Web MD has an article today, stating that low-carb diets may impair brain function. Read it here.
The article is as interesting as the study, which only followed participants for three weeks. Participants were tested on short- and long-term memory and attention three times during the study. I'm not sure you can come to any firm conclusions on anything after only three weeks, but these scientists have tried.
To me, the study's results are in line with what I know of how carbohydrates affect the human body. Read the fine print on any program recommending low-carb living, and you will see that advocates believe it takes 10 to 14 days to wean the body from reliance on simple carbohydrates. Withdrawal symptoms include irritability, sluggishness, lethargy and difficulty concentrating. Once the body has recovered from its withdrawal, usually people find themselves more alert, energetic and able to multi-task.
So if participants in this study only limited their carb intake for a week, it would stand to reason that they'd be in the midst of withdrawal symptoms when researchers tested their brain function. A week later, after resuming carbohydrate consumption, those symptoms would be gone.