INTRO-- An exhibit showcasing the art of Yoko Ono opens this weekend at the Walker Art Center in Minneapolis. Ono, who is best known for her marriage to John Lennon, has pushed the boundaries of art during her 40-year career. The Walker exhibit features more than 150 pieces of her work, including many she did in collaboration with Lennon. Barbara Haskell is a curator at the Whitney Museum of American Art in New York, and co-author of a book about Yoko Ono's work. Haskell says Ono began in the early 1960s with performance art, which was cutting edge at the time.