Directional coherence disentangles causality within the sensorimotor loop, but cannot open the loop.

More and more studies indicate that corticomuscular coherence in the beta band (15–30 Hz), which expresses the functional coupling between the cortex and the muscles, originates from the interaction within the sensorimotor loop (e.g. Witham et al. 2011). The phase of the corticomuscular coherence expresses the relative time–frequency relationship and is often explained as to result from the efferent delay between the cortex and the muscles. In a recent issue of The Journal of Physiology, Witham and co-workers (2011) demonstrated that the slope of the phase of corticomuscular coherence is less negative than would be expected of pure efferent pathways and even becomes positive in some subjects (negative slopes indicate that the muscle lags the brain). This is a clear indication of a bidirectional coupling between EEG and EMG; in other words, the signals are part of a closed-loop system. However, the authors also use the phase of the directional coherence to assess the delays in the efferent and afferent pathways, which will give erroneous results in a closed-loop system, like the sensorimotor loop. The causality of signals within a closed loop is difficult to assess. For example, in the sensorimotor loop it is not obvious whether cortical activity leads muscle activity – suggesting an efferent