Abstract:
Cerebellum-to-cerebrum functional connectivity can indirectly be assessed with
transcranial magnetic stimulation (TMS) combined with electromyography by
applying the cerebellar brain inhibition (CBI) procedure, measuring a suppression
of motor evoked potential (MEP) amplitude in hand muscles. Consequently, this
output is limited to the primary motor cortex and, depending on cortical
excitability, MEP amplitudes show high variability. As TMS combined with
electroencephalography (EEG) could address these limitations, feasibility studies
were conducted recently. Several challenges have so far precluded the
identification of EEG markers elicited by cerebellar TMS (cbTMS) - a lack of
control conditions for sensory-evoked EEG potentials and concomitant occipital
cortex stimulation. Hence, the objective of the present study was the identification
of specific EEG responses to cbTMS.
46 healthy volunteers completed two resting state experiments. To confirm
cbTMS was effectively delivered to the cerebellar cortex, CBI was assessed
individually to determine cbTMS intensity. A model of the cbTMS coil was created
utilizing radiography of the coil and individual magnetic resonance imaging (MRI)
scans from 8 randomly selected participants to simulate the electric field
generated by cbTMS. Control conditions adapted for cbTMS were applied. By
performing cluster-based permutation t-statistics, specific EEG responses could
be identified - the cb-P25 and cb-N45. EEG responses were projected into the
source space. By performing time-frequency analysis, a specific increase in left
prefrontal high beta power was observed. The method was applied in a post-
stroke patient with delayed-onset hemiataxia. An MRI scan and diffusion tensor
imaging tractography were acquired, demonstrating an asymmetry between left
and right dentato-thalamo-cortical tract (DTC). cb-P25 was absent on the
structurally impaired side and reproduced on the intact side.
Applying a novel method, specific EEG responses to cbTMS could be identified.
These responses were interpreted to reflect effective connectivity between
cerebellum and cerebral cortex mediated by the DTC. The cbTMS-EEG
technique could test pathology of the DTC, as demonstrated in one patient with
neurodegeneration of the DTC on one side.