Pretties while undergoing surgery recieve brain lesions. These lesions will be removed if they go into a mentally demanding career. Pretties with these lesions are called "bubbleheads" which can also be used to refer to stupid or dumb people.
Frizz also invented lesions to make people Radically Honest.
The lesions were made so that Pretties wouldn't rebel, but Tally cured her lesions using her own will. Zane cured them with pills. The pills were the invention of Az and Maddy, David's parents, who had created the pills to impede the lesions, in hopes the Pretties might rise up.
Specials also have lesions to make them colder towards Randoms to the point where they don't feel remorse killing them.