Dr Peter Iltis conducted a study using a functional MRI chamber at the Biomedical NMR Lab at the Max Planck Institute in Göttingen, Germany, watching hornist Sarah Willis’s tongue position as she plays in different registers, different tonguing patterns, and different dynamics. This (safely) replicates some of the fluoroscopic studies that were done with brass players. Check out some of the footage.
It’s pretty clear in this video that her tongue position raises as she ascends and lowers as she descends. I also find it interesting how there is a slight “bump” with the tongue arch when slurring. In other words, her tongue arch to slur up might jump up high and then snaps down to a slightly lower position, but still higher than it was on the lower note. This may be the equivalent of attacking the note with the tip of the tongue, giving it an extra push to slot, but with a much smoother attack to the pitch for slurred notes.
Iltis is interviewed about his research on a second video. Since he is also a horn player he has a good understanding of how brass players play as well as are taught about tonguing.