Assuming the numbers go from 0 to 9 (those included) and can be repeated, it must be 10 * 10 * 10 * 10 = 10000 combinations :-)

Could you link some of those studies? I’d be interested to read more about that

But that study was done on people aged 65+ for 11 weeks? I mean, sure, they didn’t measure any significant changes to the brain, but that doesn’t preclude changes forever. 11 weeks is not long to practice a language