In the climax of Star Wars: Return of the Jedi Darth Vader throws Emperor Palpatine over a precipice and just as he does so, a subliminal image of a skull is flashed on the screen over Vader's face. The image only appears for a fraction of a second:
Has this subliminal image been explained by any of the makers of film as to its origin or purpose?
I know that in the 1960s it was considered cool to insert single stills into movies that were either pornographic or images of death, like skulls. Is this some kind of homage to that phenomena by Lucas?