for maintaining functions, there is a struggle for stability that is only partially achieved. Better stability may be possible, but perhaps not in systems that are also designed to change and to acquire new memories, and new functions. Our brains must be capable of processing information, retrieving information, and controlling behavior in a predictable and reliable manner, and this would seem to depend on morphologically and functionally stable machinery, the circuits of the brain. And yet, the circuits cannot be completely stable if we are going to acquire new skills and abilities, and adjust to sensory loss and other impairments. We also realize, as memories fade and distort, that brain circuits change even when you do not try to change them or want them to change. It may be true that once you learn to ride a bike, you can always ride a bike; however, it is not true that skills and abilities can be maintained at a high level without practice. The professional athlete or musician fully realizes how much effort is needed just to maintain abilities. These talented individuals practice to fine-tune the neural circuits used in their professions. Experience both changes and maintains.

Practical procedures for using experience and training to achieve and maintain abilities, and recover from impairments need not depend on any knowledge of the brain, its circuits, and how they are altered. Effective procedures have been and are being empirically derived. Nevertheless, altering or maintaining brain circuits is what training, practice, and therapy is about, and in principle procedures could be improved if we understand the brain, and how it is modified. We need to learn how to modify brain circuits most effectively when we want change, and to maintain brain circuits when we do not.

In this chapter, we consider evidence that many brain circuits, perhaps all, can be modified by experience, even in adult humans and animals, although less powerfully in adults than in some stages of development. We also review some of the mechanisms of rewiring brain circuits, so that ways of using these mechanisms can be considered. This review concentrates on the somatosensory and the motor systems, where research has lead to much progress over the last 20 years. Related reviews include Buonomano and Merzenich (1998), Chino (1997), Dykes (1997), Ebner et al. (1997), Kaas (1996), Kaas et al. (1997), Nicolelis (1997), and Nudo et al. (1997). The focus is on brain plasticity that may be important in recoveries from sensory loss, brain injury, and errors in development, as well as in gaining new sensorimotor skills and perceptual abilities. Important neural aspects of learning and memory are reviewed elsewhere (Cohen and Eichenbaum, 1993; Eichenbaum, 1997; Squire and Zola, 1997; Tulving and Morkowitsch, 1997; Salmon and Butters, 1995).

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