Introduction

In the last decade, our knowledge about the mechanisms of neurologic injury and recovery has improved. There is now considerable evidence that cortical representations are continuously modulated in response to practice and skill acquisition, a process often referred to as plasticity (Kaas, 1991; Donoghue et al., 1996). Plasticity can also be elicited by lesions in the central and peripheral nervous systems and may take place in cortical as well as subcortical structures (Kaas, 1991; Donoghue et al., 1996; Nudo and Milliken, 1996; Buonomano and Merzenich, 1998; Cohen et al., 1998; Jones and Pons, 1998; see Volume I, Chapters 6, 8 and 14). Cortical plasticity may thus be defined as any enduring change in cortical properties, as, for example, in the strength of internal connections, representation patterns, or neuronal properties, either morphologic or functional (Donoghue et al., 1996). Cortical reorganization can, depending on the settings, contribute to desirable behavioral developments, such as improved performance, or can be linked with unwanted outcomes like phantom pain (Flor et al., 1995; see Volume II, Chapter 15). The primary vehicle for acquiring knowledge on plasticity in the human central nervous system (CNS) has been animal research. Beginning in the 1970s, research from different laboratories (Merzenich et al., 1984; Kaas, 1991) showed that the adult mammalian CNS has the capacity to reorganize after injury. Understanding of mechanisms, development of strategies to purposefully modulate these mechanisms, and translation into rational strategies to promote recovery of function are the goals of modern neurorehabilitation.