Cortical mechanisms underlying stretch reflex adaptation to intention: a combined EEG-TMS study

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Voluntary motor acts rely on rapid reflexes like short- and long-latency stretch reflexes (SLSR and LLSR) to counteract external forces.
• The long-latency stretch reflex (LLSR) involves a transcortical loop and can be modulated by a subject's intention.

Purpose of the Study:

• To investigate the cortical mechanisms underlying intention-related modulation of the LLSR.
• To differentiate cortical processes involved in intention-based LLSR modulation from those of voluntary reactions.

Main Methods:

• Combined Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG) were used.
• Subjects prepared for wrist extension, instructed to either 'resist' or 'let-go' of an external perturbation.
• TMS pulses were delivered during the expectation of perturbation to assess anticipatory cortical activity.

Main Results:

• Early cortical activity (45-75 ms) post-perturbation was greater in the 'let-go' condition and negatively correlated with LLSR amplitude.
• Late cortical activity (>100 ms) was greater in the 'resist' condition, indicating differences in voluntary reaction mechanisms.
• TMS-evoked N100 amplitude decreased when intending to 'let-go', suggesting reduced anticipatory intracortical inhibition.

Conclusions:

• Cortical mechanisms for intention-related LLSR modulation differ from those for voluntary reactions.
• Anticipatory processes in the sensorimotor cortex preset neural activity to adapt early responses to perturbations based on intention.