I saw an article today on what actually happens in the brain when animals (including humans and dogs, one presumes) are learning a new motor skill. The researchers, studying mice being trained to reach through a slot to get a seed, observed that new synapses in the motor cortex formed rapidly during learning. So the brain was actually remodeling to incorporate the new skill. The new synapses were still there four months later, when the mice were again asked to perform the seed-reaching activity. However, other already-existing synapses were selectively eliminated so that the overall density remained the same.
This seems to prove the old adage that "you never forget how to ride a bike," unless of course those paticular synapses are replaced by some other more newly learned motor activity. There was no mention of how the brain decides which connections to delete.
But this is an excellent recommendation for the idea that keeping a brain working helps to keep it active and helps prevent cognitive loss in older age. So when you're training your dog, consider that you may be helping both of you (as training is often somewhat challenging for the trainer as well as the subject)! I can testify that due to cold and snowy conditions here in the northwest, Nestle and I spent the weekend mostly indoors. He demands exercise, so though we played some tug games, we also worked on some training, and it left both of us happily tired. As my co-author Mandy Book and I are working on a new edition of our clicker training book, Quick Clicks, it looks like Nestle is in for somewhat more training than usual. He gets so excited when I pick up a clicker that I always feel guilty that I don't indulge in teaching him more new things. But we're including lots of new behaviors in the book, so his repertoire is about to increase. We're looking forward to our brains expanding!