Lag Dependence Research Articles

Epileptic seizures are temporally interdependent under certain conditions

The possibility that seizures may be intercorrelated has not been sufficiently investigated. A handful of studies, the majority based on patient seizure diaries, provide disparate results: some claim that seizures are serially correlated and others that they are random events. This study investigates the effect that a seizure may have on the time of occurrence and severity of subsequent ones in subjects undergoing invasive surgical evaluation. The Savit-Green statistic, a measure of time series lag dependency, was applied to seizure sequences derived from the ECoGs of 26 epilepsy surgery candidates. Seizure onset times, intensities and durations were obtained using a validated seizure detection algorithm, and from these, inter-seizure intervals (ISI) and severities were computed and their lag dependencies were compared to suitably randomized and amplitude-scaled linear surrogate sets. The null hypothesis (seizures are uncorrelated) was rejected (p<0.05) for ISI in 12/26 subjects and for seizure severity in 13/26. The temporal correlations spanned up to three preceding seizures and were nonlinear in 7/12 subjects for ISI and in 8/13 for severity. An important finding is that dependencies may be related to the frequency of seizures in the sample. This study demonstrates that under certain conditions, there are linear and nonlinear seizure dependencies of low order and at small time scales (minutes to hours), for ISI and seizure severity. This observation has important implications for studies of seizure predictability, which de facto treat seizures as independent occurrences. Given the study subjects' conditions, it is not clear if the dependencies reflect innate brain dynamics, drug withdrawal, local trauma or a combination of these.

The catalytic transfer hydrogenation on Ni electroless alloys

The catalytic transfer hydrogenation (CTH) was investigated on Ni electroless alloys, using H2PO2− ion as the H-donor and 3-buten-1-ol as a model organic compound. Ni electroless alloys films containing 7, 9 and 14 at.% of P were prepared and characterized by SEM, EDS and XRD. Cyclic voltammetry allowed to evaluate qualitatively the catalyst ability to co-adsorb the H-donor and the organic compound and revealed that the adsorption ability of 3-buten-1-ol is stronger on the highest P content alloy film. The CTH performance was characterized in terms of yield, H-donor efficiency and selectivity and the results were compared to those for the electrocatalytic hydrogenation. The dependence of the catalyst activity on the P content of the Ni alloy was found to follow different trends on the two hydrogenation methodologies. This behaviour was qualitatively explained on the basis of the competitive adsorption of 3-buten-1-ol and the supplier of hydrogen. It was concluded that hydrogenation of a strong adsorbable unsaturated organic compounds is more successful through the use of an H-donor than through the electrocatalytic hydrogenation. In the course of the CTH run, the Ni alloy deactivation was observed, but re-activation of the catalyst by applying a rather negative potential was attained. Dependence of the time lag for deactivation with the P content of the Ni alloy was found. The source of the deactivation was investigated and an explanation for this phenomenon was proposed.

Analysis of buffer‐related lag phenomena and current collapse in GaN FETs

Drain-current responses of GaN MESFETs with a semi-insulating buffer layer are calculated when the drain voltage and/or the gate voltage are changed abruptly, and pulsed I-V curves are derived from them. It is shown that so-called current collapse or current slump is not so dependent on the gate length LG (0.3 μm – 1 μm), and this is essentially determined by a deep-acceptor density NDA in the buffer layer. LG and NDA dependence of gate lag is also studied, indicating that the gate lag is weaker for longer LG and the lag rate becomes high with NDA, although it may show a saturation behaviour. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Diffusion anomaly and blind pore character in carbon molecular sieve membrane

The structure of a carbon molecular sieve (CMS) membrane is characterized by the through pores and blind pores with non-linear sorption isotherms inside. Time-lag analysis was conducted for gas permeation in such a structure and mathematical formulations were derived for two cases. It is found that the pressure dependence of the time lag is dominated by the ratio of sorption affinities in the two types of pores. The experimental permeation data of pure component CO 2 and N 2 measured on a CMS membrane were used to validate the model. It is found that, for the adsorbing species ( CO 2 ) , the model is able to well describe the diffusion anomalies over a wide range of permeation pressure, while for the weakly adsorbing species ( N 2 ) , the model is inadequate to cope with the anomalies at the low end of permeation pressure.

CO2 Permeation in Carbon Membranes with Different Degrees of Carbonization

CO2 adsorption and permeation were measured on two carbon molecular sieve membranes over a wide pressure range. The two membrane samples were fabricated under similar conditions but with different degrees of carbonization. The pressure dependence of the permeation time lag was investigated, and it is found that the diffusion coefficient takes a stronger functional dependence on loading than the Darken relation and this dependence increases with the degree of carbonization. The experimental data and diffusion models from the literature were employed for the purpose of analysis and comparison.

Observation of the phase lags among the molecules in the coherent phase control of multiphoton ionization

The phase lags among the molecules have been observed in the coherent phase control of multiphoton ionization. Coherent phase control using three- and one-photon excitation scheme has been applied to the multiphoton ionization of the (CH 3) 3N, (CH 3) 2S, C 2H 5SH, CH 3I, and CD 3I molecules. The phase lags of methyliodide (CH 3I and CD 3I) relative to other molecules showed considerable dependence on the excitation wavelength, especially in the vicinity of the absorption peak of CD 3I. The energy dependence of the phase lag indicates that the molecular phase for the ionization of methyliodide is not zero.

Soft Lag and Its Implications for X‐Ray Time Variations in GRS 1915+105

The X-ray time variation of GRS 1915+105 is analyzed, focusing on the phase-lag properties in class θ. The energy and frequency dependence of the soft lag are derived. This energy dependence can be interpreted as Comptonization of high-energy photons by relatively soft electrons with temperatures of less than ~2 keV. A Monte Carlo simulation of the Comptonization of power-law photons by soft electrons is presented. This model can explain the energy spectrum, the energy dependence, and the frequency dependence of the soft lag. The size of the Compton cloud is estimated to be ~2 × 109 cm by comparing the simulation with the data. The time evolution in class θ, including the dip, is discussed.

Effects of Climate on Incidence of Campylobacter spp. in Humans and Prevalence in Broiler Flocks in Denmark

Campylobacter infections are increasing and pose a serious public health problem in Denmark. Infections in humans and broiler flocks show similar seasonality, suggesting that climate may play a role in infection. We examined the effects of temperature, precipitation, relative humidity, and hours of sunlight on Campylobacter incidence in humans and broiler flocks by using lag dependence functions, locally fitted linear models, and cross validation methods. For humans, the best model included average temperature and sunlight 4 weeks prior to infection; the maximum temperature lagged at 4 weeks was the best single predictor. For broilers, the average and maximum temperatures 3 weeks prior to slaughter gave the best estimate; the average temperature lagged at 3 weeks was the best single predictor. The combined effects of temperature and sunlight or the combined effects of temperature and relative humidity predicted the incidence in humans equally well. For broiler flock incidence these factors explained considerably less. Future research should focus on elements within the broiler environment that may be affected by climate, as well as the interaction of microclimatic factors on and around broiler farms. There is a need to quantify the contribution of broilers as a source of campylobacteriosis in humans and to further examine the effect of temperature on human incidence after this contribution is accounted for. Investigations should be conducted into food consumption and preparation practices and poultry sales that may vary by season.

Influence of metallic and lanthanide dopants on the thermal diffusivity of lithium niobate crystals

The influence of crystal doping on thermal diffusivity of LiNbO 3 crystal has been studied. Thermal diffusivities of investigated samples have been measured by modified Angstrom method. This method requires that phase lag of a thermal wave propagating in the sample be determined. One plane of rectangular prismatic sample is periodically heated and cooled while others remain thermally insulated. Periodical changes in the temperature of heated plane and the plane opposite to it are measured and used to determine phase lag between the two signals. The measurements have been taken for different frequencies and thermal diffusivity value has been found from frequency dependence of phase lag. Examined crystals have been grown using Czochralski's method. One of them has been pure LiNbO 3 , others have been doped with the following elements: Cu (0.05 %at.), Fe (0.1 %at.), Dy (0.1 %at., 1.0 %at., and 1.7 %at.), Er (0. %at.), and Er + Yb (1.0 %at. Er + 0.1 %at. Yb, 1.0 %at. Er + 0.5 %at. Yb, 1.0 %at. Er + 1.0 %at. Yb). The measurements have been carried out for crystallographic directions [100], [120] and [001]. Measured value of thermal diffusivity of pure lithium niobate is in good agreement with values reported by other researchers, values obtained for samples doped with Cu and Fe are comparable to those obtained for pure crystals. Crystals doped with rare earth elements have considerably lower thermal diffusivities.

A simple method for calibrating the temperature in dynamic mechanical analysers and thermal mechanical analysers

An intrinsic requirement in reliable dynamic mechanical analysis (DMA) or thermal mechanical analysis (TMA) is the precise determination of the sample temperature. In this work a straightforward method is proposed and implemented that allows correction of the average sample temperature for a variety of probe geometries (flexural, tensile and compression). Hydrophilic polymeric-based matrices, with geometries similar to the sample to be tested, are swelled with a pure liquid (in this case water) and placed in the correct testing place of the equipment; a static force is applied at low temperature and the probe position is monitored in a temperature scan, where the melting of the standard is clearly detected and compared with the expected melting temperature. This allows the average sample temperature to be obtained, using a sample that exhibits similar thermal resistance to the sample to be tested, instead of probing specific locations inside the furnace, as it is usually proposed in calibration routines with pure metals. For the specific configurations analysed, a strong and similar dependence of the thermal lag on the scanning rate was found for both flexural and compression modes that also depends weakly on the sample dimensions. More complicated dependence of the thermal lag on the scanning rate is found for the tensile mode, probably due to the complex thermal environment existing inside the furnace with this arrangement. However, the results indicate that there is a significant contribution from the thermal resistance of the sample in the temperature correction. It is suggested that the proposed method may be employed in other situations such as isothermal and non-isothermal creep/stress relaxation experiments or in rheometers.

Spatial Effects within the Agricultural Land Market in Northern Ireland

The importance of dealing properly with spatial effects, such as spatial autocorrelation, in cross‐sectional econometric estimation has become more widely recognised in recent years. Spatial autocorrelation is similar in many ways to serial correlation, but while the latter is ordered on a one‐dimensional time axis, the former is ordered in two dimensions. The multi‐directional nature of spatial dependence means that specialised techniques are needed for diagnostic testing and estimation purposes. This paper uses these specialised diagnostics to test for spatial effects within a hedonic pricing study of the agricultural land market. The tests indicate that spatial autocorrelation (in the form of spatial lag dependence) and spatially distinct sub‐markets (or spatial heterogeneity) are present. Ignoring these effects in the estimation process is likely to lead to biased parameter estimates. Consequently, we re‐specify the hedonic model to allow for these spatial effects. The presence of spatial lag dependence suggests that there is circularity of price setting within the agricultural land market. This means that agricultural land prices are not solely determined by the inherent characteristics of the land, but tend to reflect also the average local price per acre.

Understanding China?s recent growth experience: A spatial econometric perspective

This study reconsiders the question of China’s recent growth experience from a spatial econometric perspective. An empirical model of Chinese output growth using cross provincial data over the 1978–1998 period is specified, but a spatial econometric analysis of the specification reveals strong evidence of misspecification due to ignored spatial lag dependence. The subsequent estimating using Anselin’s spatial lag model determines the important sources of growth to be the growth of non-farm labor force, manufactured products, capital stock, and realized direct foreign investment. On the other hand, the estimated coefficient for the spatial lag variable suggests a polarizing process undergoing within the Chinese spatial economy, and the resulting change in the estimates of causal factors implies that as marketization progresses, a variety of spillover effects due to factor mobility, transfer payments and technological diffusion become operational, which actually improve the marginal productivity of factor inputs for labor (GL) and capital (GK, GDFI) and bring national output closer to its frontier of the Chinese economy.

Tests for spatial lag dependence based on method of moments estimation

In this paper, we formulate GMM versions of the Wald, the Likelihood ratio and the Lagrange multiplier test statistics for spatial lag dependence in the spatial lag model with autocorrelated errors. The tests are based on the GMM estimator suggested by Kelejian and Robinson [Papers in Regional Science: J. RSAI 72 (1993) 297] and on the work of Newey and West [Int. Econ. Rev. 28 (1987) 777]. We analyze their finite-sample properties in a Monte Carlo experiment along with those of two alternative tests for spatial lag dependence based on least squares and generalized least squares estimation, respectively. Results indicate that the small sample distribution of the GMM tests is well approximated by their asymptotic chi-squared distribution for relatively small sample sizes, but only for small values of the spatial error parameter. However, we find variations across different spatial weighting schemes for given sample size and given values of the spatial error parameter.

Effective laser field influence on the transverse triggering of a spark gap in air

Nd : YAG laser triggering of a centimetric air gap in a homogeneous electric field is studied for pressures ranged from 0.5 to 5 bar. Breakdown transient voltage and current are measured and the formative time lag dependence on reduced field and laser intensity is analysed. It is quantitatively shown that the laser field strongly influences the time lag to breakdown through the effective reduced field.

Study of isotope effects in the photoionization of HI and DI using phase lag spectroscopy

Phase lag spectroscopic measurements are performed for HI and DI in the 5d(π,δ) resonance region. Measurements of the phase lags for both molecules reveals a strong isotopic effect, especially pronounced in the ionization channel. The energy dependence of the phase lag provides clear examples of continuum and resonance coupling effects that were previously predicted theoretically but have not yet been observed. It is demonstrated that phase lag spectroscopy is a sensitive technique for studying properties of molecular continua, furnishing information that is not available from phase-insensitive observables.

DOUBLE LENGTH ARTIFICIAL REGRESSIONS FOR TESTING SPATIAL DEPENDENCE

This paper derives two simple artificial Double Length Regressions (DLR) to test for spatial dependence. The first DLR tests for spatial lag dependence while the second DLR tests for spatial error dependence. Both artificial regressions utilize only least squares residuals of the restricted model and are therefore easy to compute. These tests are illustrated using two simple examples. In addition, Monte Carlo experiments are performed to study the small sample performance of these tests. As expected, these DLR tests have similar performance to their corresponding LM counterparts.

Evaluation of Viscoelasticity of Biological Cells in Liquid by Scanning Probe Microscopy.

An analytical method is proposed to evaluate the viscoelasticity of a biomaterial in water by the scanning probe microscope. The proposed method utilizes the dependence of amplitude and phase lag of cantilever vibration on the viscoelasticity when the tip on the cantilever cyclically penetrates into the cell. The cyclical penetrations are carried out in two ways; one is by excitation of the cantilever fixed end, and other by that of the table. The analysis takes the drag force of water due to the cantilever vibration into account. The stiffness and the viscosity of the cell are analytically related with the amplitude and the phase lag of the cantilever vibration. The results shows that the relations among the amplitude, the phase lag, stiffness and viscosity vary significantly depending on the excitation frequencies of the cantilever fixed end and the table.

Acoustic observation of hydrothermal flows

Acoustic backscatter has been used to observe hydrothermal flows at several sites on the Juan de Fuca Ridge at frequencies of 200 and 330 kHz. In the case of plume discharges (‘‘smokers’’), backscatter from particulates is used to obtain three-dimensional images and to measure flow via Doppler shift. For diffuse flows, acoustic scintillation of the signal backscattered from the seafloor is detected using ping-to-ping correlation. Mapping of diffuse flows using both fixed and moving platforms will be discussed. The magnitude and time lag dependence of ping-to-ping correlation have been linked to flow parameters using the Rytov scattering approximation. [Work supported by NSF Grant OCE 98-18841.]

1118 水中に置かれた生体の原子間力顕微鏡による粘弾性評価法(OS-5 リモデリング・マイクロバイオメカニクス)

This paper shows that the viscoelasticity of a biomaterial in water is strictly evaluated by the atomic force microscope (AFM). The evaluation method utilizes the dependence of amplitude and phase lag of cantilever vibration on the viscoelasticity when the tip on the cantilever cyclically penetrates into the material. The penetration motion is given by the vibration of the fixed end of the cantilever. The vibration analyses are made, taking the effects of drag force by water on its natural frequency and damping coefficient into account. Equations which derivate the surface stiffness and the surface viscosity are expressed by functions of the amplitude and the phase lag of the cantilever vibration at its tip end.

Statistical analysis of atmospheric forcing of North Pole ice draft variations

A series of correlation experiments and regression models are used to explore the atmospheric forcing of observed interannual variability in North Pole ice drafts as measured by U.S. Navy submarine sonar from 1982 to 1992. Mean ice drafts as well as ice draft distributions are used in the analysis. Nearby drifting buoys are used to approximate a full 2‐year motion history of the North Pole ice parcels prior to submarine underpass. Drawing from theoretical work and empirical observation, predictors of North Pole ice draft variation are selected which embody accumulated ice exposure to subfreezing air temperature, ice divergence, and patterns of ice motion. The timescales of the predictors are based on systematic evaluations of the lag dependence of the predictor‐predictand relationships. Linear correlation of each predictor with the observed ice draft distribution shows accumulated ice exposure to subfreezing air temperature is weakly correlated with reductions in shallow (0 to 2.5‐m) drafts. Ice divergence a week prior to draft measurement correlates most closely with the redistribution of ice drafts shallower than 3.5 m. Ice divergence and ice deflection from its wind‐forced motion during the 6 months prior to observation correlate with deeper ice drafts (deeper than 6.8 and 4.6 m, respectively). Together, these two parameters explain more than 80% of the variance of the observed mean ice draft. The accumulated ice exposure to subfreezing air temperatures colder than the 1980–1992 mean in the 2 years prior to ice arrival at the North Pole adds little to an explanation of the interannual variance of the mean ice draft. The results imply that (1) a realistic formulation of sea ice dynamics is a first‐order priority for model simulations of ice draft and (2) North Pole ice thickness is not a simple function of Arctic air temperature.