There is some good info in this thread and some mis-information.
As has been stated, the percentage of oxygen in the air is the same at all levels (in and outside the cabin) but the partial pressure will reduce at altitude. At 10,000 ft the pressure is about 85% of sea level pressure and at 18,000 ft it is about half.
Cabin altitude in cruise is typically 6-7000 ft and never above 8000 ft unless something has gone wrong.
Your body - and the pilot's - won't have any significant drop in blood oxygen levels in spite of the reduced pressure, because the body's need for oxygen is quite low as we are mainly sitting. There is not any significant or measureable effect on mental performance.
This is also why altitude sickness is not a concern at these levels. Smokers handicap themselves with equivalent of a few thousand feet extra altitude, but that has never shown significant negative effects on mental performance at these altitudes.
Even at 14,000 ft most people are "fully functional", although they may be losing some mental performance. Back in the early days of aviation pilots regularly flew at these altitudes without oxygen.
For a healthy person on an airplane, your brain can get all the oxygen it needs during a normal flight.
The effects of fatigue are much more significant on pilot performance. The airline business with its industrial attitude to crew duty times and rest requirements is a thousand times more significant in putting a "slow" pilot in the seat.
Last edited by AtoBsafely; 4th Aug 2012 at 02:36.