Illustration: Pierre Le Tan
I'd barely crossed the threshold of middle age. As a journalist, I was invested in staying smart and quick, mistress of my good brain and sardonic tongue. But almost overnight, I found that I was missing critical information—the names of people and places, the titles of books and movies. Worse, I had the attention span of a flea. I was having trouble keeping track of my calendar, and my sense of direction had disappeared. The change was so dramatic that sometimes I felt foreign to myself.

Over the course of a few years, as friends and relatives moved into their 40s and 50s, I realized that I was part of a large group of people who were struggling to keep up. I was determined to find a plausible explanation for what was happening to my brain and, by extension, to middle-aged minds in general.

As a first step, I began to study and categorize midlife mental lapses as if they were so many butterflies. There was Colliding-Planets Syndrome, which occurs when you fail to grasp, until too late, that you've scheduled a child's orthodontist appointment in the suburbs for the same hour as a business meeting in the city. Quick-Who-Is-She Dysfunction surfaces when you are face-to-face with someone whose name stubbornly refuses to come to mind. What-Am-I-Doing-Here Paranoia leaves you standing empty-handed in a doorway, trying to figure out what you've come for. The Damn-It-They-Were-Just-in-My-Hand Affliction leads to panicky moments spent looking for your favorite new sunglasses, when all the while they're on top of your head. And Wrong-Vessel Disorder results in placing the ice cream in the pantry rather than the freezer.

In the past decade, cognitive neuroscientists have learned that much of what we blame on fading memory in midlife can be more accurately attributed to failing attention. Physiological changes in the brain's frontal lobes make it harder to maintain attention in the face of distractions, explains Cheryl Grady, PhD, a neuroscientist and assistant director of the Rotman Research Institute in Toronto. When the frontal lobes are in top form, they're adept at figuring out what's important for the job at hand and what's irrelevant blather; a sort of neural “bouncer” automatically keeps out unnecessary information. In middle age, that bouncer takes a lot of coffee breaks. Instead of focusing on the report that's due, you find yourself wondering what's for dinner. Even background noise—the phone chatter of the coworker in the next cubicle—can impair your ability to concentrate on the task before you.

When the neural bouncer slacks off, the cognitive scratch pad called working memory (which allows us to manipulate and prioritize information, and remember the thread of an argument) is quickly overwhelmed. You know the feeling: You can't absorb one more shred of information, so you erect a sturdy wall, neatly deflecting your husband's announcement that he'll be working late—an announcement you later swear he never made. “Metaphorically speaking,” writes social theorist David Shenk in his book Data Smog, “we plug up our ears, pinch our noses, cover our eyes...and step into a bodysuit lined with protective padding.”

As you age, you may also notice that information that once popped into your head in milliseconds now shows up in its own sweet time. Denise Park, PhD, a cognitive neuroscientist at the University of Illinois Urbana-Champaign, has found that while processing speed begins to decline in your late 20s, typically you don't feel the effect until your 40s or 50s. And then you feel as though you're wading through mental Jell-O.

It's tough to acknowledge that your brain is aging right along with your abs, but in both cases you can put up a fight.