An elderly woman named Rosalie was sitting in …

An elderly woman named Rosalie was sitting in her nursing home when her room suddenly burst to life with twirling fabrics.

Through the elaborate drapings, she could make out animals, children and costumed characters. Rosalie was alarmed, not by the intrusion, but because she knew this entourage was an extremely detailed hallucination. Her cognitive function was excellent, and she had not taken any medications that might cause hallucinations. Strangest of all, had a real-life crowd of circus performers burst into her room, she wouldn’t have been able to see them: she was completely blind.

Rosalie had developed a condition known as Charles Bonnet Syndrome, in which patients with either impaired vision or total blindness suddenly hallucinate whole scenes in vivid color. These hallucinations appear suddenly, and can last for mere minutes or recur for years. We still don’t fully understand what causes them to come and go, or why certain patients develop them when others don’t.

For more on the science and research of hallucinations, check out the TED-Ed Lesson What causes hallucinations? – Elizabeth Cox

Animation by Nerdo