Influence of Parietal Cortex on Motor Surround Inhibition in Healthy Adults - A TMS Study

Surround inhibition (SI) in the motor cortex is a phenomenon necessary for controlling fine, focal movements wherein the excitability of synergist muscles is enhanced and that of surrounding muscles is inhibited. Patients with focal hand dystonia (FHD) have less/absent SI, suggesting that SI might contribute to the disease pathophysiology. Hence studying this phenomenon in detail is likely to enhance our understanding of the pathophysiology of FHD. It has been postulated that the parietal-premotor-motor network is crucial in FHD. As a result, past studies from our group1,2evaluated the role of the premotor cortex in SI, yet neither the dorsal nor ventral premotor cortex influenced SI. In this exploratory study, we aimed to determine the influence of the posterior parietal cortex, specifically the anterior inferior parietal lobule (aIPL) on SI using transcranial magnetic stimulation (TMS) and electromyography (EMG). We recruited 11 healthy volunteers, out of which 4 completed the study. Participants performed an auditory-cued simple index finger movement task and single-pulse TMS was applied either at rest or movement onset. Motor-evoked potentials (MEPs) were recorded from the synergist and surround muscles. Peak-peak MEP amplitude served as our primary outcome measure. Our preliminary results show that the posterior parietal cortex is probably not involved in SI, as SI did not change by conditioning the aIPL. We are recruiting more subjects to obtain more reliable results