The forensic dude is wrong based on information I have read revolving around the brain striking the interior of the skull, in a coup(the brain is bruised on the side of impact) or contra coup(The brain is bruised on the side oppsite initial impact, usually from smacking the impact side and bouncing back against the opposite side) incident. The job of the helmet is not to, so much, keep your head from touching the ground as it is to lengthen the time of impact. To test this, and this will use your hand, not your brain, so permanent damage is less likely, Take a cinder block. Punch it as hard as you can with your non dominant hand(That is the one you don't write with). Now got to the urgent care and get it casted. On your way home pick up a cheap styrofoam cooler and some duct tape. Once home break off enough foam to make about a two inch thick foam block about twice the size of your squared fist(bigger if you can't aim). Tape it to the cinder block and hit it with your other hand, the one not in a cast. The force of the impact will be spread over a longer period of time reducing the peak impact force and lowering the area under the curve. This is what keeps you from suffering a brain injury. Your brain still smacks against the inside of your skull but not so hard as the impact is not sudden but spread out over, a short, period of time. This is the same reason auto makers build crumple zones into cars and make the steering column collapse as it hits your chest. Or like when you trip and fall into a bush as compared to tripping and falling on a pile of bricks.
I am not a trained forensic specialist (I am a trained EMT) And I have worked with guys racing cars, motorcycles, and bicycles since I was 18. They all wear helmets and are still alive. Even tho some of the moto guys have had some nasty highsides, I have seen two cars destroyed, and one of my bike guys has broken three frames in the last year(He is working on a Banshee now)
Merriwether, Some of us wear helmets when driving a car,