COMPOSER Barry Vercoe sits at a keyboard, as composers have been doing as long as keyboards have existed. But there is a difference.

In the Experimental Music Studio Technology, where Vercoe is the director, the keyboard is attached to a computer and the computer is attached to a video-display screen. When Vercoe hits a C-sharp, he produces not a piano tone but a C-sharp neatly printed on a staff and displayed on the screen.

Vercoe can type in a melody and its harmonies, a short motif, a six-part fugue or a whole sonata. And when he asks the computer, it will play it back as instructed -- sounding like an orchestra, a piano, or no musical instrument ever before played by human hands.

What Vercoe has developed is essentially a communication system. His piano-style keyboard allows composers to use a language they understand and translate it into a language the computer -- a sort of ultimate musical instrument -- can use. It gives the composer, in effect, a resident orchestra that will allow him to hear his music instantly, make changes immediately and play them back, store the work in its memory and print out a copy for reference. It eliminates the routine drudgery of composing by automating it, and enormously enhances the composer's control of his material.

Vercoe's score-editor exemplifies the encounter between art and technology, which is happening in many places but with special intensity at MIT, one of the few institutions in the world that have both the resources and the interest to promote such interaction on a grand scale and in many disciplines. Ground will be broken later this year for a new, $15-million center for arts and media technology where the encounter will certainly be intensified.

"At any given time," says MIT president Jerome Wiesner, "about half of our students are active in photography, music or another artistic discipline. I don't know of any other school, except art schools, that can say that."

Until the new complex is completed (probably in 1983), these students and the faculty members who are finding artistic potential in everything from computers to laser beams are scattered over acres of campus.

In the Center for Advanced Visual Studies, artist Harriet Casdin-Silver pushes a button and an image appears in mid-air. It looks solid, and if you take a few steps to the side, you can see it from another angle. But if you reach out to touch it, there is nothing there.

What Casdin-Silver does is called "holography," and it is hard to find a more usable word in everyday English. She is a sculptor, because her work has three dimensions, although it is not solid. She is photographer, because her art is produced by light's impact on a sensitized plate, although the image, suspended in mid-air like ectoplasm, is not what you would usually call a photograph. She is certainly an artist, although he work is in a high-technology field and her shop-talk can rapidly become very technical, because the images that spring into the air when she pushes a button are art: an abstraction, a still-life, a small, three-dimensional image of a women model. Each has an emotional impact; each embodies an artist's vision.

Nicholas Negroponte sits in a small room that is really a computer terminal. The room is programmed to recognize his voice and follow spoken instructions. One wall is a giant video screen, and on that screen Negroponte can summon up nearly any image he wants -- a page of a favorite book, a map of Denver or a detailed picture of any point on that map, a Rembrandt reproduced in meticulous detail and larger than life, a custom-made television drama. He can screen the Mona Lisa and tell the computer to add a moustache. Or if he doesn't like the plot of a story on the screen, he can change it.

He looks at a video screen where a student is examining a composite image of a city street, partly based on stored photographic data and partly constructed by a computer according to instructions. He is talking about his program on how to repair a bicycle.

"The computer is given information on the user. It knows that you are left-handed and what tools you have available, and it knows that you have experience repairing a brake, so it doesn't have to tell you so much about that. But you have never repaired a seat and need detailed instructions. With this kind of data, it can select from all the information it has to tell this person how to repair this bicycle using this equipment.

"On the same principle, you can have a television show suited to your tastes: If you want a blonde heroine, she is a blonde. If you want a happy ending, you get one. And if you want to write yourself in as the hero, you can do that, too."

Richard Leacock's film studio lies on the outer fringes of the sprawling MIT campus, beyond a building now occupied by Polaroid and scheduled for demolition in the next few years when construction begins on the center for arts and media technology. Leacock closes a cabinet full of super-8 equipment that he played a large part in developing, walks through an editing room where a student is busily at work, takes a seat in the video room and looks with mingled love and exasperation at the small screen that can be fed either by tape or by disc.

"I've decided that TV is here to stay," he says, "and whether we like it or not, we had better see what can be done with it."

The real problem, he says, is that "the networks and I seldom agree on what's worth watching."

His proposed solution is "a national video jukebox."

Leacock was one of the leaders in developing the hand-held camera with synchronous sound, and thus a father of the "cinema verite" school of movie-making. His own credits range from "Monterey Pop" to "Hidden Empire," an Emmy-winning film about the Ku Klux Klan. One of his specialties, before he joined the MIT faculty, was musical documentaries on such subjects as Casals, Cliburn, Bernstein and Stravinsky.

He has much use for the media, little use for the mass media. "I used to work for a show called 'Omnibus,' which was considered a failure because it could only attract 9 million viewers," he recalls. "Where would the book publishing industry be, if a book that only sold 9 million copies was a failure?"

The answer, he feels, is not to damn the media but to use them and recognize their special potentials: "I remember a documentary film I made about Leonard Bernstein. He was conducting the Beethoven concerto with Isaac Stern, and Stern was playing the first-movement cadenza with his eyes closed -- in a world all his own. When he reached the end of the cadenza, he gave Bernstein the cue with his left eyelid. You can't catch a detail like that far out in the audience, but a video camera can -- and I don't know why anyone should have to miss it."

Leacock's dream of a national video jukebox -- a system of home terminals wired to a central storage, allowing the customer to dial any material he wants when he wants it -- interacts neatly with Negroponte's. They see the video disc as a publishing medium (the most revolutionary since Gutenberg's press) as well as a way of storing data -- or works of art -- and they anticipate a time when computer memory (now rapidly being expanded through chip technology) will be abundant and cheap -- perhaps free for owners of terminals. This capacity will allow the storage of material for all kinds of specialized tastes.

"A very important audience is being largely ignored by the large television organizations -- for their own good reasons. I don't know how large this audience is, but it had to be millions. I happen to love the films of Luis Bunuel. I would like to own them all, or at least be able to see them when I like. But unless you happen to live in New York or one or two other large cities, the odds are overwhelmingly against your ever being able to see a single Bunuel film." The answer, he feels, is video disc or the national video jukebox, either of which can reach a worthwhile potential audience spread thinly over a whole continent.

He puts a video disc on and starts it in the quick-scanning mode; detached, still images leap to the screen, linger a moment and give way to the next. "I have a video disc of 'Smokey and the Bandit' -- a film that does not interest me greatly," he explains, "except that its cast includes Jackie Gleason, whom I like to see. On disc, I can scan through the picture at a rate of 20 seconds for every half-hour, and when I see Gleason on the screen, I can slow down."

Not slowing down at all, he talks about the greater publishing flexibility offered by the disc (or computer memory) as a storage medium: "As compared to a television special, which is the equivalent of a magazine article, we could use discs to produce the video equipment of a book. For example, I recall one documentary I worked on, 'Anatomy of a Murder,' which was produced as a two-hour television special. That was taken from 160 hours of material, and a lot of the material would be interesting and useful to serious students of law enforcement and judicial processes."

The same can be true of performing arts, he believes: "Our performing arts are locked into a format that consumes 1 1/2 to 2 hours, which is dictated by extraneous factors -- the availability of baby-sitters, working hours, transportation, etc. -- that have nothing to do with the needs to a work of art. What's wrong with a 45-second masterpiece? Why not have a film that takes as long to see as a novel takes to read -- you don't just sit down and read straight through 'A Tale of Two cities.' With a national video jukebox, you wouldn't have to watch it all at once, the way you do when you go out to the movies. You could call it up on your screen when you're ready for the next part, as you do with a book. And you could be going through four or five at the same time, the way we do with books."

After talking to some of the electronics wizards at MIT, one begins to wonder what will happen to old-fashioned books in the age when everything can be called up on a screen. Muriel Cooper of the Visible Language Workshop (alias graphics department) is ambivalently reassuring. "Paper has a future," she says. "I'm not so sure about ink." Something of what she means can be seen in the laboratories adjoining her office, where her associates are busily at work using computers to refine or modify visual images and to fix them on paper in a variety of ways, in which the use of ink is only one of many possibilities.

Across the street in the photography department, a once-revolutionary technology (including ink-free images on paper) has become a venerable tradition. Starr Ockenga, the head of the department, welcomes technological improvements that make the work of photography less of a bother, but approaches it basically as an art. "Engineers and physicists come in to learn how to take better pictures -- to get the technology of photography," she says, "and they learn that photography is a language, that they can say things through photography that cannot be said in any other way. We teach them the grammar, the syntax and vocabulary of that language. They add their own individuality, their style and vision."

Samples of that style and vision are displayed all around her. The department has an exhibit gallery as well as classrooms and laboratories, but at present it is located on the third floor of an old building, above a student gymnasium and the headquarters of the institute's security forces. It is crowded, not really designed for its present specialized purpose, and the long staircases with no elevators make access difficult for the handicapped.

The photography department will be better served in the new building, as will the institute's innovative art gallery, which mounts a dozen exhibitions per year -- often focusing on the relation between art and technology but venturing into many other themes and sometimes presenting artists at work. "We could show microphotography over and over again," says Kathy Halbreich, the projects director for the institute's committee on the visual arts, "but I don't know how much that will stretch people's minds."

When the new building opens in a few years, a remarkable variety of interests and skills will be assembled under one roof. People who are fascinated with nuts-and-bolts, with new ways of getting things done, will be side-by-side with artists seeking new tools to give form to their visions. "With five different groups interacting in visual studies besides the music people, one plus one will add up to more than two," predicts Starr Ockenga. "The possible interactions are unlimited and unpredictable, except that they are sure to happen."