Christopher Nolan’s 2010 blockbuster Inception is set in a distant future where military technology enables one to infiltrate and surreptitiously alter other people’s dreams. Leonardo Di Caprio plays Dom Cobb, an industrial spy tasked with planting an idea into the mind of a powerful businessman. The film has a complex, layered structure: Cobb and the other characters create dreams within dreams within dreams, but they cannot distinguish between reality and the dream states they fabricate.

Most of us distinguish between real and imagined events using unconscious processes to monitor the accuracy of our experiences. But these processes can break down in some psychiatric conditions. Patients with schizophrenia, for example, can experience auditory and visual hallucinations that they believe are real, while some brain damaged and delusional patients live in a world of perpetual false memories. Japanese researchers have developed an ‘Inception helmet