...People with high folate and low B12 status were found to be at a
disadvantage when compared to those with normal folate and low B12
status; the former group was more likely to exhibit both anemia and
cognitive impairment, according to Jacques. A single cognitive
function test was used to assess aptitudes such as response speed,
sustained attention, visual-spatial skills, associative learning and
memory....

http://www.sciencedaily.com/releases/2007/11/071126151019.htm

This could explain the poor mental health long-term vegans experience.
Folic acid too high and B-12 too low.