Modulatory actions of inputs from the visual system to cervical interneurons (IN) for arm muscle control are poorly understood in humans. In the present study, we examined whether visual stimulation modulates the excitation of cervical IN systems mediating corticospinal tract (CST) inputs to biceps brachii (BB). Twenty-eight healthy volunteers were seated, and electromyogram recordings from the BB were performed across six experiments, each with discrete objectives. A flash stimulator for visual stimulation (50-μs duration) was placed 60 cm from the participant's eye. The CST was stimulated with transcranial magnetic/electrical stimulation (TMS/TES, respectively) contralateral to the recording site. Visual stimulation with TMS/TES was randomly delivered during weak tonic BB contractions. Single TMS/TES-induced motor-evoked potentials (MEPs) were markedly enhanced from 60-100 ms after visual stimulation compared with the control condition. The MEPs were significantly increased by combining the electrical stimulation of the ulnar nerve at the wrist [7.5-12 ms of nerve stimulation (NERVE)/TMS interval] with and without visual stimulation compared with the algebraic summation of responses obtained with either TMS or NERVE. Interestingly, the combined stimulation-induced MEP facilitation was significantly increased after visual stimulation compared with the control. Single motor unit (MU) recording also revealed the further enhancement of combined stimulation effects on the firing probabilities of MU during visual stimulation, which was observed in the peaks of the peristimulus time histogram, 1-2 ms later than the onset latency. The present findings suggest that visual stimulation facilitates the oligosynaptic CST excitation of arm motoneurons mediated by the cervical IN system.NEW & NOTEWORTHY To date, little is known about how visual information modulates the human cervical motor systems, including the presumed interneuron (IN) circuitry. This study demonstrates that photic visual stimulation influences presumed oligosynaptic corticospinal transmission to arm motoneurons, which are mediated by cervical INs. In animals, these systems are known to be crucial for visually guided switching movements, and similar visual input systems to INs may exist in humans.
Keywords: cervical interneurons; corticospinal tract; motor unit; transcranial magnetic/electrical stimulation; visual stimulation.