Gretchen Reynolds summarizes several experiments demonstrating that exercise stimulates synthesis of brain derived neurotropic factor (BNDF), which promotes the health and multiplication of brain cells, and apparently also cognitive health. An Irish group has shown that strenuous aerobic exercise on a stationary cycle boosts BNDF levels and also performance on memory tests. A Brazilian group found that after sedentary elderly rats ran for approximately five minutes several days a week for five weeks, BNDF production increased in memory center of their brains. The old, exercised animals then performed almost as well as much younger rats on rodent memory tests. A study at UCLA showed that if adult rats were allowed to run at will for a week, the memory centers of their brains afterward contained more BDNF molecules than those of sedentary rats. Finally a Stanford study found that the normally occurring decay of performance with age in skilled airline pilots was more pronounced in those who carried a common genetic variation that is believed to reduce BDNF activity in their brains.