Somewhere between the metal aggression of Black Sabbath and the guitar grind of the Swans, Zeni Geva was the rock equivalent of opera, a full-throttle exploration of the emotional spectrum's dark side. Long hair flying and vocals growling, guitarist and vocalist Kazuyuki Ishino, a k a K.K. Null, channeled the group's heavy energy into performances that were dynamic and brutally physical. For many people, though they were also intimidating.

"It wasn't my intention," says Null of the performances. "I have to be intense and aggressive when I'm playing, but afterward, I'm smiling."

Zeni Geva, one of Japan's more successful independent rock exports -- with former Hanatarashi drummer, Taketani and ex-Boredoms member Mitsuru Tabata on guitar -- toured the world, put out loads of records, and played with many of the most important musicians on the avant-rock scene. Despite critical acclaim and cult status both at home and abroad, the rock life began to wear thin.

"I was sick of just being a rock musician, and just repeating the same thing: making records and touring," he says. "I lost the inspiration."

The long hair is gone now, replaced by a skinhead. It's more monk than punk, however.

"Another reason I got sick of playing rock music was that it was too human. The lyrics for Zeni Geva had too much emotion: anger, despair, aggression, love and hate. I stopped listening to rock music or anything with a human voice."

Null's desire to step back from the rigors of rock coincided with business difficulties for their label, U.S.-based Alternative Tentacles (of the Dead Kennedys fame). A new record to be produced by Steve Albini, producer of the Pixies, Nirvana and a slew of more challenging though less well-known groups, has been put on permanent hold.

The break has given Null the opportunity to move beyond the strictures of the rock idiom into an exploration of the less-constrained world of "improvised music." In Gugan, his duo with fellow Zeni Geva member Tanaka, he mixes the theremin and guitar. A prolific creator, he has also produced numerous albums in the past year that document his solo sonic experiments.

Unlike Zeni Geva, his solo work is calm and emotionally inert. On "Inorganic Orgasm," he constructs textural, droning washes of guitar sound that, unlike a lot of music in this genre, can actually be listened to at length without pain or the feeling that one must persevere it because it's art. It is very close to being, as Null describes it: relaxing.

"The Extasy of Zero-G Sex" is slightly more challenging. Buzzing white noise dissolves into rhythmic patterns that could be either tribal or digital.

Like a label on a package of organic vegetables, Null's solo CDs are clearly marked "analog only." They sound much like the noise/ambient releases at the fringe of electronica, yet are produced only with guitars and Null's arsenal of effects dubbed Nullsonic.

"I want to show that you can do this kind of thing without a computer. People can get a computer so easily and use it easily too, but they don't use their imagination because the computer can do so many things for you. Just touch the keyboard. But I want to use my head and my imagination; it's more interesting."

Recently the heavens have been something of a consuming interest for Null, who spends much of his reading time perusing tomes on astronomy and astrophysics. It's part of his continuing obsession with the dichotomy between physicality and the "purity and neutrality" of inorganic matter. Hence the titles of his recent albums, which couple sexual allusions with the scientific, or that of his upcoming release, "0.0004," the diameter of the cosmos directly after the Big Bang.

"Really I want to be a star. This kind of music is magic to me, a possibility to experience another world. When I am creating or listening to music, I can stop being human," he says.

An ambivalent relationship to physicality goes back to the early '80s when Null studied in butoh dancer Min Tanaka's body workshops, an experience that would influence his aggressive, physical performances with Zeni Geva.

More recently, he has collaborated with musicians such as Polish artist Zbigniew Karkowski's project Sensorband, who seek to resurrect the physical aspects of music-making while still using a computer. Connecting computer sensors to metal bars that can then be struck to cause the computer to produce sound, Karkowski has integrated physical movement into computer music.

Though Null is continuing his solo career with vigor, recently playing solo shows in Australia, including a few gigs as the warm-up act for Sonic Youth, and an upcoming sound festival in Los Angeles in May, his recent forays into other genres outside rock have reignited his enthusiasm for Zeni Geva.

"When I stopped Zeni Geva, I had lost my passion. Since then, I've been concentrating on my solo stuff, playing with many different people from many different genres, and listening to a lot of different music," he says. "Actually at the moment I'm into salsa music, so Zeni Geva will play salsa. Maybe."