Anodal Direct Current Stimulation of the Cerebellum Reduces Cerebellar Brain Inhibition but Does Not Influence Afferent Input from the Hand or Face in Healthy Adults

© 2015, Springer Science+Business Media New York. Background: The cerebellum controls descending motor commands by outputs to primary motor cortex (M1) and the brainstem in response to sensory feedback. The cerebellum may also modulate afferent input en route to M1 and the brainstem. Objective: The objective of this study is to determine if anodal transcranial direct current stimulation (tDCS) to the cerebellum influences cerebellar brain inhibition (CBI), short afferent inhibition (SAI) and trigeminal reflexes (TRs) in healthy adults. Methods: Data from two studies evaluating effects of cerebellar anodal and sham tDCS are presented. The first study used a twin coil transcranial magnetic stimulation (TMS) protocol to investigate CBI and combined TMS and cutaneous stimulation of the digit to assess SAI. The second study evaluated effects on trigemino-cervical and trigemino-masseter reflexes using peripheral nerve stimulation of the face. Results: Fourteen right-handed healthy adults participated in experiment 1. CBI was observed at baseline and was reduced by anodal cerebellar DCS only (P < 0.01). There was SAI at interstimulus intervals of 25 and 30 ms at baseline (both P < 0.0001), but cerebellar tDCS had no effect. Thirteen right-handed healthy adults participated in experiment 2. Inhibitory reflexes were evoked in the ipsilateral masseter and sternocleidomastoid muscles. There was no effect of cerebellar DCS on either reflex. Conclusions: Anodal DCS reduced CBI but did not change SAI or TRs in healthy adults. These results require confirmation in individuals with neurological impairment

Modulation of cortical motor networks following primed theta burst transcranial magnetic stimulation

To investigate whether priming stimulation influences the responses of intracortical inhibitory and facilitatory motor circuits to a subsequent plasticity-inducing inhibitory theta burst TMS paradigm. Using standard transcranial magnetic stimulation (TMS) procedures, MEP amplitude, short-interval intracortical inhibition (SICI), and short-interval intracortical facilitation (SICF) were assessed at baseline and 5, 20 and 30 min following continuous theta burst stimulation (cTBS), intermittent TBS (iTBS), and iTBS-primed cTBS. SICI was assessed using paired-pulse TMS at inter-stimulus intervals (ISI) of 3 ms (SICI(3)) and the latency corresponding to the latency at which SICF was minimal in each individual. SICF was assessed at ISIs corresponding to Peak 1, Trough 1, Peak 2, and Peak 3 of each individual's SICF curve. When applied alone cTBS inhibited and iTBS facilitated MEP amplitudes. iTBS-primed cTBS resulted in greater MEP inhibition than cTBS alone. There were no changes in SICF and only marginal changes in SICI following any intervention. Synapses mediating MEP generation undergo modification following iTBS-primed cTBS, possibly through mechanisms related to metaplasticity or synaptic depotentiation. A lack of substantial changes in SICI and SICF under all experimental conditions suggests that the tested rTMS paradigms may be non-optimal for inducing robust modulation of the neural elements mediating SICI and SICF across subjects. Priming stimulation may provide an approach which facilitate neuroplastic change within the human motor cortex at least in circuits responsible for MEP generation.Sebastian H. Doeltgen, Michael C. Riddin