yeah , i agree with papercut , i remember seeing a video a few years back , American i think (70's or 80's), where they studied the effects of decompression and hypoxia . they put people in a chamber and did decomprssions , they then did certain mental exersizes with these folk . I believe that all the people involved have now suffered serious brain damage or have passed away ( due to starvation of oxygen to the brain during the tests ). so i certainly wouldn't want to do it for fun !!!