A couple of years ago, my best friend in Silicon Valley used his personal computer to turn himself into a woman.

"I was signed on to a bunch of {computer} bulletin boards trying to talk with people, but without too much success," said Rob, describing the telephone networks that link computer users together.

As a lark, Rob signed on to the system as Susan B. (not his real pseudonym) and "all of a sudden, I'm the most popular person on the network. ... I made up a whole character: a 22-year-old Stanford coed majoring in English. When somebody asked me if I was cute, I typed, 'Everybody's cute in 40 characters,' " the alphanumeric screen size of the Commodore 64 computer.

This digital masquerade went on for several weeks as Susan became, to use Rob's word, a teleflirt. "I found out a lot; the bulletin boards are the equivalent of a bar," Rob recalled. "There are a lot of lonely, unhappy people on line, and I didn't want to become their fantasy figure. When a guy asked me for my address to send me a valentine, I knew I had gone too far."

The network as gender-bender? My buddy as a computational Christine Jorgensen? Well, not necessarily ... but why not?

On some of these networks, you can wear any mask you want. Allucquere Stone, a sociologist at the University of California-San Diego, tells of a quadriplegic woman on a computer network who could communicate on line by tapping out the alphabet with her headstick. Her courage and strength -- not to mention her advice -- was a source of healing and inspiration to other women on the network. Except that this network-eloquent quadriplegic woman was really a male psychiatrist. "Women who had been healed by {this person} actually recanted their cures," Stone says.

The issue isn't just gender and deception. That's just the tease to lure you into a very sticky web of questions spun by our expanding repertoire of multimedia, the rise of networks and the fragmentary nature of time and bandwidth (the range of frequencies, or room, needed to communicate).

These questions strike at the heart of who we are and who we think we are. Do we use media to express ourselves? Or do we use these media as tools to construct our personalities? Maybe the medium isn't just the message; maybe media are the Tinkertoys of personality. Some people take great care in recording their answering machine message to convey the full flavor of their interest. Other people couldn't care less.

Who doesn't have several telephone conversational styles depending on who's on the other end and what the conversation requires? Do you handwrite fax notes or insist that they be typed? Would you rather send an electronic mail message or leave a voice-mail message? What do those choices reveal? What do you want them to conceal? Are those decisions being made consciously or automatically? How do the constraints of the medium -- the absence of visual cues, for example -- shape the way we use it?

"We're all 'bandwidth structuralists,' " says Stone, who has written such papers as "Will the Real Body Please Stand Up?: Boundary, Interface and Possession." She argues that we use media to construct and reconstruct ourselves.

With computers, video, audio and telecommunications networks all breeding and interconnecting, we have more opportunities to define and project ourselves in different ways. (Even today, women occasionally use initials on those old-fashioned corporate memos to conceal their gender from chauvinistic senior managers.) Who doesn't have a friend who's more articulate on the phone than in person? Today's computer networks have limited bandwidth, so it's easy to construct a mask of text that conceals age and gender.

Phone mail allows inflections and nuance that convey far more than content. Increasingly, the personalities we present to the world are bandwidth-driven and bandwidth-constrained. This isn't McLuhanacy -- this is how we use state-of-the-art technology to manufacture ourselves.

"We've always worn masks," says Paul Saffo, a research fellow at the Menlo Park-based Institute for the Future. "And now there are new electronic masks that people can play with ... but we haven't figured out the right roles yet. There is the potential for a Wizard of Oz syndrome ... building electronic facades."

Indeed, even in the future, when high-definition video-conferencing overwhelms text-based computer conferencing, Saffo insists, "there's no question that people will control their own screen personae ... there will be a privacy switch that will show only a photo."

Jaron Lanier, a pioneer in using technology to create "virtual realities" that enable people to play any one of a number of roles in simulated environments, flatly asserts: "The definition of yourself in the virtual world is fuzzy because you're so redesignable." It's the postmodern technological definition of the self-made man.

This has enormous implications. If we use media to construct personalities, then we can use media to deconstruct personalities as well (just as semioticians like Derrida, Foucault and DeMan have deconstructed philosophical and literary texts to determine their underlying influences). Maybe therapists should pay as much attention to the media that people use to express and construct themselves as they do to family background. In our own personal lives and certainly in our professional lives, we are going to have to deconstruct the multimedia personalities we face.

Silly? OK, who wouldn't look twice at a man wearing a dress to a company picnic or a woman in a sarong at a management committee? In this so-called Information Age, why won't people make judgments based on media styles as readily as we do today based on clothes and fashion?

In fits and starts, we're growing to recognize that multiple media often mean multiple personalities. That can be true in the (ominous) Jekyll & Hyde, "Three Faces of Eve" and Sibyl sense. Technology can breed fragmentation and schizophrenia.

Conversely, it can be a medium to better manage how we interact with both the real world and the virtual world. The power to use technology to construct the masks that we show the world should never be underestimated -- because we often become the masks we wear. Michael Schrage is a columnist for the Los Angeles Times.