Descartes was a really great scientist who was one of the first to carry out systematic examination of cadavers. His approach was to study the physical, leaving what was left to a spiritual mind. It set in place a materialist paradigm that felt it could do away with the spiritual mind altogether--until "the empire struck back."
Beginning with the cognitive revolution of the late sixties--and parallel neurophysiological studies--scientific study shifted to the other side. The mind could be studied with new methods. I think Descartes would have been thrilled.
Some of those discoveries did involved those with damaged brains. One that I once worked with was Roger Sperry, who won a Nobel prize for his work on the "split brain."
Here's an interesting case: