What is a memory? Science writer Jonah Lehrer tells us is it’s a physical thing in the brain… not some ephemeral flash. It’s a concrete thing made of matter. And NYU neuroscientist Joe LeDoux, who studies fear memories in rats, tells us how with a one shock, one tone, and one drug injection, you can bust up this piece of matter, and prevent a rat from every making a memory. LeDoux’s research goes sci-fi, when he and his colleague Karim Nader start trying to erase memories. And Nader applies this research to humans suffering from PTSD.