I am sure that I have had a few sub-concussive impacts previously in life, but none were enough to make me have any symptoms save for thinking "ow" for a few seconds and then getting on with the task at hand.

Only the one really, lost consciousness (I am told) for 10-15 seconds, then got up and acted normal for about an hour, and then... I fell down? Yeah. Where? West 8. West 8, that's a weird place to fall down? Yup. This the first time I've crashed this year? Yeah.... I fell down? (wash, rinse, repeat). Ha.