Read about this research here.
For those who wish to delve further into the vitamin D/brain connection, the American philanthropic organization, Dana Foundation published an excellent article in 2009 on vitamin D and the brain. Authors R. Douglas Shytle and Paula C. Bickford said: "We find compelling the scientific evidence that vitamin D supports healthy brain function throughout life. Vitamin D appears to be a “multi-potent” brain-cell-protective hormone, working through diverse and complex mechanisms, including brain calcium regulation, anti-oxidative properties, immune system regulation and enhanced brain cell signaling."
The article also points out that recommended dosages for this essential hormone may be too low: "...recent research has detected signs of vitamin D toxicity only after daily doses exceed 50,000 IU for several days or weeks." They point out that the current recommended daily allowance for elderly people – 600 IU – is clearly not enough, “particularly (for) those who are inactive”.
Another good source for studies on vitamin D, including the subject of dosage, can be found at the Pure North S'Energy Foundation. This Calgary-based organization helps people achieve optimal vitamin D status and supports research into nutritional supplements. Check out their literature page here.