Here is a great presentation by Tim Lundeen from the recent QS Show&Tell. Tim is running some interesting self-experiments on diet and cognition.
Diet and cognition is a topic of such obvious interest that it regularly breaks through into the popular press and the science blogs. For instance, eating blueberries and walnuts make rats smarter.
Tim was specifically inspired by the many posts Seth Roberts has made about the benefits of omega-3 fatty acids. Tim uses a simple test of cognitive function as his dependent variable: he gives himself 100 very simple math problems and records the time it takes to complete them.
Here is one of the graphs Tim made in his self-study. The y-axis is the time it take to complete his 100 problems. On about day 80, he upped his dose of DHA from fish oil.