Each week, “Cerebral Matter” highlights articles, anecdotes, and analyses related to the psychological aspects of performance.Mets pitcher Brian Stokes understands the importance of the mental game:As much as last season tested him, Stokes said he never grew discouraged. He returned home to California and spent some of the off-season performing exercises to improve his mental toughness. “It wasn’t physical, I knew, because I had gotten to the major leagues,” Stok...