A research study to find out if people with Primary Generalized Tonic-Clonic Seizures, by taking perampanel in addition to their normal epilepsy medicine(s), have fewer seizures and feel better. Either perampanel (real medicine) or placebo (pretend medicine that looks the same as the real medicine) will be given in addition to ordinary epilepsy medicines. Who gets which is decided randomly. There is an option to continue into the Follow-on (Extension) Phase of the study