Historically neurosurgeons were taken to be better dealing with minute details, i.e., using microscope / loupes for surgery like decompression and discectomies and spine surgeons (orthopedics) in bony work like fusions and deformity correction.
This behavior relates to Charles Law of Gases (Gay-Lussac's Law also applies in terms of pressure changes in a rigid container). You may have noticed that car tires and balloons shrink in cold weather. This is due to the molecular motion of gas particles. The gas particles slow down as temperature decreases. This, in turn, decreases their force exerted on the container walls and surrounding particles. The reverse is also true if temperature is increased. Youtube has a 90 minute TV special that relates temperature and absolute zero to the behavior of matter.
Click on the menu on the left to see the range of altitude.org interactive calculators. At sea level there is a pressure equivalent to 10 metres of water pressing down on all of us all the time. This is because of the weight of the air above us in the atmosphere. The important effect of this decrease in pressure is this: in a given volume of air, there are fewer molecules present. This is really just another way of saying that the pressure is lower. (This is called Boyle's law.) The percentage of those molecules that are oxygen is exactly the same: 21%.