Research ReportCortical representation of rhythmic foot movements
Introduction
In a recent study voluntary rhythmic hand motor patterns have been shown to have a cortical correlate (Pollok et al., 2004). This is compatible with the strong and in large parts monosymptomatic corticospinal projections to the motoneurons of the distal forearm and hand muscles (Lawrence and Kuypers, 1968a, Lawrence and Kuypers, 1968b). For proximal arm and leg muscles the corticomotoneuronal interaction is more indirect via spinal interneuronal circuits, and these interneuronal pathways differ significantly between the forearm and leg segments (Baldissera et al., 1981). This difference in corticospinal connectivity is thought to be one basis for the more delicate and fine motor capacities of the hands as compared to the feet (Boczek-Funcke, 1998, Hultborn and Illert, 1991). Thus the question arises whether the cortical involvement in the generation of rhythmic motor patterns is unique to the distal upper limbs or may play a role in cyclic foot movements as well. There is emerging evidence in non primate mammals that rhythmic gait movements of the lower limbs are paralleled by rhythmic changes in motor cortical activity (Armstrong and Drew, 1984). However, a cortical representation of cyclic foot movement patterns as for rhythmic hand movements has never been shown in humans. Since movement and muscle artefacts usually preclude a recording of cortical (EEG) activity during actual locomotion in humans we asked ten healthy volunteers (6 male, 4 female; age-range: 25–38 yrs.) to perform rhythmic foot stepping movements while seated, and investigated coherent rhythmic modulations of EEG activity in relation to rhythmic calf muscle (EMG) activity at the frequency of the stepping movements. The topography of this coherence on the scalp was analysed by constructing isocoherence maps based on a 64 channel arrangement. In order to differentiate between pure reafference and corticomuscular projection we estimated the delay between the EEG and EMG signal at the movement frequency using a new method (maximising coherence method) for estimation of delays between narrow band coherent signals (Govindan et al., 2006Govindan, 2005, Muthuraman, 2008, Raethjen, 2007).
Section snippets
Results
All subjects showed a significant coherence between the anterior tibial muscles on both sides and the EEG at the stepping frequency that is the frequency of muscle bursts or its first harmonic. This coherence was present for all recording conditions (see below). An example of EEG and EMG raw data with the corresponding power spectra and corticomuscular coherence spectrum is shown in Fig. 1. In 2 of the subjects we also found a weaker 15–30 Hz coherence during the rhythmic movements as displayed
Discussion
In the present paper we show for the first time that there is rhythmic EEG activity directly related to rhythmic foot movements.
Coherent EEG electrodes were mainly found in the central midline region, and there was a clear overlap with the electrodes showing the physiological 15–30 Hz coherence during isometric contractions. As there is converging evidence that the 15–30 Hz rhythm mainly originates from the motor cortex (Hansen and Nielsen, 2004, Salenius and Hari, 2003) this overlap may
Experimental procedures
Ten healthy volunteers (6 male, 4 female; age-range: 25–38 yrs.) were recruited from hospital staff and their relatives. They were seated comfortably with their feet safely reaching the ground in front of the chair. During the recordings they were asked to keep their eyes open and fix their view on a point straight ahead on their eyes' level. In a first set of recordings all subjects were asked to perform rhythmic alternating stepping movements with their feet by taking turns in lifting their
Acknowledgment
This work was supported by the German research council (Deutsche Forschungsgemeinschaft, DFG, Grant RA 1005,1-1).
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