During the 1920s-1930s, American artists traveled to Europe for inspiration. However, during WWII many European artists migrated to New York City, which became a center for art. This led to the development of the "New York School" style of abstract expressionism. Key movements within the New York School included action painting by Jackson Pollock and color field painting which used large fields of color. In the 1960s, new movements like pop art, conceptual art, and op art emerged, focusing on popular culture, ideas over objects, and optical illusions.