Dr. Meltzoff said that in a phenomenon called “cortical magnification,” the lips may be “overrepresented, there’s more neural tissue devoted to the baby’s lips than to hands and feet.” The cortical magnification may reflect the feeding the babies have already done, he said, but “it’s perfectly possible that for evolutionary reasons, baby humans have a very prominent lip representation because lips are used not only for survival but for emotional expressions and language.”

And the hands are going to be used to explore the world — these babies are still too young to be intentionally reaching and grabbing, but the hand is already well represented in the brain, which may be a precursor to the reaching behavior soon to develop.

In another study published earlier in 2018, 7-month-old babies were shown a film of somebody else’s hand being touched, and the hand areas of their brains became active. “Babies as young as 7 months are able to connect self and other,” Dr. Meltzoff said, and speculated on the implications of the baby understanding, “their hand is like your hand, their lips are like your lips; I am like you, you are like me.” This process in which the baby looks out at another person’s body and sees it as being “like me,” he said, could be an important foundation for social and cognitive development.

“Before humans have language, they have the language of touch, they communicate through the language of touch,” Dr. Meltzoff said. Parents understand the importance of skin-to-skin contact with babies, he said, and recognize both that it can calm the baby, and that it can be very rewarding for the parent. “There’s something like a touch hunger in young babies,” he said. And from touch comes important developmental stimulation, including a sense of the baby’s own body.

Dr. Saby, who is a postdoctoral fellow in radiology at the Children’s Hospital of Philadelphia, said she has been studying babies in the research lab for 10 years, but now with a 5-month-old of her own, “I’m finally seeing everything through the parent’s eyes.”

She finds herself feeding her baby, “thinking about every time I touch her hand, there’s activity sent to that part of her brain that represents her hand,” she said. “Constant stimulation going to her brain helping develop that area, helping their brains connect with their bodies, and eventually they’re going to be able to use those body parts for other things, not only motor things, but helping to understand other people’s body parts as well.”