Dynamic Changes in Upper-Limb Corticospinal Excitability during a 'Pro-/Anti-saccade' Double-Choice Task

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Visually guided eye movements influence cortico-spinal system (CSS) excitability in upper-limb muscles.
• This modulation may suggest a covert motor program for manual movement.
• The study investigates if this oculo-manual coupling is context-dependent.

Purpose of the Study:

• To determine if implicit oculo-manual coupling occurs with every saccade or depends on behavioral context.
• To investigate the timing and specificity of CSS excitability changes during oculomotor tasks.

Main Methods:

• Twenty-two healthy adults performed a double-choice eye task (prosaccade/antisaccade) cued by color.
• Single-pulse transcranial magnetic stimulation (TMS) assessed motor cortex excitability at nine time points.
• Motor evoked potentials (MEPs) in three upper-limb muscles were analyzed.

Main Results:

• A facilitation peak in CSS excitability occurred 120 ms before saccade onset in two muscles.
• A generalized reduction in corticomotor excitability followed eye movement.
• Direction-specific modulation of MEPs was observed in the first dorsal interosseous muscle ~150 ms before saccade onset, irrespective of saccade type.

Conclusions:

• Implicit oculo-manual coupling is contingent on the cognitive aspects and motor set of the oculomotor task.
• Direction-specific CSS excitability modulation is linked to perceptual and decision-making processes, not a pre-programmed hand movement.
• Findings challenge previous notions of a fixed coupling between eye and hand motor systems.