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Abstract

The oscillatory suppression of sensorimotor-mu power (i.e., 10-12 Hz) is a robust EEG correlate of motor control. Simply imagining voluntary limb movement can result in consistent suppression of mu-power, especially in contralateral electrode sites. This is typically exploited by neuroprostheses (e.g., BCI-controlled wheelchairs; Huang et al., 2012) that seek to restore movement to spinal-cord injury patients. In some examples, levels of mu-suppression have also been treated as an index of motor control effort (e.g., Mann et al., 1996). However, mu-suppression in contralateral sites can also be observed during passive limb movements, namely in the absence of voluntary control effort (Formaggio et al., 2013). In this study, we investigate whether patterns of oscillatory EEG activity across contralateral (C3) and ipsilateral (C4) sites discriminate for voluntary control and limb movement. In our study, EEG measurements were taken of ten participants who were required to either actively follow or resist the deflections of a control-loaded side-stick, this respectively required voluntary control in the presence and absence of limb movement. In contrast, they were also tested in conditions with passive or no limb movements, which respectively required them to simply hold on to a moving or stationary side-stick. A repeated-measures 2 x 2 x 2 ANOVA for the factors of electrode site (contralateral vs. ipsilateral), control (active vs. passive), and movement (movement vs stationary) revealed the following. To begin, there was a significant main effect of lateralized mu-suppression. Suppression of mu-power is larger in the contralateral site compared to the ipsilateral site (F(1,9)=5.10, p=0.05). More importantly, three significant interactions were found, movement x control (F(1,9)=13.1, p<0.01), electrode x movement (F(1,9)=5.78, p=0.04) and for electrode x control (F(1,9)=5.81, p=0.039). Limb movement resulted in selective mu-suppression of only the contralateral electrode. Voluntary control resulted in mu-suppression in both contralateral and ipsilateral electrodes, albeit to a lesser extent in the ipsilateral site. Overall, active resistance against side-stick deflections resulted in the largest levels of mu-suppression. The current results suggest that active voluntary resistance can result in high levels of mu-suppression that do not exhibit strong lateralization. This might go unnoticed in brain-computer-interface and experimental paradigms that estimate control effort by contrasting contralateral to ipsilateral mu-suppression.