A subcortical origin for rapid, target-oriented corticospinal excitability changes during visually guided reaching

During visually guided reaching, proximal limb muscles can be activated within 80 ms of target appearance. Such “express” visuomotor responses are temporally aligned with target appearance rather than movement onset, and invariably tuned towards the direction of the visual target regardless of the instructed reach direction. These features prompt the hypothesis that express visuomotor responses are driven by a subcortical pathway. We tested this by measuring the changes in Motor Evoked Potentials (MEPs) following Transcranial Magnetic Stimulation (TMS) or Transcranial Electrical Stimulation (TES) of the motor cortex as participants reached either towards or away from visual targets. We found that 70-80 ms after target presentation, MEPs in primary shoulder flexor muscle (pectoralis major) were oriented towards the target direction regardless of whether the participant subsequently reached towards or away from the target. Similar target-oriented MEP modulations were seen in posterior deltoid and biceps brachii, whereas MEPs in a finger muscle were affected neither by target nor reach direction. Critically, there were no significant differences in modulation of responses to TMS and TES across all reaching conditions, which suggests that the target-oriented modulation occurs downstream of the motor cortex output neurons. Combined, our results show that rapid target-oriented corticospinal excitability changes are tuned to the location of the visual target and at least partly driven by a subcortical pathway. A prime candidate for such subcortical modulation involves the superior colliculus and reticular formation.