In reply to Badgers:
I've read many of the media articles since this study came out.
I see here the IFLS article says "It was also noted that it only occurs at heights exceeding 7,000 meters" - whereas the source article says "...Episodes reported above 3500 m altitude with possible psychosis were collected."
That's a major difference, even though in a technical medical sense 'high' altitude usually starts at 10,00ft / 2800m or sometimes 3500m, as here.
I've had some pretty major hallucinations on a climb, the strongest of them after 30+hrs on the go, climbing up and down a 2300m new route in soft snow, single push. This article mentions the lesser ones I had on that climb, not the wildest ones:
http://www.alpinist.com/doc/web07-08w/newswire-antarctica-omega-epperly-gps
But although some altitude - around 4000m - was a factor in that situation, I have always put it down to a combination of dehydration, exhaustion, stress etc. Other polar expeditioners have experienced some types of 'Third Man' syndrome, usually at low altitudes. Personally I doubt the primacy of 'high altitude' as a cause of such phenomena.