I don't think there is a substantial decline due to disuse unless that disuse is due to being in a coma. The tests are not geared to high levels of performance where 'academic or cognitive laziness' would show up.

Plus, if one lost memory skills due to an injury but then learned new ways to access memory with brain training, the skills lost to injury may not show up. I believe that much of the improvements achieved from using Lumosity and other brain training programs are not due to neuro-plasticity developing new pathways. In my experience, most brain training is developing or strengthening under-developed skills that were always present but not adequately utilized. The observation skills, discretionary skills, focus skills, and many others have just been under-utilized.

As I said, I learned how to do the Trail Making Test after doing it multiple times. If I did not use my work-arounds, I would do much worse.

It would be interesting to see what kinds of improvements are quantifiable by an NPA after doing a Lumosity training program. The studies released do not get into much detail.