From Science Daily
Psychologists are finding out that even when people try to focus on a task they tend to lose concentration within 40 minutes, and sometimes as little as 10 minutes. The studies are based on a new technique, called transcranial Doppler sonography, that uses ultrasound to monitor blood flow velocity in the brain. The technique could be turned into a warning system for workers who perform critical tasks -- such as pilots or air traffic controllers -- or even for drivers.
I'd love to see the results if they did this kind of scan on long-time meditators. My hunch, naturally, is that they'd be able to pay attention longer.