Low doses are very stimulating. It probably increases creativity. I don't know what low doses do with IQ.

As ron and i said, though: high doses definately make you dumber, temporarily. Your short term memory doesn't function normally anymore and the simplest mental task become incredibly complex.

That's one of the reasons why LSD has such a 'brainfuck' potential; your thinking is so impaired that you can easily believe everything.

You can compare it with the movies: movies create an atmosphere whereïn weird things can happen; nobody ever walks out of a cinema saying "hah, who believes that shit?". LSD does something similar in the sense that 'atmosphere' becomes more important for your judgement.

You could make people on acid believe the most ridiculous things, as long as the 'atmosphere' is right.

I once freaked out completely because i fell and i started thinking about all the injuries i could have had, that i didn't notice yet..so a few minutes later i was speculating about all the possible internal bleedings and pierced organs, etc. While normally when you accidentally stumble and it doesn't hurt, you know that you're alright, and the chance that you broke your neck without noticing it and you could get paralized forever, and so on, is just very small and implausible as well.