Now, I’ve always ascribed to the dictum ‘use it or lose it’. If you don’t use your muscles they become weaker and if you don’t use them at all, as in the case of someone wheelchair bound, they atrophy. Getting yourself out of breath expands the capacity of your lungs. Swimming, and particulary swimming underwater is especially good in this respect. Putting stress on your bones strengthens them. Those whose bones remain unstressed for a length of time, like astronauts, rapidly lose bone mass.
I’ve always thought that the same rules apply to the brain. If you don’t make any mental effort, your ability to make mental effort declines. I feel that this equally applies to those rather vague cognitive functions like imagination. And this article lays it all out very neatly.