Reorganization of Cortical Maps

[From Bruce Abbott (2013.12.01 12:15 EST)]

Lately I’ve been reading a book by Norman Doidge (2007), The Brain that Changes Itself. It’s about a currently hot topic, brain plasticity, and reviews a number of case studies that support the thesis that the brain is far more plastic – able to rewire itself – than was formerly believed. The term the author uses to label this rewiring process is — wait for it — reorganization.

The first case reviewed concerns a woman whose vestibular system was poisoned by excessive use of the antibiotic gentamicin to treat a post-operative infection. Lacking perceptual signals from the organs of balance, she felt as though she were perpetually falling. The problem was treated by fitting the woman with a helmet that was equipped with accelerometers that sent electrical signals to device shaped like a stick of gum that was placed on her tongue. This device produced sensations like fizzy bubbles whose positions on the tongue changed with the accelerations of her head. Within a short time she was able to use this substitute feedback to maintain her balance. But what was more amazing, after sufficient exposure to the device, she was able to continue maintaining her balance without the device.

Other cases described how folks who had suffered strokes that resulted in limb paralysis regained the use of their paralyzed limbs by forcing the patients to use them. For example, when an arm was paralyzed, the “good” arm was restrained so that the patient had to use the “bad” arm. Patients were encouraged whenever they were able to show some progress toward completing a desired move (such as grasping a cup) and the criterion for success was gradually made closer to the desired movement as the patient successfully reached the previous criterion (a technique known as shaping by successive approximations).

The explanation given for these recoveries is that the cortical “maps” normally active when the limb was moved prior to the stroke had begun to degenerate through disuse after the stroke. Cortical mapping studies showed that these maps become less differentiated and may to some extent be “taken over” by adjacent cortical areas serving a different function. By forcing the patient to use the affected limb, these areas were being reorganized to serve reestablish control over that limb.

Other chapters in the book deal with other examples of treatments being devised based on the idea that the brain can be induced to rewire itself (such as a treatment for “phantom limb” in patients who have suffered limb amputations and now experience unpleasant sensations that seem to be located in the missing limb. The latter work was initiated by V. S. Ramachandran using a “mirror box.” A patient with, say, a missing hand places his normal arm in the box and the arm with the missing hand is placed in position so that the reflection of the hand seems to be the patients missing hand. The patient is instructed to move the fingers etc. of his hand and sees those same fingers move on his now magically restored missing hand. A patient who suffered from the feeling that the fingers of her missing hand were clenched painfully into her palm soon learned to unclench it and relieve the pain.

These examples show that reorganization does indeed go on in the brain and that reorganization goes on throughout life, although sometimes special conditions must be arranged to induce reorganization to occur in the right pathways.

Ramachandran is quoted in the book as saying something that is a bit off the topic of reorganization but which I think is worth repeating here. Doidge sets up the quote as follows:

He [Ramachandran] is a sleuth, solving mysteries one case at a time, as though utterly unaware that modern science is now occupied with large statistical studies. He believes that individual cases have everything to contribute to science. As he puts it, “Imagine I were to present a pig to a skeptical scientist, insisting it could speak English, then waved my hand, and the pig spoke English. Would it really make sence for the skeptic to argue, ‘But that is just one pig, Ramachandran. Show me another, and I might believe you!’

I haven’t finished the book yet. Next chapter up: Imagination.

Bruce A.