Two fascinating studies which challenge two "sacred cows" of behavior change and skill development (especially as related to pronunciation teaching!)
(A) Use it or lose it (forever)!
(B) Habit change requires some positive reinforcement (or good feelings)
Study A, by Schwartz of University of Massachusetts, short version: Found that you don't lose it (muscle memory), not really; it can be reawakened faster than learning it first time.
Details: Studies with "rodents and insects" establish that (from the Neurosciencenews summary) " . . . nuclei are not lost from atrophying muscle fibers, and even remain after muscle death has been initiated . . .This suggests that once a nucleus has been acquired by a muscle fiber, it belongs to the muscle syncytium — probably for life."
Implications for (pronunciation) teaching: If learners can pronounce a sound right sometimes, even if only in oral reading carefully, they can be guided into using it spontaneously . . . believe it or not! The muscle "memory" for the action is nearly permanent; you just have to get back to it. There are a myriad of ways to do that, regular, disciplined practice being one!
Study B by Ludvig of Warwick University and colleagues, short version: Found that establishing a good habit depends more on how often you do the action rather than any inherent satisfaction you might get from it.
Details: Another study with (digital) rodents, established that (from the Neurosciencenews summary) " . . . habits themselves are a product of our previous actions, but in certain situations those habits can be supplanted by our desire to get the best outcome.”
Implications for (pronunciation) teaching: Regular drill and practice, done rationally and with strong "felt sense" (focused awareness on what it feels like to say the targeted words or processes.), develops effective habits and improvement, NOT whether or not it feels good prior to when the habits are firmly established. In other words, trust the method or instructor, at least temporarily, until sitting down (or standing up in haptic work) is nearly automatic, something you are just committed to.
How long does it take to establish a habit? Generally about a month in fitness training (See James Clear!), maybe a little less in pronunciation work, but not much . . . trust me.
Learners need to be motivated to practice, in part by being informed about and understanding this and related research--and practicing what you assign them--all the way to progress and the satisfaction and warm feeling that comes with it.
Frontiers (2019, January 25). Muscle Memory Discovery Ends ‘Use It or Lose It’ Dogma. NeuroscienceNews. Retrieved January 25, 2019 from http://neurosciencenews.com/muscle-memory-dogma-10637/
University of Warwick (2019, January 28). Train the Brain to Form Good Habits Through Repetition. NeuroscienceNews. Retrieved January 28, 2019 from http://neurosciencenews.com/repetition-habit-training-10652/
*On this blog, "Tigger warning" refers to "paper tigers", such as the Tigger of Winnie the Pooh!