Muscle-specific modulation of indirect inputs to primary motor cortex during action observation

Area of Science:

• Neuroscience
• Motor Control
• Human Motor System

Background:

• Single-pulse transcranial magnetic stimulation (spTMS) shows movement observation enhances corticospinal excitability in primary motor cortex (M1) muscle-selectively.
• Motor evoked potentials (MEPs) from spTMS involve multiple descending volleys (indirect waves or I-waves), but their specific roles in action observation remain unclear.

Purpose of the Study:

• To investigate how action observation modulates different components of corticospinal excitability, specifically focusing on short-intracortical facilitation (SICF) peaks.
• To determine which SICF components are associated with muscle-specific M1 modulation during observation of different actions.

Main Methods:

• Participants observed precision and whole-hand grip actions.
• Motor evoked potentials (MEPs) were recorded from index (FDI) and little finger (ADM) muscles.
• Short-intracortical facilitation (SICF) was assessed using paired-pulse TMS at interstimulus intervals of 1.3, 2.5, and 4.1 ms.

Main Results:

• Both early (1.3 ms) and late (2.5, 4.1 ms) SICF peaks were modulated by movement observation.
• Only the late SICF peak at 2.5 ms interstimulus interval showed a significant association with muscle-specific corticospinal excitability modulation.
• This finding suggests the 2.5 ms SICF peak reflects cortico-cortical pathway activity influencing M1 muscle representations.

Conclusions:

• Movement observation activates diverse neural circuits within M1.
• Cortico-cortical communication plays a crucial role in the muscle-specific M1 modulation observed during action observation.
• Late SICF peaks, particularly at 2.5 ms, are key indicators of these cortico-cortical interactions in M1 during action observation.