I think this is the money quote -- noting that these effects are noted on neurotypcial, and not on ADHD brains and therefore may or may not be generalizable:

These results can only be interpreted in the context on normal brain structure and function, and thus would have direct implications for the illicit/neurocognitive use of MPH. Since the underlying anatomy and biochemistry of ADHD has not been definitively characterized, our findings may or may not be generalizable to the vast majority of humans who are properly diagnosed with ADHD and are prescribed methylphenidate. Nevertheless, this work supports studies [51], [57], [58], [59] that demonstrate that drugs shown to increase the levels of dopamine in the synaptic cleft can contribute to degenerative changes in the basal ganglia.