Does Marijuana impair cognition?

A recent study in the Nature Group journal Neuropsychopharmacology investigates the acute effects of smoking marijuana on cognition. I've always assumed that smoking makes you dumb while you're high, and certainly had never seen any study on the acute effects. Yet it seems those assumptions might be misguided. The double-blinded study looked at a group of 8 females and 10 males who regularly use the drug (average of 24 joints a week!), which were given a ride to the lab then given cigarettes with various concentration of THC, underwent a battery of tests and were finally fed. Great care was taken in preparing the joints:

"During each session, participants smoked a single one-gram marijuana cigarette (0, 1.8, 3.9% Delta9-THC w/w, provided by the National Institute on Drug Abuse). Participants took three standardized puffs from the marijuana cigarette: each puff consisted of a 5-sec preparation interval, followed by 5-sec of inhalation, 10-sec of breath-hold, and 40-sec of exhalation and rest. Cigarettes were tightly rolled at both ends and were smoked through a hollow plastic cigarette holder so that the contents were not visible."

The authors first remark that other than memory recall very little has been investigated about the acute effects of THC, and the only other metric investigated, reaction time in psychomotor tasks, seems unaffected.
The tests they tried was a reaction time test, using both auditory and visual cues on a computer where the test subjects had to press a key. Memory test involved recalling details about stories and sequences of digits. There were also visuo-spatial tasks, mental calculations and various other standardised tasks. The results were somewhat surprising:

" In summary, while subjective ratings and heart rate were significantly elevated in a Delta9-THC concentration-related fashion, cognitive performance was minimally affected following acute marijuana smoking. Participants experienced greater difficulties inhibiting inappropriate responding following the high Delta9-THC concentration cigarette and they required more time to complete several cognitive tests, but their test accuracy rates were unaffected. The finding that accuracy was unaffected by smoked marijuana indicates that heavy, daily, marijuana smokers will not fulfill the DSM-IV criterion for marijuana intoxication that requires impairment of complex cognitive functioning. The consistent slowing of cognitive performance may have significant behavioral effects under some circumstances requiring complex operations that must be accomplished in a limited time frame, such as certain workplace tasks and the operation of machinery and automobiles."

So I guess it doesn't make you dumber, but it does slow you down...