The operation known as a hemispherectomy—the removal of half the brain—sounds too radical to ever consider, much less perform. In the past century, however, surgeons have done it hundreds of times for disorders that cannot be controlled any other way. Perhaps surprisingly, the surgery has no apparent effect on personality or memory. Does that mean a person needs only half a brain? Yes and no. People can survive and function pretty well after the procedure, but they will have some physical disabilities.

The first known hemispherectomy was performed on a dog in 1888 by German physiologist Friedrich Goltz. Neurosurgeon Walter Dandy pioneered the use of the procedure on humans at Johns Hopkins University in 1923, operating on a patient who had a brain tumor. (That man lived for more than three years before ultimately succumbing to cancer.)

In 1938, after performing a hemispherectomy on a 16-year-old girl, Canadian neurosurgeon Kenneth McKenzie reported that it could stop debilitating seizures. And today brain surgeons perform hemispherectomies on patients who undergo dozens of seizures daily that resist all medication and stem from conditions that primarily afflict one hemisphere. “These disorders are often progressive and damage the rest of the brain if not treated,” explains neurosurgeon Gary W. Mathern of the University of California, Los Angeles.

The surgery takes two forms. Anatomical hemispherectomies involve the removal of an entire hemisphere, whereas functional hemispherectomies take out only 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 up with cerebrospinal fluid over time.

Doctors often prefer anatomical hemispherectomies because “leaving even a little bit of brain behind can lead seizures to return,” says neurologist John Freeman of Johns Hopkins, which specializes in the procedure. 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. Neurosurgeons have performed the functional operation on children as young as three months old. In these tiny patients, memory and personality develop normally.

Most Johns Hopkins hemispherectomy patients are older than five years. A recent study found that 86 percent of the 111 children who underwent the procedure at Johns Hopkins between 1975 and 2001 are either seizure-free or have nondisabling seizures that do not require medication. Another study found that children who underwent a hemispherectomy 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 notes.

But Can You Dance?

Of course, removal of half the brain does have its downside—notably significant loss of function on one side of the body. “You can walk, run—some dance or skip—but you lose use of the hand opposite of the hemisphere that was removed,” Freeman says. Vision is impaired as well.

Also, if the left side of the brain is taken out, most people have problems with their speech. But, Freeman notes, the younger a person is when he or she undergoes hemispherectomy, the less speech disability the person is likely to have.

Mathern and his colleagues have recently conducted the first functional magnetic resonance imaging study into hemispherectomy patients, investigating how their brain changes with physical rehabilitation. Probing how the remaining cerebral hemispheres of these patients acquire language, sensory, motor and other functions “could shed a great deal of light on the brain's plasticity, or ability to change,” Freeman observes. Still, he says, hemispherectomy is among the most drastic kinds of brain surgery and is “something that one does only when the alternatives are worse.”