels, transmitted to his tongue via a corresponding grid of four hundred tiny electrodes on the lollipop. Dark pixels provide a strong shock; lighter pixels merely tingle. The resulting vision is a sensation that Weihenmayer describes as “pictures being painted with tiny bubbles.”

Reading the cards before his climb helped Weihenmayer calibrate the intensity of the electrical stimulation and make sure that the camera was pointing where he thought it was pointing. When he was done, he tied himself into his harness and set off up Mad Dog, a difficult route marked by small blue plastic holds set far apart on the wall. Without the BrainPort, Weihenmayer’s climbing style is inelegant but astonishingly fast—a spidery scramble with arms and feet sweeping like windshield wipers across the wall in front of him in order to feel out the next hold. With the device on his tongue, he is much slower, but more deliberate. After each move, he leans away from the wall, surveys the cliff face, and then carefully reaches his hand out into midair, where it hovers for a split second before lunging toward a hold several feet away. “You have to do the hand thing, because it’s hard to know where, exactly, things are in space,” Weihenmayer explained, as I prepared to tackle Cry Baby, a much simpler route. “Once my hand blocks the hold, I know I’m in front of it, and then I just kind of go in there.”

Weihenmayer told me that he wouldn’t take the BrainPort up Everest—relying on fallible electronics in such extreme conditions would be foolhardy. But he has used it on challenging outdoor climbs in Utah and around Colorado, and he loves the way that it restores his lost hand-eye coördination. “I can see the hold, I reach up, and I’m, like, ‘Pow!’ ” he said. “It’s in space, and I just grabbed it in space. It sounds so simple when you have eyes, but that’s a really cool feeling.”

The BrainPort, which uses the sense of touch as a substitute for sight, is one of a growing number of so-called sensory-substitution devices. Another, the vOICe, turns visual information into sound. Others translate auditory information into tactile sensation for the deaf or use sounds to supply missing haptic information for burn victims and leprosy patients. While these devices were designed with the goal of restoring lost sensation, in the past decade they have begun to revise our understanding of brain organization and development. The idea that underlies sensory substitution is a radical one: that the brain is capable of processing perceptual information in much the same way, no matter which organ delivers it. As the BrainPort’s inventor, the neuroscientist Paul Bach-y-Rita, put it, “You don’t see with the eyes. You see with the brain.”

Bach-y-Rita, who died in 2006, is known as “the father of sensory substitution,” although, as he liked to point out, both Braille and white canes are essentially sensory-substitution systems, replacing information that is typically visual—words on a page, objects at a distance—with tactile sensation. He even argued that writing ought to be considered the original precursor, because it enabled the previously auditory experience of the spoken word to be presented visually.

Bach-y-Rita began his medical career in visual rehabilitation, gaining a reputation as a specialist in the neurophysiology of eye muscles. In 1959, his father, Pedro Bach-y-Rita, a Catalan poet who had immigrated to the Bronx and taught at City College, suffered a catastrophic stroke. Doctors said that he would never speak or walk again, but Paul’s brother, then a medical student, designed a gruelling rehabilitation regimen: Pedro had to crawl around on kneepads until he could walk, and to practice scooping up coins until he had learned to feed himself. After a year, Pedro went back to work as a teacher and, after two, he was able to live independently. When he eventually died—in 1965, of a heart attack—he was hiking up a mountain in Colombia. And yet, as his autopsy revealed, his brain was still severely damaged; the areas responsible for motion and involuntary muscle movements had been all but destroyed. “How could he have recovered so much?” Bach-y-Rita marvelled. “If he could recover, why didn’t others recover?”

Bach-y-Rita had already begun tinkering with devices that substituted tactile sensation for vision, but, encouraged by this personal evidence of the brain’s ability to adapt to loss, he completed his first prototype in 1969. It was built from castoffs—a discarded dentist’s chair, an old TV camera—and weighed four hundred pounds. A blind person could sit in the chair and scan the scene by using hand cranks to move the camera. The analog video stream was fed into an enormous computer, which converted it into four hundred gray-scale dots. These points of information were then transferred not to four hundred electrodes, as in the BrainPort, but to a grid of vibrating, Teflon-tipped pins mounted on the back of the chair. The pins vibrated intensely for dark pixels and stayed still for light ones, enabling users to feel the picture pulsing on their backs. After just a few hours’ practice, Bach-y-Rita’s first six volunteers, all blind from birth, could distinguish between straight lines and curved ones, identify a telephone and a coffee mug, and even recognize a picture of the supermodel Twiggy.

Bach-y-Rita published his results in Nature, in 1969. During the following decade, he continued to refine the system, testing his blind subjects with more and more complex tasks while trying to shrink the enormous contraption into something more manageable. The bulk of cameras and computers at the time wasn’t the only challenge. He also ran up against a tactile constraint known as “two-point discrimination”—our ability to tell that two things touching the skin are indeed discrete objects, rather than a single large one. The skin’s spatial resolution varies widely; on the back, the stimuli had to be quite far apart, and Bach-y-Rita spent years looking for a better spot. Some of the most point-sensitive areas are on the hand, but if blind users had their hands stuck in a device they wouldn’t be able to manipulate the objects they were newly capable of seeing. Bach-y-Rita’s colleagues scoffed when he settled on the tongue, pointing out the difficulty of making the device work in a wet environment. But the tongue’s moisture makes it an excellent transmitter of electrical energy, and it is as sensitive to two-point discrimination as a fingertip.