Rhythmic cortical activity and its relation to the neurogenic components of normal and pathological tremors

The work of Stiles and Randall and that of Elble and Randall established that the spectrum of a tremor signal consists of two components: an inertial load dependent component and a neurogenic component. The former is a property of the structure exhibiting the tremor and shifts toward lower frequencies with increased inertial loading. The neurogenic component is load independent and was first attributed to central descending processes in the frequency range 8–12 Hz. During a maintained postural task, an additional 15–30-Hz neurogenic component of tremor was identified and associated with motor unit synchronization. Farmer et al. inferred, on the basis of indirect evidence, that the common drive to the synchronized motor units was of descending origin. Conway et al., using magnetoencephalographic (MEG) techniques, and Halliday et al., with conventional electroencephalographic (EEG) techniques, demonstrated a correlation between localized cortical activity in the 15–30-Hz frequency band and the neurogenic component of tremor associated with motor unit synchronization. These results imply that the localized rhythmic cortical activity in the 15–30-Hz frequency band that occurs during maintained postural tasks accounts for the 15–30-Hz component of neurogenic tremor, which in turn is a consequence of motor unit synchronization. The analysis of the correlation between the 15–30-Hz rhythmic cortical oscillations and the neurogenic component of tremor is equivalent to an investigation of the descending drive that synchronizes motor units during maintained postural tasks. The application of spectral and coherence analyses therefore provides a powerful procedure for characterizing the central drive associated with rhythmic cortical activity and motor unit synchrony in normal and pathological tremors. The application of these analytical methods reveals significant differences in the correlation between cortical drive and tremor in normal subjects with that observed in the cases of essential tremor and Parkinsonian tremor.