The effect of altitudinal hypoxia on night vision is primarily one of an elevation of the rod and cone threshold. Although decreased cone function is clearly demonstrated by the loss of color vision at hypoxic altitudes, the decrement in central VA is usually insignificant. However, scotopic night vision at altitude can be significantly reduced. Scotopic vision has been reported to decrease by 5% at 3,500 feet, 20% at 10,000 feet, and 35% at 13,000 feet, if supplemental oxygen is not provided. Thus, the use of oxygen, even at low pressure altitudes, can be very important at night.