Excitability of human motor cortex inputs prior to grasp

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Corticospinal excitability modulates during movement preparation.
• Transcranial magnetic stimulation (TMS) can probe motor cortex excitability.
• Previous work showed paired-pulse TMS facilitates motor-evoked potentials (MEPs) during grasp preparation.

Purpose of the Study:

• To investigate the time course of single- and paired-pulse TMS modulation of corticospinal excitability.
• To determine when object-specific hand shaping information reaches the primary motor cortex (M1).
• To understand the neural mechanisms underlying visually guided grasping preparation.

Main Methods:

• Delivered single- and paired-pulse TMS over the primary motor cortex (M1) in healthy subjects.
• Measured motor-evoked potentials (MEPs) in hand muscles.
• Varied the time interval between object presentation (handle or disc) and TMS delivery.
• Instructed participants to prepare for a visually guided grasp.

Main Results:

• Paired-pulse TMS facilitated MEPs, while single-pulse TMS suppressed MEPs.
• Object-specific facilitation patterns emerged for TMS delivered ≥150 ms after object presentation.
• This object-specific modulation was absent when TMS was delivered immediately after object presentation or decoupled from the grasp cue.

Conclusions:

• Object-related information for hand shaping reaches M1 shortly before grasp execution.
• Anticipatory modulation of corticospinal excitability reflects visuomotor guidance of hand shape.
• The timing of information processing in M1 is critical for precise motor preparation.