Yet twice a week or so, Hamel puts on an adorable red bonnet decorated with white polka dots and fastens its strap under his chin. He does so to exercise what has become the most powerful muscle in his body: his mind. With the help of that bonnet -- actually a high-tech piece of headgear that is wired to equipment developed by scientists in Albany, N.Y. -- Hamel has learned how to move a cursor around on a computer screen just by thinking about it.