I was told a story about a neuroscience study that involved paralyzing the lead author with curare and manually manipulating his eyeball to study his perception as a result of the manipulation; I was told it was never repeated, although a quick Google to find a reference revealed at least one similar study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3232712/), so I guess it's more popular than it used to be.

Another famous one was Werner Forssmann, the first doctor to perform a cardiac catheterization, who performed the operation on himself first (then walked over to the X-ray room to get a picture to prove he succeeded).