1. Prolonged (> 10h) exposure to hypoxia and high altitude (> 5000 m) invariably have detrimental effects on cognitive performance. Paradoxically, mild improvements in cognitive function in patients with chronic obstructive pulmonary disease after cessation of oxygen therapy have been reported.

2. We studied in each of 10 healthy subjects the effect of an acute altitude challenge [rapid helicopter transport to the Jungfraujoch (3450 m), experiment 1] and of an acute exposure to mild hypoxia (fractional inspiratory oxygen concentration 14.5%, experiment 2) on a simple test of cognitive performance (the time needed to read briefly displayed letters).

3. Under both hypoxic conditions the time needed to read briefly presented letters decreased, from 12.1 ± sd 3.8 ms to 8.3 ± 1.5 ms (P<0.01) in experiment 1, and from 11.9 ± 1.9 ms to 8.1 ± 1.1 ms (P<0.01) in experiment 2.

4. A rapid and mild hypoxic challenge seems to improve a simple measure of cognitive performance above normal values. The common notion that exposure to hypoxia and altitude invariably impairs cognitive performance may have to be re-evaluated.

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