96. Read the following passage, then answer the question below.

For much of the twentieth century, scientists believed that the adult brain was essentially fixed: neurons could die but not be replaced, and the brain's wiring was settled in early childhood. This view began to erode in the 1960s and 1970s, when researchers observed that animals deprived of sight in one eye early in life developed brains in which the unused regions were taken over by the functioning eye. The brain, it seemed, was capable of reorganizing itself in response to experience, a property now called neuroplasticity.

Studies of adults soon supplied further evidence. London taxi drivers, who must memorize some twenty-five thousand streets to earn a license, were found to have a larger hippocampus, a region involved in spatial memory, than the average person. Moreover, the size of the region correlated with the number of years spent driving, suggesting that the difference was a result of the work rather than something drivers were born with. Similar findings emerged for musicians, whose brain areas controlling finger movement are expanded.

These discoveries have encouraged new approaches to rehabilitation. Stroke patients, for example, can sometimes regain the use of a paralyzed limb if the healthy arm is restrained, forcing the patient to practice with the weak one. Over weeks, neighboring brain regions appear to take over functions lost to damage.

Popular accounts, however, sometimes overstate the case. Plasticity is not unlimited, and it is not always beneficial: the same mechanism that allows recovery can also strengthen the pathways of chronic pain. Commercial brain training games, many scientists note, improve performance on the games themselves but have shown little transfer to everyday abilities. The lesson of neuroplasticity may be that the brain changes most when it is challenged in specific, sustained ways, and that no shortcut exists.

Why is the healthy arm of a stroke patient sometimes restrained?