They're portable, low oxygen chambers used by many of the world's best athletes to improve their endurance and aerobic performance.

NPR: Anti-Doping Agency Reviews Low-Oxygen Tents

Such transfusions, which can improve performance in endurance competitions, are banned by antidoping rules.

WSJ: Cycling Doctor Under a Microscope

They want to improve career performance by boosting attention and concentration, rather than the boosts in endurance, speed, or strength athletes seek.

FORBES: When 'Study Drugs' Kill (Part 2): Reducing The Risks From Brain-Doping