According to research appearing in the Archives of Internal Medicine (2007 Nov12;167(20)2184-90) found that long term beta carotene supplementation may help with mental clarity. Harvard researchers followed two groups of subjects for 18 years. One group was given a placebo; the other was given 50 mg of beta carotene. The group receiving the supplement had higher scores on cognitive tests than did the placebo group.
Short term the supplementation did not seem to have much effect. Another group was tested after one year; there was no difference between the placebo group and the group receiving the supplementation.