Why do we not have any recent scientific studies with the Buddhist monks who participated in the paper «Long-term meditators self-induce high-amplitude gamma synchrony during mental practice»?

https://www.pnas.org/doi/10.1073/pnas.0407401101 Richard Davidson says this paper was confirmed in a second study. We sah Mattieu Ricard participating in such a study in a video. But why are there no further studies? Where there special terms and conditions with these people not to do this, not to repeat this study with the same people, or to …