From Story: Substance, Structure, Style and the principles of screenwriting by Robert McKee:
In 1965 Ingmar Bergman contracted labryinthitis, a viral infection of the inner ear that keeps its victims in a ceaslessly swirling vertigo even while sleeping. For weeks Bergman was bedridden, his head in a brace, trying to keep vertigo at bay by staring at a spot his doctor had painted on the ceiling, but with each glance away the room spun like a whirligig. Concentrating on the spot, he began to imagine two faces intermingled. Days later, as he recovered, he glanced through a window and saw a nurse and patient sitting comparing hands. Those images, the nurse/patient relationship and merging faces, were the genesis for Bergman’s masterpeice PERSONA.