Low-intensity sub-threshold rhythmic 10Hz median nerve stimulation modulates mu-band EEG oscillations in individuals with Tourette syndrome

Tourette syndrome (TS) is a neurological disorder characterised by the occurrence of vocal and motor tics. Rhythmic median nerve stimulation (MNS) at 10Hz has been shown to cause a substantial reduction in tic frequency in individuals with Tourette Syndrome. The mechanism of action is currently unknown but has been hypothesised to involve entrainment of cortical oscillations within the sensorimotor cortex linked to the initiation of movement. An important methodological detail of these studies is that MNS is delivered at or above threshold (i.e., the minimum stimulation level required to elicit a visible muscle twitch). This is important issue as it means that the observed effects of rMNS could be driven primarily by afferent signals in response to stimulation, the re-afferent signals arising from the muscle, or a combination of these signals. To examine this further, we used electroencephalography (EEG) to investigate the effect of delivering 1s trains of sub-threshold rhythmic 10Hz MNS in a group of 15 adults with TS compared to a matched group of 20 neurotypical control participants. The results demonstrate that the EEG response (somatosensory evoked potential (SEP) to rMNS increased linearly with increasing stimulation amplitude. This was paralleled by substantially increased inter-trial coherence (ITC) during rMNS. Importantly, the duration of increased ITC was reduced for the TS group compared to controls. Importantly, these results were largely similar when analyses were restricted only to sub-threshold trials in which no visible muscle twitch was elicited by MNS. These results confirm that sub-threshold rhythmic MNS is sufficient to modulate somatosensory physiology and may also be sufficient to elicit the clinical benefits previously observed for MNS.