In the mid-1980s, when Will Wright was just getting started as a game designer, he realized that the process of constructing a game—building out the individual levels—was fun in and of itself. Why not share the joy of creation with players? He conceived of a new game in which people …

In retrospect, it might have been a clue. But in early 2010, when Kamni Vallabh first began to complain that her eyesight was failing, there didn’t seem to be much cause for concern. She was 51; maybe middle age was catching up with her. Maybe the harsh western Pennsylvania winter—two …