I have also wondered about this and was going to make it my first post on this forum at some point
After reading "The Attention Revolution" by Alan Wallace, I made a goal of 'attaining' Shamatha (I failed due to my concentration bringing up lots of 'stuff'). Mr Wallace makes it very clear in his book that Shamatha is the culmination of a process; the result of training in concentration, not concentration itself. After training and reaching the higher states of abiding (ninth stage) and practicing concentration there, one may attain to a tenth stage called "shamatha" which is in practice irreversible. He specifically mentions that the attainment is accompied by a sensation on the top of the head, like pressing a ball onto the top of the brain if I remember correctly, or something to that effect. He believes there is some kind of definitive neurophysiological change when one attains Shamatha.
In fact in some lectures I've heard Wallace say that AFTER one attains Shamatha, THEN one can practice jhana. So it is a very different view from what is discussed on this forum.
I am curious about this too because we talk about stages in insight practice which are reversible and stages which are not reversible. Perhaps the same is true in concentration practice?