Hemispherectomy (removal of one half of the brain) is a treatment for Rasmussen's encephalitis when it's sufficiently severe (or the seizures are not responding

Hemispherectomy (removal of one half of the brain) is a treatment for Rasmussen's encephalitis when it's sufficiently severe (or the seizures are not responding to drugs). If done very early in life, the plasticity of brain makes it such that those patients grow up to be normally functioning adults. We believe this is because the regions that specialize for different tasks are able to combine their function on the same half. Think of, for instance, the "language" areas (Broca's area and Wernicke's area). (The following over-simplified explanation assumes typical right-hand dominant individuals for the sake of brevity.) If you inactivate that area in the left hemisphere (using drugs or transcranial magnetic stimulation), you'll lose "language" processing of data that occurs around 1 Hz (in English, this corresponds to individual words, phonemes; in tonal languages, this includes tones of lexical significance). If you inactivate that area in the right hemisphere instead, you'll lose "language" processing of data that occurs an order of magnitude slower than that (in most languages, this corresponds to pitch and volume changes over the level of the sentence, and can convey prosody information such as speaker intent as well as speaker identity). Due to specializations like this, hemispherectomies are not performed later in life. You would get symptoms similar to stroke patients. But for all areas, not just language.