The Interplay of Sensory Feedback, Arousal, and Action Tremor Amplitude in Essential Tremor

Essential tremor (ET) amplitude is modulated by visual feedback during target driven movements. In a grip force task, tremor amplitude increased during large scale visual feedback compared to a condition with low scale visual feedback. It has not been examined whether visual feedback exclusively modulates target force tremor amplitude or if other afferent inputs like auditory sensation has a modulatory effect on tremor amplitude as well. Also, it is unknown whether the enhanced sensory feedback causes an increase of arousal in persons with ET (p-ET). We hypothesized that (1) amplitude of tremor is modulated by variation of auditory feedback in the absence of visual feedback in a force tremor paradigm; (2) increase of tremor amplitude coincides with pupillary size as a measure of arousal. 14 p-ET and 14 matched healthy controls (HC) conducted a computer-based experiment in which they were asked to match a target force on a force sensor using their thumb and index finger. The force-induced movement was fed back to the participant visually, auditory or by a combination of both. Results showed a comparable deviation from the target force (RMSE) during the experiment during all three sensory feedback modalities. The ANOVA revealed an effect of the scaling factor on the tremor severity (Power 4-12Hz) for the visual- and also for the auditory feedback condition in p-ET. Pupillometry showed a significantly increased pupil diameter during the large scale auditory involved feedback conditions compared to the low scale feedback conditions in p-ET. Our findings suggest that action tremor in ET is firstly modulated not only by visual feedback but also by auditory feedback in a comparable manner. Therefore, tremor modulation seems to be modality independent. Secondly, enhanced feedback causes an increase of arousal as measured here by the pupil size. Further work including neurophysiological measures is required to better understand the interaction between arousal and target-related tremor.