The effect of transcranial magnetic stimulation (TMS) induced electric field highly depends on its orientation regarding the brain. The brain here, acts as a "pickup" coil. The magnetic field induces perpendicular electrical currents (parallel to the induction coil) in the brain tissue underneath the coil.
The 8-digit coil with MagVenture System in NYU CBI
After I chose to perform a TMS study, I learned from the experience of Grace, a former post-doc in Curtis Lab, on how to position the TMS Coil for those regions of interest (ROIs) of visual retinotopic maps.
Because the region of maximum stimulation of the figure 8 coil used at CBI is under the center, the only thing uncertain in positioning is picking up orientation. Here is her advice:
The coil should be positioned tangentially on the scalp, with the figure-of-eight of the coil itself parallel to the ROI of interest (in other words, with the handle of the coil perpendicularly bisecting the middle most portion of the ROI).
Shout out to Grace. She was truly resourceful and a pro of TMS! She was correct about it. I didn't think deeply about the reason behind it until recently when I met some issues when I am collecting my own TMS data massively:
IPS1 on the inflated surface
IPS1 on the pial surface (invisible)
Therefore, I went on the quest of what defines the best orientation of the TMS coil and why.
In searching for optimal TMS coil orientation, the hit topic was the resting-state motor thresholding by TMS, where scientists try to find the highest or most stable motor-evoked potential amplitude per individual. Based on Janssen et al. (2015):
In general, the optimal coil orientation for M1 induces a primary electric field directed at an angle of approximately 45 degrees to the medial-sagittal plane of the subjects head. This orientation induces a posterior-anterior (P-A) directed electric field perpendicular to the central sulcus.
It’s reasonable that 45-degree angle is the standard, considering the orientation of the primary motor cortex gyrus is also often at 45 degrees. This orientation induces a posterior-anterior (P-A) directed electric field perpendicular to the central sulcus.
AFNI-SUMA view of a subject’s pial surface. The primary motor cortex lies on the gyrus anterior to the central sulcus (black line).
A prevailing explanation for the coil orientation preference of M1 stimulation is given by the theoretical cortical column cosine model of TMS efficacy ($C_3$-model), based on the cortical column as the functional unit (Fox et al., 2004).
The cortical-column aligned electric field (perpendicular to and directed into the cortical surface) contributes most to the TMS-induced brain activation because the field will be longitudinal and orthodromic to the greatest possible number of cortical neurons at the site of interest.