On the Effective Resolution of TMS, its Tradeoff with Signal-to-Noise Ratio, and the Experimental Implications

Experimental data have indicated that transcranial magnetic stimulation (TMS) has high spatial specificity. For instance, even a millimeter-scale movement of the TMS coil above the motor cortex can change the recorded response in peripheral muscles drastically. However, such a small coil displacement induces a cortical electric field that overlaps considerably with the original electric field. Here, we describe how the basic electrophysiological properties of neurons can explain the apparent high spatial resolution of TMS. We show with simulations that a TMS intensity close to the neuronal threshold yields the highest spatial resolution, albeit with a possible tradeoff with the signal-to-noise ratio. The simulations suggest that with such a TMS intensity, the activated cortical area is significantly smaller than the area in which the electric field exceeds its half-maximum strength.