That familiar feeling comes from deep in the brain

It’s happened to all of us at one time or another: You’re walking through a crowd, and suddenly a face seems incredibly familiar — so much so that you do a double-take. Who is that? How do you know them? You have no idea, but something about their face nags at you. You know you’ve seen it before.

The reason you know that face is in part because of your perirhinal cortex. This is an area of the brain that helps us to determine familiarity, or whether we have seen an object before. A new study of brain cells in this area finds that firing these neurons at one frequency makes the brain treat novel images as old hat. But firing these same neurons at another frequency can make the old new again.

“Novelty and familiarity are both really important,” says study coauthor Rebecca Burwell, a neuroscientist at Brown University in Providence, R.I. “They are important for learning and memory and decision making.” Finding a cache of food and knowing it is new could be useful for an animal’s future. So is recognizing a familiar place where the pickings were good in the past.

But knowing that something is familiar is not quite the same thing as knowing what that thing is. “You’re in a crowd and you see a familiar face, and there’s a feeling,” Burwell explains. “You can’t identify them, you don’t know where you met them, but there’s a sense of familiarity.” It’s different from recalling where you met the person, or even who the person is. This is a sense at the base of memory. And while scientists knew the perirhinal cortex was involved in this sense of familiarity, how that feeling of new or old was coded in the brain wasn’t fully understood.

When exposed to something new, groups of brain cells in the perirhinal cortex fire quickly, at a rate of about 30 pulses per second, or hertz. As the object gets familiar, this oscillation — or pattern of firing from a group of brain cells — decreases, to around 11 Hz. To investigate just how this change affects an animal’s sense of novelty, Burwell and her colleagues infected brain cells in rats’ perirhinal cortex with a virus containing a light-activated channel. When the cells were exposed to blue light, they would fire. The frequency of the blue light stimulation determined how quickly the brain cells fired.

Then, the researchers took advantage of rats’ natural tendency to look longer at novel objects, and showed them images of, say, cloverleaves, swans or bunnies. In the presence of a familiar bunny, the rats spent less time looking at the picture. After all, they had seen that animal before.

But when the scientists stimulated the rats’ brains at 30 Hz, the rats looked longer at the bunny, as if seeing it for the first time. Conversely, when presented with a new image, such as a cloverleaf, the rats stared at it for longer than they gazed at a familiar swan shape. But when their perirhinal cortex was stimulated at 11 Hz, the rats didn’t find cloverleaves so fascinating. The new object had suddenly become familiar, Burwell and her colleagues report September 30 in the Journal of Neuroscience.

The results suggest that novelty and familiarity are two sides of the same brain cells. Turn them down, and even the new is boring and old. Turn them up and the old is new again. Many other studies have associated these oscillations with specific kinds of recognition memory, says Howard Eichenbaum, a neuroscientist at Boston University. But this study goes beyond association, he notes, and actually gets at a cause, showing that specific frequencies cause rats to treat an object as new or old.

“It’s a very neat paper,” says John Aggleton, a neuroscientist at Cardiff University in Wales. “And there’s a parallel to what’s going on in humans. We know in fMRI, if you repeat [images] you get decreased activity in the perirhinal cortex, and we know in animals if you look at expression of genes you get a decrease in activity. All of these come together in a very neat way.” He says this study indicates the signals of activity are “not a by-product, but a key component” of how we process the old and the new.

So the next time you see a face in the crowd, and you know that person is familiar, thank the brain cells in your perirhinal cortex. But when you can’t remember their name or how know you them? Well, I’m afraid the rest of your brain is to blame.