ATLANTA — Following his second day of work as an intern in the mental health program at the Carter Center last month, Chris Borland was driving home past a high school. On a field situated along the road, he saw a football team in the middle of a spring practice. Borland pulled over and watched for 10 minutes, not out of nostalgia for a game he left behind, but rather fixating on the players as their helmets collided repeatedly during a series of contact drills.