I think there is more evidence it crosses (two studies with spinal measures) than it does not (0 studies). For (almost) direct measures check out Neumann, Inga D., et al., 2013 and Born, 2002. There are great many studies showing effects that could only be caused by encephalic neuromodulation. If it does not cross it, then it should cause increased encephalic levels of some neurochemical with the exact same profile, but that would be really weird.
I think there is more evidence it crosses (two studies with spinal measures) than it does not (0 studies). For (almost) direct measures check out Neumann, Inga D., et al., 2013 and Born, 2002. There are great many studies showing effects that could only be caused by encephalic neuromodulation. If it does not cross it, then it should cause increased encephalic levels of some neurochemical with the exact same profile, but that would be really weird.