Conflicting results on the existence of tremor related cortical activity in essential
tremor (ET) have raised questions on the role of the cortex in tremor generation.
Here we attempt to address these issues.
We recorded 64 channel surface EEG and EMG from forearm muscles in 15 patients with
definite ET. EEG and EMG power spectra, relative power of the rhythmic EMG activity,
relative EEG power at the tremor frequency and EEG-EMG and EEG-EEG coherence were
calculated and their dynamics over time explored. Corticomuscular delay was studied
using a new method for narrow band coherent signals.
Corticomuscular coherence in the contralateral central region at the tremor frequency
was present in all patients in recordings with a relative tremor EMG power exceeding
a certain level. However, the coherence was lost intermittently even with tremors
far above this level. Physiological 15–30Hz coherence was found consistently in 11
patients with significantly weaker EMG activity in this frequency range. A more frontal
(mesial) hot spot was also intermittently coupled with the tremor and the central
hot spot in 5 patients. Corticomuscular delays were compatible with transmission in
fast corticospinal pathways and feed back of the tremor signal.
On the one hand our data show that different cortical motor areas are involved in
the oscillations of ET and their transmission to the periphery. On the other hand,
this cortical involvement seems to be dispensable to the peripheral tremor remaining
even in phases without coherent activity in the cortex. We hypothesize that tremor
oscillations build up in different subcortical and subcortico-cortical circuits only
temporarily entraining each other.
The importance of the cortex within this widespread oscillating network can be tested
by modulating cortical excitability e.g. by transcranial direct current stimulation
and monitoring its influence on the peripheral tremor. First results in a subgroup
of these ET patients will be presented.
