Characterizing off-target corticospinal responses to double-cone transcranial magnetic stimulation

Area of Science:

• Neuroscience
• Biomedical Engineering

Background:

• The double-cone coil (D-CONE) is widely used for transcranial magnetic stimulation (TMS) targeting the motor cortex's lower-limb area.
• Previous studies suggest D-CONE TMS has off-target effects, but their physiological extent is not fully understood.

Purpose of the Study:

• To quantify the off-target effects of D-CONE TMS by examining corticospinal responses in bilateral leg and hand muscles.
• To compare the stimulation patterns of D-CONE TMS with a figure-of-eight coil.

Main Methods:

• Thirty participants underwent stimulus-response curve procedures with D-CONE TMS applied to the vastus lateralis (VL).
• Motor-evoked potentials (MEPs) were recorded from bilateral VL and first dorsal interosseous (FDI) muscles.
• A figure-of-eight coil served as a control, targeting the FDI with MEPs recorded from bilateral FDI and VL.

Main Results:

• Off-target responses were observed in all recorded muscles.
• MEP maximums (MEPMAX) were similar between the targeted and off-target leg muscles and comparable to the control condition in the hand.
• Stimulation latencies were shorter with the figure-of-eight coil but consistent across muscles with D-CONE TMS.

Conclusions:

• Lower-limb D-CONE TMS induces significant, distance-dependent off-target effects on corticospinal excitability.
• Real-time monitoring of off-target muscle responses can improve TMS targeting accuracy.
• Future research should integrate off-target effects into statistical models for more precise analysis.