Any mental exercise helps cognition by building, lengthening or strengthening the pathways that carry information between neurons. Generally, the more pathways you have, the better your cognition.
When you carry out a particular mental skill, connective tissue builds up in the part of the brain responsible for it, just like arm exercises build your biceps. For all-round cognitive improvement, therefore, you should do lots of everything: motor skills (ie, physical activity), talking, socialising, planning, game-playing, calculating, writing, reading and talking. But the problem is that we tend not to do everything, especially as we get older.
This is where brain training comes in. Systems like Lumosity, Brain HQ and SmartMind claim to exercise all parts of your brain, and thus to raise your cognitive abilities generally, rather than in one particular area. Alas, the proof of this is just not there. Scientists reviewed the literature that brain-training companies cite to support their products and found that, while people got better at individual tests, there was no general improvement in cognition.
Brain-training apps only improve the brain’s performance at particular tasks