While brain training exercises in general are not receiving a very good press these days, experiments testing effects of BrainHQ's speed of processing exercise called "Double Decision" have been the most convincing. I've tried it out several times, feel like it perks me up quite a bit for awhile, then get really bored repeating it and stop.
In the exercise, you see an image in the center of your vision–for example, either a car or a truck–and at the same time, you see another image way off in your peripheral vision. The images are only on the screen for a brief period of time–well under a second. You then have to say whether you saw the car or the truck in the center of your vision, and then you have to show where you saw the image in your peripheral vision. This challenges the speed and the accuracy of your visual system. And as you get faster and more accurate, the speed increases and the peripheral vision task gets more demanding–pushing your brain further.
Edwards et al. (open source) now report a randomized controlled trial among 2,802 initially healthy older adults, which examined the efficacy of three cognitive training programs (memory, reasoning, or speed of processing) relative to a no-contact control condition. They found that healthy older adults randomized to the Double Decision speed of processing cognitive training, but not memory or reasoning training, had a 29% reduction in their risk of dementia after 10 years of follow-up compared to the untreated control group.