LONG DURATION INTRACORTICAL MICROSTIMULATION AS A DEVICE TO CHARACTERIZE MOTOR CIRCUITS AND MUSCULAR RECRUITMENT

The mechanisms by which the cerebral cortex represents the world are mostly unknown. The study of cortical representations and their underlying neural circuits is fundamental for the development of new therapies in neurological rehabilitation, including the improvement of prostheses. How can we map these areas and the circuits that control the movement, however? Intracortical Microstimulation is a powerful tool for mapping brain motor representation. By applying short-duration microstimulations, Penfield and collaborators developed a somatotopical map that reflects, precisely, the relation between the cortex and the muscular groups. Most recent studies, however, have shown that by increasing the stimulus duration, it is possible to recruit muscles in multiple members, triggering complex movements. This project aims to characterize the obtained motor responses from long-duration intracortical microstimulation (LD-ICMS) in the marmoset primary motor, parietal, and premotor cortices. To represent each site's recruitability, we are developing a metric called the Motor Recruitment Index (MRI), which will be sensitive to the extension amplitude of recruited muscles and the relative distance of evoked members' cortical representation. In parallel, to reflect the movement's complexity, the Motor Complexity Index (MCI), sensitive to the movement's directionality and smoothness, is being developed. Each of these variables is being acquired by a simple and cheap computational setup in Python developed in the laboratory. In this study, we’ll be using four marmosets anesthetized with ketamine hydrochloride (CEUA-CCS, Protocol 137/23). The cortex will be microstimulated with currents between 5µA and 500µA. During the microstimulation, the evoked movements will be captured on video for posterior analysis. Offline, we will correlate the movements with the stimulus site’s cortical area, identified by histology. Preliminary results indicate that larger amplitudes and longer durations evoke progressively more complex movements (i.e., higher MCI or MRI), which can be bilateral, broad, and ethologically coherent. We also observed motor responses driven by microstimulation in the parietal cortex, including the primary somatosensory area (S1).