inhumandecency says:

The press releases for the marijuana say that permanent impairment was only observed in "heavy users," which they defined as people who smoke more than *70* joints per week. The study isn't published yet, so I haven't been able to check up on this rather implausible number. But it's not reasonable to say that marijuana permanently impairs cerebral blood flow, when their results suggest that there's no impairment in someone who smokes, say, 50 joints per week -- still an unbelievably large amount and far beyond what most regular users (or even abusers) consume.

Chris Yu says:

The study of long term effects of marijuana you refer to has been refuted by a couple other sources.

Vaughan says:

I've uploaded the paper here:
http://bak.spc.org/tha/mjstudy.pdf

It doesn't seem to claim anything about permanent effects, as it only used a one-month measure.

The main findings are given as :

"The main findings of the study are that, in comparison to control subjects, marijuana users show 1) elevated systolic and mean blood flow velocity in the MCA [middle cerebral artery] and ACA [arterior communicating artery]; 2) higher PI [pulsatility index; measure of blood flow resistance] values in both the MCA and ACA; 3) no significant improvements in systolic velocity during a month of monitored abstinence; and 4) improvement in PI values only for the light and moderate marijuana users during the month of monitored abstinence. The present measurements, obtained during a month of observed abstinence from marijuana users, document potentially prolonged marijuana-mediated changes in vascular hemodynamics."

The groups are defined like so:

"The light group smoked 11.0 +/- 3.5 joints per week (n = 11; range 2.2 to 15.0). The moderate group smoked the equivalent of 43.7 +/- 16.4 joints per week (n = 23; range 17 to 70). The heavy group smoked the equivalent of 130.8 +/- 73.0 joints per week (n = 20; range 78 to 350)."