Simultaneous Recordings of Head and Hand Tremor in Subjects With Essential Tremor: an Investigation of Coherence

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MacDonald, Morgan C.
Essential Tremor , Frequency Coherence , Central Oscillator
The pathophysiology of essential tremor (ET) is not clearly understood but is thought to involve multiple brain regions. The purpose of this study was to describe in greater detail head tremor in ET and to investigate the possible relationship between head and hand tremor. Ten ET subjects were recruited (1 male, 9 female) and compared to three control subjects (1 male, 2 female). Head and hand tremors were recorded simultaneously with surface electromyography (EMG) of the wrist extensors and various neck muscles, laser displacement sensors (hand tremor), a load cell (hand tremor) and an accelerometer (head tremor). While seated, subjects performed four tasks: 1) constant force (10% maximum) wrist extensions (with and without visual feedback); maintenance of the hands in a horizontal posture against gravity while 2) seated upright in a chair, 3) seated in a reclined chair (20° backward, head not supported); and 4) seated upright in a chair and producing steady submaximal hip adduction forces. Head tremor spectral peaks were found between 3.5 and 7 Hz in neck muscle EMG and the accelerometer signal. Wrist tremor (EMG and kinematic data) was slightly higher in frequency with a range of 4 -10 Hz. Of the ten ET subjects recruited for this study, 60% (n = 6) demonstrated significant levels of coherence (p < 0.05) in at least one neck-wrist muscle comparison at the fundamental frequency of their tremor. The results demonstrated an obvious bias of the trapezius descendens (TD) muscles over the more axial neck muscles to demonstrate significant coherence with the extensor carpi radialis (ECR) muscles. Of the six neck muscles investigated, the SPLs and the SCMs were commonly driven at the same frequencies (change in frequency < 0.5), although this seldom resulted in coherence. There is indication that the oscillatory activity driving more distal muscles is different from that in the SPL and the SCM. Due to the multifunctional nature of the TD, it may be the recipient of two descending neural commands. These commands may each be of a different oscillatory frequency originating from different central oscillators.
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