Estimated Time Constant Research Articles

Parallel spike trains analysis using positive definite kernels

Since spikes are considered to be the fundamental element in neural systems, there has been a significant amount of work that analyzes the sequence of spike timings, or spike trains [1,2]. One important research question is to know how information is encoded in a spike train. There is an approach to construct a feature space that captures characteristics of spike trains, as is usually done in the field of pattern recognition. Another approach, which is increasingly becoming popular in recent years, is to find an appropriate similarity measure between spike trains [3,4]. This corresponds to constructing a model of information decoding done by the postsynaptic neurons. One promising way to define such similarity measures is to use positive definite kernels [5-7]. Since positive definite kernels are generalizations of inner products, they enable application of various linear methods used in machine learning (including regression, classification, and dimension reduction) to spike trains. Positive definite kernels are extensively used in kernel methods. While positive definite kernels on spike trains obtained from a single neuron have already been proposed [5-7], the extension of this to parallel spike trains obtained from multiple neurons has not been explored yet. Therefore, we defined such a kernel by extending the memoryless cross intensity kernel (mCI kernel) proposed by Paiva et al. [5]. To make the extension as natural as possible, we used a linear combination of cross-neuron interactions. We name the new kernel the linear combination of interactions kernel (LCIK) [9]. The parameters of the kernel can be set to make it positive definite. We applied this kernel to publicly available in vivo recordings taken from the primary visual field in macaque monkey brains [8]. The LCIK performed better than other possible kernels defined on a set of spike trains. When the parameters of the kernel were estimated using spike trains obtained from real neurons, we obtained biologically plausible values. For example, the estimated time constant was near the value commonly used in neural modeling. This indicates the possibility of using such kernels to look for an appropriate model of neurons based on observed spike recordings. We also simulated a simple neural network and generated spike trains to see the effect on the estimated values of the kernel parameters by changing the synaptic connection parameters. The result indicated that the kernel indirectly represents some of the internal parameters of the neural networks.

Effects of plasma-induced charging damage on random telegraph noise in metal–oxide–semiconductor field-effect transistors with SiO2 and high-k gate dielectrics

We clarified in this study how plasma-induced charging damage (PCD) affects the so-called “random telegraph noise (RTN)” — a principal concern in designing ultimately scaled large-scale integrated circuits (LSIs). Metal–oxide–semiconductor field-effect transistors (MOSFETs) with SiO2 and high-k gate dielectric were exposed to an inductively coupled plasma (ICP) with Ar gas. Drain current vs gate voltage (Ids–Vg) characteristics were obtained before and after the ICP plasma exposure for the same device. Then, the time evolution of Ids fluctuation defined as Ids/μIds was measured, where μIds is the mean Ids. This value corresponds to an RTN feature, and RTN was obtained under various gate voltages (Vg) by a customized measurement technique. We focused on the statistical distribution width of (Ids/μIds), δ(Ids/μIds), in order to clarify the effects of PCD on RTN. δ(Ids/μIds) was increased by PCD for both MOSFETs with the SiO2 and high-k gate dielectrics, suggesting that RTN can be used as a measure of PCD, i.e., a distribution width increase directly indicates the presence of PCD. The dependence of δ(Ids/μIds) on the overdrive voltage Vg–Vth, where Vth is the threshold voltage, was investigated by the present technique. It was confirmed that δ(Ids/μIds) increased with a decrease in the overdrive voltage for MOSFETs with the SiO2 and high-k gate dielectrics. The presence of created carrier trap sites with PCD was characterized by the time constants for carrier capture and emission. The threshold voltage shift (ΔVth) induced by PCD was also evaluated and compared with the RTN change, to correlate the RTN increase with ΔVth induced by PCD. Although the estimated time constants exhibited complex behaviors due to the nature of trap sites created by PCD, δ(Ids/μIds) showed a straightforward tendency in accordance with the amount of PCD. These findings provide an in-depth understanding of plasma-induced RTN characteristic changes in future MOSFETs.

Velocity storage mechanism in zebrafish larvae

The optokinetic reflex (OKR) and the angular vestibulo-ocular reflex (aVOR) complement each other to stabilize images on the retina despite self- or world motion, a joint mechanism that is critical for effective vision. It is currently hypothesized that signals from both systems integrate, in a mathematical sense, in a network of neurons operating as a velocity storage mechanism (VSM). When exposed to a rotating visual surround, subjects display the OKR, slow following eye movements frequently interrupted by fast resetting eye movements. Subsequent to light-off during optokinetic stimulation, eye movements do not stop abruptly, but decay slowly, a phenomenon referred to as the optokinetic after-response (OKAR). The OKAR is most likely generated by the VSM. In this study, we observed the OKAR in developing larval zebrafish before the horizontal aVOR emerged. Our results suggest that the VSM develops prior to and without the need for a functional aVOR. It may be critical to ocular motor control in early development as it increases the efficiency of the OKR.

Performance of a new dynamic model for predicting diurnal time courses of stomatal conductance at the leaf level

Under natural conditions, plants are subjected to continuous changes of irradiance that drive variations of stomatal conductance to water vapour (g(s)). We propose a dynamic model to predict the temporal response of g(s) at the leaf level using an asymmetric sigmoid function with a unique parameter describing time constants for increasing and decreasing g(s). The model parameters were adjusted to observed data using Approximate Bayesian Computation. We tested the model performance for (1) instant changes of irradiance; or (2) continuous and controlled variations of irradiance simulating diurnal time courses. Compared with the two mostly used steady-state models, our dynamic model described daily time courses of g(s) with a higher accuracy. In particular, it was able to describe the hysteresis of g(s) responses to increasing/decreasing irradiance and the resulting rapid variations of intrinsic water-use efficiency. Compared to the mechanistic model of temporal responses of g(s) by Kirschbaum, Gross & Pearcy, for which time constants were estimated with a large variance, our model estimated time constants with a higher precision. It is expected to improve predictions of water loss and water-use efficiency in higher scale models by using a small number of parameters.

Temporal summation of airborne tones in a California sea lion (Zalophus californianus)

The trade-off between sound level and duration on hearing sensitivity (temporal summation) was investigated in a California sea lion (Zalophus californianus) using airborne pure-tone stimuli. Thresholds were behaviorally measured using the method of constant stimuli at 2.5, 5, and 10 kHz for nine signal durations ranging from 25 to 500 ms. In general, thresholds decreased as duration increased up to 300 ms, beyond which thresholds did not significantly improve. When these data were fitted separately to two versions of an exponential model, the estimated time constants (92-167 ms) were generally consistent between the two fits. However, the model with more free parameters generated fits with consistently higher R(2) values, while avoiding potential arbitrary decisions about which data to include. The time constants derived for the California sea lion were generally consistent with those reported for other mammals, including other pinnipeds. The current study did not show a clear correlation between time constant and test frequency. The results should be considered when conducting audiometric work, assessing communications ranges, and evaluating potential noise impacts of airborne tonal signals on California sea lions.

Cr(CO)6 photochemistry: Semi-classical study of UV absorption spectral intensities and dynamics of photodissociation

The UV absorption spectrum of Cr(CO)(6) (chromium hexacarbonyl) in gas phase is investigated by theoretical methods with focus on the absorption intensities. It is shown that in spite of good predictions for the excitation energies, the most frequently employed methods for excited-state calculations produce poor predictions for oscillator strengths and absorption cross sections. In particular, time-dependent DFT predicts relative intensities for the two main spectral bands to be up to five times larger than the experimental results depending on the functional. The best results are obtained by a multireference configuration interaction method based on DFT (DFT/MRCI). Spectral shoulders caused by vibronic-coupling absorption are assigned based on symmetry-restricted spectrum simulations. The dynamics of Cr(CO)(6) photodissociation was also considered at TDDFT/B3LYP level. The estimated time constants for the Cr(CO)(6) relaxation and dissociation are in excellent agreement with experimental values. The time constant for internal conversion, however, is longer than the experimentally observed by factor 2, presumably due to an underestimation of the experimental analysis.

Gas-Wall Partitioning of Organic Compounds in a Teflon Film Chamber and Potential Effects on Reaction Product and Aerosol Yield Measurements

Gas-wall partitioning of organic compounds (OC) that included C 8 –C 16 n-alkanes and 1-alkenes and C 8 –C 13 2-alcohols and 2-ketones was investigated in two Teflon FEP chambers whose walls were either untreated, oxidized in sunlight, or previously exposed to secondary organic aerosol (SOA). Partitioning was nearly independent of chamber treatment, reversible, and obeyed Henry's law. The fraction of an OC that partitioned to the walls at equilibrium ranged from 0 to ∼ 65%. Values increased with increasing carbon number within an OC class and for OC with similar vapor pressures increased in the order n-alkanes <1-alkenes <2-alcohols <2-ketones. Estimated time constants for achieving partitioning equilibrium ranged from ∼ 60 min for n -alkanes to ⩽ 8 min for 2-ketones. The observations are consistent with a sorption mechanism in which OC dissolve into the film but are restricted to the near-surface region by a sharp permeability gradient that develops in response to OC-induced stresses in polymer chains. When the results were analyzed using a model analogous to one commonly employed for gas-particle partitioning, it was estimated that the sorption properties of the chamber walls were equivalent to organic aerosol mass concentrations of 2, 4, 10, and 24 mg m– 3 with respect to the partitioning of n -alkanes, 1-alkenes, 2-alcohols, and 2-ketones. These values are up to ∼ 4 orders of magnitude larger than concentrations used in most laboratory studies of SOA, which are typically 1–10 3 μ g m– 3 , meaning that if full partitioning equilibrium is established in the chamber then semi-volatile OC will reside overwhelmingly in the chamber walls. Model simulations of gas-particle-wall partitioning were also carried out using the experimental data, and demonstrate quantitatively the large potential effects of Teflon walls on measured yields of gas-phase OC products and SOA.

AC Losses in Poloidal Coils of the Large Helical Device

AC losses in poloidal coils of the Large Helical Device (LHD) have been measured during single-pulse operations. The superconductors of the coils are Nb-Ti cable-in-conduit conductors (CICC) cooled by supercritical helium. The enthalpy increase due to the losses was observed at the inlet and outlet of the helium coolant. Time constants of the coupling losses can be estimated using analytical expressions with a circuit model and by considering the magnetic field distribution in the coil. The estimated time constants are 80 and 200 ms for two of the poloidal coils. We found a clear difference in the time constants, even though the two coils were fabricated with exactly the same design.

Effects of Unsteady Trailing-Edge Blowing on Delta Wing Aerodynamics

The effects of unsteady trailing-edge blowing on delta wing aerodynamics were investigated experimentally to understand the aerodynamics-propulsion interaction for dynamic thrust vectoring. Two models with sweep angles of A = 50 and 65 deg, representing nonslender and slender delta wings, respectively, were tested in a water tunnel. Flow visualization and velocity and force measurements were conducted at stall and poststall incidences. For the periodic blowing, it was found that the dynamic response of leading-edge vortex breakdown and wing normal force coefficient exhibit phase lags for both nonslender and slender delta wings. The estimated time constants are larger than those reported in the literature for unsteady wings undergoing pitching or plunging. In the case of transient blowing, the time delay for the decelerating jet is significantly larger than that for the accelerating jet. The variation of the circulation and the reattachment process near the wing surface were studied by means of velocity measurements. The range of the estimated time constants is similar at the stall and poststall incidences for both the slender and nonslender wings.

Estimations of climate sensitivity based on top-of-atmosphere radiation imbalance

Abstract. Large climate feedback uncertainties limit the accuracy in predicting the response of the Earth's climate to the increase of CO2 concentration within the atmosphere. This study explores a potential to reduce uncertainties in climate sensitivity estimations using energy balance analysis, especially top-of-atmosphere (TOA) radiation imbalance. The time-scales studied generally cover from decade to century, that is, middle-range climate sensitivity is considered, which is directly related to the climate issue caused by atmospheric CO2 change. The significant difference between current analysis and previous energy balance models is that the current study targets at the boundary condition problem instead of solving the initial condition problem. Additionally, climate system memory and deep ocean heat transport are considered. The climate feedbacks are obtained based on the constraints of the TOA radiation imbalance and surface temperature measurements of the present climate. In this study, the TOA imbalance value of 0.85 W/m2 is used. Note that this imbalance value has large uncertainties. Based on this value, a positive climate feedback with a feedback coefficient ranging from −1.3 to −1.0 W/m2/K is found. The range of feedback coefficient is determined by climate system memory. The longer the memory, the stronger the positive feedback. The estimated time constant of the climate is large (70~120 years) mainly owing to the deep ocean heat transport, implying that the system may be not in an equilibrium state under the external forcing during the industrial era. For the doubled-CO2 climate (or 3.7 W/m2 forcing), the estimated global warming would be 3.1 K if the current estimate of 0.85 W/m2 TOA net radiative heating could be confirmed. With accurate long-term measurements of TOA radiation, the analysis method suggested by this study provides a great potential in the estimations of middle-range climate sensitivity.

The effect of water content on the electrochemical impedance response and microstructure of Ni-CGO anodes for solid oxide fuel cells

Using electrochemical impedance spectroscopy, the high frequency (HF) and low frequency (LF) impedance of nickel–gadolinium-doped ceria (Ni-CGO) symmetrical cells (Ni-CGO/YSZ/Ni-CGO) in dry and moist atmospheres were studied. The HF component of the impedance response in moist H 2 varied with operating temperature, while the LF component varied with the H 2 content in the fuel. The EIS response in dry fuel behaved differently. The LF impedance in dry fuel (97% H 2, 3% N 2) showed a large increase, and varied significantly with both temperature and H 2 content in the fuel, while the HF component varied with temperature in a similar manner to that observed in moist fuel. This suggests that the HF impedance in both moist and dry fuel is associated with the charge transfer resistance. The LF impedance in the case of moist fuel can be reasonably attributed to mass transport effects, while that in the case of dry fuel cannot be attributed to the same mass transport process. The estimated time constant of the LF component was considerably larger in dry atmospheres, compared to that in moist conditions. Scanning electron microscopy (SEM) images showed crack formation in the anode cermets exposed to dry atmospheres, which were not evident in cermets exposed to moist conditions.

Muscle phosphocreatine and pulmonary oxygen uptake kinetics in children at the onset and offset of moderate intensity exercise

To further understand the mechanism(s) explaining the faster pulmonary oxygen uptake (p(VO)(2)) kinetics found in children compared to adults, this study examined whether the phase II p(VO)(2) kinetics in children are mechanistically linked to the dynamics of intramuscular PCr, which is known to play a principal role in controlling mitochondrial oxidative phosphorylation during metabolic transitions. On separate days, 18 children completed repeated bouts of moderate intensity constant work-rate exercise for determination of (1) PCr changes every 6 s during prone quadriceps exercise using (31)P-magnetic resonance spectroscopy, and (2) breath by breath changes in p(VO)(2) during upright cycle ergometry. Only subjects (n = 12) with 95% confidence intervals <or=+/-7 s for all estimated time constants were considered for analysis. No differences were found between the PCr and phase II p(VO)(2) time constants at the onset (PCr 23 +/- 5 vs. p(VO)(2) 23 +/- 4 s, P = 1.000) or offset (PCr 28 +/- 5 vs. p(VO)(2) 29 +/- 5 s, P = 1.000) of exercise. The average difference between the PCr and phase II p(VO)(2) time constants was 4 +/- 4 s for the onset and offset responses. Pooling of the exercise onset and offset responses revealed a significant correlation between the PCr and p(VO)(2) time constants (r = 0.459, P = 0.024). The close kinetic coupling between the p(VO)(2) and PCr responses at the onset and offset of exercise in children is consistent with our current understanding of metabolic control and suggests that an age-related modulation of the putative phosphate linked controller(s) of mitochondrial oxidative phosphorylation may explain the faster p(VO)(2) kinetics found in children compared to adults.

The contribution of tympanic transmission to fine temporal signal evaluation in an ultrasonic moth

In lesser waxmoths Achroia grisella, pair formation and female mate choice involve very fine discrimination of male ultrasonic signals. Female A. grisella prefer male signals with longer pulses and longer ;asynchrony intervals', and evaluate differences in these characteristics in the range of 80-260 mus. The first step in the evaluation of these characteristics is the tympanic transmission of stimuli. We used laser vibrometry to describe the mode of vibration, frequency tuning and stimulus transmission of the tympana of A. grisella. The tympanic response consisted of a rotational mode of vibration, in which the anterior and posterior sections moved out of phase; the posterior section of the tympanum vibrated with all points moving in phase and maximum displacement at the attachment point of the scoloparium that contains the receptor cells. The tympana of A. grisella were tuned to high ultrasonic frequencies and had an estimated time constant (i.e. the limit to their temporal acuity) of about 20-50 mus. Pulse length and all but the shortest asynchrony interval were thus well resolved by the tympanum. We discuss implications for the evaluation of pulse length and asynchrony interval.

Evaluation of Surface States of AlGaN/GaN HFET Using Open-Gated Structure

We analyzed passivation film and the AlGaN surface states using open-gated structures of AlGaN/GaN HFETs by numerical simulation and experiments. From the analyses, we confirmed that insulating film conductivity plays the prominent roles in device performances of the wide bandgap semiconductor device. Device simulation confirmed that the difference in I D -V G characteristics is due to the trapping type of the surface states; electron-trap type or hole-trap type. For electron-trap type surface states, the surface potential pinned at electron quasi-Fermi level, which is the same as the channel potential in the open-gated FETs. As a result, surface potential of ungated region is equal to the channel electric potential resulting in the uncontrollability of the channel current by the edge placed gate electrode. For hole-trap type surface states, the surface potential is pinned at hole quasi-Fermi level, which must be the same as the edge placed gate electrode potential. Then, the AlGaN surface potential varies with the electrode potential variation allowing the control of channel current as if the whole channel is covered with a metal electrode. Experiments for open-gated FET with unpassivated surface show no current variation. This corresponds to electron-trap type surface states from the simulation. On the other hand, SiO x evaporated open-gated FET show current control by the gate electrode. The I D -V G characteristics resembles in simulated I D -V G characteristics with hole-trap surface states. However, the estimated time constants for the trap reactions are incredibly long due to the deep energy level for the surface states in wide bandgap semiconductors. In addition, the open-gated FET showed reverse threshold shift to the value expected from the hole-trap pinning levels. So, we concluded that the no current variation for the unpassivated open-gated FET can be attributed to electron traps in the surface states, but the control of the drain current for SiO X deposited open-gated FET is not by surface hole-traps, but by slightly conductive passivation film is SiO X .

Evaluation of the Dropsonde Humidity Sensor Using Data from DYCOMS-II and IHOP_2002

Abstract The dropsonde humidity data have not been fully utilized due to lack of knowledge on performance of the dropsonde humidity sensor. This study evaluates the performance of the dropsonde humidity sensor using data collected from two field experiments, the Dynamics and Chemistry of Marine Stratocumulus Phase II: Entrainment Studies (DYCOMS-II) and the International H2O Project (IHOP)_2002. During DYCOMS-II, 63 dropsondes were dropped above marine stratocumulus clouds. It provides a unique opportunity to evaluate the performance of the dropsonde humidity sensor within clouds. Relative humidity (RH) inside clouds did not reach 100% all the time, but the maximum RH reached 100% for 28% of soundings and was within the sensor accuracy range (94%–100%). This suggests that the dropsonde humidity sensor displays no systematic dry bias near saturation. The dropsonde humidity sensor experienced large time-lag errors when it descended from a dry environment above clouds into clouds. The mean estimated time constant of the sensor is 5 s at 15°C, which is much larger than 0.5 s at 20°C given by the manufacturer. The humidity sensor still reported near-saturation RH after it exited clouds because of water on the sensor. The approximately coincident dropsonde and aircraft temperature data during DYCOMS-II show that the dropsonde underestimates temperatures inside and below clouds by averages of 0.21° and 0.93°C, respectively. Seventy-one pairs of dropsonde and radiosonde soundings during IHOP_2002 were launched within a half hour and 50 km and sampled the same air mass based on the visual examination. The comparisons show that the dropsonde and radiosonde RH data agree with each other within ±2% RH, suggesting no systematic dry bias in dropsonde humidity data. However, dropsonde-measured temperature is consistently colder than that by radiosonde by ∼0.4°C.

In situ tritium recovery behavior from Li 2TiO 3 pebble bed under neutron pulse operation

A binary pebble bed of lithium titanate (Li 2TiO 3) was irradiated in the Japan Materials Testing Reactor (JMTR), and its tritium recovery characteristics bed was studied under pulsed neutron operations. The temperature at the outside edge of the pebble bed increased from 300 to 350 °C immediately after the window of hafnium (Hf) neutron absorber was turned toward the reactor core, while the tritium recovery rate increased gradually. The ratio of tritium recovery rate to generation rate at the high-power, ( R/ G) high, approached the saturated value of unity at about 20 h of operation. Overall tritium recovery behavior under the pulsed operation was similar to that under the steady state power operation. An estimated time constant of about 3 h for the tritium recovery was much longer than the thermal time constant of about 100 s.

Evaluation of glassy-state dynamics from the width of the glass transition: Results from theoretical simulation of differential scanning calorimetry and comparisons with experiment

The purpose of this study is to investigate the quantitative relationship between the width of the glass transition, ΔTg, and glass fragility or activation energy for structural relaxation. The ultimate objective is the estimation of structural relaxation time as a function of temperature from the width of the glass transition region, allowing characterization of glass dynamics by a single simple measurement. The Moynihan correlation indicates that activation energy for structural relaxation should be inversely proportional to the width of the glass transition, but recent experimental studies suggest this relationship is a poor approximation for glasses of pharmaceutical interest. The present study is an effort to better understand the validity of the Moynihan correlation by selected experimental studies and a theoretical analysis of those factors that impact the glass transition width. Experimental data for glass transition widths for (poly)vinylpyrrolidone, sucrose, and trehalose are obtained using a variety of procedures, and relaxation time data are obtained using the thermal activity monitor. The theoretical analysis begins by simulating the temperature dependence of the heat capacity by breaking the cooling and heating scans into a large number of temperature steps followed by isothermal holds, during which relaxation of the material is calculated. Here, the modified VTF equation is used for relaxation time and the generalized Kohlraush-Williams-Watts stretched exponential function is used to describe the relaxation kinetics. Simulations are performed for materials of varying fragility and varying “stretched exponential” constants, β, and the width of the glass transition region, ΔTg, is evaluated from the simulated heat capacity versus temperature curves as one would do with experimental data. Agreement between the theoretical simulations and experimental ΔTg data is excellent. The simulations demonstrate that although the Moynihan correlation is not valid for variable β, a modification of the Moynihan correlation which includes variation in β is a good approximation. Thus, an estimate of fragility may be obtained from glass transition width data provided an estimate of β is available. Furthermore, it is shown that a first approximation for β may be obtained from the magnitude (i.e., height) of the differential scanning calorimetry thermal overshoot. We also find that using the modified VTF equation to evaluate the temperature dependence of the structural relaxation time at the glass transition, and integrating this expression to lower temperatures, it is possible to obtain an evaluation of the relaxation time constant, τβ, in the glass at any temperature, using only the ΔTg and β values obtained from a single differential scanning calorimetry scan. These estimated time constants correlate very well with the values directly measured with the thermal activity monitor.

Loss of quantum yield in extremely low light.

It has generally been assumed that the photosynthetic quantum yield of all C3 plants is essentially the same for all unstressed leaves at the same temperature and CO2 and O2 concentrations. However, some recent work by H.C. Timm et al. (2002, Trees 16:47-62) has shown that quantum yield can be reduced for some time after leaves have been exposed to darkness. To investigate under what light conditions quantum yield can be reduced, we carried out a number of experiments on leaves of a partial-shade (unlit greenhouse)-grown Coleus blumei Benth. hybrid. We found that after leaves had been exposed to complete darkness, quantum yield was reduced by about 60%. Only very low light levels were needed for quantum yield to be fully restored, with 5 micromol quanta m(-2) s(-1) being sufficient for 85% of the quantum yield of fully induced leaves to be achieved. Leaves regained higher quantum yields upon exposure to higher light levels with an estimated time constant of 130 s. It was concluded that the loss of quantum yield would be quantitatively important only for leaves growing in very dense understoreys where maximum light levels might not exceed 5 micromol quanta m(-2) s(-1) even in the middle of the day. Most leaves, even in understorey conditions, do, however, experience light levels in excess of 5 micromol quanta m(-2) s(-1) over periods where they obtain most of their carbon so that the loss of quantum yield would affect total carbon gain of those leaves only marginally.

A Synthetic Peptide Ligand of Neural Cell Adhesion Molecule (NCAM), C3d, Promotes Neuritogenesis and Synaptogenesis and Modulates Presynaptic Function in Primary Cultures of Rat Hippocampal Neurons

The neural cell adhesion molecule (NCAM) plays a key role in morphogenesis of the nervous system and in remodeling of neuronal connections accompanying regenerative and cognitive processes. Recently, a new synthetic ligand of NCAM, the C3-peptide, which binds to the NCAM IgI module, has been identified by means of combinatorial chemistry (Rønn, L. C. B, Olsen, M., Ostergaard, S., Kiselyov, V., Berezin, V., Mortensen, M. T., Lerche, M. H., Jensen, P. H., Soroka, V., Saffell, J. L., Doherty, P., Poulsen, F. M., Bock, E., Holm, A., and Saffells, J. L. (1999) Nat. Biotechnol. 17, 1000-1005). In vitro, the dendrimeric form of C3, termed C3d, disrupts NCAM-mediated cell adhesion, induces neurite outgrowth, and triggers intracellular signaling cascades similar to those activated by homophilic NCAM binding. The peptide may therefore be expected to regulate regeneration and synaptic plasticity. Here we demonstrate that in primary cultures of hippocampal neurons: 1) C3d induces a sustained neuritogenic response, the neuritogenic activity of the compound being dependent on the dose, starting time, and duration of peptide application; 2) the peptide triggers the neuritogenic response by forming an adhesive substratum necessary for NCAM-mediated neurite formation and elongation; 3) C3d promotes synapse formation; and 4) C3d modulates the presynaptic function, causing a transient increase of the function at low (2 and 5 microm) doses and a reduction when applied at a higher concentration (10 microm). The effect of the peptide is dependent on the activation of the fibroblast growth factor receptor. We suggest that C3d may constitute a useful lead for the development of compounds for treatment of various neurodegenerative disorders.

Correction of cold-wire response for mean temperature dissipation rate measurements

This work is aimed at improving dissipation rate measurements using cold-wire anemometry. The mean dissipation rate of the temperature variance is measured on the axis of a heated turbulent round jet. Measurements are performed with a constant current anemometer (CCA) operating fine Pt–10%Rh wires at very low overheat. The CCA developed for this purpose uses a current injection method in order to estimate the time constant of the wire. In the first part of the paper, it is shown that the time constants obtained for two wire diameters ( d=1.2 and 0.58 μm) compare well with those measured at the same time using two other methods (laser excitation and pulsed wire method). Moreover, for these two wires, the estimated time constants are in good agreement with those obtained from theory. In the second part of the paper, a compensation procedure, involving post-processing filtering, is developed in order to improve the frequency response of the cold-wire probes. Measurements on the jet axis (Re D=16 500 , Re λ =167) show that the frequency response of the 1.2 μm wire is indeed significantly improved after compensation. The spectrum of the compensated signal for the 1.2 μm wire compares fairly well with that for the 0.58 μm wire. The results also indicate that the compensation should be applied when the cut-off frequency of the cold-wire f C is less than about 2 f K, where f K is the Kolmogorov frequency. When f C≈0.6 f K, the compensation can reduce the error in the estimated mean dissipation rate by more than 20%. When f C=2 f K, the reduction is about 5%.