Motor cortex activation by H-coil and figure-8 coil at different depths. Combined motor threshold and electric field distribution study

• Efficacy of stimulation of hand and leg motor cortex is compared between an H-coil and a figure-8 coil.
• The motor thresholds were measured in healthy subjects, and electric field distribution induced by both coils was measured in a head model.
• The combination of the neurophysiological findings and the head model field measurements consistently point towards a more efficient activation of deeper cortical regions using the H-coil.

ObjectiveTo compare the ability of an H-coil and figure-8 coil to stimulate different motor cortex regions.MethodsThe resting (rMT) and active (aMT) motor thresholds were measured for the right hand APB and leg AHB muscles in 10 subjects, using an H-coil and a figure-8 coil. The electric field distribution induced by the coils was measured in a head model. The combination of the hand and leg MTs with the field measurements was used to determine the depth of hand and leg motor areas via the intersection points.ResultsThe rMT and aMT of both APB and AHB were significantly lower for the H-coil. The ratio and difference between the leg and hand rMT and aMT were significantly lower for the H-Coil. Electric field measurements revealed significantly more favorable depth profile and larger volume of stimulation for the H-coil. The averaged intersection for the APB was at a distance from coil of 1.83 ± 0.54 cm and at an intensity of 97.8 ± 21.4 V/m, while for the AHB it was at a distance of 2.73 ± 0.44 cm and at an intensity of 118.6 ± 21.3 V/m.ConclusionThe results suggest a more efficient activation of deeper motor cortical regions using the H-coil.SignificanceThe combined evaluation of MTs by H- and figure-8 coils allows measurement of the individual depth of different motor cortex regions. This could be helpful for optimizing stimulation parameters for TMS treatment.