Evidence for early activation of primary motor cortex and SMA after electrical lower limb stimulation using EEG source reconstruction.

Compared to median nerve somatosensory evoked potentials (SEP), less is known about activity evoked by nerve stimulation of the lower limb. To understand the mechanisms and the physiology of sensor- and motor control it is useful to investigate the sensorimotor functions as revealed by a standardized functional status. Therefore, we investigated SEPs of the lower limb in 6 healthy male volunteers. For each side, tibial and peroneal nerves were stimulated transcutaneously at the fossa poplitea. The tibial nerves were also stimulated further distally at the ankle joint. Source localization was applied to 64-EEG-channel data of the SEPs. In contrast to somatosensory areas, which are activated after median nerve stimulation, we found dipoles adjacent to motor areas near Brodmann area 4 (BA 4) for SEP components P 32/40 and P 54/60 and near the supplementary motor area (SMA) for the N 75/83 component. These sources could reliably be distinguished for each individual subject as well as for the grand mean data set. Our data show that afferent projections from the lower limb mainly reach primary motor areas (BA 4) and only subsequently, with a delay of 40 ms, higher order motor areas such as SMA. We conclude that a focused view on SEP of the lower limb could be a useful tool to investigate pathological states in motor control or peripheral deafferentiation.