It’s an early symptom in the aging process—a nagging concern about memory loss and mental slow-downs. At first we joke and then we fret over real or imagined declines in mental agility. But the brain may not be as fragile as we think. To keep those cells alert and active as we age, just give them a shock now and then, advises neuroscientist Michael Merznich. Not an electric shock. He means the shock of change. “The brain is a surprise machine,” Merznich says. It wants variety