Effects of force–load on cortical activity preceding voluntary finger movement: Whole-scalp magnetoencephalography of the Bereitschaftsfeld

Neural activity preceding force-loaded voluntary finger movement (the Bereitschaftsfeld) was recorded using 143-channel whole-scalp magnetoencephalography (MEG) in order to determine how the level of force produced during voluntary finger movement is represented in activity over different premovement time intervals localized to different cortical areas. Eighteen healthy subjects performed voluntary right index-finger extension movements against an inertial load of either 0, 100, or 200 g. Results showed that the earliest component of premovement activity, beginning between 1.5 and 1.0 s prior to movement and localized to the central midline around the region of supplementary/cingulate motor areas, was not modulated by the level of force required for movement. However, later premovement activity, occurring between 500 and 200 ms prior to movement onset, was significantly greater for the highest force movements compared with both intermediate (p < 0.05) and no weight-load conditions (p < 0.01). This component was localized to primary sensorimotor cortical areas, with greater source strength on the left side contralateral to movement. Results indicate that, although early premovement activity of the supplementary/cingulate motor areas does not appear to encode movement force, later premovement activity of the primary motor cortex is significantly greater for movements made with more force, not only during movement execution but also up to 500 ms prior in readiness for intended movements of greater force.