ootsore visitors to the Austin Museum of Art in Texas may have been tempted by a recent installation called SeatSale, a plain wooden chair with a scrolling electronic sign and a credit card reader. But the chair has something else special: at the spot where a cushion normally goes are dozens of sharp metal spikes.

"Weary travelers no longer need to stand for hours on end," a sign says. "Use your government-issued photo ID card to download a free seating license."

Sliding a credit card through the slot makes the spikes retract, but it also commits the user to certain terms and conditions. "If you don't agree to these terms and conditions, remain standing," the device says.

It is good advice, in any case. If someone sits down, the chair issues a warning in about a minute that the time will soon be up; the person must stand or be speared by the spikes.

If Dr. Steve Mann offers you a seat, you had better read the fine print. Dr. Mann, 39, a professor of electrical engineering at the University of Toronto, who calls himself one of the world's first cyborgs, uses his oddly prescient technological vision to create artifacts from a possible, if not necessarily desirable, future.

The inspiration for SeatSale and for many of Dr. Mann's other recent inventions is his growing alarm at what he calls the totalitarian uses of technology: the omnipresent surveillance cameras hidden behind "ceiling domes of wine-dark opacity" or the daunting licensing agreements anyone has to accede to before running new software or trying out new online services.

Dr. Mann fights technology with technology, wearing computers on his body and cameras in his glasses so he can "shoot back" by recording everything he sees. The billboards and advertisements posted on every public surface are a form of "attention theft," he says, so he has invented technology that replaces these messages with whatever he would like to see. When he is wearing his "eyetap" glasses, which project an image onto the retina of his eye, a condom ad in a bathroom becomes a picture of a waterfall.

"If the eye is the window of the soul," he argues, "then that window needs a shade. If the brain is a computer, then the eye is an open port, an unsecured opening against hackers."

Dr. Mann greets a visitor to his laboratory wearing his everyday headset, an aluminum frame form- fitted to his brow, which is studded with lasers, cameras and tiny electronic parts. A thick bundle of colored wires runs from his headset to a computer in a fanny pack at his waist. In one hand he clutches a small "chording keyboard" that allows him to type commands or compose messages with one hand.

A wireless connection provides a constant Internet link. With his wearable computer, Dr. Mann can see and hear things invisible to his visitor.

Still, he can manage to look almost normal. Dressed in a vintage sport coat over a sweater and wrap- around shades, and with a perpetual two-day beard, Dr. Mann could easily pass as an artist, a philosopher or (his favorite guise) an assistant mail room clerk. But mounted on the frame of the glasses are the tiny video cameras, and beneath the sports coat, strapped against his body, are the half-dozen computers that help him record, reinterpret, augment and share his experiences.

His father worked in the fashion industry, so Dr. Mann's childhood toys in Hamilton, Ontario, were bobbins of thread and knitting needles. Perhaps it was inevitable that he would weave wire and thread together to create wearable computers. He started more than 20 years ago, while he was a teenager in Canada.

It took years of invention for Dr. Mann to transform himself into a cyborg. Only then did he try to figure out exactly what he had invented. Then, he tried to figure out what he had invented. Dr. Mann, who coins words with the same fervor and facility with which he designs circuits, calls this method of inventing "existential technology" or "existech." (He has even founded a company called Existech, in which he is assistant mail room clerk.)

If Edison had been an existential technologist, he would have invented the light bulb not to create light but to give intellectual illumination. In existech, as in existential philosophy, an inventor builds something, then tries to figure out what it is.

In the early 1980's, as an undergraduate in physics at McMaster University in Hamilton, he attracted (or, as he says, repulsed) a lot of attention by walking around the streets in full cyborg attire.

"People would walk across the street to avoid me," Dr. Mann recalled, especially his fellow science students.

His headset recorded everything he saw and assembled it into intriguing mosaics. It was a new form of first-person photography, and he was asked by an art gallery to give an exhibition of his work.

"I sort of unwittingly became an artist," Dr. Mann recalled. "I had two lives: as a student and as a cyborg-artist. And the two were quite incompatible. Except on one occasion. I was once asked by the dean to do a series of pictures of the engineering building. They liked my genre. It felt strange, wearing all my machines, walking around there doing a picture of an engineering building. The same building where people were laughing at me with a computer attached to my body - a lot of those pictures are framed and still hanging up to this day."

In 1991, he went to the Massachusetts Institute of Technology to study at the Media Lab. He sneaked up to the roof of the tallest building on campus and connected an antenna to an unused network hub so he could have a permanent wireless Internet connection, long before such things became common. For several years, anyone could log in on the Internet to see whatever he saw or to send him e-mail notes that would be superimposed over his view of the world.

"Having 30,000 people inside my head drove me a little crazy, maybe," Dr. Mann said. "So I had to limit the number of people who had write- permission on my retina."

In the Media Lab, Dr. Mann worked in the architecting surveillance laboratory, which had cameras everywhere. Some were hooked up to face-recognition software to open doors; others recorded the activity in the lab. These cameras bothered him. "When I complained about these cameras in the lab," he said, "the only response I got was `They're here to help you, not to hurt you.' "

The strongest defender of these cameras was a professor who objected when Dr. Mann tried to wear a camera to class, he said. That asymmetry - they can film us but we can't film them - struck him as the definition of totalitarian technology.

Over the past few years, Dr. Mann has critiqued what he calls the totalitarian possibilities of technology through inventions like SeatSale and documentary films like "Shooting Back," which is part Marshall McLuhan and part Michael Moore. While some people call these activities performance art, Dr. Mann prefers to call them experiments and publishes essays about them in technological journals alongside his more conventional, equation-laden work.

To make "Shooting Back," Dr. Mann, equipped with his hidden "wearcam," went into chain stores and innocently asked why there were domes on the ceiling. He secretly filmed the employees' evasiveness and their defensive responses - "They're here for your own good" - and then reached into his bag and pulled out a video camera of his own. At that point, he was usually kicked out of the store.

"Of all places I go, it's the casinos and department stores and police stations that object most strongly to my apparatus," he said. "And yet those are the places where I feel it's most needed to prevent what would be a one-sided totalitarian regime."

Lately, Dr. Mann has been working to design chips to run some of the algorithms he has developed for his wearcam so they will be cheaply and readily available.

His most important innovation is a mathematical technique that he calls video orbits. The images that flow through Dr. Mann's eyetap camera are used to record his moving gaze. The video orbit algorithm takes these images and automatically pastes them together to make a seamless whole. By pasting together many overlapping images, the low- resolution camera behind the glasses effectively becomes a camera of far higher resolution.

More important, the video orbit algorithm tracks head movement well enough to allow the computer to superimpose a fixed display on the world. So, equipped with face recognition software, the display could attach name tags to faces. Or when the software recognizes an advertisement that had been earlier placed in a "kill file," it can superimpose something more suitable.

This technology also makes it possible for Dr. Mann's wife to "accompany" him (via the Internet) when he is shopping. "My wife can pop inside my head," he said. "And if I reach for whole milk, she can draw an X through it to give me a hint or suggestion. If she thinks a used car salesman is lying to me, she could draw a Pinocchio nose on him."

The algorithm now runs on a rather bulky computer that barely fits the description of portable. But Dr. Mann predicts that within a year it will run on a custom chip that can be implanted in the eyetap glasses.

He expects wearable computers to become common in 10 years. "I just see it as the thing that replaces all the things you normally carry around: cellphones, pagers, wristwatches, all those things become subsumed into one item," he said.

Dr. Mann is less fascinated by the equipment than with the changes it represents. "People are amazed that I can read my e-mail while I'm walking down the street," he said. "Big deal - it's just a side effect of having a computer blasting right into your eye. The real powerful thing is that you're experiencing the world through it, and that's something none of the other things that we carry do. It's allowing a new form of communication, namely communication by modifying the visual perception of reality."