The human central nervous system (CNS) can change in response to new environmental challenges or lesions. While such changes are more pronounced in the developing brain, they are also present in adults. It has been a widely held belief that these alterations underlie behavioural modifications such as learning new skills or recovery of lost function after injuries. However, until recently there has been little evidence to support this assertion. The development of neuroimaging and neurophysiological techniques such as functional magnetic resonance imaging (fMRI), positron emission tomography (PET), event-related potentials, electroencephalography (EEG), magnetoencephalography (MEG) and transcranial magnetic stimulation (TMS), have demonstrated that neuroplastic modifications have functional implications.
TMS is a non-invasive technique that allows focal delivery of currents into the brain. It is possible to apply TMS to a specific cortical region and disrupt cortical activity there. Evaluation of the behavioural consequences of this disruption describes some of the functions of that part of the brain. TMS can therefore produce a ‘virtual lesion’ that lasts for milliseconds (Gerloff et al., 1997; Amassian et al., 1989). In the presence of brain reorganization, TMS could be applied to the reorganized cortical regions, while the subject performs a specific task. If TMS, by disrupting the activity of that part of the brain, results in altered performance, it could be inferred that the reorganized cortex plays an adaptive role. In this chapter, we will discuss experimental evidence leading to the identification of the functional role of neuroplasticity using TMS.