Transcranial magnetic stimulation (TMS) studies have shown that the motor system is facilitated when we imagine performing motor actions. However, it is not clear whether the individual's motor system modulates bilaterally and selectively for task parameters, such as movement direction and amplitude. To investigate this issue, we applied single-pulse TMS over the left and right primary motor cortex (M1) of healthy subjects, who had to imagine grasping and rotating a clock hour hand, having a starting position at noon, towards four different times: 2, 5, 7 and 10 o'clock. Rotations could be in clockwise (2 and 5 o'clock) or counter-clockwise (7 and 10 o'clock) directions and could require small (2 and 10 o'clock) or large (5 and 7 o'clock) rotation angle. TMS motor-evoked potentials were recorded for three muscles, and movements were imagined with the right and left hands. Results showed that during motor imagery a mirroring pattern was present between the right and the left motor cortices, showing selective activation of the hand-intrinsic muscles spatially close to the direction of the imagined movement. Overall a higher activation for large and a lower activation for small rotation angle were found, but no selective muscle activity was present within the hand-intrinsic muscles for this parameter. Following these results we propose that during action imagination an internally coded covariance between movement parameters is present with a muscle-specific activation for movement direction.
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