Evaluation of postural tremor of finger for neuromuscular diseases and its application to the classification

The purpose of this study is to verify the features of the power spectrum of postural tremors for neuromuscular disease patients and to classify the postural tremors. The subjects were 88 neuromuscular disease patients (30 Parkinson disease (PD), 25 cerebellar disease (CER), 7 multiple sclerosis (MS), 7 neuropathy (NEU), 10 motor neuron disease (MND), 9 myopathy (MYO)). The control subjects were 12 normal young persons and 10 normal aged persons. Postural tremor was detected by accelerator sensor. Postural tremor was recorded under the two postural conditions: The subjects maintained the index finger without or with a weight load of 50 g in a horizontal position while looking at a visual target in front of the tip of the index finger. The power spectrum was calculated by an auto-regressive model (AR model). The peak frequency and the peak power were evaluated under the two conditions. Two frequency components of 8-12 Hz and 20-25 Hz appeared in the postural tremor of both normal subjects and neuromuscular disease patients. The difference of the postural tremor between the subjects mainly appeared in the 8-12 Hz component during the postural tremor with a weight load. MYO patients belonged to one group (called as group P1) due to lower peak power, CER patients belonged to one group (called as group P2) due to higher peak power, and PD and MS patients belonged to one group (called as group P3) due to lower peak frequency and higher peak power. NER and MND patients belonged to one group (called as group N which meant normal group). These results suggested that the peak frequency and the peak power of the 8-12 Hz component were changed by the conditions of both spinal reflex system and central nervous system. An oscillator within the central nervous system produced the underlying frequency of 8-12 Hz component, while the amplitude of 8-12 Hz component was governed by both spinal reflex system and central nervous system. In conclusion, the classification of postural tremor for neuromuscular disease patients was a useful index to elucidate the mechanism of tremor oscillation and to assist in clinical diagnosis of neuromuscular disease.     

Judgment of disability stages in Parkinson disease patients due to pathological tremor of index finger

The purpose of this study is to investigate the disability stages of Parkinson disease (PD) patients by wave analysis of parkinsonian pathological tremor. Physiological tremor and pathological tremor for the index finger are detected by accelerator sensor. The power spectrum is calculated by an auto-regressive model (AR model). The peak frequency and the peak power of the tremor for the index finger are evaluated under two conditions: (1) maintaining the index finger in a horizontal position using visual feedback with or without a weight load of 50 g, in which tremor is referred to as postural tremor, (2) resting the index finger with eyes closed, in which the tremor is referred to as resting tremor. The physiological tremor and the pathological tremor for the finger are characterized by two peak frequency components in which the first peak frequency component is 8-12 Hz, and the second is 20-25 Hz under the two conditions stated above. The peak frequency and the peak power for the PD patients at the two peak frequency components show the characteristics for the disability level of the PD patients. The first peak power especially around 8-12 Hz in the postural tremor without and with the weight load significantly increases as the disability stage determined by the Hoehn-Yahr method worsens, and there is significant difference of the peak powers for the stages. These results suggest that the first peak power around 8-12 Hz for the postural tremor without and with the weight load is a useful index to judge PD disability stage. It is found that postural tremor without and with the weight load gives the significant peak frequency and peak power compared with those for normal persons. It is pointed out that the first peak frequency component of 8-12 Hz originates from the central nervous system, and the first peak power reflects the degrees of disorder in the central nervous system, since PD is caused by the functional lowering of the central nervous system.     

Effects of aging on the regularity of physiological tremor

The purpose of this investigation was to determine the effects of healthy aging on the regularity of physiological tremor under rest and postural conditions. Additionally, we examined the contribution of mechanical reflex factors to age-related changes in postural physiological tremor. Tremor regularity, tremor-electromyographic (EMG) coherence, tremor amplitude, and tremor modal frequency were calculated for 4 age groups (young: 20-30 yr, young-old: 60-69 yr, old: 70-79 yr, and old-old: 80-94 yr) under resting and loaded postural conditions. There were 6 important findings from this study: 1) there were no differences between the young and elderly subjects for any of the dependent variables measured under the rest condition; 2) postural physiological tremor regularity was increased in the elderly; 3) postural physiological tremor-EMG coherence was also increased in the elderly, and there was a strong linear relation between peak tremor-EMG coherence in the 1- to 8-Hz frequency band and regularity of tremor. This relation was primarily driven by the increased magnitude of tremor-EMG coherence at 5.85 and 6.83 Hz; 4) enhanced mechanical reflex properties were not responsible for the increased magnitude of tremor-EMG coherence in the elderly subjects; 5) tremor amplitude was not different between the 4 age groups, but there was a slight decline in tremor modal frequency in the oldest age group in the unloaded condition; and 6) despite the increases in postural physiological tremor regularity and the magnitude of low frequency tremor-EMG coherence with age, there was a clear demarcation between healthy aging and previously published findings related to tremor pathology.     

The effects of weight load and joint immobilization on reorganization of postural tremor

To investigate change in coordinative strategies due to wrist immobilization and index loading, postural tremors from the index, hand, and forearm were recorded during different postural holding tasks. The wrist joint was immobilized with a thermoplastic splint in the constrained condition, and a copper mass of 100 grams was applied to the index finger in the loaded condition. The structures of the postural tremors of all upper limb segments among the unloaded-unconstrained, unloaded-constrained, loaded-unconstrained, and loaded-constrained conditions were compared. Index loading exaggerated index/forearm postural tremor, while the load-induced tremor enhancement was no longer evident for wrist immobilization. In the unloaded condition, wrist immobilization resulted specifically in enhancement of carpal postural tremor, rather than in the index and forearm. Index loading induced a marked tremor peak and relative power in the range of 5-8 Hz. Wrist immobilization potentiated the carpal tremor peak of 1-4 Hz in association with enhancement of carpal-forearm mechanical coupling. In light of structural changes in postural tremor, our data suggest that (1) a wrist splint is effective to counteract load-induced enhancement of postural tremor, and (2) freezing of the wrist joint might facilitate compensatory strategies to minimize passive fluctuation transmission from the carpal to index.     

Load-independent contributions from motor-unit synchronization to human physiological tremor.

This study describes two load-independent rhythmic contributions from motor-unit synchronization to normal physiological tremor, which occur in the frequency ranges 1-12 Hz and 15-30 Hz. In common with previous studies, we use increased inertial loading to identify load-independent components of physiological tremor. The data consist of simultaneous recordings of tremor acceleration from the third finger, a surface electromyogram (EMG), and the discharges of pairs of single motor units from the extensor digitorum communis (EDC) muscle, collected from 13 subjects, and divided into 2 data sets: 106 records with the finger unloaded and 84 records with added mass from 5 to 40 g. Frequency domain analysis of motor-unit data from individual subjects reveals the presence of two distinct frequency bands in motor-unit synchronization, 1-12 Hz and 15-30 Hz. A novel Fourier-based population analysis demonstrates that the same two rhythmic components are present in motor-unit synchronization across both data sets. These frequency components are not related to motor-unit firing rates. The same frequency bands are present in the correlation between motor-unit activity and tremor and between surface EMG activity and tremor, despite a significant alteration in the characteristics of the tremor with increased inertial loading. A multivariate analysis demonstrates conclusively that motor-unit synchronization is the source of these contributions to normal physiological tremor. The population analysis suggests that single motor-unit discharges can predict an average of 10% of the total tremor signal in these two frequency bands. Rectified surface EMG can predict an average of 20% of the tremor; therefore within our population of recordings, the two components of motor-unit synchronization account for an average of 20% of the total tremor signal, in the frequency ranges 1-12 Hz and 15-30 Hz. Our results demonstrate that normal physiological tremor is a complex signal containing information relating to motor-unit synchronization in different frequency bands, and lead to a revised definition of normal physiological tremor during low force postural contractions, which is based on using both the tremor spectra and the correlation between motor-unit activity and tremor to characterize the load-dependent and the load-independent components of tremor. In addition, both physiological tremor and rectified EMG emerge as powerful predictors of the frequency components of motor-unit synchronization.     

Genetical components of physiological tremor.

Pysiological tremor was measured in 14 pairs of monozygotic twins, 14 pairs of dizygotic twins, and 14 pairs of control subjects matched for age and sex. Postural finger tremor was measured in each pair using a sub-miniature accelerometer and subjecting the amplified signals to power spectral analysis. Significant genetical influences were found for the proportion of tremore at frequencies near the peak frequency of 9-10 Hz. No genetical effects were found at other frequencies or for the absolute amount of tremore. The results suggest that physiological tremor between 8 and 13 Hz is genetically influenced and that tremor at these frequencies is determined by different mechanisms than those at other frequencies.     

The influence of dynamic visual cues for postural control in children aged 7–12 years

Young children rely heavily on vision for postural control during the transition to walking. Although by 10 years of age, children have automatic postural responses similar to adults, it is not clear when the integration of sensory inputs becomes fully developed. The purpose of this study was to examine this transition in the sensory integration process in children aged 7–12 years. Healthy children and adults stood on a fixed or sway-referenced support surface while viewing full-field optic flow scenes that moved sinusoidally (0.1 and 0.25 Hz) in an anterior–posterior direction. Center of pressure was recorded, and measures of sway amplitude and phase were calculated at each stimulus frequency. Children and adults had significant postural responses during approximately two-thirds of the trials. In adults, there was a 90% decrease in sway on the fixed surface compared with the sway-referenced surface, but only a 50% decrease in children. The phase between the optic flow stimulus and postural response in children led that of adults by 52° at 0.1 Hz and by 15° at 0.25 Hz. Adults and children aged 7–12 years have similar ability to use dynamic visual cues for postural control. However, 7–12-year-old children do not utilize somatosensory cues to stabilize posture to the same extent as adults when visual and somatosensory cues are conflicting.     

Longitudinal characterization of EEG power spectra during eyes open and eyes closed conditions in children

This study is the first to examine spectrum-wide (1 to 250 Hz) differences in electroencephalogram (EEG) power between eyes open (EO) and eyes closed (EC) resting state conditions in 486 children. The results extend the findings of previous studies by characterizing EEG power differences from 30 to 250 Hz between EO and EC across childhood. Developmental changes in EEG power showed spatial and frequency band differences as a function of age and EO/EC condition. A 64-electrode system was used to record EEG at 4, 5, 7, 9, and 11 years of age. Specific findings were: (1) the alpha peak shifts from 8 Hz at 4 years to 9 Hz at 11 years, (2) EC results in increased EEG power (compared to EO) at lower frequencies but decreased EEG power at higher frequencies for all ages, (3) the EEG power difference between EO and EC changes from positive to negative within a narrow frequency band which shifts toward higher frequencies with age, from 9 to 12 Hz at 4 years to 32 Hz at 11 years, (4) at all ages EC is characterized by an increase in lower frequency EEG power most prominently over posterior regions, (5) at all ages, during EC, decreases in EEG power above 30 Hz are mostly over anterior regions of the scalp. This report demonstrates that the simple challenge of opening and closing the eyes offers the potential to provide quantitative biomarkers of phenotypic variation in brain maturation by employing a brief, minimally invasive protocol throughout childhood.     

The origins of physiological tremor as deduced from immersions of the finger in various liquids

To investigate the influence of gravity on physiological tremor during a holding stretch of the finger, tremor during immersion of the finger in liquids was measured. Liquids with various densities and various coefficients of viscosity were used. Tremor was detected using an acceleration sensor, and power spectrum analysis was performed on the acceleration signal of the tremor. The total power of the tremor spectrum decreased with an increase of the density and an increase of the coefficient of viscosity, the high frequency domain of the tremor spectrum showing a larger decrease than the low frequency domain. Linear regression analysis showed that the viscosity of the liquid had a larger effect on tremor than the buoyancy due to the liquid. A model was proposed for tremor during immersion of the finger in liquid. The effect of the buoyancy and the viscosity on tremor was examined using the proposed model. The origin of two frequency bands in the tremor spectrum was verified by both the immersion experiment and the proposed model. The stretch-reflex system via the spinal cord produced a high frequency band around 25 Hz, while the supraspinal system caused a low frequency band around 10 Hz. The neuromuscular function of the human body was evaluated using the amplitude and the frequency of tremor. Electronic Publication     

Neuronal mechanisms underlying physiological tremor

1. Tremor force was recorded during stationary isometric contractions of intrinsic hand muscles of normal subjects. Subjects maintained a steady force level between their thumb and forefinger for 30 s. The force level varied from weak (0.2 kg) to strong contractions (7 kg). These experimental conditions were the same as those in two preceding studies, where single motor-unit activity (14) and the correlation between the discharges of two simultaneously recorded motor units and physiological tremor (11) have been investigated. 2. Two alterations of the power spectra were observed at successively stronger contractions: increase of tremor amplitude and changes in the shape of the power spectrum. At all force levels, the power spectra of tremor force show the well-known decay of tremor amplitude from the lower to the higher frequencies with a local peak at 6--10 Hz. This peak does not show a significant change with respect to frequency when the force level is varied. It is shifted toward lower frequencies in a pathological condition (Parkinsonism) where the recruitment firing rates of the motor units are significantly lower than in the normal. 3. Higher frequencies (greater than 20 Hz) are barely present in the power spectrum during the very weak contractions. They become significant as the contractions become stronger. 4. The steep decay of the power spectrum toward higher frequencies has a similar slope (--43 dB/decade) as the reduction in amplitude of the unfused part of the muscle contractions with increasing stimulus rates (--38 dB/decade). The cutoff of the power spectrum above 25 Hz parallels the achievement of total fusion of muscle twitches above this rate. 5. The results are consistent with the hypothesis that the power spectrum over the range of 6--25 Hz is mainly caused by the unfused parts of the twitch contractions of motor units firing between recruitment (6--8/s) and total fusion of the twitches (25--30/s). The decline of the power spectrum toward higher frequencies can be explained by mechanical damping, which results from increasing fusion of the twitch contractions. The low-frequency part of the power spectrum is assumed to be the result of the slow force deviations produced by changes in the net output of the motoneuron pool. 6. These assumptions were supported by additional animal experiments where the number and rate of force-producing elements could be controlled. Bundles of ventral root filaments innervating cat soleus and gastrocnemius muscles were stimulated synchronously and asynchronously at a number of different rates. The force output of the strain gauge was recorded, filtered, and analyzed in the same way as the human force records. 7. Stimualtion of one nerve bundle at one fixed frequency led to a sharp peak in the power spectrum at that frequency plus peaks of decreasing height representing the harmonics of the stimulation frequency. The height of the peaks decreased at --37 dB/decade. 8...     

Evidence that ventrolateral thalamotomy may eliminate the supraspinal component of both pathological and physiological tremors

Ventrolateral (VL) thalamotomy produced a marked reduction of oscillations related to the supraspinal components of Parkinson's disease tremor (4-7 Hz) and physiological tremor (8-12 Hz). Finger tremor was examined in nine patients undergoing unilateral VL thalamotomy and in nine age-matched controls. In comparison to the preoperative state, the relative percentage of power within the 7.6-12.5 Hz band did not increase after the surgical procedure. Furthermore, the amount of absolute power within the 7.6-12.5 Hz band was much lower for post-surgical patients in comparison to matched controls when periods of tremor having equal amplitudes were compared. These results suggest that VL thalamotomy interrupts a common circuit involved in the supraspinal component of both physiological and pathological tremors. We provide evidence that the thalamus may be involved in circuits generating physiological tremor in humans.     

The relationship between physiological tremor and the performance of rapid alternating movements in healthy elderly subjects

The power distribution in the frequency spectrum of tremor is known to vary among individuals and its median power frequency declines with ageing. The purpose of the present study was to determine whether a reduction of the central component of physiological tremor would correlate with a reduction of motor performance. Then, the power distribution in the frequency spectrum of tremor from limb extremities might serve as an index of neural drive in healthy elderly subjects. Rest tremor, postural tremor from the finger, and pronation-supination at the wrist were recorded in 102 healthy nuns living in a convent (mean of 72±12 years). Results reveal that several elderly subjects possessed a power distribution of tremor very similar to that of much younger subjects (mean 27 years±3 SD), showing a preponderance of power within the 7.6- to 12.5-Hz band. Duration of pronation-supination cycles of these elderly subjects was, however, similar to that of other elderly subjects who had a preponderance of power within the 3.6- to 7.5-Hz band. Consequently, healthy elderly subjects who possessed a predominance of power within higher frequencies were not at an advantage over other healthy elderly subjects when performing a pronation-supination task. The age of subjects was, however, a better predictor or motor performance. In conclusion, the present findings suggest that, under normal physiological conditions, a reduction of the central component of physiological tremor does not induce a reduction of motor performance. Consequently, tremor recorded at limb extremities cannot be used as an index of neural drive. Electronic Publication     

Age-related differences in force variability and visual display

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.     

Pulsatile control of the human masticatory muscles

Spectral analysis of jaw acceleration confirmed that the human mandible 'trembles' at a peak frequency around 6 Hz when held in its rest position and at other stationary jaw openings. The 6 Hz tremor increased during very slow movements of the mandible, but other lower-frequency peaks became prominent during more rapid jaw movements. These lower-frequency peaks are likely to be the result of asymmetries in the underlying, voluntarily produced, 'saw-tooth' movements. In comparison, finger tremor at rest and during slow voluntary movements had a mean peak frequency of about 8 Hz: this frequency did not change during rhythmical finger flexion and extension movements, but the power of the tremor increased non-linearly with the speed of the movement. The resting jaw tremor was weakly coherent with the activity of the masseter and digastric muscles at the tremor frequency in about half the subjects, but was more strongly coherent during voluntary movements in all subjects. The masseter activity was at least 150 deg out of phase with the digastric activity at the tremor frequency (and at all frequencies from 2.5–15 Hz). The alternating pattern of activity in antagonistic muscles at rest and during slow voluntary movements supports the idea that the masticatory system is subject to pulsatile control in a manner analogous to that seen in the finger.     

Does Greater Low Frequency EEG Activity in Normal Immaturity and in Children with Epilepsy Arise in the Same Neuronal Network?

Greater low frequency power (<8 Hz) in the electroencephalogram (EEG) at rest is normal in the immature developing brain of children when compared to adults. Children with epilepsy also have greater low frequency interictal resting EEG activity. Whether these power elevations reflect brain immaturity due to a developmental lag or the underlying epileptic pathophysiology is unclear. The present study addresses this question by analyzing spectral EEG topographies and sources for normally developing children and children with epilepsy. We first compared the resting EEG of healthy children to that of healthy adults to isolate effects related to normal brain immaturity. Next, we compared the EEG from 10 children with generalized cryptogenic epilepsy to the EEG of 24 healthy children to isolate effects related to epilepsy. Spectral analysis revealed that global low (delta: 1–3 Hz, theta: 4–7 Hz), medium (alpha: 8–12 Hz) and high (beta: 13–25 Hz) frequency EEG activity was greater in children without epilepsy compared to adults, and even further elevated for children with epilepsy. Topographical and tomographic EEG analyses showed that normal immaturity corresponded to greater delta and theta activity at fronto-central scalp and brain regions, respectively. In contrast, the epilepsy-related activity elevations were predominantly in the alpha band at parieto-occipital electrodes and brain regions, respectively. We conclude that lower frequency activity can be a sign of normal brain immaturity or brain pathology depending on the specific topography and frequency of the oscillating neuronal network.     

Contribution of motor unit activity enhanced by acute fatigue to physiological tremor of finger.

The contribution of motor unit activity to a physiological tremor (hereafter called as tremor) in a middle finger is studied by both a power spectrum and a correlation analysis in which the correlation coefficient and the coherence spectrum are obtained when five kinds of loads, 0, 50, 100, 150, and 200 g, are added to the middle finger for two minutes in a loading experiment on twelve male subjects. A weight of 200 g is applied to the subjects for ten minutes in a fatigue experiment. Throughout both experiments, the middle finger remains stretched from the load of the weight. The tremor is measured by an accelerometer (MT-3T, Nihon Kohden, Japan) attached to the middle finger, and the surface electromyogram (EMG) is measured by bipolar electrodes placed on m. extensor digitorum communis. A power spectrum analysis is carried out on the tremor and EMG, and a correlation analysis is performed on the relationship between the tremor and the demodulated EMG. It is found in the loading experiment that when the weight on the finger increases, the amplitude of the tremor oscillation increases since the activity of the motor units of the muscle is enhanced by the phenomenon of recruitment. Two frequency components of the tremor spectra at 10 Hz and 25 Hz under a no load condition reflect the components of the activity of the motor units of the muscle because the tremor shows a significant correlation in the frequency zone of 10 Hz and 25 Hz with the demodulated EMG. The lower frequency component of the tremor spectrum at 10 Hz results in synchronized activity of the motor units, while the higher frequency at 25 Hz occurs from the stretch reflex loop via the motoneurons of the spinal cord. The shift of the higher frequency component to the lower frequency domain due to the load of the weight originates from the prolongation of the response time of the finger mechanical system because the lag time at the peak of the correlation coefficient increases with the load of the weight. It is found in the fatigue experiment that the amplitude of the tremor oscillation increases with the progress of fatigue. The increase is caused by the recruitment of the motor unit activity of the muscle holding the finger as well as by the synchronization of the firings of the motoneurons. The progress of the synchronization is verified by the fact that the mean power frequency (MPF) of the EMG spectrum decreases and the correlation between the tremor and the demodulated EMG increases with the progress of fatigue. The mechanisms of the increase of the amplitude of the tremor oscillation under the load of the weight to the finger and under the state of fatigue of the finger are elucidated by the analysis of the tremor and EMG.     

Neural control of the lips differs for young and older adults following a perturbation

Aging impairs the control of many skilled movements including speech. The purpose of this paper was to investigate whether young and older adults adapt to lower lip perturbations during speech differently. Twenty men (10 young, 26 ± 3 years of age; 10 older, 60 ± 9 years of age) were requested to repeat the word (“papa”) 300 times. In 15% of the trials, the subjects experienced a mechanical perturbation on the lower lip. Displacement and neural activation (EMG) of the upper and lower lips were evaluated. Perturbations to the lower lip caused a greater increase in the maximum displacement of the lower lip for older adults compared with young adults (34.7 ± 19% vs. 13.4 ± 17%; P = 0.017). Furthermore, young adults exhibited significantly greater 30–100 Hz normalized EMG power for the lower lip compared to the upper lip (P < 0.005). In young adults, changes from normal to perturbed trials in the 30–50 Hz frequency band of the EMG were negatively correlated to the changes from normal to perturbed trials in the lower lip maximum displacement (R ² = 0.48; P = 0.025). It is concluded that young adults adapt better to lower lip perturbations compared with older adults and that the associated neural activation strategy of the involved muscle is different for the two age groups.     

The effects of unilateral muscle fatigue on bilateral physiological tremor

The aim of this study was to examine the post-exercise effects of fatiguing the wrist extensor muscles of a single arm on postural tremor and muscle activity in both arms. Previous research has shown that, for neurologically normal subjects, the tremor seen within a single limb segment is uncorrelated to that seen contralaterally. However it has been speculated that some bilateral relation does exist, and that the nature of the relation may only become evident under conditions where the neuromuscular system is perturbed. To further investigate this potential bilateral relation, seven healthy subjects were required to adopt a bilateral postural pointing position after exercise-induced fatigue of the wrist extensor muscles of a single arm. Tremor from the forearm, hand and finger segments of each arm, surface EMG activity from extensor digitorum (ED) of each arm, and blood lactate data were collected prior to and after the exercise intervention. The main result was that fatiguing the distal muscles of one arm resulted in a bilateral increase in both the physiological tremor and ED activity. The change in tremor was confined to the index finger with no change in the tremor for the hand or forearm segments of either arm. While three peaks were seen in the frequency profile of the finger tremor, the effects of fatigue were confined to an increase in the peak power of the neurally generated 8–12 Hz tremor component. The contralateral increase in muscle activity was also reflected by a change in the frequency profile of the EMG output, with an increase in the peak power of both muscles following exercise of the wrist extensors of a single arm. The bilateral increases in physiological tremor and EMG activity of ED were only observed during the bilateral pointing task, with no changes in tremor or EMG activity seen for the non-exercised limb during the unilateral exercise protocol. The specificity of the resultant increases in the neurally generated 8–12 Hz component of finger tremor amplitude and EMG activity, coupled with the lack of any changes in tremor for the more proximal arm segments, indicate that these bilateral effects were mediated by an increase in the central neural drive to both limbs. Together this set of results challenges the general assumption of bilateral independence of tremor production, and further illustrate the task dependent nature of exercise-induced fatigue.     

Orthostatic response of plasma renin activity and urinary kallikrein excretion in children with difficulty awakening in the morning

Twenty-six children, aged 10-15 years, with difficulty awakening in the morning, showed significantly lower plasma renin activity (PRA) before and after standing (15 and 60 min) and significantly greater postural fall in mean blood pressure than the age-matched control children. They also showed high urinary kallikrein excretion, but this was not statistically significant. These results suggest that low PRA and a readily decreasing blood pressure may contribute to difficulty awakening in the morning in teen-aged children.     

LFP power spectra in V1 cortex: the graded effect of stimulus contrast

We recorded local field potentials (LFPs) and single-unit activity simultaneously in the macaque primary visual cortex (V1) and studied their responses to drifting sinusoidal gratings that were chosen to be "optimal" for the single units. Over all stimulus conditions, the LFP spectra have much greater power in the low-frequency band (< or = 10 Hz) than higher frequencies and can be described as "1/f." Analysis of the total power limited to the low, gamma (25-90 Hz), or broad (8-240 Hz) frequency bands of the LFP as a function of stimulus contrast indicates that the LFP power gradually increases with stimulus strength across a wide band in a manner roughly comparable to the increase in the simultaneously recorded spike activity. However, the low-frequency band power remains approximately constant across all stimulus contrasts. More specifically the gamma-band LFP power increases differentially more with respect to baseline than either higher or lower bands as stimulus contrast increases. At the highest stimulus contrasts, we report as others have previously, that the power spectrum of the LFP typically contains an obvious peak in the gamma-frequency band. The gamma-band peak emerges from the overall broadband enhancement in LFP power at stimulus contrasts where most single units' responses have begun to saturate. The temporal/spectral structures of the LFP located in the gamma band-which become most evident at the highest contrasts-provide additional constraints on potential mechanisms underlying the stimulus response properties of spiking neurons in V1.