On the day of, though we had protective head gear, mouth guards, and other such stuff, this dude banged me up pretty bad for three one minute rounds that was allotted for our bout. At the time I was happy that I wasn't knocked out (we had technical knockouts if one dude was getting banged up pretty badly), but in retrospect that would have been a better outcome.
For the next few days/weeks, though I looked normal, my head felt swollen, and it would pulsate randomly when I put my head down on a pillow or try and take notes (similar the the feeling when trying to find your pulse on your wrist times.10). The school doc gave me some pain killers, but even when not on them, my attention span was gone and I couldn't stay focused at all for the next few days. Foolishly I boxed again next year, but was better at defending my self, so wasn't hit much.
I recently graduated from 4 years of college, and looking back it had a significant affect on my memory/retention my junior and senior years of school, and my focus was never the same and god knows what else was damaged. Perhaps I am attributing my decline in academic performance to that single incident, when there could be a bunch of other reasons (correlation doesn't imply causation), but if I could take those three minutes back at the cost of a year of my life, I probably would.
I recently saw a TED talk[1] by Amy Cuddy, a professor @ HBS, on an unrelated subject, where she talks about suffering head trauma in a car accident, that resulted in her having to take significantly longer to graduate college, because of the accident's affect on her mental cognition.