The Impact of Finger Stimulation on Corticospinal Excitability: Insights from the DuoMAG MP-Dual. 

Transcranial magnetic stimulation (TMS) has revolutionised our understanding of brain function, enabling non-invasive exploration of neural pathways and their interactions. In a recent study, researchers delved into the intricacies of how stimulating different finger regions affects the modulation of corticospinal excitability of hand muscles. A key tool for Bao et al. (2024) was the DuoMAG MP-Dual TMS system, which played a crucial role in elucidating these complex neural dynamics. 

 

The Core Question: Homotopic vs. Heterotopic Finger Stimulation 

 

At the heart of this research lies a fascinating inquiry: How does the stimulation of homotopic (same side) versus heterotopic (opposite side) fingers, or the simultaneous stimulation of multiple fingers, influence the modulation of corticospinal excitability of the hand muscles? Furthermore, Bao et al. (2024) sought to determine if tactile inputs from finger areas with higher innervation densities, such as the fingertips, result in more pronounced modulation of corticospinal excitability compared to areas with lower innervation densities. The study involved 31 right-handed, healthy participants, including both males and females, all of whom were new to the experimental paradigm. They underwent a series of experiments where surface electromyography (EMG) recorded activity from the first dorsal interosseous (FDI) muscle of the hand. This muscle, crucial for finger movement, served as the target for observing changes in corticospinal excitability. 

 

To deliver tactile stimulation, researchers used a novel multichannel pneumotactile stimulation device, which closely mimics natural touch. This device selectively activates cutaneous mechanoreceptors without causing discomfort, providing a realistic sensory experience akin to "taps" or "raindrops" on the skin. EMG signals were amplified, filtered, and digitized for precise analysis. The DuoMAG MP-Dual TMS system was instrumental in this study. This system allows for precise magnetic stimulation of the brain, crucial for targeting specific cortical areas. The TMS coil was positioned over the scalp to induce currents in the brain's primary motor cortex (M1), effectively activating this region and eliciting motor-evoked potentials (MEPs) in the FDI muscle. 

 

Experiment 1: Single vs. Multiple Finger Stimulation 

 

In the first experiment, tactile stimuli were applied to different finger regions: the index fingertip, base of the index finger, ring fingertip, all five fingertips, or no stimulation. The TMS-induced MEPs were then measured to assess changes in corticospinal excitability. Stimulation of the index fingertip, the base of the index finger, or all five fingertips significantly suppressed MEP amplitudes compared to no stimulation. Notably, stimulating all five fingers simultaneously led to greater suppression than stimulating individual fingers. This suggests that the combined sensory input from multiple fingers has a more profound impact on corticospinal excitability. 

 

Experiments 2-4: Intracortical Mechanisms 

 

These experiments explored how tactile stimulation affects intracortical facilitation (ICF), short-interval intracortical inhibition (SICI), and long-interval intracortical inhibition (LICI) in M1. 

  • ICF: Tactile stimulation enhanced ICF, indicating that sensory input boosts excitatory neural circuits in M1. 
  • SICI: No significant modulation by tactile stimulation, suggesting that this inhibitory mechanism is less influenced by peripheral sensory input. 
  • LICI: Tactile stimulation reduced LICI, highlighting a complex interaction where sensory input diminishes longer-lasting inhibitory processes. 

 

Experiment 5: The Role of S1 

 

The final experiment probed the influence of the primary somatosensory cortex (S1) on tactile-motor interactions. Using continuous theta burst stimulation (cTBS) to modulate S1 activity, researchers observed changes in afferent inhibition. Downregulation of S1 via cTBS reduced afferent inhibition, underscoring S1's pivotal role in integrating tactile and motor functions. 

 

This study sheds light on the nuanced ways tactile inputs from the hand influence corticospinal excitability and intracortical mechanisms. The findings highlight the importance of the spatial characteristics of tactile stimuli and their potential to shape skilled hand movements. The DuoMAG MP-Dual TMS system proved to be an invaluable tool in uncovering these complex interactions, paving the way for future research into sensory-motor integration and its applications in neurorehabilitation and motor control therapies. 



Bao, S., Wang, Y. and Escalante, Y.R. et al. (2024). Modulation of motor cortical inhibition and facilitation by touch sensation from the glabrous skin of the human hand. eNeuro 11(3). DOI: https://doi.org/10.1523/ENEURO.0410-23.2024