Most of his best-known artworks are relatively close, but not exact, copies of comic book panels, a subject he largely abandoned in 1965. (He would occasionally incorporate comics into his work in different ways in later decades.) These panels were originally drawn by such comics artists as Jack Kirby and DC Comics artists Russ Heath, Tony Abruzzo, Irv Novick, and Jerry Grandenetti, who rarely received any credit. Jack Cowart, executive director of the Lichtenstein Foundation, contests the notion that Lichtenstein was a copyist, saying: "Roy's work was a wonderment of the graphic formulae and the codification of sentiment that had been worked out by others. The panels were changed in scale, color, treatment, and in their implications. There is no exact copy." However, some have been critical of Lichtenstein's use of comic-book imagery and art pieces, especially insofar as that use has been seen as endorsement of a patronizing view of comic by the art mainstream; noted comics author Art Spiegelman commented that "Lichtenstein did no more or less for comics than Andy Warhol did for soup."