“The Great Heart Split,” writer Gail Godwin calls it: that moment, about 400 years ago, when our knowledge of how the physical heart works leaped forward, sending ancient beliefs about the heart as spiritual headquarters backward, to be filed under folklore and mythology. News flash: the powerful, tangible pumping of the heart is what keeps our bodies alive. The heart’s emotional value, its mystical properties? Not actually located in the center of our chests. Ever since, rational knowledge has trumped what used to be called, simply, heart. And then December comes along, and people start doing things that make no sense. We string colored lights from rooftops and balconies. We feverishly bake cookies, as if eating sweets mattered more than eating anything else. And, strangest of all, we cut trees and prop them up in basins of water in our living rooms. Even scientists and doctors do these things. And the scientists and doctors who study the brain—that mysterious organ where the intangible version of the heart has been hiding all along—they know that the protean behavior in which we indulge during this strange season called the Holidays can be both wonderful and awful for our brains, often at the very same time. From Thanksgiving through New Year’s Day, our hearts and heads are bombarded with memories. Many are good. Some are not. If you’ve lost someone who used to be a big part of your holiday season, you’ll be feeling that pain. If you have a family member or two who ever excelled in causing [...]