Curiously, both sides of the body are "represented" in the motor cortex of each hemisphere, but the cells are wired up to to control only the muscles on the opposite side of the body. Neuroscientists hope to one day have a "brain-machine interface" that can capture this trapped knowledge in the good hemisphere and direct it to the muscles controlled by the damaged one.
Reassignment and rewiring under certain circumstances may be more successful in rehabilitating language use.
Normally, the language centers in the left hemisphere suppress the activity of the areas in the right hemisphere corresponding to them. When the left hemisphere is damaged, the suppression is lifted and the corresponding areas on the right become more active. Whether that is always helpful for recovery, however, is uncertain.
Only in small children or the very rare adult can the right hemisphere fully take over language. For a good recovery, some of the language function on the left must survive.
Calling on the undamaged side of the brain appears to work even better for complicated activities such as watching and remembering a changing group of objects.
Recent research by Bradley Voytek and Robert T. Knight at the Helen Wills Neuroscience Institute at the University of California at Berkeley has shown that a damaged left frontal lobe can handle this kind of task as long as it's simple. But the harder the task gets, the more the brain turns to the right frontal lobe for help.
"What these studies show you is that the brain is moving things around," Knight said. "It is taking a piece of information and getting it to the part of the brain that can solve the problem."
Targeted rehab and hard effort are essential to maximizing all of this. That's what's behind "constraint-induced therapy" in which the patient's good limb is restrained, forcing use of the weak one - and driving neuroplasticity. It's also the basis for aggressive speech therapy, and the need for more of it.
"Patients need eight or nine hours of speech therapy a week, and typically they don't get that as outpatients," said Washington University's Corbetta. "That's why it's very important to supplement what you get in a clinic with home exercises and computer exercises."
Therapy is just beginning for even subtler functions that are less hard-wired than vision and movement and may be even more recoverable.
"The rehab world focuses on motor activity, but that's the hardest thing to fix," Knight said. "We are now seeing increasing interest in rehabilitation of more abstract things, such as how good a patient is in focusing attention, in problem solving, in 'executive control.' "