The complete frequency spectrum of physiological tremor can be recreated by broadband mechanical or electrical drive (original) (raw)
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There is a debate in the literature about whether the low- and high-frequency peaks of physiological finger tremor are caused by resonance or central drive. One way to address this issue is to examine the consequences of eliminating, as far as possible, the resonant properties or the voluntary drive. To study the effect of minimizing resonance, finger tremor was recorded under isometric conditions and compared with normal isotonic tremor. To minimize central drive, finger tremor was generated artificially by broad-band electrical stimulation. When resonance was minimized, tremor size declined almost monotonically with increasing frequency. There was no consistent large peak at a frequency characteristic of tremor. Although there was sometimes a peak around the tremor frequency during some isometric conditions, it was extremely small and variable; therefore, any contribution of central drive was minimal. In contrast, there was always a prominent peak in the isotonic frequency spectra. Resonance was, therefore, necessary to produce the characteristic tremor peaks. When central drive was minimized by replacing voluntary muscle activation with artificial stimulation, a realistic tremor spectrum was observed. Central drive is, therefore, not required to generate a characteristic physiological tremor spectrum. In addition, regardless of the nature of the driving input (voluntary or artificial), increasing the size of the input considerably reduced isotonic tremor frequency. We attribute the frequency reduction to a movement-related thixotropic change in muscle stiffness. From these results we conclude that physiological finger tremor across a large range of frequencies is produced by natural broad-band forcing of a nonlinear resonant system, and that synchronous central input is not required.
The resonant component of human physiological hand tremor is altered by slow voluntary movements
Limb resonance imparts a characteristic spectrum to hand tremor. Movement will alter the resonance. We have examined the consequences of this change. Rectified forearm extensor muscle EMG and physiological hand tremor were recorded. In postural conditions the EMG spectrum is relatively flat whereas the acceleration spectrum is sharply peaked. Consequently, the gain between EMG and acceleration is maximal at the frequency where the tremor is largest (∼8 Hz). The shape of the gain curve implies mechanical resonance. Substantial alterations in posture do not significantly change the characteristics of the tremor or the shape or size of the gain curve. By contrast, slow or moderately paced voluntary wrist flexion–extension movements dramatically increase the hand tremor size and lower its peak frequency. These changes in size and frequency of the tremor cannot be attributed to changes in the EMG. Instead they reflect a very large change in the size and shape of the gain curve relating EMG to acceleration. The gain becomes larger and the peak moves to a lower frequency (∼6 Hz). We suggest that a movement-related (thixotropic) alteration in resonant properties of the wrist provides a simple explanation for these changes. The mechanism is illustrated by a model. Our new findings confirm that resonance plays a major role in wrist tremor. We also demonstrate that muscles operate very differently under postural and dynamic conditions. The different coupling between EMG and movement in posture and when moving must pose a considerable challenge for neural predictive control of skeletal muscles.
The amplitude of physiological tremor can be voluntarily modulated
Experimental Brain Research, 2009
The objective of this study was to determine whether it was possible to voluntarily modulate physiological tremor (PT), i.e., reduce its amplitude. We recorded the postural index finger tremor of 30 healthy participants with a laser in four conditions: (A) eyes closed, without any attempt to modulate PT amplitude, (B) no visual feedback, trying to reduce PT amplitude, (C) visual feedback, trying to reduce PT amplitude. For conditions B and C, subjects were asked to avoid using muscle contraction as a means to stabilize the finger. Finally, (D) subjects were asked to reduce PT amplitude using voluntary muscle contraction to stabilize the finger. We used electromyography to monitor the extensor digitorum communis and flexor digitorum superficialis. Total amplitude of PT did not change significantly between conditions A and B. In condition C, a significant decrease of PT amplitude was observed. A significant increase in tremor amplitude was observed in D compared with other conditions, confirming that co-contraction was not used to modulate the amplitude of PT in other conditions. Subsequently, we formed three subgroups based on their ability to modulate PT: Most Improved (n = 7), Least Improved (n = 16) and Not Improved (n = 7). Although oscillations within the low frequency bands increased only in the Not Improved group, oscillations within the 8–12 and 16–30 Hz bands either remained stable or decreased for all participants, supporting a disassociation between mechanical-reflex and central components of PT. Our results show that it is possible to voluntarily modulate PT. Therefore, a cortical influence is being exerted on tremor.
Quantifying the importance of high frequency components on the amplitude of physiological tremor
Experimental Brain Research, 2010
The objective of this study was to determine the importance of every frequency component on total physiological tremor (PT) amplitude. We suspect that since high frequencies of PT are of lower amplitude in displacement, removing them will have little to no impact on PT amplitude. PT of the index finger was measured with a laser displacement sensor while the finger was held horizontally. Amplitude of tremor was calculated in displacement, velocity and acceleration. PT amplitude was also calculated within five frequency bands. Although displacement amplitude of oscillations within the 7.5–12.5 and 16.5–30 Hz frequency bands represent 24 and 10% of total PT oscillation amplitude, respectively, their removal reduced PT amplitude by less than 3%. Conversely, the removal of the oscillations within 1–3.5 Hz band from the PT signal reduced the amplitude of the original PT signal by 56% in displacement. This suggests that when a task to be studied involves the measurement of a reduction in tremor, focus should be on the oscillations in the 1–3.5 Hz band.
Bilateral effect of a unilateral voluntary modulation of physiological tremor
Clinical Neurophysiology, 2010
Objective: Verify whether a unilateral physiological tremor modulation attempt has a bilateral effect on central oscillators responsible for physiological tremor. Methods: Bilateral finger physiological tremor was recorded in 35 right-handed participants with laser displacement sensors in: (A) control condition, (B) modulation attempt of physiological tremor of the right index finger, (C) modulation attempt of physiological tremor of the right index finger by using co-contraction as a means to stabilize the finger. Results: Physiological tremor amplitude was significantly reduced between the control and modulation conditions for the right index finger. Physiological tremor amplitude was also significantly reduced for the left index finger. Regression analysis of oscillation amplitudes showed little relationship between both fingers, even during modulation attempts. The lack of relationship between fingers is also evident by low coherence values obtained in the control and modulation conditions. The coherence in the co-activation condition was significantly higher, albeit still low, in each frequency band. Conclusion: Our results confirm that physiological tremor can be voluntarily reduced, and this reduction is bilateral. The modulation attempt did not however increase the frequency relationship between both sides. Significance: A central command aiming at modulating tremor amplitude will not increase the synchronization between oscillators responsible for the central components of physiological tremor.
Experimental Brain Research, 2005
The goal of this study was accurate quantification of the amplitude of high-frequency components of physiological tremor (PT) in units of displacement, velocity, and acceleration. In addition, changes of amplitude with finger loading were compared within specific frequency bands. Index finger tremor was measured for 20 healthy subjects using a high-resolution laser, simultaneously with an accelerometer, under two conditions, unloaded and loaded (70 g). By use of an accurate filtering technique, oscillations within six predetermined frequency bands were isolated. Results showed that overall mean tremor amplitude under the unloaded condition was 0.0973 mm in displacement units, 4.525 mm s À1 in velocity units, and 301.526 mm s À2 in acceleration units. Although the mean amplitude of oscillations located within the 16.5-30 Hz band was 0.009 mm and represented only 10% of total tremor amplitude, amplitude of acceleration within the 16.5-30 Hz band was 191 mm s À2 and represented 60% of total acceleration amplitude. Mean amplitude increased significantly with loading (displacement, t=À2.67, P=0.015; velocity, t=À4.33, P=0.000; acceleration, t=À3.48, P=0.002) but the magnitude of that change was different in each frequency band and its relative importance depended on the level of signal differentiation. Velocity was the only measure that retained sensitivity to changes in amplitude with loading in the low and high-frequency components of PT. In conclusion, this study provides, for the first time, accurate quantification of the amplitude of oscillation of highfrequency components of PT. In addition, it provides clear evidence that the velocity of tremor oscillation is more suitable for detection of the impact of finger loading because it enables detection of amplitude changes in both the low and high-frequency components of PT.
Entrainment to extinction of physiological tremor by spindle afferent input
Experimental Brain Research, 2006
In this study the systematic modulation of wrist flexor muscle activity by imposed joint movement was examined. Ten subjects maintained a constant contraction level (25% of maximum; trial duration: 20 s) in flexor carpi radialis while their wrists were perturbed with 50 different quasi-sinusoidal signals (frequency range: 0.5 -9.5 Hz; amplitude: 0.3° -4.2°). Frequency spectra of wrist position and the rectified and filtered electromyogram (EMG) were determined. The muscle activity was only weakly entrained to imposed movements of small amplitude and low frequency, as shown by a small peak in the EMG spectrum at the frequency of movement, while the most prominent peak in the spectrum was between 9 -15 Hz, corresponding to the frequency range of physiological tremor. The entrainment of muscle activity increased markedly as the amplitude and frequency of the imposed movement increased, to the point of saturation of modulation and harmonic peaks in the spectrum. In parallel with this increase in entrainment, the 9 -15 Hz tremor peak was progressively extinguished. The results are consistent with a coupled oscillator model in which the central oscillatory source(s) of tremor became fully entrained to the imposed movement at the highest amplitudes and frequencies. Such coupling depends on communication between the external forcing oscillator and the central oscillator(s), the Ia afferent signal from the imposed movement being the most likely candidate to provide the entraining signal for the central oscillator(s).
The Dynamics of Voluntary Force Production in Afferented Muscle Influence Involuntary Tremor
Frontiers in computational neuroscience, 2016
Voluntary control of force is always marked by some degree of error and unsteadiness. Both neural and mechanical factors contribute to these fluctuations, but how they interact to produce them is poorly understood. In this study, we identify and characterize a previously undescribed neuromechanical interaction where the dynamics of voluntary force production suffice to generate involuntary tremor. Specifically, participants were asked to produce isometric force with the index finger and use visual feedback to track a sinusoidal target spanning 5-9% of each individual's maximal voluntary force level. Force fluctuations and EMG activity over the flexor digitorum superficialis (FDS) muscle were recorded and their frequency content was analyzed as a function of target phase. Force variability in either the 1-5 or 6-15 Hz frequency ranges tended to be largest at the peaks and valleys of the target sinusoid. In those same periods, FDS EMG activity was synchronized with force fluctuati...
Journal of Neurophysiology, 2010
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