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Two eyes, aligned horizontally, above a nose, above a mouth. These are the basic elements of a face, as your brain knows quite well. Within about 200 milliseconds of seeing a picture, the brain can decide whether it’s a face or some other object. It can detect subtle differences between faces, too — walking around at my family reunion, for example, many faces look similar, and yet I can easily distinguish Sue from Ann from Pam. Our fascination with faces exists, to some extent, on the day we’re born. Studies of newborn babies have shown that they prefer to look at face-like pictures. A 1999 study showed, for example, that babies prefer a crude drawing of a lightbulb “head” with squares for its eyes and nose compared with the same drawing with the nose above the eyes.

Two new studies tried to get at this brain biology with the help of a rare group of participants: children who were born with dense cataracts in their eyes, preventing them from receiving early visual input, and who then, years later, underwent corrective surgery. After recording the brain waves of these children with electro- encephalography (EEG), the researchers suggest that there is a “sensitive period” in brain development for face perception — a window of time during the first two months of life in which the brain requires visual input in order to fully acquire the skill. If the brain doesn’t get this input, it can still learn the crude aspects of face processing — identifying a face as a face, for example — but lacks the fine-tuning ability of distinguishing one face from another. These differences show up not only in the patients’ behaviors, but in their brain waves.

None of the patients, even those who were blind for years before having surgery, had any trouble distinguishing faces from houses. But the way their brains performed the task was different. Whereas healthy controls only elicited the N170 marker after seeing faces, the patients showed it after seeing any kind of visual stimuli. This makes sense given what we know about early brain development, Röder says. “We are born with a lot of connections in the brain, and these connections are pruned down to 50 percent of the original number,” she says. “This pruning makes a functionally specialized system. It requires input during a particular phase of life, and it seems not to have taken place in these patients.” What’s more, she says, these deficits seem to persist for a long time, maybe forever. “Some of the individuals we’ve studied have been seen for more than 20 years, and they didn’t show this face sensitive response.”