Just as a personal experience. About 2 months ago I had an accident on my motorcycle (rain+sand+corner = helmet on pavement). I survived fine, but the very last thing I remember was trying to brake to a stop with both tires skidding over the pavement.
The very next thing I could remember is why is there a cat on my chest and more importantly where am I? As it turns out I was fully cogent during this time, called my buddy multiple times etc... I had lost memory formation entirely and repeated the same questions over and over.
While this incident was quite honestly minor in scope, it really helped me to understand at a personal level that consciousness really seems to just be a higher order brain function. All I remember in the time in between is.... black, no other way to describe it. I am hoping we can prove consiousness first before we go insane trying to figure out if machines are conscious too. Let alone have "souls".
I know anecdotal personal experience isn't worth a whole lot as proof, but it doesn't seem to take much to lose the ability to exist.