I'm not certain but it may have been the AMA (American Medical Assc) that printed the stat. I'm not sure. I'll see if I can track it down.
1st of all I want to be perfectly clear that I am making an educated GUESS about why that might be.
Clearly animal products are generally higher in fats. A good deal of your brain and nerves are made of "Lipids" (which unfortunately are operationally defined by what kind of solvents they'll disolve in).
I'm guessing that without enough of the right kind of fat that your brain eventually wigs out(notice the highly technical medical term).
But whether that is a credible appraisal .... I dont' know.