The hypoxia threshold for a decrement in perceptual-motor performance was determined with six subjects using a serial choice response time task at two levels of stimulus brightness. Low-oxygen mixtures were used to reduce SaO] (arterial oxyhemoglobin saturation) to hypoxic levels ranging from 86% to 76% in steps of2%. These values correspond to altitudes ranging from 8900 ft to 11 400 feet. Response time was slowed in a dose-dependent manner with a significant effect becoming apparent at an SaO] of 82% (10 000 feet). The slope of the dose-response function was steeper for the low than for the high brightness condition. These results provide a threshold estimate of9750 feet for performance decrements due to hypoxia and point to the disruption of vision as a factor influencing this decrement.

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