Determine if somatosensory disabilities in patients with essential tremor (ET) are related to impaired somatosensory change detection (SCD) using fMRI oddball paradigms in which the predictability of incoming tactile stimuli is manipulated. 

ET is a common cerebellar neurodegenerative disorder characterized by an action tremor that mainly affects the upper limbs [1, 2].  ET patients also present impaired somatosensory processing that contributes to their disability [3, 4]. SCT in our environment is necessary for appropriate behavioral reactions. Higher response for less predictable somatosensory stimuli is described in the secondary somatosensory cortex (S2) [5,6] and the cerebellum [7, 8] suggesting that efficient cerebello-S2 interplay supports SCD. Here, we postulate that such cerebello-S2 interplay is affected in ET.

Seventeen patients with ET and matched for age and gender healthy subjects participated in the study.  

Each subject underwent fMRI tactile oddball paradigms consisting in pneumatic tactile stimulation applied in the right index fingertip as standard stimuli and simultaneous pneumatic tactile stimulation to both right index fingertip and the middle phalanx as deviant stimuli [7].  The predictability of the deviant stimuli was manipulated to generate a predictable deviant condition (Predictable) and an unpredictable deviant condition (Random) [7]. For both condition and through a region of interest (ROI) analysis, S2 activation were compared between groups using paired T-test. Cerebello-S2 connectivity was studied using psychophysiological interaction analysis (PPI). 

In Random, S2 activation levels were higher than in Predictable in healthy participants (contrast estimate : 0,95±0,10 vs 0,40±0,15, p=0,036) and similar in ET patients (0,65±0,20 vs 0,45±0,15, p=0,09).  Lower S2 activation in Predictable was associated to increased cerebello-S2 connectivity in PPI analysis only in healthy subjects.

SCD is underpinned by an underlying cerebello-S2 interplay which allow a modulation of S2 activity according to the predictability of the stimulation. SCD is impaired in ET and likely contribute to somatosensory disabilities.