The amplitude of physiological tremor can be voluntarily modulated

Abstract

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.

Changes in tremor as a function of type of augmented visual information

“…Past studies have had varying success in finding an effect of augmented visual feedback on tremor. Some studies observed the expected positive correlation (Morrison and Keogh 2001;Carignan et al 2009) but others have not (Stephens and Taylor 1974;Feys et al 2004;Daneault et al 2010).…”

“…In both experiments 1 and 2, the acceleration power spectra of the computer visual feedback conditions were shifted towards lower frequencies compared to the natural vision condition, which suggests that these feedback conditions involved greater levels of voluntary control (Freund and Hefter 1993). This effect has been attributed to increased synchronization at the motor unit (Carignan et al 2009), cortex (Babiloni et al 2004), and cerebellum (Houweling et al 2008) levels during finger movements that require attention under visual feedback.…”

“…In studies comparing no vision to an augmented visual information condition in which gain is amplified, subjects have successfully reduced the amount of tremor when provided with augmented feedback (Feys et al 2006;Carignan et al 2009;Carrie 1966). In addition, the power spectrum of the tremor profile broadened dependent on the degree of subjects' success in reducing the amount of tremor, and all spectral peaks in the most successful group decreased (Carignan et al 2009).…”

“…In addition, the power spectrum of the tremor profile broadened dependent on the degree of subjects' success in reducing the amount of tremor, and all spectral peaks in the most successful group decreased (Carignan et al 2009). The lesser-improved group's power spectrum only showed a reduced mid-range peak (8-12 Hz), and the unimproved group's power spectrum did not change significantly.…”

“…However, the role of vision, including the relative influence of different kinds of augmented visual information in regulating tremor, is less well understood (Beuter et al 1995;Stephens and Taylor 1974;Sutton and Sykes 1967;Vasilakos et al 1998;Keogh et al 2004). There are predominantly three categories of experimental methods that have been used to investigate the influence of visual information on tremor: (1) comparing natural vision to no vision (Feys et al 2006;Carignan et al 2009), (2) introducing augmented visual information typically through a computer aided representation (Feys et al 2004), and (3) manipulating the gain of the augmented visual information feedback (Carignan et al 2009;Stephens and Taylor 1974;Morrison and Keogh 2001).…”