Healthy, thinking, reasoning person before the operation, and a vegetable afterwards. Sometimes a baby is starved of oxygen during birth, and suffers irreparable brain-damage because of this. That is, the soul floats along inside our heads and use the brain the drive all the various muscles etc. A mouse has a similar set of bodily organs and muscles to a human. Many dinosaurs had large bodies and tiny brains, so a big brain is not required to move big muscles.
Some people partially lose their memory when their brain is injured.
The answer is quite simple, and not to the liking of theists. Your personality is a direct product of that physical organ in your head, your brain. When your brain dies, your soul dies with it. Nothing else makes sense and, as usual, the theistic explanations only raise more problems than they solve.