Effects of subthalamic nucleus (STN) stimulation on motor cortex excitability

To the Editor: We agree with Duuper et al.,1 that STN stimulation may modulate cortical excitability.2 However, the study by Duuper et al.1 raises several concerns related to methodology, results, and discussion. Regarding the methodology, information on the age of the control group is not provided—age, however, is a crucial factor for motor evoked potentials. For a valid comparison the age group of the controls should be matched with the age of the patients. Besides, the reproducibility of the total voltage time integral may be questionable, considering the intertrial variability (see figure 1 in their article) and considering that only a few trials were averaged (possibly preselected from more trials by an unblinded observer). Movement of coil position and angle against the individual’s vertex may also have increased data variability. Moreover, patients with predominantly akinetic-rigid PD are not able to cooperate as well as controls in silent period (SP) studies requiring maintenance of a constant muscle tone.3 Consequently, …