Microstimulation mapping of precentral cortex during trained movements.

1. The precentral cortex of three Macaca mulatta monkeys were mapped with intracortical microstimulation (ICMS) while the monkeys performed alternating wrist flexion and extension movements. A forearm cocontraction task was also employed with one monkey. Electromyogram (EMG) recordings from forearm muscles were used to evaluate the results of ICMS. 2. We have found that the results of ICMS can be misleading if EMG activity is not recorded from the responding muscle. Inhibition can be interpreted as excitation if muscle palpation or joint movement are the only response criteria. 3. Movement of the stimulating electrode by as little as 200 microns in a single radial column sometimes changed EMG responses from inhibition to excitation or vice versa, indicating that cortical inhibitory areas for a muscle can be located very close to excitatory zones. 4. Both excitation and inhibition of muscles could be produced with ICMS of precentral cortex when the animal was performing a task involving the muscles being mapped. EMG responses to ICMS were stable, provided that the stimulation was applied at the same time during a repetitive task such that the motoneurons were at a given level of excitability. 5. Zones where ICMS produced inhibition of a particular forearm muscle were interspersed among zones that produced excitation for that muscle. 6. Regions exist in precentral cortex where ICMS activates antagonistic wrist muscles producing cocontraction. 7. The extensive cortical region from which any individual muscle can be activated or suppressed with ICMS and the various combinations of muscles that are activated from within this region suggest that different types of movements involving a single muscle are represented at different locations within this region. 8. At a few locations in precentral cortex, the EMG responses to ICMS were not just a function of the level of excitation of the motoneuron pool at the time of stimulation but were also dependent on the specific task the monkey was performing at the time of stimulation.