Population-based corticospinal interactions in macaques are correlated with visuomotor processing.

Visuomotor transformation is a fundamental process in executing voluntary actions. The final steps of this transformation are presumed to take place in the corticospinal (CS) system, yet the way in which the motor cortex (MC) interacts with spinal circuitry during this process is unclear. We studied neural correlates of visuomotor transformation in the MC and cervical spinal cord while monkeys performed an isometric wrist task. We recorded 2 measures of population activity: local field potential (LFP), reflecting local synaptic inputs and multi-unit activity (MUA), reflecting spiking activity emitted by nearby neurons. We found robust cortical and spinal responses locked to visual and motor events. In motor cortex, LFP responses were predominantly visually related; MUA responses were mostly motor related. Spinal LFP responses were generally weak, yet spinal MUAs showed visual and motor responses with distinctive patterns. For both structures, amplitudes of visual responses were positively correlated with amplitudes of motor responses and negatively correlated with reaction times. The temporal relations of cortical and spinal responses shifted from weak coactivation before movement to increased coupling following torque onset, with cortical leading spinal activity. Thus, ongoing CS interactions may exist at early stages of movement preparation. These interactions are dynamic and may shape the executed motor action.