I think this effect was pioneered in Jacob's Ladder, which is a great film if you haven't seen it! The director was originally inspired by Francis Bacon's paintings, which you might study too for inspiration on your picture. ('Heads' I presume)
In the DVD commentary he mentions that because of the extreme motion blur and distortion etc, it's hard to make out if it's human or not, and gets a lot of it's horror from that sort of uncertainty.