Motor demand dependent improvement in accuracy following low - frequency 1 transcranial magnetic stimulation of left motor cortex 2

21 The role of primary motor cortex (M1) in the control of voluntary movements is still 22 unclear. In brain functional imaging studies of unilateral hand performance, bilateral M1 23 activation is inconsistently observed and disruptions of M1 using repetitive transcranial 24 magnetic stimulation (rTMS) lead to variable results in the hand motor performance. As 25 the motor tasks differed qualitatively in these studies, it is conceivable that M1 26 contribution differs depending on the level of skillfulness. The objective of the present 27 study was to determine whether M1 contribution to hand motor performance differed 28 depending on the level of precision of the motor task. Here we used low frequency rTMS 29 of left M1 to determine its effect on the performance of a pointing task that allows the 30 parametric increase of the level of precision thereby increase the level of required 31 precision quantitatively. We found that low frequency rTMS improved performance in 32 both hands for the task with the highest demand on precision, while performance 33 remained unchanged for the tasks with lower demands. These results suggest that the 34 functional relevance of M1 activity for motor performance changes as a function of 35 motor demand. The bilateral effect of rTMS to left M1 would also support the notion of 36 M1 functions at a higher level in motor control by integrating afferent input from non37 primary motor areas. 38 39