If a couple of batteries can cause brain damage, then they better get rid of all flashlights...fast! Just think of all the things that run on batteries...we should be a world filled with brain damaged people. Someone is taking you for a fool if they told you that.
As for the headaches, you could if you leave it vibrating in your mouth for a long time. But since brushing your teeth thoroughly is considered one of the best ways to keep yourself healthy, I think its worth the headache you might get. They really do get more gunk out of your mouth that the hand worked ones(unless you are very thorough). And that makes for good health.....it doesn't take much for that bacteria in your mouth to get into your blood stream.
And just to make it even better, they now have UV light sanitizers for toothbrushes that will kill the bacterial growth in your brush in just a few minutes. Plan to give them for Christmas this year.
Here's to good dental health!
J