One 25-year Harvard Medical School study of more than 13,000 women showed that the participants who ate relatively high amounts of vegetables over the years had less age-related decline in memory. Cruciferous vegetables and leafy green vegetables had the biggest effect. In another study, the phytochemicals, and quercetin, has reversed some of the age-related memory deficits in animals. Tufts University in Boston found that eating foods rich in folic acids protects memory. Also, an Australian study found that eating plenty of foods rich in folic acid was associated with faster information processing and memory recall. A study conducted by Rush University Medical Center in Chicago followed more than 3,000 men and women for six years to see how diet affected memory. The found that people who ate fish at least once a week had a 10 percent slower decline.
So, eat your brain food and stay sharp! Personally I think organic is best, because it doesn't have the pesticides that can have counter-production side-effects.