Paired Associative Stimulation Targeting the Tibialis Anterior Muscle using either Mono or Biphasic Transcranial Magnetic Stimulation

Abstract

Paired associative stimulation (PAS) protocols induce plastic changes within the motor cortex. The objectives of this study were to investigate PAS effects targeting the tibialis anterior (TA) muscle using a biphasic transcranial magnetic stimulation (TMS) pulse form and, to determine whether a reduced intensity of this pulse would lead to significant changes as has been reported for hand muscles using a monophasic TMS pulse. Three interventions were investigated: (1) suprathreshold PA_bi-PAS (n = 11); (2) suprathreshold PA_mono-PAS (n = 11) where PAS was applied using a biphasic or monophasic pulse form at 120% resting motor threshold (RMT); (3) subthreshold PA_bi-PAS (n = 10) where PAS was applied as for (1) at 95% active motor threshold (AMT). The peak-to-peak motor evoked potentials (MEPs) were quantified prior to, immediately following, and 30 min after the cessation of the intervention. TA MEP size increased significantly for all interventions immediately post (61% for suprathreshold PA_bi-PAS, 83% for suprathreshold PA_mono-PAS, 55% for subthreshold PA_bi-PAS) and 30 min after the cessation of the intervention (123% for suprathreshold PA_bi-PAS, 105% for suprathreshold PA_mono-PAS, 80% for subthreshold PA_bi-PAS. PAS using a biphasic pulse form at subthreshold intensities induces similar effects to conventional PAS.

Keywords: biphasic current; human; paired associative stimulation; plasticity; tibialis anterior; transcranial magnetic stimulation.

Figure 1

Experimental protocol.

Figure 2

Change in tibialis anterior (TA) motor evoked potential (MEP) amplitude after suprathreshold PA_bi-paired associative stimulation (PAS) compared to suprathreshold PA_mono-PAS at rest. (A) TA MEP changes prior to and following the suprathreshold PA_bi-PAS intervention for one participant. Data are the average of 16 trials. (B) TA MEP changes in mV, prior to, immediately following, and 30 min post suprathreshold PA_bi-PAS (n = 11) compared to the suprathreshold PA_mono-PAS (n = 11) intervention. Across both interventions, MEPs were significantly increased immediately after PAS (p = 0.018) and 30 min after PAS (p < 0.001) compared to before PAS. (C) TA MEP immediately following and 30 min post suprathreshold PA_bi-PAS and suprathreshold PA_mono-PAS interventions expressed as a percent change from values prior to the intervention. Asterisks in (B,C) denote significant differences. Error bars represent standard deviation.

Figure 3

Change in TA stretch reflex after suprathreshold PA_bi-PAS at rest. (A) Right ankle angle (°). The vertical line indicates the onset of the imposed plantarflexion perturbation. (B) TA rectified electromyographic (EMG) trace prior to (solid line) and following (dashed line) the suprathreshold PA_bi-PAS intervention. Data are the average for 30 traces.

Figure 4

Change in TA MEP amplitude after subthreshold PA_bi-PAS compared to suprathreshold PA_bi-PAS at rest. (A) TA MEP changes prior to and following the subthreshold PA_bi-PAS intervention for one participant. Data are the average of 16 trials. (B) TA MEP changes in mV, prior to, immediately following, and 30 min post suprathreshold PA_bi-PAS compared to subthreshold PA_bi-PAS intervention (n = 10). Across both interventions, MEPs were significantly increased immediately after PAS (p = 0.028) and 30 min after PAS (p = 0.002) compared to before PAS. (C) TA MEP immediately following, and 30 min post suprathreshold PA_bi-PAS and subthreshold PA_bi-PAS interventions expressed as a percent change from values prior to the intervention. Asterisks in (B,C) denote significant differences. Error bars represent standard deviation.