Note that in the Discussion section of the paper they themselves admit lack of cross-matching in the subject groups for Tobacco use as well as Education, both of which could correlate to reduced "grey matter", if for instance the vast majority of frequent K users are heavy smokers with low education levels as compared to the non-K users, so the causality of the results are not exacly crystal clear.
On another Ketamine-related research topic:
just for kicks I did this search:
http://www.google.com/#sclient=psy&...oq=&pbx=1&fp=c820984782c38d0&biw=1602&bih=778
Among the result was this:
http://www.ncbi.nlm.nih.gov/pubmed/11818769
In which we read that in general "isomers induce less tiredness and cognitive impairment" than racemic, and specifically, that S-isomer users had less decline in concentration and were "more brave." Don't often see medical research studies evaluating "bravery"!!! HAHA!
RESULTS:
Transient increases in blood pressure, heart rate, and catecholamines were similar after administration of all drugs. At 20 min after injection, subjects felt less decline in concentration and were more brave after S(+)- than racemic ketamine. They reported being less lethargic but more out-of-control after R(-)- than racemic ketamine. Ketamine isomers induced less drowsiness, less lethargy, and less impairment in clustered subjective cognitive capacity than racemic ketamine for the 60-min study. Objective concentration capacity [test time, S(+): 25.4 +/- 15.2 s, R(-): 34.8 +/- 18.4 s, racemic ketamine: 40.8 +/- 20.8 s, mean +/- SD] and retention in primary memory [test time, S(+): 4.6 +/- 1.2 s, R(-): 4.2 +/- 1.4 s, racemic ketamine: 4.0 +/- 1.4 s, mean +/- SD] declined less after S(+)- than either R(-)- or racemic ketamine at 1 min. At 5 min, immediate recall, anterograde amnesia, retention in primary memory, short-term storage capacity, and intelligence quotient were less reduced after the isomers than racemic ketamine. Speed reading and central information flow decreased less after S(+)- than racemic ketamine.
CONCLUSIONS:
Early after injection, ketamine isomers induce less tiredness and cognitive impairment than equianalgesic small-dose racemic ketamine. In addition, S(+)-ketamine causes less decline in concentration capacity and primary memory.