"I know it's the right side," says composer Phillip Glass, "because my wife is a doctor, and she told me about it. Sometimes we ask one another, 'Am I talking to the right side or the left side of your brain?"

The leading composer and performer of the newest wave in music is speculating on which side of the human brain is affected by his work. He thinks it is and should be the right side, which is sensual and intuitive, rather than the left, which is intellectual.

Glass, 43, was in town with his ensemble (their first visit to Washington in six years) for a concert last night at the Old Pension Building and a lecture and video program at 1 today at d.c. space. They brought along two tons of electronic equipment. If that makes him sound like a rock musician, the impression is not entirely wrong. His audience is partly a rock audience, both in numbers and tastes. The sound level of his music is what you expect at a rock concert, and the fees are the kind paid to fairly successful rock performers (or very successful classical performers): $2,000 for a solo appearance by Glass with a Yamaha organ; $4,500 if he brings along his six colleagues who perform on flute, saxophone, voice, organ and amplifiers.

The music is something else, and if you ask whether it is popular or classical, the most realistic answer is: "Yes." It also has some affinities with the music of Ravi Shankar, with whom Glass has collaborated.

It is not hard to grasp; if anything, it seems oversimple on first hearing. Small motifs are repeated almost endlessly, so that the music seems to hang motionlessly; then the structure changes slightly, or sometimes it may be only the listener's perception that changes. It functions on a time scale enormously different from that of most other Western music, growing slowly through gradual additions to the texture. Sometimes it is called "pulse music," because of its emphasis on rhythmic structures.

Whatever you call it (and Glass isn't sure himself), it is reaching a large and growing audience. His first opera, "Einstein on the Beach," had two standing-room New York performances at the Met in 1976. Some of the patrons left before the 4 1/2-hour opera really got rolling, but they were met by hundreds of eagar fans, standing outside and waiting to beg for ticket stubs.

Glass has had all sorts of adjectives thrown at his music. "I don't like to hear it called 'trance music,' that sounds kind of druggy," he says. "It bothers me less to hear it called 'minimal music.' The music I was writing from 1965 to '75 could be called minimal, but now it has expanded into too rich a vocabulary for that term.

"At the outset, it seems to be a very visceral kind of music -- it has a steady pulse, high amplification and a gross similarity to more popular kinds of music. If you want to look at it in its own terms, there are a lot of mathematical and structural refinements and elaborations, but you don't have to be able to read the recipe to enjoy the meal.

"Some new-music people see what I am doing as an alternative language -- a radical alternative to Berio, Crumb and Stockhausen," Glass says. "At the beginning, my work polarized the new-music audience, and I was vilified in academeic circles. I am still vilified in some academic circles, but it's beginning to change. This year, I'm playing on campuses for the first time. What's happened is that audiences have broadened their ideas of what music is."

Glass is interested in the suggestion that his music is directed at the non-intellectual right side of the brain. "Couldn't we say that a lot of the trouble with avant-garde music in the '60s was that it was written for the wrong side of the brain? Composers like Irving Babbitt writing music for composers like Irving Babbit."

Glass spent some time writing for the "wrong side of the brain" before going through a crisis and a complete change of style in his late 20s. "I had a strong academic background in flute, piano and composition at Peabody and Juilliard. I guess my ambition was to be Elliott Carter II, and I had a lot of academic music being performed and more than 20 pieces published," he says. "I was very proud. I didn't realize that I would throw them all away when I was 25. Then it became impossible for me to compose any more in that style -- not for lack of ability but for lack of belief. I could no longer hide in a corner and pretend that even if people weren't ready to listen to my music now, they would be listening to it in 50 or 60 years."

His solution, he says, was "to change the rules. I became familiar with non-Western music from kIndia, Japan, Africa, and around 1965 I began to borrow some of its principles and work them into a new musical vocabulary. When I discovered that people weren't going to perform it. I became a performer. Performing my own music has been decisive for me. I like the audience feedback, and I like to go into a town and see what else is happening musically. I wouldn't be happy unless I could measure my music against everything else that's around -- find myself in competition with 'Rigoletto,' for example."

His third opera has been commissioned for production in Stuttgart in 1983. It is based on the life of the Pharaoh Akhenaten, whom Glass calls "a terrific character." The second, "Satyagraha" (commissioned for the city of Rotterdam), was composed last year and is now touring Europe in a Netherlands Opera production. It is based on the life of Gandhi and has a libretto in Sanskrit. "The audience doesn't understand it," he says, "but the audience isn't supposed to understand it.

"I'm coming out of a non-literary theater tradition, and I don't tell stories in the usual way. At bottom, music is not really a narrative structure, and I think we're coming to understand that. My music is more like a painting than a story."