 Constraint-induced movement therapy CI therapy is a highly efficacious treatment for residual motor disability in chronic stroke. Its effectiveness is believed to be due, at least in part, to the therapys ability to aid the brain in rewiring itself. For example, CI therapy produces increases in the amount of grey matter the parts of the brain where neuron cell bodies are most closely clustered in certain areas of the human brain Gauthier et al., 2008. The cellular and molecular mechanisms that are responsible for this increase in grey matter volume are not known, however. Thus, it is unclear how the therapy helps brains rewire themselves. This study aims to better understand the timecourse and cellularmolecular nature of brain changes during CI therapy. Because there is currently no way to directly measure cellularmolecular changes in the brain noninvasively, this study will infer what is happening on a microstructural level using new MRI techniques three dimensional pictures of the brain. For example, by charting the timecourse of grey matter changes during CI therapy, and cross-comparing this to what is known about the timecourses of different cellularmolecular processes, the investigators can gain a greater understanding of what cellular processes may be responsible for increases in grey matter. The investigators will gain additional information about which cellular processes are important for rehabilitation-induced improvement by measuring larger-scale changes e.g., amount of blood flow through different brain areas that accompany cellular changes. The investigators are hopeful that by better understanding how CI therapy can change the brain, the effectiveness of rehabilitation can be improved upon. For example, insight into the mechanisms of rehabilitation-induced brain change may suggest particular drug targets to increase brain plasticity. This study will help us better understand how the brain repairs itself after injury. 