The film never had an original prologue. It was added when the film was sold to TV and a further few minutes were required to pad out the running time. Only Luana Anders from the original cast was available so an extra scene of her in a madhouse was filmed and tacked on to the beginning. This scene does not really tie in with the rest of the film.
Actor John Kerr was worried about being strapped down to the table with the pendulum above him for the movie's climax. In order to demonstrate that it was perfectly safe, director Roger Corman stood in for Kerr while the scene was being set up.