I wonder how cognitive functions and our personality would be affected? Two halfs make a whole, yin and yang and all that, if half the brain is asleep, would we even be able to think straight?
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.
So the brain can function normally without a whole half? That's very interesting. Now I'm wondering how much can be removed without adverse effects :-)
A non-precise answer: a surprisingly large amount if you're careful about which parts to remove and if you do it really, really early in life. ;)
In adults (or even adolescents) though, not as much luck.
In the case of hemispherectomies on Rasmussen's patients, you can analogize it as: people are dual-core by default. If you remove one of the cores early enough in life (like <5 yrs old), you can re-route the wiring so that processing can done by one of the cores, and still be pretty much normal. But whatever subtle thing is lost when if we don't do parallel processing like we usually do -- that things is pretty subtle to test for as well.
We would be able to think reasonable normally, apparently, assuming the sleeping half doesn't send confusing signals.
There have been quite a bit of research on people with severed brain stems, where the brain halves effectively are cut off from each other and limited to communicating through external senses. In those situations the brain halves are actually exceedingly good at trying to obscure that anything is wrong by making inferences of actions carried out by one brain half that the other one could not possibly have knowledge of.
The bigger problem is that because the brain halves control different parts of the body, we'd likely lose a lot of our physical function.
Probably would not be very useful. You would lose motor control of half of your body. When the left half of the brain sleeps, you would be a total mute as well.
