It was very obvious in term of decline, my father had trouble with doing some tasks on his computer (when he never did before having been an early adopter and programming some educational software for schools in the 80s), he no longer could concentrate enough when reading and became more extreme in his politics (he went from voting from the parti socialiste and being mostly center left to being very left wing). He also had less control over his emotions and would sometimes explode in a rage unlike anything he had before.
He himself felt diminished mentally and that greatly bothered him during the last years of his life.
On my side, I've refused general anaesthesia as much as I can because it scares me deeply. When I was a kid, I had an operation under general anaesthesia and I woke up with a very strong asthma attack, completely unable to breathe. Reading this, it does give argument towards my stance to asking for local anaesthesia as much as possible.