I am currently taking one liquid gel a day of the following blueberry extract:

Amazon.com: Wild Blueberry Concentrate - Made from Organic Berries | GMO & Gluten Free - Packed with Antioxidants and Phytonutrients | 6 Liquid Softgels - 2 Month Supply!: Health & Personal Care

I have noticed a discernible difference with this product on my daily function. Although I am by no means "normal" with this, I notice I can think more clearly, have fewer reading problems, and have better memory, and am also better able to tolerate caffeine more frequently. This effect is equal for me to eating an entire container of blueberries each day.

I can't find a citation for this, but I have read elsewhere that blueberries contain substances which activate a particular enzyme which improves the flow of oxygen to the brain. Given that caffeine causes reduced blood flow in the brain, that would explain my increased tolerance for caffeine while taking this product.