In conclusion, the regular marijuana users in our study showed cognitive impairment in the maze task and in the WCST on 17mg THC... This evidence is compatible with previous studies showing impaired cognitive function in regular users of marijuana.
Weinstein et al, "A Study Investigating the Acute Dose-response Effects of 13 Mg and 17 Mg Δ 9- Tetrahydrocannabinol on Cognitive-motor Skills, Subjective and Autonomic Measures in Regular Users of Marijuana," Journal of Psychopharmacology, Vol. 22 Issue 4, June 2008, 441-451.
A quick read through the conclusions and/or the abstracts of a few other papers suggests that, while marijuana (or, to be more precise, the THC in marijuana) can and does impair cognitive and motor skills in most users, there are some users who show little or no effect from using the drug. This is generally explained by a combination of genetic predisposition (i.e., some people seem to have a sort of natural resistance to the effects) and accumulated tolerance (i.e., some people who smoke a lot can build up their tolerance for it, and avoid some of the worst effects).
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But a blanket statement that it doesn't impair cognitive or motor function is simply unsupported by the recent medical literature.