The future of TMS
The recent technical achievement of combining TMS with PET and MRI has been useful in studying the connectivity of the human brain, in validating the specificity of TMS and in guiding the location of TMS application. Looking to the future, the next step is to combine TMS, fMRI and PET in behavioral studies. One method that has already been used successfully is the application of low frequency TMS (for example, 1 Hz for 15 minutes) 20 minutes before the subject performs a task in the scanner. Low frequency rTMS reduces blood flow in the region stimulated for several minutes and can also produce a concomitant reduction in behavioral performance of tasks that rely on that region.
Figure 1 | Spatial and functional specificity of TMS.
This specificity is evident in the correspondence between blood flow changes induced by TMS over the motor cortex to produce a finger movement, and the activity produced by an intentional movement, which also produces supplementary motor area activity.
TMS has been used in studies of cognitive and sensory functions for over a decade, although much less extensively than functional neuroimaging by PET or fMRI. Its period of being a ‘new and exciting’ technique is over and work with TMS must now be judged within the theoretical frameworks used to evaluate other neurocognitive approaches.
TMS has clearly made a contribution to the understanding of perception, attention awareness and plasticity.
Further progress, however, will depend on the application of TMS in other areas such as the neuropsychology of language and memory.
The combination of TMS with other techniques, in conjunction with formal predictions on the basis of lesions of neural networks should provide fruitful avenues of research.
Many procedural and methodological hurdles remain, however, and the reliability and replicability of TMS effects in cognitive studies is a long way from that achieved in neurophysiological studies. The effects of TMS at anatomically connected areas also require careful consideration when developing good control procedures. It is to be hoped that the accumulation of experience from different groups will allow the further development of this method. One possibility for ‘fast tracking’ the methodological advance of TMS in studies of cognition might be some form of data sharing, as recently suggested for other areas of neuroscience. There are good reasons to approach this with caution, but in some cases, for example, mapping phosphenes on individual MRI scans or comparing the effects of different coils in similar experimental situations, data sharing may lead to faster and more efficient methodological advancement”.
