Functional overlap between hand and forearm motor cortical representations during motor cognitive tasks

Objective The aim of this study was to verify whether motor imagery (MI) and observation of a movement (MO) enhanced cortical representations of the hand/forearm muscles not primarily involved in the task. We also explored the existence of functional overlaps in the upper-limb cortical representations during the aforementioned tasks. Methods Focal transcranial magnetic stimulation (TMS) was used to map out the cortical representation of the opponens pollicis (OP, target muscle) and other hand and forearm muscles at rest and during MI and MO. Results The MI and MO tasks induced similar changes in the area and volume of both the OP and synergic muscles. No significant changes were observed in the cortical excitability of the remaining muscles. The superimposition of different muscle maps revealed extensive functional overlaps in the hand/forearm cortical territories. Conclusions This study demonstrates that neither the MI nor MO changes single muscle motor responses and that the hand/forearm muscle maps extensively overlap during motor cognitive tasks. Significance The data reported in this study support the notion that the basic unit of cortical output is not the mere activation of a given muscle. This flexible organization may have important implications in motor learning and plasticity.