Moving Hands or Moving Feet: Cortical Connectivity during isolated limb movements

Introduction: Any motor action results from dynamic interactions of various brain regions involved in the various aspects of movement preparation and execution. The most important cortical motor output is provided by the primary motor cortex (M1) in which foot movements are represented on the mesial surface whereas hand movements are generated on the lateral surface at the "hand knob" formation. We here used fMRI and dynamic causal modelling (DCM) to investigate effective connectivity in the cortical motor system during voluntary movements of the hands and feet.

Methods: 13 healthy right-handed subjects were scanned with fMRI while performing short blocks (2 s) of movements with one hand (left- or right-sided fist closures) or with one foot (left-or right-sided ankle flexion) in a randomized sequence according to the frequency (1.5 Hz) of a blinking visual cue. Dynamic causal modeling (DCM) as implemented in SPM8 was used to assess interregional coupling between cortical motor areas.

Results: The analysis of connectivity showed that movements of either limb increased the promoting influence of all premotor areas (SMA, PMC) on the respective M1 representation area, while the M1 representations of the "inactive" limbs received extensive inhibitory influences from motor areas in both hemispheres. Of note, interhemispheric M1-M1 influences among the hand representations were strictly inhibitory while foot representations were positively coupled during movements of the respective limb.

Conclusions: The main difference between movements of the hand and feet was found for the transcallosal interactions of the M1 representations which are strictly inhibitory for hand movements but excitatory for foot movements. This difference in cortical connectivity might account for stronger lateralization in the use of the hands allowing for independent skilled movements as compared to movements of the lower limb.