Rebecca Saxe’s first son, Arthur, was just a month old when he first entered the bore of an MRI machine to have his brain scanned. Saxe, a cognitive scientist at the Massachusetts Institute of Technology, went headfirst with him: Lying uncomfortably on her stomach, her face near his diaper, she stroked and soothed him as the three-tesla magnet whirred around them. Arthur, unfazed, promptly fell asleep.All parents wonder what’s going on inside their baby’s mind; few have the means to find out. When Saxe got pregnant, she’d already been working with colleagues for years to devise a setup to image brain activity in babies. But her due date in September 2013 put an impetus on getting everything ready. Over the past couple of decades, researchers like Saxe have used functional MRI to study brain activity in adults and children. But fMRI, like a 19th-century daguerreotype, requires subjects to lie perfectly still lest the image become hopelessly blurred. Babies are jittering bundles of motion when not asleep, and they can’t be cajoled or bribed into stillness. The few fMRI studies done on babies to date mostly focused on playing sounds to them while they slept.