Take the now-famous seagull experiment. When a mother seagull brings her chicks food, the babies peck at her beak, which has a red dot on it. Chicks will peck just as much at both the disembodied head ...
Sean Scully, "Heart of Darkness" (1982), oil on canvas, 8 x 12 feet. Art Institute of Chicago, Gift of Society for Contemporary Art (image courtesy the Art Institute ...