Split brain surgery has some interesting side effects. I've seen examples where a test subject has been shown a picture of something to one eye following the surgery. Depending on which eye saw the image of an object, he might not be able to speak the object's name but could write it or draw a picture of it.

There is a procedure called hemispherectomy where half of the brain is removed. It is still a rare procedure but has proven useful in treating people who suffer many seizures every day. It is most often performed on young children. IIRC, the recovery is more problematic after puberty.

One side effect Canadian neurosurgeon Kenneth McKenzie reported in 1938 after a hemispherectomy on a 16-year-old girl who suffered a stroke was that her seizures stopped. Nowadays, the surgery is performed on patients who suffer dozens of seizures every day that resist all medication, and which are due to conditions that mostly afflict one hemisphere. "These disorders are often progressive and damage the rest of the brain if not treated," University of California, Los Angeles, neurosurgeon Gary Mathern says. Freeman concurs: "Hemispherectomy is something that one only does when the alternatives are worse."

Anatomical hemispherectomies involve the removal of the entire hemisphere, whereas functional hemispherectomies only take out parts of a hemisphere, as well as severing the corpus callosum, the fiber bundle that connects the two halves of the brain. The evacuated cavity is left empty, filling with cerebrospinal fluid in a day or so.

The strength of anatomical hemispherectomies, a specialty of Hopkins, lies in the fact that "leaving even a little bit of brain behind can lead seizures to return," Freeman says. On the other hand, functional hemispherectomies, which U.C.L.A. surgeons usually perform, lead to less blood loss. "Our patients are usually under two years of age, so they have less blood to lose," Mathern says. Most Hopkins hemispherectomy patients are five to 10 years old.

Neurosurgeons have performed the operation on children as young as three months old. Astonishingly, memory and personality develop normally. A recent study found that 86 percent of the 111 children who underwent hemispherectomy at Hopkins between 1975 and 2001 are either seizure-free or have nondisabling seizures that do not require medication. The patients who still suffer seizures usually have congenital defects or developmental abnormalities, where brain damage is often not confined to just one hemisphere, Freeman explains.

Another study found that children that underwent hemispherectomies often improved academically once their seizures stopped. "One was champion bowler of her class, one was chess champion of his state, and others are in college doing very nicely," Freeman says.

Of course, the operation has its downside: "You can walk, run—some dance or skip—but you lose use of the hand opposite of the hemisphere that was removed. You have little function in that arm and vision on that side is lost," Freeman says.