I'm not an expert, but I don't imagine any study that could be designed that would determine this to any practical degree of certainty--B12 is correlated with s

I'm not an expert, but I don't imagine any study that could be designed that would determine this to any practical degree of certainty--B12 is correlated with so many other risk factors, like smoking for example. People with healthier brains possibly take more risks, but it's just a vague guess too.