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We've all felt the tap to the soul you get from driving by your old high school or hearing a tune you once danced to. But why is that feeling so universal?

Nostalgia, a combination of the Greek words nostos (homecoming) and algos (pain), was a special type of homesickness associated with soldiers fighting far-off wars. Seventeenth century physicians worried such thoughts put health at risk. In the 19th century, doctors believed it could cause irregular heartbeat, fever, and death.

Our understanding of nostalgia has developed since then. "It's a very mixed emotion," says Frederick Barrett, a neuroscientist. That makes it hard to fit into existing theory, which typically categorizes emotions as either positive or negative. And triggers - the cars, music or smells - are extremely personal. Therefore. designing a standardized study is difficult.

But we do know nostalgia has a marked effect on us: brain imaging studies show that those experiences have their own neural signature. Neuroscientists argued that the emotion is co-produced by the brain's recall and reward systems. They found that nostalgic images use the memory-managing hippocampus (海马区) more than other sights, as people mine autobiographical (个人经历的) details deep in the past. This mental effort pays off:as the hippocampus activates, so does one of the brain's reward centers.

That longing for the past might be a protective mechanism, says Tim Wildschut, a professor. His work also suggests a more primitive purpose for the feeling: it developed to remind our ancient ancestors of pleasant physical feelings during periods of discomfort and pain.

Recent research suggests the occasional look backwards can give us a boost in unnoticeable ways: by increasing self-respect and protecting against depression. Nostalgia's apparent power to jump-start one's memory also seems to improve recall ability in people with Alzheimer's disease.

New flavors of "reminiscence therapy" (回忆疗法) are emerging around the world. In 2018, the George G. Glenner Alzheimer's Family Centers opened its first Town Square, an adult daycare facility designed to look like a small town in 1950s America. Though Town Square has yet to publish peer-reviewed data on the success of the program, clients say it has helped seniors access dusty memories and reconnect with loved ones.

Scientists need a lot more information to adequately characterize this complex and bittersweet feeling. But while centuries of doctors considered nostalgia a deadly disease, we now know: it can help us make it through today.