If lab mice run as much as they like, their brain function improves; if they’re forced to run harder than that, it improves even more, according to researchers from National Cheng Kung University in Taiwan. In fact, mice who were pushed to race on treadmills show evidence of molecular changes in several parts of their brains. Meanwhile, those that could run at their own pace on the treadmill, an activity most mice enjoy, saw brain function change in only one area, the New York Times reports. Researchers have long known that exercise changes brain structure, and affects thinking; ten years ago, California scientists showed that exercise stimulates the creation of new brain cells. But should the exercise be strenuous? Must it be aerobic? And are cognitive changes permanent? Many questions remain. Answers, though, are starting to emerge: in one study, 21 students at the University of Illinois were asked to memorize a series of letters, then pick them out from a list of flashcards. After either sitting quietly, running on a treadmill or lifting weights, they were asked to perform the task again; after running, they did better on the test, and continued to perform better after cooling down. “There seems to be something different about aerobic exercise,” says Charles Hillman, an author of the study.