(p. 537) TMS and Brain Mapping
Why should we combine TMS with brain mapping? The answer is threefold: (1) to localize the target of stimulation; (2) to measure local and distal response of the brain to the stimulation; and (3) to assess long-term (hours, days, weeks) effects of repetitive TMS. The following four chapters provide a detailed overview of the approaches in this domain, focusing on the combination of TMS with three imaging modalities, namely positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and electroencephalography (EEG).
Over 20 years ago, two very different approaches to brain mapping emerged. In 1985, Barker and colleagues described their system for noninvasive stimulation of the human cerebral cortex. At the same time, the first series of PET studies appeared in which various perceptual, motor, and cognitive processes were mapped into distinct regions using regional cerebral blood flow as an index of brain activity.
In the next 10 years, these two brain-mapping approaches lived rather independent lives. For a variety of reasons and motivations, however, they began to converge in the mid-1990s. The functional neuroimaging community perhaps realized the need for injecting causality into their otherwise correlational approach afforded by measuring changes in brain activity during a task performance, while the TMS community was interested in other-than-motor measures of TMS effects on the brain.
Introduction of repetitive TMS made it possible to test behavioral effects of brain stimulation in healthy volunteers. But those interested in targeting nonmotor cortical regions in their cognitive studies needed a tool allowing them to localize precisely the region of interest. This need was answered by the use of frameless stereotaxy, which uses structural MRI for this purpose.
The interest of the TMS community in measuring more than motor-evoked potentials coincided with the growing interest of the functional neuroimaging community in developing new techniques for studying functional (or effective) connectivity. This common goal was achieved by combining TMS with PET and EEG and, a few years later, with fMRI. An overview of these initial efforts, as well as the description of the technical know-how and the latest developments in this area, is the main focus of this section. (p. 538)