Transcranial magnetic stimulation--a new tool for functional imaging of the brain

Recent progress in the theory and technology of transcranial magnetic stimulation (TMS) is leading to novel approaches in brain mapping. TMS becomes a powerful functional brain mapping tool when other imaging methods are used to record TMS-evoked activity or when peripheral effects are observed as a function of stimulus location. TMS-evoked activity currently can be recorded by EEG, PET, and fMRI. In addition to providing indices of cortical excitability, these methods allow one to study brain connectivity directly, without the need for behavioral activations. When the coordinate systems in the different imaging modalities are combined, anatomical structures seen in MRI and activation sites determined by PET, fMRI, or MEG/EEG can be used for the selection of target areas in the brain. PET and fMRI can be used to map the spatial distribution of TMS-evoked activity. On the other hand, the combination of TMS and high-resolution EEG may often be the method of choice for basic neuroscience and for clinical diagnosis, for example, in the assessment of brain connectivity in patients suffering from neurodegenerative diseases or head injuries.