Here's one: http://www.pnas.org/content/105/19/6829.full http://www.pnas.org/content/105/19/6829.full The full text is online. The study has a control group.

Here's one: http://www.pnas.org/content/105/19/6829.full http://www.pnas.org/content/105/19/6829.full The full text is online. The study has a control group. The subject had to perform the task for about 25 minutes per day, every day. The effect is dose-dependent. The longer the subjects trained, the better they fared on an unrelated fluid intelligence test.