In April, when Barack Obama announced the launch of the BRAIN Initiative, a well-funded drive to better understand the brain, some neuroscientists raised a sceptical eyebrow. The project’s fuzzy goals included mapping the activity of vast numbers of neurons in the brain as they fired—effectively, watching thoughts in real-time. Some people talked about mapping hundreds of thousands of neurons. Others spoke about recording activity from every single one—all 86 billion of them.

It seemed far-fetched. Today’s technology makes it difficult to record the activity of a handful of neurons, let alone thousands or billions. For example, you could use a technique called patch-clamping, which involves impaling neurons with microscopic electrodes. It’s arcane, cumbersome, very difficult to pull off