The amplitude of physiological tremor can be voluntarily modulated

The objective of this study was to determine whether it was possible to voluntarily modulate physiological tremor (PT), i.e., reduce its amplitude. We recorded the postural index finger tremor of 30 healthy participants with a laser in four conditions: (A) eyes closed, without any attempt to modulate PT amplitude, (B) no visual feedback, trying to reduce PT amplitude, (C) visual feedback, trying to reduce PT amplitude. For conditions B and C, subjects were asked to avoid using muscle contraction as a means to stabilize the finger. Finally, (D) subjects were asked to reduce PT amplitude using voluntary muscle contraction to stabilize the finger. We used electromyography to monitor the extensor digitorum communis and flexor digitorum superficialis. Total amplitude of PT did not change significantly between conditions A and B. In condition C, a significant decrease of PT amplitude was observed. A significant increase in tremor amplitude was observed in D compared with other conditions, confirming that co-contraction was not used to modulate the amplitude of PT in other conditions. Subsequently, we formed three subgroups based on their ability to modulate PT: Most Improved (n = 7), Least Improved (n = 16) and Not Improved (n = 7). Although oscillations within the low frequency bands increased only in the Not Improved group, oscillations within the 8-12 and 16-30 Hz bands either remained stable or decreased for all participants, supporting a disassociation between mechanical-reflex and central components of PT. Our results show that it is possible to voluntarily modulate PT. Therefore, a cortical influence is being exerted on tremor.