When Mathew Wilson left Chicago three years ago, taking his wife and new baby to his native England, he thought he'd left his adopted city and his days as a performance artist behind for good. "I came to think that I had been deluding myself--surrounded by other artists deluding themselves--and that one day I'd grow up," he says. "When I had my kid I thought, that's it." But a year away from everything that had inspired a decade of convention-defying performance work was "like living in an oxygen-free environment. I realized I am what I am." So he charged back to Chicago to resume his self-described "bizarre life" as an impresario of poetic banality. Since then he's mounted a few small-scale pieces--last May, 15 people stood along LaSalle Street projecting amplified heartbeats through megaphones--but on Monday he'll stage one of his signature mob events. Conceived with sculptor Adam Brooks, Dropping 163 Pounds on Daley Plaza brings Wilson back to one of his favorite venues: 100 volunteers fell over and lay there as if dead for an hour in 1994. This time he and Brooks will assemble more than 100 performers in honor of Patriots' Day to execute one simple ensemble gesture. They won't say what, though they offer this tantalizing clue: according to their calculations, 163 pounds is the weight of the average American. Daley Plaza, 50 W. Washington. Monday, April 21, 9 AM.