P 10. Acute state-dependent changes in corticomotor threshold in the absence of overt motor activity

Abstract

The cortical motor threshold (CMT) is often used to individually adjust the intensity of transcranial magnetic stimulation (TMS). It reflects the integrated excitability of the corticomotor projection, including the excitability at the spinal level. The CMT reflects axonal and synaptic excitability as CMT can be modified by drugs that voltage-gated sodium channel blockers or non-NMDA glutamatergic drugs. The CMT is not a static measure but is subject to state-dependent fluctuations. To test this hypothesis we used an adaptive stair-case procedure to track acute changes in CMT associated with motor imagery (MI) or shifts in visuospatial attention (VA). Eight healthy subjects were asked to imagine a continuous thumb-index movement with the left or right hand (MI-left and MI-right), to shift their visuospatial attention to the left or right FDI (VA-left and VA-right) or to fixate a cross in the central visual field. Using a fast (<15 pulses) threshold-hunting procedure (Borckardt, 2006), we assessed task-dependent CMT changes in the left primary motor hand area (M1) during each task. CMT was defined as the stimulus intensity, at which a single TMS pulse elicited a MEP of 50 μV in the relaxed right first dorsal interosseus muscle (FDI) with a probability of 0.5. The CMT measurements during the VA and MI task were normalized to the control condition. All participants underwent two experimental sessions on separate days, in which the TMS pulse either induced a posterior-to-anterior (PA) or anterior-to-posterior (AP) current in M1. The number of pulses needed to determine rMT did not significantly differ across conditions (average: 13.5). Using the normalized CMT values as dependent variable, we computed a repeated measures ANOVA with the factors Current (AP/PA), Task (MI/VA) and Site (Left/Right). We found a significant Task-by-Site interaction ( p = 0.004). Post hoc testing confirmed that CMT during MI-right was significantly lower than during MI-left or during VA-right. This was the case for AP and PA current direction. The resting CMT is subject to rapid state-dependent changes in the absence of overt motor activity. We show that these acute CMT changes can be captured with a threshold-hunting procedure. The “CMT tracking” approach might be particularly useful to assess the temporal dynamics of corticomotor excitability changes induced by interventional brain stimulation protocols.