(p. 409) TMS in Perception and Cognition
The ability to interfere with the activity of cortical regions by using TMS in cognitive neuroscience experiments has opened a new world of lesion methodology. In describing the functions of brain areas in cognition, lesions have the last word: whatever correlations can be made between cognitive functions and brain activity measured by electrode recordings or brain imaging, the test of the contribution of a brain area to a task lies in either testing patients who have damage to specific areas, removing or deactivating the area in animals, or using TMS to disrupt the normal functioning of the area.
Since Amassian's experiments on vision (e.g. Amassian et al. 1989), TMS has been seized upon as a means of making inferences about brain function and testing neuropsychological theories. The advantages of TMS over testing patients and lesions in animals are many. The ability to use subjects as their own controls allows one to study the effects of learning and plasticity while avoiding the problems caused by reorganization in patients. An important consequence of this difference between TMS interference and real lesions is that it leads to ways in which TMS can give usefully different results from the findings with neuropsychological patients. For example, despite years of assumption that the right posterior parietal cortex is important for visual search, it is clear that this is only the case when the search is novel rather than familiar to the subjects (Walsh et al. 1998). To discover this requires being able to train subjects—almost impossible for experimental purposes with neuropsycholgical patients—and test them at different levels of expertise. Equally important to the value of TMS is the temporal precision with which interference can be induced. To test psychological or physiologically inspired theories of cognitive functions requires one to be able to dissect the time course of neural processes. This is all but impossible with patients, but a range of paradigms have been developed for TMS timing studies (see Walsh and Pascual-Leone 2003).
The chapters in this section address most of the core subject areas of cognitive neuroscience—language, vision, awareness, numerical processing, action, plasticity, and memory—and they provide a snapshot of where the field stands at this point in time. For anyone embarking on TMS studies in cognition this survey is a good place to start.
Amassian VE, Cracco RQ, Maccabee PJ, Cracco JB, Rudell A, Eberle L (1989) Electroencephalography and Clinical Neurophysiology 74, 458–462.
Walsh V, Pascual-Leone A (2003) Transcranial magnetic stimulation: a neurochronometrics of mind. Cambridge, MA: MIT Press.
Walsh V, Ashbridge E, Cowey A (1998) Neuropsychologia 36, 363–367. (p. 410)