How can active cycling produce less brain activity than passive cycling?

@article{Duysens2013HowCA,
  title={How can active cycling produce less brain activity than passive cycling?},
  author={Jacques Duysens and M. Severens and B. Nienhuis},
  journal={Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology},
  year={2013},
  volume={124 2},
  pages={217-8}
}