What Is the Head-Twitch Response?

How do you know if a mouse is on a psychedelic? It might just shake its head. This behavior, known as the head-twitch response, signals whether a molecule is a psychedelic-like hallucinogen. Head twitch is one tool used by pharmaceutical companies racing to develop new mental health drugs. But does a mouse experience an altered state like a human? Do animals have mystical experiences? And what can we really learn about the brain, or even human consciousness, from these unwitting rodent psychonauts?