Improving a specific skill is not hard. Or at least knowing what to do (practice) is not hard, even if actually doing it is not so easy. But improving at very general skills, the sort of skills that underlie many tasks we take on, has proven much more difficult. The grail among these general skills is working memory, as it’s thought to be a crucial component of (if not nearly synonymous with) fluid intelligence. Brain training programs that promise wide-ranging cognitive improvements usually offer tasks that promise to exercise working memory, and so increase its capacity and or efficiency.
Source: Bad news for brain training