Increase In Low-frequency Power Research Articles

Eye closure causes widespread low-frequency power increase and focal gamma attenuation in the human electrocorticogram

ObjectiveWe sought to characterize the effects of eye closure on EEG power using electrocorticography (ECoG). Specifically, we sought to elucidate the anatomical areas demonstrating an eye closure effect, and at which frequencies this effect occurs. MethodsECoG was recorded from 32 patients undergoing invasive monitoring for seizure focus localization. Patients were instructed to close and open their eyes repeatedly. ECoG power was compared in the epochs following eye closure and opening, for various frequency bands and brain regions. ResultsWe found that at low frequencies, eye closure causes widespread power increases involving all lobes of the brain. This effect was significant not only in the α (8–12Hz) band but in the δ (2–4Hz), θ (4–8Hz), and β (15–30Hz) bands as well. At high frequencies, eye closure causes comparatively focal power decreases over occipital cortex and frontal Brodmann areas 8 and 9. ConclusionsEye closure (1) affects a broad range of frequencies outside the α band and (2) involves a distributed network of neural activity in anatomical areas outside visual cortex. SignificanceThis study constitutes the first large-scale, systematic application of ECoG to study eye closure, which is shown to influence a broad range of frequencies and brain regions.

Time-dependent effects of isoflurane and dexmedetomidine on functional connectivity, spectral characteristics, and spatial distribution of spontaneous BOLD fluctuations

Anesthesia is often necessary to perform fMRI experiments in the rodent model; however, commonly used anesthetic protocols may manifest changing brain conditions over the duration of the study. This possibility was explored in the current work. Eleven rats were anesthetized with 2% isoflurane anesthesia; four rats were anesthetized for a short period (30 min, simulating induction and fMRI setup) and seven rats were anesthetized for a long period (3 h, simulating surgical preparation). Following the initial anesthetic period, isoflurane was discontinued, and a dexmedetomidine bolus (0.025 mg/kg) and continuous subcutaneous infusion (0.05 mg/kg/h) were administered. Blood-oxygen-level dependent resting state imaging was performed every 30 min from 0.75 h post dexmedetomidine bolus until 5.75 h post-bolus. Evaluation of power spectra obtained from time courses in the primary somatosensory cortex revealed, in general, a monotonic increase in low-frequency power (0.05-0.3 Hz) in both groups over the duration of resting state imaging. Greater low-band spectral power (0.05-0.15 Hz) is present in the short isoflurane group for the first 2.75 h, but the spectra become highly uniform at 3.25 h. The emergence of a ~0.18 Hz peak, beginning at the 3.75 h time point, exists in both groups and evolves similarly, increasing in strength as the duration of dexmedetomidine sedation (and time since isoflurane cessation) extends. In the long isoflurane group only, bilateral functional connectivity strengthens with anesthetic duration, and correlation is linearly linked to low-band spectral power. Convergence of connectivity and spectral metrics between the short and long isoflurane groups occurs at ~3.25 h, suggesting the effects of isoflurane have subsided. Researchers using dexmedetomidine following isoflurane for functional studies should be aware of the duration specific effects of the pre-scan isoflurane durations as well as the continuing influences of long-term imaging under dexmedetomidine.

Study of high radio frequency plasma discharge effects on carbon fiber using Raman spectroscopy

The goal of this research is to evaluate the potential of single and dual high RF reactors for the treatment of carbon fiber by Raman and FT-IR spectroscopies. Unsized poly-acrylonitrile (PAN) carbon fiber was treated with single high frequency (40.68MHz) and double radio frequency capacitively coupled plasma (RF-CCP) discharge reactor (40.68/2.1MHz) using pure nitrogen gas. The changes in the structure of the carbon fiber surfaces due to the variations of the D and G band intensity ratio (ID/IG), crystallite size (La) and Full-Width-at-Half-Maximum with plasma power, gas pressure, and treatment-time were described. It was found that the disorder of carbon fiber increases (the ID/IG ratio increased) with increasing high RF power and the process time. In contrast, the degree of the order (La) increases with increasing the gas pressure. It was also observed that the degree of graphitization accrued due to the shift of the D and G bands to the lower wave number. And, from FT-IR result, it was found that the CC bond disappeared, and only carbon to nitrogen double and triple bonds are available by increasing pressure and RF power. In dual RF reactor, at high pressure and long process time, the disorder values (ID/IG) decrease with the increase of low frequency power. Furthermore, this mode has faster effects on the surface of carbon fiber than single RF mode.

Physiologic Effects of Rhythmical Massage: A Prospective Exploratory Cohort Study

This study was performed to characterize the physiologic effects of rhythmical massage (RM), an anthroposophic therapy whose effectiveness is supported by empirical observations and a prospective observational study. Patients referred to RM at the Paracelsus Hospital Richterswil for any indication were continuously enrolled. They received an average of 10 RM sessions, which included not only the massage but also therapeutic rest in the supine position immediately thereafter. Effects of RM on surface temperature and on heart rate variability (HRV) were determined with infrared imaging (IRI) and electrocardiography (ECG), respectively. IRI of the patients' dorsal region was performed without clothes, in standing position, at the start and at the end of two waiting periods before and after RM. ECG was performed from the beginning of the first waiting period until the end of the second one. Results on IRI from 9 and ECG from 11 patients could be analyzed. RM led to an immediate increase in dorsal temperature. Furthermore, comparison of the IRI images for consecutive RM sessions showed a tendency toward improved warmth distribution as a progressive therapy effect. Analysis of the EGC results recorded during the waiting periods-in the sitting position--showed a significant increase of HRV after RM, as detected by the standard deviation of the beat-to-beat periods and a relative increase of low-frequency power. During the course of the RM sessions, the change in HRV during the therapeutic rest period depended on the initial value: Low initial values were enhanced, whereas high ones were reduced. RM led to an immediate increase in the patient's dorsal surface temperature, as well as increased HRV and sympathetic stimulation. In the long term, RM resulted in a progressive improvement of warmth distribution and regulation of the resting HRV.

Human hippocampal increases in low-frequency power during associative prediction violations

Environmental cues often trigger memories of past events (associative retrieval), and these memories are a form of prediction about imminent experience. Learning is driven by the detection of prediction violations, when the past and present diverge. Using intracranial electroencephalography (iEEG), we show that associative prediction violations elicit increased low-frequency power (in the slow-theta range) in human hippocampus, that this low-frequency power increase is modulated by whether conditions allow predictions to be generated, that the increase rapidly onsets after the moment of violation, and that changes in low-frequency power are not present in adjacent perirhinal cortex. These data suggest that associative mismatch is computed within hippocampus when cues trigger predictions that are violated by imminent experience.

The Effects of Levodopa and Deep Brain Stimulation on Subthalamic Local Field Low-Frequency Oscillations in Parkinson's Disease

New adaptive systems for deep brain stimulation (DBS) could in the near future optimize stimulation settings online so as to achieve better control over the clinical fluctuations in Parkinson's disease (PD). Local field potentials (LFPs) recorded from the subthalamic nucleus (STN) in PD patients show that levodopa and DBS modulate STN oscillations. Because previous research has shown that levodopa and DBS variably influence beta LFP activity (8-20 Hz), we designed this study to find out how they affect low-frequency (LF) oscillations (2-7 Hz). STN LFPs were recorded in 19 patients with PD during DBS, after levodopa medication, and during DBS and levodopa intake combined. We investigated the relationship between LF modulations, DBS duration and levodopa intake. We also studied whether LF power depended on disease severity, the patient's clinical condition and whether LF modulations were related to electrode impedances. LF power increased during DBS, after levodopa intake and under both experimental conditions combined. The LF power increase correlated with the levodopa-induced clinical improvement and the higher the electrode impedance, the greater was the LF power change. These data suggest that the LF band could be useful as a control neurosignal for developing novel adaptive DBS systems for patients with PD.

Quantitative and Qualitative Analysis of Ambulatory Electroencephalography During Mild Traumatic Brain Injury

To characterize the neurophysiological changes in a patient with mild traumatic brain injury (mTBI) and to compare these changes with a small cohort of patients with neurocardiogenic syncope, an analogous cause of transient neurological dysfunction. Case report and quantitative analysis of a small electroencephalography (EEG) cohort. University-affiliated teaching hospital. A 64-year-old man with mTBI recorded on ambulatory EEG. The comparison group was 4 patients with spontaneous neurocardiogenic syncope during continuous video EEG recording. Quantitative and qualitative analysis of EEG. Changes in quantitative EEG measurements between the patient with mTBI and the comparison group. In the patient with mTBI, there was an abrupt decrease in high-frequency (beta) power and alpha-delta ratio immediately after the injury and a corresponding increase in lower-frequency (alpha, theta, delta) power. The change in beta power resolved within 5 minutes of the injury, but the increases in low-frequency power persisted up to 20 minutes after the injury before resolving. Similar but smaller changes were seen in the patients with syncope, but these changes resolved within 5 minutes, with no intermediate or long-term changes. The quantitative EEG changes in mTBI are initially similar to those in syncope, suggesting acute transient cortical dysfunction. However, there are longer-lasting increases in low-frequency power during mTBI, suggesting ongoing disruption of cortical-thalamic circuits.

Nitrous Oxide Paradoxically Modulates Slow Electroencephalogram Oscillations

Nitrous oxide (N(2)O) is one of the oldest analgesics/adjuvant agents still in use today; however, its effects on the human electroencephalogram (EEG) remain unclear. It has been proposed that N(2)O may enhance higher-frequency EEG activity (often indicative of alert states and cognition) duration sedation. This possibly paradoxical effect has been used to explain the failure of many EEG monitors to capture the effects of N(2)O on patient state during anesthesia. To better understand the poor efficacy of current EEG approaches to monitoring N(2)O action, we quantitatively studied the sole effect of N(2)O on the resting EEG in healthy volunteers using multichannel EEG recordings under noise-minimized laboratory conditions. Healthy male volunteers were administered 20% (n = 10), 40% (n = 10), or 60% (n = 5) inspired N(2)O mixed with oxygen during noise-shielded EEG recordings. N(2)O was administered over a 20-minute period involving a 5-minute equilibration period and 5-minute washout. EEG spectral edge frequency (95%), median power frequency, total power, and band-limited power (δ, , α, β, and γ) were used as quantitative EEG parameters. The changes in these EEG parameters were quantified throughout N(2)O inhalation and compared between predrug baseline, peak drug effect, and washout. Quantification of changes in spectral power during N(2)O inhalation showed only minor changes in estimates of spectral edge and median power frequency, whereas significant reductions in total power were observed at frontal sites during peak gas effect (P = 0.001; mean reduction [95% confidence interval]: 41.90 μV(2) [18.19-65.61 μV(2)]) that rebounded during N(2)O washout. Such changes in total power were driven by shifts in low-frequency power (δ/), which were most elevated at frontal sites. Rather than directly enhancing high-frequency EEG power (β or γ bands), N(2)O seems to preserve the awake features of resting EEG (α band) and suppress power in those bands in which increases are typically associated with sedation/hypnosis (δ and ). These data suggest that N(2)O's suppression of low-frequency EEG power may help to explain previously reported difficulties in attempting to monitor patient state with the EEG during anesthesia involving N(2)O. Because increases in low-frequency power typically indicate increasing anesthesia, N(2)O's suppression of such activity and its rebound during washout would paradoxically influence EEG monitoring parameters. Therefore, correcting for such effects is expected to improve future monitoring methods.

The Influence of Carbon Dioxide on Brain Activity and Metabolism in Conscious Humans

A better understanding of carbon dioxide (CO(2)) effect on brain activity may have a profound impact on clinical studies using CO(2) manipulation to assess cerebrovascular reserve and on the use of hypercapnia as a means to calibrate functional magnetic resonance imaging (fMRI) signal. This study investigates how an increase in blood CO(2), via inhalation of 5% CO(2), may alter brain activity in humans. Dynamic measurement of brain metabolism revealed that mild hypercapnia resulted in a suppression of cerebral metabolic rate of oxygen (CMRO(2)) by 13.4% ± 2.3% (N=14) and, furthermore, the CMRO(2) change was proportional to the subject's end-tidal CO(2) (Et-CO(2)) change. When using functional connectivity MRI (fcMRI) to assess the changes in resting-state neural activity, it was found that hypercapnia resulted in a reduction in all fcMRI indices assessed including cluster volume, cross-correlation coefficient, and amplitude of the fcMRI signal in the default-mode network (DMN). The extent of the reduction was more pronounced than similar indices obtained in visual-evoked fMRI, suggesting a selective suppression effect on resting-state neural activity. Scalp electroencephalogram (EEG) studies comparing hypercapnia with normocapnia conditions showed a relative increase in low frequency power in the EEG spectra, suggesting that the brain is entering a low arousal state on CO(2) inhalation.

Functional differences of low- and high-frequency oscillatory dynamics during illusory border perception

Neuronal oscillations are an essential part of several brain processes like perception, memory and attention, but little is known about the simultaneous dynamics of different frequency bands in such processes. Here, we studied the low- and high-frequency dynamics of neuronal oscillations in a task that involves covert searching and visual perception of an illusory contour figure (Kanisza square). Subjects were cued to a particular region of the space, and then a matrix of illusory contour inductors appeared. Illusory contour could appear at the cued position, in a different position or not appear at all. We analyze the condition in which an illusory contour appears and those in which the illusory borders do not appear. During the visual perception of the illusory contour, we found a significant increase in a narrow and sustained gamma frequency band (55–65 Hz). We also found a transient increase in the theta band (4–10 Hz), 0.3 s after stimuli onset. This low-frequency power increase correlates strongly with the detection of the illusory contour and it is inversely correlated with the gamma-band response. When illusory contours were absent, a sustained low-frequency response in the same frequency range was present until appearance of the motor response. The intensity of this low-frequency power response was also related to the speed of the motor response. We hypothesize that theta and gamma interact to process different aspects of the task. Gamma would be related to perception of the stimuli, while theta would be involved in search mechanisms or the initiation of a motor response.

Effect of low-frequency power on etching of SiCOH low- k films in CHF 3 13.56 MHz/2 MHz dual-frequency capacitively coupled plasma

This work investigates the etching characteristics of SiCOH low dielectric constant (low- k) films in the CHF 3 13.56 MHz/2 MHz dual-frequency capacitively couple plasma (CCP). The effect of low-frequency (LF) power on etching behavior is analyzed. The results show that the increase of LF power can leads to a transition of etching behavior from films deposition to etching. By Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis on the etched SiCOH films and optical emission spectroscopy (OES) analysis on the plasma radicals, the transition behavior is found to relate to the suppression of C:F deposition due to the energetic ions sputtering and the increase of F concentration at higher LF power.

Control of the discharge chemistry of CHF3 in dual-frequency capacitively coupled plasmas

The discharge chemistry of CHF3 in 27/2, 60/2, and 60/13.56 MHz dual-frequency capacitively coupled plasmas (DF-CCPs) is studied with actinometric optical emission spectroscopy and mass spectrometry. The frequency effect on the generation of reactive species was investigated. The reactive radicals and the density ratio of F/CF2 could be controlled by the 2 MHz rf power in 27/2 and 60/2 MHz DF-CCPs. The density ratios of F/CF2 in 27/2 and 60/2 MHz DF-CCPs are observed to increase with an increase in low-frequency power. However, this control could not be obtained in 60/13.56 MHz DF-CCP.

Orthostatic influence on heart rate and blood pressure variability in trained persons with tetraplegia

To examine the orthostatic influence on heart rate and blood pressure variability in persons with tetraplegia playing wheelchair basketball, ten trained persons with tetraplegia, ten untrained persons with tetraplegia, and ten able-bodied participated in this study. Spectrum analysis of the ECG R-R interval and blood-pressure on a beat-by-beat basis during head-up tilt 60 degrees sitting were performed. The ratio of the high frequency to total frequency (HF/TF) in the R-R interval decreased from supine (0.5 +/- 0.2) to sitting (0.3 +/- 0.2), and the low frequency (LF) power in systolic blood pressure increased from 4.7 +/- 9.1 to 15.0 +/- 13.1 mmHg(2) only in the untrained persons with tetraplegia (P < 0.01). The decrease in the HF/TF ratio in the untrained persons with tetraplegia indicates attenuated parasympathetic activity to the orthostatic challenge and the similar increase in LF power indicate that parasympathetic activity was reduced and sympathetic activity increased only in these persons. These results suggest that training enhances cardiovascular stability in tetraplegic subjects.

Subthalamic local field potential oscillations during ongoing deep brain stimulation in Parkinson's disease

How deep brain stimulation (DBS) acts and how the brain responds to it remains unclear. To investigate the mechanisms involved, we analyzed changes in local field potentials from the subthalamic area (STN-LFPs) recorded through the deep brain macroelectrode during monopolar DBS of the subthalamic nucleus area (STN-DBS) in a group of eight patients (16 nuclei) with idiopathic Parkinson's disease. Monopolar STN-DBS was delivered through contact 1 and differential LFP recordings were acquired between contacts 0 and 2. The stimulating contact was 0.5 mm away from each recording contact. The power spectral analysis of STN-LFPs showed that during ongoing STN-DBS whereas the power of beta oscillations (8–20 Hz) and high beta oscillations (21–40 Hz) remained unchanged, the power of low-frequency oscillations (1–7 Hz) significantly increased (baseline = 0.37 ± 0.22; during DBS = 7.07 ± 15.10, p = 0.0003). Despite comparable low-frequency baseline power with and without levodopa, the increase in low-frequency oscillations during STN-DBS was over boosted by pretreatment with levodopa. The low-frequency power increase in STN-LFPs during ongoing STN-DBS could reflect changes induced at basal ganglia network level similar to those elicited by levodopa. In addition, the correlation between the heart beat and the low-frequency oscillations suggests that part of the low-frequency power increase during STN-DBS arises from polarization phenomena around the stimulating electrode. Local polarization might in turn also help to normalize STN hyperactivity in Parkinson's disease.

Heating of a dual frequency capacitively coupled plasma via the plasma series resonance

The behavior of dual frequency capacitively coupled plasma discharges (2f-CCP) is experimentally studied by Langmuir probe and rf current measurements and is compared with simulations from the literature. The driving frequency ratio, system pressure, high frequency (HF) power and low frequency (LF) power are varied in the experiments. An increase in LF power causes a moderate increase in electron density but a significant decrease in electron temperature. An increase in HF power causes a strong increase in electron density and populates the high energy part of the electron energy distribution function. These dependences can be explained on the basis of a global model. It is shown that the ratios of HF/LF power and driving frequency are the most important parameters. At integer frequency ratios a significant increase in electron density was found, which is explained by the indirect heating at the plasma series resonance. Several design guidelines are derived which address industrial applications and process stability.

Persistence of baroreceptor control of cerebral blood flow velocity at a simulated altitude of 5000 m

To assess the effects of acute exposure to simulated high altitude on baroreflex control of mean cerebral blood flow velocity (MCFV). We compared beat-to-beat changes in RR interval, arterial blood pressure, mean MCFV (by transcranial Doppler velocimetry in the middle cerebral artery), end-tidal CO2, oxygen saturation and respiration in 19 healthy subjects at baseline (Albuquerque, 1779 m), after acute exposure to simulated high altitude in a hypobaric chamber (barometric pressure as at 5000 m) and during oxygen administration (to achieve 100% oxygen saturation) at the same barometric pressure (HOX). Baroreflex control on each signal was assessed by univariate and bivariate power spectral analysis performed on time series obtained during controlled (15 breaths/min) breathing, before and during baroreflex modulation induced by 0.1-Hz sinusoidal neck suction. At baseline, neck suction was able to induce a clear increase in low-frequency power in MCFV (P<0.001) as well as in RR and blood pressure. At high altitude, MCFV, as well as RR and blood pressure, was still able to respond to neck suction (all P<0.001), compared to controlled breathing alone, despite marked decreases in end-tidal CO2 and oxygen saturation at high altitude. A similar response was obtained at HOX. Phase delay analysis excluded a passive transmission of low-frequency oscillations from arterial pressure to cerebral circulation. During acute exposure to high altitude, cerebral blood flow is still modulated by the autonomic nervous system through the baroreflex, whose sensitivity is not affected by changes in CO2 and oxygen saturation levels.

Longitudinal assessment of heart rate variability in very low birth weight infants during their NICU stay

Background Maturation of the autonomic nervous system has not been studied in high-risk very low birth weight (VLBW) infants in the first few weeks of life. Aim To characterize developmental changes in autonomic nervous system activity of high-risk VLBW infants from 23 to 38 weeks post-menstrual age by measuring heart rate variability (HRV). Study design and subjects In this prospective cohort study 38 infants admitted to Children's Memorial Hermann Hospital NICU were longitudinally followed weekly or biweekly. Heart period data were recorded while infants were resting in active sleep. Outcome measures Growth of spectral power of HRV in low-frequency (0.05–0.25 Hz) and high-frequency (0.25–1.00 Hz) bands was modeled with linear mixed-effects models. The high-frequency power provides a measure of respiratory sinus arrhythmia (RSA). Results Low-frequency power increases with post-menstrual age, and intubated infants have lower HRV. The increase in low-frequency power is faster (0.50 ± 0.12 dB/week) than the increase in RSA (0.17 ± 0.09 dB/week). Conclusion This longitudinal data exhibits developmental maturation of the RSA and of the low-frequency power of HRV in high-risk VLBW infants.

Diurnal spectral EEG fluctuations in narcoleptic patients during rest and reaction time tasks

Narcolepsy is associated with lowered vigilance. Diurnal variation in vigilance appears altered, but the exact nature of this change is unclear. It was hypothesized that the homeostatic sleep drive is increased in narcolepsy. Decreased levels of vigilance are reflected in low frequency band power in the electroencephalogram (EEG), so these frequencies were expected to be increased in the narcolepsy group. Furthermore, it was expected that low frequency power should increase over the day. Narcoleptic patients and healthy controls participated (36 participants in total); they were not allowed to take medication or naps on the experimental day. EEG was measured at 9:00, 11:00, 13:00, 15:00, and 17:00 hours, during rest and during reaction time tasks. In the narcolepsy group, alpha power was lower at rest at all times. Delta and theta power during rest and task performance increased steadily over the day in this group, from 11:00 hours onwards. Additionally, in the narcolepsy group beta1 and beta2 power during rest appeared increased at the end of the day. The effects in the lower frequency bands strongly suggest that vigilance is low at all times. The progressive increase in low frequency power indicates that the sleep drive is enhanced. It is not clear whether this pattern reflects an extreme state of low vigilance, or a pathological brain condition. The effects in the higher frequencies suggest that narcoleptic patients may make an effort to counteract their low vigilance level.

Heart Rate Variability Assessment of Overreaching in Active and Sedentary Females

Resting heart rate variability (HRV) profile was compared between sedentary and active females before, during, and after a 2-week overreaching protocol. Twenty currently active and 20 sedentary females were subdivided within their groups to either an experimental or control group. The active and sedentary experimental groups completed an overreaching protocol; endurance exercise, 40 minutes per session at an intensity of 70-90 % heart rate reserve, 7 d.wk(-1). The active experimental group showed an increase in low frequency power and a decrease in high frequency power over the intervention weeks (LF : HF ratio; Baseline = 2.0 [1.1], End week 1 = 2.4 [1.9], End week 2 = 3.2 [1.9], Recovery = 1.9 [1.0] mean [SD]) although this was not statistically significant (p > or = 0.05) for within group effects. The sedentary experimental group however, showed a significant (p < or = 0.05) increase in low frequency power and a decrease in high frequency power during the overreaching phase suggesting a move towards a predominance of resting sympathetic cardiac modulation (LF:HF ratio; Baseline = 1.6[1.1], End week 1 = 3.4 [1.6], End week 2 = 4.4 [3.0], Recovery = 1.8 [0.9]). These findings arose in the absence of any significant change in resting heart rate. Time domain analysis however failed to provide similar findings in either experimental group. Neither control group showed any significant changes. These data suggest a more marked change in sympathovagal balance in sedentary people when exposed to an overreaching protocol than in active individuals.

Measures of Steadiness Are influenced by the Frequency Content of the Signals

0077 Fluctuations in limb acceleration during a steady contraction are often not associated with the fluctuations in limb position. Although the two signals often have two major peaks in the frequency range of 0–4 Hz (low frequency) and 8–12 Hz (high frequency), the low-frequency component is more prominent in the position signal. PURPOSE: To determine the influence of the low-frequency component on the association between simulated fluctuations in position and acceleration. METHODS: Position signals (20 s at 1024 Hz) were generated by summing 1,000 sinusoidal waves of varying frequency (< 15 Hz) and phase. The amplitude of each sinusoidal wave was adjusted so that the power of the low-frequency component in the position signal varied, whereas the amplitude of the high-frequency component remained constant. The position signal was double-differentiated to yield the acceleration signal. The low-frequency component was varied from 0.24 to 93.96% of the total power in the frequency spectrum. Fifty pairs of position and acceleration signals were simulated at each level of low-frequency power. Correlation coefficients between the standard deviations of position and acceleration were calculated at each level. RESULTS: The low- and high-frequency power in the acceleration signal ranged from 0.006% to 5.70% and from 86.77% to 79.56% total power, respectively. The correlation coefficients between the standard deviation of position and acceleration was distributed from −0.184 to 0.967. The correlation coefficient was high when the low-frequency component was small and it decreased continuously with an increase in low-frequency power of each signal: up to ∼ 60% in position and ∼ 0.5% in acceleration. For the signals with greater low-frequency power, the correlation coefficient was low (< 0.4) and variable. CONCLUSION: The results suggest that the association between the two measures of steadiness (fluctuations in position and acceleration) depend primarily on small changes in low-frequency power in the signals, particularly the acceleration signal.