When Valeria Della-Maggiore first scanned the brains of 29 volunteers learning to tap out a five-digit sequence with their non-dominant hand, she expected to see the same blurry signals researchers had seen before. What she found instead was two separate physical changes happening at completely different speeds — one that vanished within hours and another that stuck around.
"You could compare it to what happens when you exercise," Della-Maggiarec EXPLAINED. "Your muscles swell during the workout from all the fluid rushing in, but that's different from the actual muscle growth that happens later. The brain works the same way."
The study, published in the journal PLOS Biology, used a new imaging technique called SANDI that can peer inside brain cells for the first time. Traditional brain scans blend everything together into one fuzzy picture. SANDI separates the signal into three parts: the cell bodies themselves, the branching tendrils called neurites that connect brain cells, and the fluid around them.
Della-Maggiarec's team at Argentina's National University of San Martin and the University of Buenos Aires put volunteers through a simple task. They practiced tapping five digits in a specific pattern with their left hand — the one most people use less — for about 15 to 20 minutes. The researchers scanned their brains before the practice session, again 30 minutes later, and once more a full day afterward.
The first scan, taken just half an hour after learning, showed something striking. Every brain region involved in the task — the hippocampus, the primary motor cortex, the posterior parietal cortex, and the precuneus — had swollen cells. The cell bodies had puffed up like balloons, filled with extra water. By the next day, all of that had disappeared. The swelling was gone.
The second scan told a different story. Twenty-four hours later, only two regions still showed changes: the precuneus and the posterior parietal cortex. These areas had actually grown new neurites — the tiny branches and fibers that brain cells use to talk to each other. And here's the striking part: people who improved the most on the task showed the biggest growth in those regions.
Meanwhile, the hippocampus — a region famous for learning and memory — showed no lasting structural changes at all. It got involved early, sure, but whatever memory of the finger-tapping sequence formed, it wasn't stored there.
"This tells us that when you learn something, your brain isn't just doing one thing," Della-Maggiarec said. "It's running two completely separate processes at the same time, in different parts of the cell, on different timescales."
The researchers hope this new imaging technique could eventually help doctors distinguish between healthy brain changes — like what happens when you learn a new skill — and harmful ones, like the damage seen in aging or disease. Right now, there's no way to tell those apart in a living person. This might change that.