Just did a little cursory research. Black garlic is made by aging garlic at high heat, which would seem to argue for decreased rather than increased medicinal effect, since heat destroys many of the sulphur compounds that make garlic healthful. Given the description of black garlic as having a very mild flavor, that would seem to confirm a breakdown of the sulphur products. Now, there are other things in garlic that make it nice to use, but the most important health benefits are from these sulphur products, so I'd say I'd need to see a lot more to say this is better than raw garlic. Now, some say this is a fermented product, and maybe it was originally, but other articles say it isn't fermented, it's just aged at high heat to give it a different mellower flavor. Who knows? Maybe there's an old way and a new way of making it. As to the memory issue, never heard of it -- people have been using garlic a long long long long time, but garlic is used in so many different ways, which garlic are they talking about? Cooked garlic, which destroys most of the active medicinal parts? Aged garlic? Raw garlic? Garlic juice? A lot of garlic? A little garlic? Do onions, a relative of garlic and containing many of the same compounds, cause memory loss? Can't say if it's true, but you'd think we'd have learned about this centuries ago if it were true.
http://www.drweil.com/drw/u/QAA400833/Is-Black-Garlic-Better.html