Commentary on: Deng et al., Electric field depth–focality tradeoff in transcranial magnetic stimulation: Simulation comparison of 50 coil designs

* Corresponding author. Tel.: þ972 8 934 4415; fax: E-mail address: azangen@bgu.ac.il (A. Zangen). 1935-861X/$ e see front matter 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.brs.2012.04.003 The review by Deng et al. in this edition modeling 50 theoretical and practical coils [1] is a comprehensive and important article that compares the electric field depth-focality profile of various coils using a spherical head model. The use of a spherical model facilitates the definition of a simple and universal metric framework for a convenient comparison of the depth and focality characteristics of all TMS coils. Yet, the gain in generality and simplicity of this approach comes at the expense of losing some valuable features that must be acknowledged. First, the human brain geometry is obviously significantly different from the ideal spherical form, hence the induced electric field distributionwill have different features in the two cases. In the realistic head geometry, the resulting electric field distribution will be much more sensitive to the coil position and orientation since the head surface is non-uniform and with a variable curvature. The analogousmetric definition of d1⁄2 in a realistic head should be based on the distance from the deepest point having field at or above E1⁄2, relative to its closest point on the brain surface. Second, in most practical clinical applications, a certain degree of symmetry break is required. For example, a spherically-symmetric stimulation of a cortical ring around the brain is usually undesired, but instead a complex distribution inducing suprathreshold electric field in certain targeted brain regions e including deep regions e with simultaneous minimization of the effect at other brain regions. It is imperative tominimize adverse side effects such as excessivemotor activation (which may increase the risk of seizure), facial muscle activation, and subject pain or discomfort. The degree and type of