How would a helmet protect against that?
It is rather unlikely--if your head never hit the ground--that you would receive serious brain damage. You're not attached to your motorcycle therefore you are not going to receive massive de-acceleration like in a car. If you do massively de-accelerate it's because you hit a solid object..and if that happens..your head will be hitting the same object.
All of that said, helmets are designed, if your head does hit the ground to absorb some of that impact and protect your brain. They also do wonders for abrasion which is also a major problem at high speed.
As far as thinking that you can stop your head from hitting the ground. Good luck with that. If a car hits you someday it won't work like that.
When I was 6 years old my dad was hit by a car on his motorcycle with his girlfriend. He was traveling about 20 mph and the car around the same. (Car screwed up).
Neither of them were wearing a helmet. His girlfriend was killed instantly upon impact and his skull was cracked. He had received some fairly serious brain damage that the medical folk didn't think it was going to be recoverable. It happened so fast he didn't know what happened he just woke up in intensive care after several surgeries. He was not in-experienced, and like I, had been riding since he was 5 years old.
Somehow he survived. He appears fully normal--no obvious side effects. Everyone that I know said he is not the same and his life went rapidly down-hill since then. It is likely that I never have, nor never will really know my real dad.
Wear a damn helmet. I ride about 10,000 miles per year and simply cannot comprehend why someone wouldn't wear a good full-face helmet. It provides so much more than protection from a crash. It will protect you from rain, massive bugs, birds, flying road debris. etc.
I once had a rock kicked up by a semi that did so much damage to my helmet that I bought a new one. I really don't want to know how I would have went through that without one. If you ever run into a big thunderstorm on a trip--good luck--HEAVY thunderstorm rain hurts my hands through my armored gloves. 8 hours of that to the face? No thanks.
Don't stop with the helmet either. Get a good armored jacket with some type of energy reducing material on the impact points like your elbows. Also make sure it has something to protect your spine. From there you want good armored gloves, pants, good boots..etc