Films and videos began to be understood as artworks—or at least part(s) of them—in the 1950s, when artists such as Nam June Paik and Wolf Vostell began incorporating televisions into their works. In 1965, with the invention of the Sony Portapack, the ability to create moving images became much more accessible than ever before; subsequently, artistic experimentation with video increased markedly. During the 1960s and '70s, Steina and Woody Vasulka experimented early on with technical manipulations of the medium, and Vito Acconci, Bruce Nauman, and Richard Serra turned the camera on themselves, in many ways challenging the myths surrounding artists. Chris Burden also recorded a number of iconic performances involving self-inflicted violence, while artists such as Martha Rosler recorded videos that called into question the traditional roles of women. In recent years, a number of ambitious film projects have been realized, among them Matthew Barney's Cremaster Cycle, consisting of five elaborate feature-length films, and Christian Marclay's The Clock, a looping 24-hour montage of appropriated film clips.