P06.06.B EXPLORING HAND MUSCLE DISRUPTIONS FROM STIMULATION OF THE PREMOTOR AND PARIETAL CORTICES: A NEW INSIGHT INTO MOTOR CONTROL

Abstract BACKGROUND Transcranial magnetic stimulation (TMS) is an established tool for brain mapping but shows limitations for higher-order motor functions due to relatively high variability. In this study, we demonstrate TMS’s potential to systematically disrupt motor activity via premotor and parietal cortex stimulation, offering a novel approach to mapping motor functions beyond the corticospinal system. MATERIAL AND METHODS We analyzed data from 10 patients with surgically treated extra-axial infratentorial lesions, all undergoing radiological follow-up at least two years post-surgery, without motor impairments. Using navigated TMS, we applied single-pulse biphasic stimulation at 120% of resting motor threshold (rMT) in blocks of 30 stimulations per site. Stimulation targeted 10-12 points in the dominant hemisphere, including M1, the ventrolateral premotor cortex, the inferior parietal lobule, and the frontal and parietal opercular cortices. Electromyographic (EMG) activity was recorded bilaterally from the abductor pollicis brevis (APB) and first dorsal interosseous (FDI) muscles. Patients were instructed to clench their fingers and hand with submaximal strength. Single EMG recordings were high-pass filtered (20 Hz) and rectified. Silent periods (SPs) were identified and single traces averaged within each condition. Significant changes in post-stimulus activity were computed by massive univariate comparisons using t-tests. RESULTS We observed silent periods (SPs) not preceded by motor-evoked potentials (MEPs) following stimulation of the ventrolateral premotor area, the pars opercularis of the inferior frontal gyrus (Brodmann area 44), and the parietal opercular region. CSPs evoked from frontal regions exhibited latencies of approximately 20 ms, whereas parietal stimulation produced CSPs at latencies closer to 50 ms, often preceded by a small peak in EMG activity. CONCLUSION We describe a novel TMS-induced phenomenon—a short-latency silent period in ongoing voluntary EMG activity that is not preceded by a motor-evoked potential (MEP). The timing of this effect suggests interference with ongoing motor planning processes within the studied cortical regions. Similar effects observed with direct cortical stimulation (DCS) further underscore the potential clinical relevance of this phenomenon for reducing perioperative morbidity, especially in tumour surgery. Our findings highlight the importance of identifying functional motor areas beyond the primary motor cortex and provide thought-provoking insights into the subcortical functional connectivity between the motor cortex and regions involved in motor control. The unique advantage of our approach is the non-invasiveness, making it a potentially valuable tool for pre-surgical mapping.