There's a word for when a band takes a scrap of music and repeats it over and over with little or no variation, and even though Mogwai's end product bears little or no resemblance to the common understanding of the term ''groove," the Scottish band's performance Sunday at Avalon caught the group laying down one after another. But instead of aiming the grooves at listeners' feet and hips, Mogwai targeted the brain, and the result was mood-altering and hypnotic.
With drumbeats, spindly, interlocking guitar lines, and the occasional icy keyboards combining to generate a knotty heft, the mostly instrumental band hardly needed vocals to push the songs forward. Instead, it was repetition and dynamics that built and maintained momentum. ''May Nothing But Happiness Come Through Your Door" began with a simple, delicate guitar figure doubling back on itself, with a middle section punctuated by stabs of much louder guitar that always withered away after a few bars before starting again.
Other songs varied the volume. ''Ithica 27-9" shifted from soft to loud to pulverizing and then back again, and ''Glasgow Mega-snake" ended the main set by swelling into an overpowering, almost visceral noise before stopping dead, as though a switch had been thrown.
The few songs that featured vocals treated them as little more than instruments added to the mix, with keyboardist Barry Burns's processed quaver in ''Hunted by a Freak" and Stuart Braithwaite's dispassionate drone in ''Acid Food."
Opening act Torche played grinding metal at ear-crushing volume. The Miami band's guitars, tuned well below standard pitch, served as percussive companions to the pounding of the drums.