• DOI:10.3389/fpsyg.2018.01171
• Corpus ID: 49573037

Fancies and Fallacies of Spatial Sampling With Transcranial Magnetic Stimulation (TMS)

@article{Cattaneo2018FanciesAF,
  title={Fancies and Fallacies of Spatial Sampling With Transcranial Magnetic Stimulation (TMS)},
  author={Luigi Cattaneo},
  journal={Frontiers in Psychology},
  year={2018},
  volume={9}
}

• L. Cattaneo
• Published 5 July 2018
• Biology
• Frontiers in Psychology

Transcranial magnetic stimulation (TMS) is a procedure by which an electrical field is induced in the brain. The ensuing extracellular electrical current determines supra-threshold depolarization of the neuronal membrane, ultimately resulting in action potentials. Inductor coils with appropriate shapes produce peaks in the induced magnetic field that result in induced electrical fields that are referred to as “focal” because their size is over two magnitudes smaller than the cortical surface that potentially could be sampled. Focal TMS is commonly used in cognitive neuroscience as a brain mapping tool, though certain applications of TMS escape the brain-mapping logic, for example, manipulations with repetitive TMS in the domain of cortical rhythms, TMS used to produce a measurable cortical output (such as MEPs or phosphenes) or the use of TMS in statedependent paradigms. These experimental strategies will not be the object of discussion here. The focal electrical field of TMS in brain mapping is used as a probe for sampling the 2-dimensional space of the cortical surface. TMS does not produce data by itself but only if coupled with behavioral or physiological measures. Therefore, TMS experiments describe the spatial distribution of the effects of focal stimulation on any behavioral measure that has been chosen. Consequently, in cognitive neuroscience, TMS results are commonly described in cartographic terms: “cortical area x is necessary for behavioral function y.” In my opinion, there are several caveats to be considered before similar inference in the spatial domain can be drawn. As any functional brain-mapping procedure, TMS-brainmapping is a process of spatial sampling and reconstruction, similar to those employed in many communication systems as digital photography or video broadcasting (Dubois, 1985). The brain is the (analogic) native image, which is sampled, obtaining an array of points, which are then used to reconstruct the original image (the brain map and, ultimately, our model of how the brain works). An efficient sampling-reconstruction procedure produces a reconstructed image that is as similar as possible to the native image.