By BEN SISARIO

KINGSTON, New York - It was not your typical jam session, even for Mercury Rev.
The guitarist and the keyboardist played moody atmospherics, and the singer extemporized at a microphone. But next to them in a dark rehearsal room in this small Hudson Valley city stood a guest performer with electrodes hooked up to his head, staring intently at a computer readout of his brain waves.
“This is the sound of the brain at work,” said the singer, Jonathan Donahue, as a deep, stuttering fuzz bounced back and forth between the speakers. “Hundreds of billions of neurons firing or not firing, or firing and then not firing.”
Over two decades Mercury Rev has become an underground favorite with a playful and surrealistic take on psychedelic rock, but now the band is veering into science. Or at least a kind of sciencefiction music, as it collaborates with a Brooklyn artist, Alex Chechile, on altering music by using the brain’s electronic signals.
Mr. Chechile has made an improvised electroencephalograph, or EEG, along with a computer program that he says can be used to modify music in real time according to his own brain activity. When the band plays, Mr. Chechile and his device, part of which is in a plastic food container , act as a kind of effects pedal for the sound, altering pitch, tone, volume and various other aspects. In turn, the band members react to the way their music is changing, rendering the music-making process something of a random, invisible chase.
The rock band was rehearsing in a small former television studio in Kingston, where Mercury Rev has been based since the late 1990s. Using the device is helping the band’s music live up “to the potential that I always thought rock ‘n’ roll was,” Mr. Donahue said, adding, “It’s a process, not an end state where you say, ‘Oh, we reached Chuck Berryland, let’s go to sleep.’ It’s a process, and you are the process.”
The band has evolved in sometimes surprising ways from its noisy first album, “Yerself Is Steam” (1991), to the rustic “Deserter’s Songs” (1998), building an avid but small cult of fans.
The members of Mercury Rev say that constant reinvention is necessary to keep their creative juices fresh . After a string of relatively stripped-down and linear albums, the band’s new release, “Snowflake Midnight,” is awash in bubbling electronics .
During the rehearsal with the brainwave machine, Mr. Donahue picked up his laptop and loaded one of the programs used to record the new albums. He described a sound-manipulation process perhaps even harder to grasp than the brain-wave device. This one, he said, alters sound based on images of demons and witches. When asked how it worked, he grinned widely.
“I don’t know,” he said, “and that’s kind of the fun.”