P156 Ongoing EEG oscillations modulate MEP amplitudes: A possible mechanism of MEP variability

Abstract

Objective Although the resting motor threshold (RMT) is relatively stable, the variability of MEPs produced by TMS over the primary motor cortex (M1) is well known, and its precise mechanism is still unclear. We hypothesized that EEG oscillations just before TMS could modulate the MEP amplitudes that underlie the MEP variability. Thus, we evaluated the relationship between EEG oscillations and MEP amplitudes. Methods Two experiments were conducted in this study. EEGs were obtained at 19 locations (international 10–20 system) and MEPs were recorded from right FDI in Exp. 1 (n = 12). Left M1 hotspot was stimulated by TMS with the intensity to obtain about 1 mV MEP amplitudes (S-1 mV). MEPs and EEGs were recorded for 200 times in total for each subject under the eyes-open (EO) and eyes-close (EC) conditions. EEGs were segmented between −500 and 0 ms of the TMS onset, and the power values (8–30 Hz) were calculated by wavelet analysis. EEG powers were compared between the epochs of higher and lower amplitude MEPs. In Exp. 2 (n = 9), the effect of low TMS intensity was evaluated under the EO condition. EEG oscillations were compared between the high (S-1 mV) and low intensity (RMT) TMS conditions. Results EEG powers of α and β range were significantly increased (synchronization) at C3 when MEP amplitudes were larger in both EO and EC conditions. When the EEG power difference of higher/lower MEPs was compared between the EO and EC conditions, these differences under EO were more pronounced than that of EC at frontal and parietal leads. In contrast, EEG powers were reduced (desynchronization) at C3 when MEP amplitudes were larger in the low TMS condition (Exp. 2). Discussion We found that EEG synchronization of α / β bands prior to TMS was a determinant factor for the MEP amplitudes at S-1 mV (Exp. 1). Distributed networks over the frontal areas may modulate MEPs differently under EO and EC conditions. However, desynchronization of α / β bands produced MEPs at RMT (Exp. 2). From this complex interaction between EEG synchronization/desynchronization and TMS intensity, we assume that ongoing EEG oscillations of α / β bands performs the dual functions of gateway and gatekeeper. Like a push–pull system, that depends on the ongoing EEG state and TMS intensity, the dual system may allow the TMS impulses to M1 by which the MEP variability occurs.