Effects of unilateral voluntary movement on motor imagery of the contralateral limb

To investigate whether unilateral voluntary movement affects voluntary drive of motor imagery for the contralateral limb.

The subjects were asked to maintain the left index-finger movements with different directions (abduction and adduction, 5% of maximum voluntary contraction; MVC) and with different force levels (10% and 25% MVC). Under these conditions, transcranial magnetic stimulation was applied over the left motor cortex to record motor evoked potential (MEP) from the right first dorsal interosseous muscle with or without motor imagery of the right index-finger abduction.

Voluntary movement of the left index finger with isodirection, but not mirrored direction, to the imagined movement reduced the MEP enhancement induced by motor imagery. MEP was gradually increased depending on increment of the force level on the left side, while the motor imagery-induced MEP enhancement was consequently reduced.

Enhancement of the motor cortex excitability driven by motor imagery of the contralateral limb is interfered with by isodirection and forceful movement of the ipsilateral limb, which may be due to an increase in the transcallosal inhibitory effects.

Using motor imagery as a therapeutic tool, the voluntary movements on the other side of the body should be taken into account.