In Stephen R. Donaldson's world of the far future,

zone implants were first used to control grand mal epileptic seizures. They were typically inserted between skull sutures into specific areas of the brain. With imaginative use on the part of doctors and their patients, it was soon discovered that various results could be obtained by judiciously modulating the implant's emissions. "{B}y tuning the implant to different zones of the brain... Violent insanities could be tamed. Manic behaviors could be moderated. Catatonia could be relieved — or induced. Recalcitrance could be moderated. Pain could be reinterpreted as pleasure."1

Of course zone implants were illegal — and unauthorized use required the death penalty. However like many powerful tools, illegality didn't stop the asteroid miners and others who worked in space under difficult and hazardous conditions from having a few secreted away for emergencies.

Morn Hyland found that she could use her zone implant to control her gap-sickness; and she learned how to use it in other ways as well to enable her survival in situations that would have killed any other human.

1. The Gap Into Conflict -- The Real Story, by Stephen R. Donaldson; ©1991; Bantam Books.