For Henry Schipper, 49, of Venice, Calif., the first warning signs are mild and almost pleasant—a giddy light-headedness that evolves into what he describes as a "happy series of energetic moments." Then for about 15 minutes his eyes play tricks on him, and a wall of shimmering light obscures his field of vision. "There's no pain at that point," says Schipper, who produces documentaries for the History Channel. "But once the shimmer starts, the countdown begins.