Influence of Primary Somatosensory Cortex on Hand Motor Circuitry and the Role of Stimulation Parameters

Abstract

<p>The primary somatosensory cortex (SI) is important for hand function and influences motor circuitry in the primary motor cortex (M1). Areas 3a, 1 and 2 of SI have direct connectivity with M1. Much of our present knowledge of this connectivity and its relevance to hand function is based on animal research. However, less is known about the neural mechanisms that underpin hand function in humans. The present study investigated the influence of SI on corticospinal excitability as well as inhibitory and excitatory neural circuitry within M1 before and after continuous theta-burst stimulation (cTBS). Additionally, stimulation parameters influence the direction and magnitude of cTBS after-effects. Thus, current direction and frequency of cTBS were manipulated. Two experiments were performed. In Experiment 1, motor-evoked potentials (MEPs) were recorded from the first-dorsal interosseous (FDI) muscle bilaterally before and after 50 Hz cTBS over left SI. In a second condition, the orientation of cTBS was reversed. Experiment 2 measured MEPs, short-latency intracortical inhibition (SICI) and intracortical facilitation (ICF) from the right FDI following a modified 30 Hz cTBS over left SI or M1. The results of Experiment 1 and 2 demonstrate that SI influences M1 circuitry such that MEPs are facilitated following cTBS over SI. However, MEPs are suppressed when the current direction is reversed. CTBS at 30 Hz delivered over M1 suppressed excitatory circuitry that generates MEPs and ICF. The findings from the thesis suggest that SI influences hand motor circuitry and is likely a mechanism by which somatosensory information modulates hand motor function.</p>