I think that straining working memory makes activity occur in the PFC,
but also everywhere else (like the parietal cortex and Broca's area --
that could explain people making verbal fluency gains as well as Gf
gains?). What makes me believe that there are semi-permanent (heh)
motivational benefits is the fact that some studies show an increase
of dopamine receptors in the brain after DNB training. Dopamine
receptors are involved in motivation.
On a side, I really enjoy the lucid feeling I get after an hour of n-
back. I start to look at things and ideas seem to flow into my head
very vividly (I've made some of my best Lego creations after an n-back
session :p).