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

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Abstract

Introduction

The double-cone coil (D-CONE) is frequently used in transcranial magnetic stimulation (TMS) experiments that target the motor cortex (M1) lower-limb representation. Anecdotal evidence and modeling studies have shed light on the off-target effects of D-CONE TMS but the physiological extent remains undetermined.

Purpose

To characterize the off-target effects of D-CONE TMS based on bilateral corticospinal responses in the legs and hands.

Methods

Thirty (N = 30) participants (9 women, age: 26 ± 5yrs) completed a stimulus–response curve procedure with D-CONE TMS applied to the dominant vastus lateralis (cVL) and motor-evoked potentials (MEPs) recorded in each active VL and resting first dorsal interosseous (FDI). As a positive control (CON), the dominant FDI was directly targeted with a figure-of-eight coil and MEPs were similarly recorded in each active FDI and resting VL. MEPMAX, V50 and MEP latencies were compared with repeated-measures ANOVAs or mixed-effects analysis and Bonferroni-corrected pairwise comparisons.

Results

Off-target responses were evident in all muscles, with similar MEPMAX in the target (cVL) and off-target (iVL) leg (p = 0.99) and cFDI compared with CON (p = 0.99). cFDI and CON MEPMAX were greater than iFDI (p < 0.01). A main effect of target (p < 0.001) indicated that latencies were shorter with CON but similar in all muscles with D-CONE.

Discussion

Concurrent MEP recordings in bilateral upper- and lower-extremity muscles confirm that lower-limb D-CONE TMS produces substantial distance-dependent off-target effects. In addition to monitoring corticospinal responses in off-target muscles to improve targeting accuracy in real-time, future studies may incorporate off-target information into statistical models post-hoc.

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Data availability

The raw data and material supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

References

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Funding

Funding for this study was provided by the Department of Defense (Award number: W81XWH-16-PHTBIRP-CR3A).

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Authors and Affiliations

Authors

Contributions

SDF, BCN, AG, CC: funding acquisition; FP, MCC, MEB, AMS, SRE, ADL, WRC, SAJ, SDF: data collection; FP, SDF, MCC: data analysis; FP, MCC, SDF: data interpretation; FP, SDF: manuscript draft; all authors contributed to the manuscript editing.

Corresponding author

Correspondence to S. D. Flanagan.

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Conflict of interest

None of the authors report a conflict of interest.

Additional information

Communicated by Bill J. Yates.

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Cite this article

Proessl, F., Canino, M.C., Beckner, M.E. et al. Characterizing off-target corticospinal responses to double-cone transcranial magnetic stimulation. Exp Brain Res 239, 1099–1110 (2021). https://doi.org/10.1007/s00221-021-06044-5

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  • DOI: https://doi.org/10.1007/s00221-021-06044-5

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