Spatially resolved impedance spectroscopy of a Nafion polyelectrolyte membrane is performed employing a conductive and Pt-coated tip of an atomic force microscope as a point-like contact and electrode. The experiment is conducted by perturbing the system by a rectangular voltage step and measuring the incurred current, followed by Fourier transformation and plotting the impedance against the frequency in a conventional Bode diagram. To test the potential and limitations of this novel method, we present a feasibility study using an identical hydrogen atmosphere at a well-defined relative humidity on both sides of the membrane. It is demonstrated that good quality impedance spectra are obtained in a frequency range of 0.2–1000 Hz. The extracted polarization curves exhibit a maximum current which cannot be explained by typical diffusion effects. Simulation based on equivalent circuits requires a Nernst element for restricted diffusion in the membrane which suggests that this effect is based on the potential dependence of the electrolyte resistance in the high overpotential region. Proton exchange membrane fuel cells (PEMFCs) represent a promising energy conversion technology for mobile and portable applications. The development of deeper insight into the functionality of PEMFC components calls for new experimental methods. One of the most important components is the proton exchange membrane which warrants the performance of the PEMFC by its inherently high proton conductivity, and its longevity via its chemical and thermal stability. Different structural models of Nafion, mainly based on small-angle x-ray and small-angle neutron scattering experiments have been developed. A model derived by Schmidt-Rohr fits best the observed experimental data and excludes several other models . It propFor further investigations of the proton conducting domains, the so-called electrochemical atomic force microscopy (EC-AFM) was developed ,4. In thet al. [A variant of the AFM-based technique was reported by O’Hayre et al. . These aet al. . This enReaching a high spatial resolution with an AFM is difficult since the sample is strongly influenced by environmental parameters like temperature and RH. The resulting sample drift affects the resolution, and therefore a short measurement time is favorable. In the present, work, the setup of EC-AFM is extended to permit chronoamperometric experiments with high spatial and temporal resolution. The current response to a rectangular voltage step is measured as a function of time and converted to frequency space by Fourier transformation, allowing the impedance spectrum to be calculated and simulated using common equivalent circuits. For simplicity, the novel approach is tested by measurements of a hydrogen pump across a Nafion membrane as a model system. −1 with the same RH was provided for each chamber. For the measurement of transients the existing experimental setup is extended by a signal access module  which provides access to all electrical signals of the AFM-system. The chronoamperometric experiments were performed using a data acquisition card from National Instruments (PCIe-6361) which was controlled by the LabVIEW software (Version 10.2). The main features of the in-house written data acquisition software are an adjustable sampling frequency, a variable duration of the measurement and a freely selectable voltage step.The experimental setup consists of an atomic force microscope  equipped with a conductive AFM module, enabling the measurement of currents between the tip and the sample in the pA to nA range. The construction of the measurement cell is sketched in ® which was activated following a standard procedure [2O2 at 85 °C for 1 h and then rinsed with doubly distilled water for 15 min. Following this, it was placed in 0.5 M sulfuric acid at 85 °C for 1 h and subsequently rinsed again with doubly distilled water for 15 min. After drying in air, the membrane was coated on one side using a wet spraying preparation technique. The ink was prepared from platinum black , doubly distilled water and Nafion Dispersion  which was all mixed together and stirred for at least 12 h. The procedure leads to a catalyst loading of around 1 mg Pt per cm2 with 30 wt. % Nafion within the electrode. The coated membrane is stored in doubly distilled water for at least 24 h prior to use.All samples were prepared from Nafionrocedure . First, The transient current and the corresponding voltage signals are transformed to the frequency domain by Fourier transformation. The temporal resolution of the signal traces plays an important role. The highest defined frequency (with amplitude and phase) of a discrete signal is given by the sampling theorem and is equal to or smaller than half of the sampling rate. The lower frequency limit is given by the reciprocal duration of the measurement . These aTo calculate the impedance of the system, the voltage and the current signals, given by arrays of 10,000 points per second, are Fourier transformed (Equation (1)) using a numerical procedure described elsewhere . . In the present work 100 points were calculated within each frequency decade.The function H(t) represents the time dependent voltage or current signal which is transformed to the complex function H*(ν) in the frequency domain. Subsequently, the impedance (Z*) is calculated by complex division according to Equation (2), where −1. The waiting time before the next experiment was several seconds. The first voltage step was from 0 V to 0.1 V, and the upper value was incremented by 100 mV for each subsequent measurement. The AFM tip was placed on a non-conductive position of the Nafion 212 membrane in a hydrogen atmosphere at 78% RH. A series of voltage steps was applied and held for 5 s before it was set back to zero. The current and voltage signals were sampled with a rate of 100,000 sS∙CD [All current traces show a fast increase followed by an exponential decay to zero current. This is the typical behavior of an ideal polarizable electrode which can be represented by an equivalent circuit consisting of a capacitor connected in series with an Ohmic resistance. When a step voltage is applied to this circuit, the current response I(t) is entirely capacitive and described by an exponential decay with a time constant τ = RS∙CD ,D the capacity, and RS reflects the Ohmic resistance of the solid electrolyte. The dashed lines in S) value. With the extracted value, τ = 0.24 ms, it takes at least 1 ms until meaningful information can be observed from this system in a chronoamperometric experiment. In other words, the cell constant of the system restricts the high frequency limit to 1 kHz, which is chosen as the upper limit for the calculation of the impedance spectra. E represents the plateau voltage, CTo comply with this fact in accordance with the sampling theorem, the sampling rate for the subsequent measurements is chosen to be 10,000 points per second. −4 nF. Assuming a typical geometric capacitance of 30 µF cm−2 for platinum, the area is calculated to 0.37 µm2. This corresponds to a radius of 346 nm if a circular contact area is assumed. As the radius of the tip is known to be about 10 nm, this contact area seems rather large. It is therefore likely that the observed capacitance is increased by other effects as for example pseudocapacitance due to hydrogen adsorption [The total exchanged charge is obtained by calculating the areas under the curves in sorption . HoweverD and lowers the time constant according to τ = RS∙CD.The small size of this electrode has a pronounced influence on the characteristics of the electrochemical system. The measured currents are in the pA to nA range, which in principle corresponds to a very low IR drop or permits using highly resistive electrolytes giving a moderate IR drop. In any case the use of a two-electrode system is meaningful. Additionally, regarding the highly resistive electrolyte, the high temporal resolution of 1 ms is a result of the small size of the electrode, which has a small capacity CThe reproducibility of the current-time traces were investigated by performing many measurement series for several experimental parameters. The aim was to verify a systematic behavior and find the optimum experimental parameters. Apart from the applied force between the tip and the sample and the RH of the gas atmosphere, the position on the sample and the recovery time between the measurements, which is a measure for the relaxation time of the membrane, were also varied. All measurements were carried out in a hydrogen atmosphere following a voltage step from 0.4 V to 0.5 V. The duration of a single measurement is 10.5 s whereby the voltage step was applied 0.5 s after the start. Five measurements were recorded for each set of parameters. The RH values were chosen to be 47%, 61% and 80%. For each RH value the measurements were taken at two different tip positions on the membrane so that a total of 6 different positions were investigated. On each position the force was varied from 10 nN to 30 nN and 50 nN, and also the time delay within the series of five measurements was varied from 10 s to 60 s and 180 s. Overall, 270 measurements were recorded. , of each series is calculated according to Equation (4). More precisely, for each data point at time t the arithmetic mean value of all five measurements is determined. 0 ≤ i ≤ N denotes the number of the measurement within a series (N = 5):To permit a suitable comparison, the relative error based on the standard deviation of the data in this measurement series is determined in the following way. The arithmetic mean, , based on  and corresponding to one standard deviation, is calculated from equation (5) for each measured curve of this series. Here k denotes the data points of the corresponding curve and N is the total number of data points:The relative error F = 10 nN), marked by the blue triangles, leads almost always to the highest relative error, independent of RH. Between the contact force of 30 nN and 50 nN no distinct difference is observed. The relative errors (filled symbols) and the arithmetic mean of the relative errors (open symbols) are plotted in High contact forces can lead to damage of the membrane and also to high wear of the platinum catalyst layer on the tip. Hence, for the later experiments, the force was chosen to be 30 nN. x are clearly less than 0.1, whereas at 61% RH a much higher scatter of the values is present. It is known that the water content in the membrane and the proton conductivity increase with RH [The increase of the RH leads to much higher relative errors. At 47% RH all values of ε with RH . TherefoThe current signals from position V are a factor two or three higher than those from position VI, indicating that the local proton conductivity at position V is significantly higher. This difference reflects the inhomogeneity of the membrane surface and therefore the suitability of this method to obtain spatially resolved measurements. To obtain similar relative errors for position V and position VI, the deviations between the measurements have to be a factor two or three higher as well, indicating that the deviations scale with (proton) current. Due to the electro-osmotic drag a certain amount of water molecules is dragged along with the protons. If the proton current is higher, the water distribution within the membrane is much more strongly affected. It is therefore suggested that the growing perturbation of the water distribution within the membrane with increasing proton flux is the dominating origin of the relative error at higher RH. Experimentally, the influence of the relaxation of the membrane due to the perturbed water distribution can be investigated by varying the recovery time between subsequent measurements. Regarding the measurements with 30 nN and 50 nN, an overall higher relative error is observed with a recovery time of 180 s. During further experiments it turned out that a recovery time of 30 s or 60 s leads to the best results.A series of measurements was conducted at a fixed tip position on a Nafion 117 membrane in a hydrogen atmosphere with an RH of 71%. For the first measurement, the voltage step was applied from 0 V to 0.1 V, whereas for each subsequent measurement the plateau voltage is incremented by 100 mV. According to the reproducibility of the measurements discussed above, the delay time between the measurements was chosen to be 30 s and the tip was pressed onto the sample with a force of 30 nN. The corresponding current traces are shown in Shortly after the voltage step, a strong increase of the current is observed for each measurement, which is mainly based on the capacitive current of the cell. For the first 7 voltage steps up to the terminal voltage of 0.7 V, each subsequent current trace increases further. This behavior may be better seen if the polarization curves of this measurement series are extracted. Therefore different profiles at different delay times parallel to the voltage axis from From 0.1 V to at least 0.4 V all three curves follow an exponential increase. According to the theory of chronoamperometric experiments , the curThis is confirmed by the inset of If the overpotential is very high in an electrochemical system with an aqueous electrolyte, the maximum current at long times reaches the diffusion limitation. This theoretical limiting current is independent of the applied voltage and is drawn schematically as a dotted line in The polarization curves selected in As can be seen from The measurement with a voltage step to 0.3 V shows the typical behavior for a parallel RC-circuit. This behavior is representative for the voltage range from 0.1 V to 0.4 V. In the Nyquist plane the RC circuit manifests itself as a semi-circle with a radius that decreases with increasing potential for a Faradaic reaction. The simulated and experimental spectra of this voltage range are shown in the lower graph of el, shown in ct representing the charge transfer resistance for the electrochemical reaction here mostly at the cathode, and a Nernst diffusion element N which is, in contrast to the Warburg diffusion element, limited to very low frequencies due to a finite-length restricted diffusion.A distinctly different characteristic is observed for the voltage range between 0.8 V and 1.1 V. The impedance starts to increase in the low frequency region, as can be seen exemplarily in Nk which is defined as  with the diffusion coefficient D and the thickness of the Nernst layer Nd. This loop is in series with an Ohmic resistance which, as in the other equivalent circuit, represents the electrolyte resistance Rel. The inductive behavior which is observed in the high frequency region for some spectra is simulated with an inductor L in series with Rel. A complex non-linear least square regression algorithm is used for the fitting procedure.The Nernst diffusion is controlled by the Warburg coefficient W and the Nernst constant ct from the potential is expected in the kinetically controlled region [The results for the charge transfer resistance from the above measurement series (RH = 71%) are plotted in d region . In the The impair of the performance of the oxygen reduction reaction was reported for a temperature dependent study of a proton exchange membrane fuel cell . This waThe simulated capacity shows little variation in the low overpotential region and reaches values of a few pF independent of the RH. For an overpotential >0.5 V up to 208 pF are observed for 89% RH, which is indicative of pseudocapacitance. el decreases with increasing RH due to higher proton conductivity within the membrane. Another observed effect is the declining tendency of Rel with increasing voltage step. Typically the increase of the electrolyte resistance with increasing current density is reported for Nafion in a fuel cell and attributed to a dehydration of the membrane at the anode side [In ode side . HoweverAlthough the samples were conditioned using a common procedure, internal reorganization of the local membrane structure due to the proton flux is possible. This may lead to an improved water distribution within the membrane and in the end to a lower electrolyte resistance. For 89% RH in the high overpotential region Rel is almost constant near 75 MΩ. This level is also reached at high overpotential for 47% RH (1.1 V) and 71% RH (0.8 V), respectively, and it seems to be the lower limit for these experimental conditions.The fitted values of the Warburg coefficient are plotted on a logarithmic scale against the voltage in The Warburg coefficient decreases with increasing relative humidity, indicating that the diffusion is less restrictive with higher RH. According to the model of Schmidt-Rohr  this behSuch an exponential relation is also observed for the Warburg parameters in Nk reaches values between 0.2 s−1 and 3.2 s−1 for all investigated RH. A diffusion coefficient can be calculated from the definition of Nk. Based on the thickness of Nafion 117 of around 175 µm and Nk = 3.2 s−1 D is calculated to be 0.98℘10−5 cm2 s−1. This value is very close to the reported diffusion coefficients for water in fully humidified Nafion [Another interesting fact is that the Nernst coefficient d Nafion  and suppet al. [Although only absolute values are presented in this work, it should be noted that area-specific values can be obtained by normalizing with the contact area of the tip and the sample. Using the method presented by O’Hayre et al.  the contAn extension of the EC-AFM technique is developed and shown to be suitable to measure impedance spectra with good quality and high spatial resolution. In view of the acquired frequency range, this method is much faster than frequency-domain impedance techniques. A short measurement time is necessary if the system is strongly sensitive to external influences such as temperature and relative humidity and thus susceptible to drift. The characterization of the system reveals optimized parameters for the measurement technique. The limits of a meaningful Fourier transformation of the current and voltage traces are determined to range from 0.2 Hz to 1 kHz. The simulations of the impedance spectra reveal detailed insight into the system. The local impedance is restricted by the charge transfer resistance in the low overpotential region. A decreasing current in the high overpotential region is observed in the polarization curves which were extracted from the current transients measured at different RH. There is evidence that this originates from a potential dependent electrolyte resistance, which is adequately described by a Nernst impedance element for restricted diffusion.


The efficiency of the procedure was about 20 %.Targets of  Although Ni layers of such thickness can be prepared in high vacuum by vapour deposition on substrate it is difficult to mount the produced foils to frame as Ni foils in this thickness range have a strong tendency to roll into tubes when self-supporting. This makes mounting the foils on the target frames when released from substrate very difficult if not impossible. To overcome this problem the thin Ni layer produced by vapour condensation or by sputtering, has to be, when still on the substrate, either additionally strengthened/stiffened by a support with low melting point removable under the beam or has to be produced on the backing which is removed after fixing the foils to the frame. Nevertheless, a method of producing Ni foils without the stresses responsible for this rolling effect has been recently reported in .Because of the nickel properties, i.e. its relatively high melting temperature and alloying with metals commonly used as the evaporation crucible nickel should be evaporated either by e-bombardment or e-gun heating or by sputtering.Taking into account the minute amount of the Ni isotopes available for the target preparation and availability of the heating systems, the Ni foils were produced using the small home made sputtering device with sputtering voltage of 10 kV and Ar ions as sputtering projectiles available at the Target Laboratory of Physics Department, University of Jyväskylä.The cathode applied in this device is made of graphite with the cavity of 5 mm in diameter for the sputtered substance which allows the target production with minute amount of the material. A short adjustable distance of the sputtered material and the substrate assures the high efficiency of the material collection. The deposit thickness inhomogeneity that may be caused by this short distance is reduced by the substrate rotation .2/min.A pellet of compressed Ni powder was placed in the cavity of the graphite cathode Fig. . The bacThe attempt to produce a set of Ni foils by the procedure described in  ended wiFixing the foils to the frame before etching the copper backing as used for thicker Ni foils production by Arnison  gave no The approach to the deposition of the sputtered Ni on substrate heated during Ni vapour deposition as recommended by Gursky  was not Z elements an attempt of depositing the Ni layers on a thin (10–12 μg/cm2) C foil was undertaken. It was not successful either. The C foil did not withstand bombardment by the sputtered Ni or the stress built up in the Ni foil during its deposition was too high. The frame after the sputtering was empty with only a few small scraps stuck to the substrate holder.Taking into account no reproducibility of the foils production by deposition on the Cu backing further etched to obtain the self-supported foil and the fact that required targets were allowed to be backed by support of low-2 polyimide foil (PIF) prepared in air or in argon [Finally targets for the experiment were prepared by nickel deposition on ~35 and 40 μg/cmin argon . The spuThe PIF mounted to the frame was fixed to the rotating substrate holder using the mask, fixing the frame to the substrate and determining the size of the deposit Fig. . Its apeThe thickness of the deposit, as for deposition process on Cu was controlled by the time of sputtering process and the thickness of the completed deposit was estimated using the weighing method.2 and isotopic nickel, 58 and 60, with areal density of 71–115 μg/cm2  of PIFs made both types of the foil usable as backing for Ni deposition by the sputtering technique.The thickness of a few randomly chosen targets was additionally determined with measurements of the alpha particles energy loss Table . The obt20Ne9+ ions beam of ~50 MeV with intensity of 30 nA. It confirms the suitability of the PIFs as backings for targets used in the beam of heavy ions.According to the report from the end users targets prepared by the method described here survived the bombardment by


Continuous flash suppression (CFS) is a powerful interocular suppression technique, which is often described as an effective means to reliably suppress stimuli from visual awareness. Suppression through CFS has been assumed to depend upon a reduction in  neural adaptation caused by the continual updating of the contents of the visual input to one eye. In this study, we started from the observation that suppressing a moving stimulus through CFS appeared to be more effective when using a mask that was actually more prone to retinotopically specific neural adaptation, but in which the properties of the mask were more similar to those of the to-be-suppressed stimulus. In two experiments, we find that using a moving Mondrian mask  is more effective in suppressing a moving stimulus than a regular CFS mask. The observed pattern of results cannot be explained by a simple simulation that computes the degree of retinotopically specific neural adaptation over time, suggesting that this kind of neural adaptation does not play a large role in predicting the differences between conditions in this context. We also find some evidence consistent with the idea that the most effective CFS mask is the one that matches the properties (speed) of the suppressed stimulus. These results question the general importance of retinotopically specific neural adaptation in CFS, and potentially help to explain an implicit trend in the literature to adapt one’s CFS mask to match one’s to-be-suppressed stimuli. Finally, the results should help to guide the methodological development of future research where continuous suppression of moving stimuli is desired. Since Crick and Koch Since CFS was introduced as a technique to reliably suppress stimuli from visual awareness, it has been used in more than 100 studies. In these experiments, CFS has been applied in two different ways. First, it has been used to present stimuli in the absence of awareness and to study the influence of the presentation of these subliminal stimuli on a subsequent task with visible stimuli. For example, Jiang, Costello, Fang, Huang, and He Secondly, CFS has been most often put into practice in the so-called “breaking CFS paradigm” . The study was approved by the Ethical Committee of the Faculty of Psychology and Educational Sciences (EC FPPW) of the University of Leuven, and the participants gave written informed consent before starting the experiment.Five students  of the undergraduate psychology program of the University of Leuven participated in the experiment for course credit. All had normal or corrected-to-normal vision. Every participant was unaware of the goal of the study.Stimuli were shown on two 19.8-in. Sony Trinitron GDM F500-R  monitors driven by a DELL Precision T3400 computer with an Intel Core Quad CPU Q9300 2.5 GHz processor running on Windows XP. Binocular presentation was achieved by a custom made stereo set-up. Two CRT monitors, which stood opposite to each other (distance of 220 cm), projected to the left and right eye respectively via two mirrors placed at a distance of 110 cm from the screen. A head- and chin rest (15 cm from the mirrors) was used to stabilize fixation. The effective viewing distance was 125 cm. Stimulus presentation, timing and keyboard responses were controlled with custom software programmed in C# using Microsoft Visual Studio 2010.A checkerboard pattern consisting of randomly positioned black and white squares of 0.37° by 0.37° was used to aid binocular fusion. The CFS masks consisted of 150 squares of equal size (0.46° by 0.46°). The color of the squares was either red, green, blue or yellow. The target stimulus was a red circle (diameter 0.46°). The target moved horizontally across a virtual square (5.5° by 5.5°) at a speed of 3°/s embedded in a larger square (7.32° by 7.32°). The mask was presented in the other eye within a bounding square of the same size as the larger square in the other eye . Every mask element had one of these motion directions during the trial and the different motion directions were equally divided amongst the mask elements such that every motion direction was equally present in the display. For every motion direction, the colors of the individual elements were also evenly distributed. The initial position of every mask element was determined randomly with one constraint. To avoid that some parts of the display did not contain enough mask elements during a trial (creating blank spots), we divided the display into four quadrants and the positions, speeds and colors for each fourth of the mask elements were randomized within this quadrant. The size of the individual mask elements was the same as the size of the suppressed stimulus. When the positions of the mask elements overlapped, they were drawn on top of one another. Furthermore, when a mask element reached the edge of the display, it would reappear on the other side according to its motion trajectory.On each trial, the CFS mask was shown in the participant’s dominant eye. Eye dominance was determined prior to the start of the experiment with Porta’s test Mask speed consisted of six different levels  and two staircases were used for each mask speed. Participants performed 65 trials for each staircase, resulting in 780  trials in total. The number of trials per staircase was selected based on pilot testing. Staircases were randomly interleaved and participants had the opportunity to take small breaks in-between.0 is the initial response level set at 1, k1 the asymptotic response level set at 0 for decay and 1 for recovery, and τ the time constant for the exponential. Note that this is not necessarily intended to provide a full or complete ‘model’ of retinotopic adaptation. Rather this simulation intends to make explicit what a simple approximation of retinotopic adaptation could look like. It is certainly possible that the adaptation dynamics in early retinotopic areas are much more complex, but this simulation enables us to quantify whether this very basic approximation of retinotopic adaptation can already predict our current results.Since we were interested in quantifying the degree of retinotopic neural adaptation for the different masks in our experiment, we conducted a simulation which implemented an approximation of the retinotopic representation of the input using Gabor filter banks often used in models of the early visual system e.g., . As highWe simulated 999 trials of each condition and transformed each frame of the trial to a grayscale image. Next, we filtered each frame with two oriented (at 0 and 90 degrees) odd-symmetric Gabor filters with a spatial frequency set at the Nyquist frequency (412 c/image) and the standard deviation of the Gaussian set to:Each threshold was determined by taking the last 20 trials of the staircases. These trials were then averaged within every staircase and subsequently averaged over staircases within each mask speed. Because of large inter-individual differences between thresholds, we normalized the thresholds by dividing them with the mean of the thresholds per participant. These normalized thresholds were then subjected to a Bayesian version of a one-way within-subjects ANOVA. Statistical inference throughout this paper did not use the classical frequentist framework but rather a Bayesian framework see . MoreoveIn our analyses, we first do model selection using Bayes Factors. Subsequently, we use Bayesian parameter estimation to further zoom in on the posterior distributions. In both cases (model selection and parameter estimation) we have used the tools that are currently available. Note that these tools rely on different models with different, but in both cases uninformative, priors. Techniques for an integrated Bayesian approach to both model selection and parameter estimation are currently quite complicated to implement.F = 6.205, p = .001). Rouder, Morey, Speckman, and Province Since the posterior distributions for In summary, the data suggest that the normalized threshold for a mask speed of 3°/s is credibly different from the average threshold of all other mask speeds. Furthermore, pair-wise comparisons suggest that this difference holds for a mask speed of 1°/s and 8°/s. As a sanity check, In the Introduction, we suggested that the effectiveness of CFS does not entirely depend on retinotopically specific neural adaptation due to the continuous updates to the CFS mask. As the results of Experiment 1 indicate, the MMM that matched the motion properties of the suppressed target provided the most effective suppression. However, whilst it is logical to assume that the mask of 3°/s would show more retinotopically specific neural adaptation, it was important to quantify this explicitly, especially in relation to the traditional CFS mask. To address this, we computed a measure of the degree of retinotopic neural adaptation as described in the Methods section.less adaptation (as explained in the Methods section). However, our results deviate from these simulations as an increase in thresholds up to 3°/s and a decrease in thresholds for masks with faster speeds was observed.The squares in In Experiment 1, we manipulated the properties of MMMs and compared their effectiveness in suppressing a moving stimulus with regular CFS. We compared our pattern of results with that expected based on computations of the degree of retinotopic neural adaptation. If avoidance of retinotopically specific neural adaptation underlies the effectiveness of CFS, the mask with the most changes would prove to be the most effective. According to our measure of degree of retinotopic neural adaptation, the MMM and the regular CFS mask would show the least adaptation. However, the fastest mask speeds did not prove to be the most effective. It was apparent that the contrast threshold was highest for a MMM that matched the motion properties of the suppressed stimulus providing evidence for feature-selective depth of suppression during CFS  neural adaptation, we tried to replicate our finding from Experiment 1 using two different to-be-suppressed target speeds. Thus, in Experiment 2 we manipulated the speed of the suppressed stimulus to move at either 2°/s or 5°/s whilst keeping the same range of mask speeds used as in Experiment 1. This also enables us to test the role of feature-selective depth of suppression during CFS. Indeed, analogous to the results of Yang and Blake Six new participants , all students of the undergraduate psychology program of the University of Leuven participated in the experiment for course credit. All had normal or corrected-to-normal vision. Every participant signed an informed consent prior to the start of the experiment and was naive to the goal of the study.The apparatus and stimuli were the same as in Experiment 1.Mask speed again consisted of six different levels . Target speed was also manipulated and consisted of two levels (2°/s and 5°/s), yielding a 2×6 within-subjects design. Participants performed 65 trials for each staircase, resulting in 1,560  trials in total. Target speed was blocked and counterbalanced across participants. In every block, staircases were randomly interleaved and participants had the opportunity to take small breaks in-between.The procedure of Experiment 2 was the same as in Experiment 1. The targets moved on a horizontal plane from the right side to the left and disappeared from the screen after 5.5 and 2.2 seconds, respectively for the 2°/s and 5°/s target speed conditions.The data were analyzed in the same way as in Experiment 1. First, we report the results from Bayesian model selection and subsequently we elaborate on them using Bayesian parameter estimation.Bayes Factors were again computed based on Rouder et al. F = 4.066, p = .008; no main effect of target speed: F = 1.346, p = .298; interaction between mask and target speed: F = 2.156, p = .09). As is apparent from As the table shows, the model with a main effect of mask speed and an interaction between mask speed and target speed is strongly preferred. Note that a classical repeated measures ANOVA yields a similar conclusion , whereas adaptation on multiple timescales or different timescales for different features might be possible. Our simulation, therefore, should only be interpreted as a coarse approximation of retinotopic adaptation. Yet we think this operationalization captures the essence of the concept of retinotopic adaptation which has been proposed as being important for the effectiveness of CFS.Methodologically, our results highlight that one should consider using MMM instead of a traditional CFS mask in some contexts to achieve desirable suppression strength. Indeed, our most consistent finding was that a MMM, and especially one that contained motion features similar to the suppressed stimulus was more effective than a traditional CFS mask, highlighting the importance of binocular feature matching Theoretically, our results highlight that the transient nature of the mask is not always the most important aspect of CFS, in the sense that the more spatially transient the mask is, the more effective suppression will be. The initial innovation in developing CFS was exactly the introduction of a transient in one eye which indeed seems crucial for the increase in suppression strength Whilst the current results challenge the idea that the effectiveness of CFS can be predicted based on a reduction in retinotopically specific neural adaptation, they do not imply that no adaptation-based processes underlie the effect. Indeed, these could potentially be explained by an adaptation mechanism acting at the level of motion speed, for example. That is, given that the visual system can adapt to motion speed Alternatively, one could also speculate that a mask moving at 3°/s would activate parts of motion area MT that also would be required to represent the target moving at 3°/s. This explanation would be more consistent with the idea that interocular competition results from a bottleneck imposed by the selective access to higher level areas. If the target and mask in CFS share more properties, then it is possible that they compete more directly for the same neural resources. The stronger motion signals in the mask could dictate that only the mask stimulus reaches higher areas and therefore stays dominant and increases detection thresholds for the suppressed stimulus.This second explanation could potentially be related to a broader mechanism implicated in the singleton pop-out literature using visual search. In this literature, target-nontarget similarity has been shown to have an influence on the slope of the search function As is apparent from our experiments, and consistent with the work of Hong and Blake Thus, regular CFS does not seem to be a general panacea for suppressing stimuli. Indeed, one has to take into account the similarity between features that can be extracted based on the input to each eye, rather than simply increasing the transients in the mask. This finding could help to account for the (implicit) tendency in the literature for different authors to adapt the CFS mask based on the stimulus they are trying to suppress, presumably by matching more closely the characteristics of the to-be-suppressed stimuli and the mask.In this study, we introduced a MMM that was shown to be more effective in suppressing a moving stimulus than a regular CFS mask. We developed an explicit quantification of the degree of retinotopically specific neural adaptation and used this to make predictions on the effectiveness of our masks. Our results were not consistent with the predictions based on the approximation of retinotopic neural adaptation, and this questions the assumption that the most effective mask will always reflect the avoidance of neural adaptation due to the transient nature of the CFS mask. We conclude that a regular CFS mask that provides effective suppression for static stimuli is not necessarily suited for suppressing moving stimuli and that in general one has to consider the feature match between mask and suppressed stimulus when attempting to use CFS.Figure S1Graphical model for the Bayesian version of a one-way repeated measures ANOVA. The data are assumed to come from a t-distribution with a certain mean and standard deviation. The mean is equal to a linear combination of the effect of mask speed ((TIF)Click here for additional data file.Figure S2Posterior distributions for the pair-wise comparisons between 3°/s and all other levels of mask speed and the regular CFS mask.(TIF)Click here for additional data file.Figure S3Posterior predictive checks for every participant. The red cross is the mean of the predicted values, the gray line the associated 95% HDI and the black dots are the individual data points for every condition. The conditions are ordered as in the bar plots going from a mask moving at 1°/s to regular CFS.(TIF)Click here for additional data file.Figure S4Graphical model for the Bayesian version of a two-way repeated measures ANOVA. The data are assumed to come from a distribution with a certain mean and standard deviation. The mean is equal to a linear combination of the effect of mask speed ((TIF)Click here for additional data file.Figure S5Posterior distributions for the pair-wise comparisons between a mask moving at 2°/s and 8°/s and 2°/s and CFS, respectively, for the 2°/s target speed condition.(TIF)Click here for additional data file.


Blood oxygen saturation of vein (SvO2) is an important clinical parameter for patient monitoring. However, the existing clinical methods are invasive, expensive, which are also painful for patients.Based on light-absorption, this study describes a new noninvasive SvO2 measurement method by using external stimulation signal to generate cyclical fluctuation signal in the vein, which overcomes the low signal-to-noise ratio problem in the measurement process. In this way, the value of SvO2 can be obtained continuously in real time.The experimental results demonstrate that the method can successfully measure venous oxygen saturation by artificial addition of stimulation. Under hypoxic conditions, the system can reflect the overall decline of venous oxygen saturation better. When the results measured by the new method are compared with those measured by the invasive method, the root mean square error of the difference is 5.31 and the correlation coefficient of the difference is 0.72. The new method can be used to measure SvO2 and evaluate body oxygen consumption, and its accuracy needs improvement.Real-time and continuous monitoring can be achieved by replacing invasive method with noninvasive method, which provides more comprehensive clinical information in a timely manner and better meet the needs of clinical treatment. However, the accuracy of the new noninvasive SvO2 measurement based on light-absorption has to be further improved.The online version of this article (doi:10.1186/s12938-016-0208-8) contains supplementary material, which is available to authorized users. SvO2 and SaO2 (blood oxygen saturation of artery) have been established as the basic parameters to evaluate oxygen delivery process and monitor patient’s oxygen consumption. According to Fick Formula in Eq. 1], the , the 1],On the other hand, there is no existing noninvasive or continuous device for measuring SvO2 in the market, even though light absorption method has already been widely used to evaluate SaO2. Besides the high price and inconvenient, invasive method also has more risk in clinical SvO2 measurement.Due to the lack of fluctuation signal in vein, SvO2 can’t be measured in the same way as SaO2 is measured, for which light absorption method is valid because of the obvious impulse in artery. To overcome this problem a new noninvasive light-absorption method for SvO2 measurement is proposed. In this method, we innovatively create vein impulse similar to artery impulse by artificially adding external stimulation signal during measurement, which enables the extraction of the stable venous signal from normal arterial pulse signal.A represents absorbance, the natural logarithm of the ratio of the incident light intensity to the transmitted light intensity; Ii represents the incident light intensity and It represents the intensity of the transmitted light, a represents the absorption coefficient, with different values for different solutions; C represents the solution concentration; L represents the optical path and is the distance that light travels through in the solution [According to the Lambert–Beer’s law, the intensity of light through the medium has different degrees of attenuation which depending on the thickness and concentration of the solution. Because the transmission light intensity will decrease with the increasing of the concentration and the thickness of the solution, the absorbance of a single solution can be calculated by Eq. 2\documenBe similar to mixed solution, the absorbance of blood can be calculated by optical absorption model for mixed solution as shown in Eq. , and theλ is the wavelength of the original light.Equation  is the mIo and I represent the intensity of the incident light and the transmission intensity of the original light respectively. λ denotes the wavelength of the original light. t is the time. S is the oxygen saturation. β0 and βγ are the absorption coefficients for the Hb and HbO2 in the solution. l(t) is the optical path from the light source to the photoelectric sensor [In dynamic state, the intensity change of the transmission light is induced by the change of optical path due to congestion artery, as shown in Eq. 5\documenc sensor .Equation  can be eThen Eq.  can be dThen Eq. , 5) and and6\docAssuming that Eq.  holds.8\rite Eq.  to Eq. (ficient)  is definAccording to Eqs.  and 9),,9), the Usually, pulse fluctuation can lead venous oxygen signal overlap with the artery blood oxygen signal, which causes problem of low signal (SvO2)-to-noise (SaO2) ratio. Therefore, the extraction of the weak venous blood oxygen signal from artery blood oxygen signal is the key to non-invasive SvO2 measurement. Based on clinical studies, a new method has been proposed. It measures the periodic fluctuation vein signal, whereas a new system has been designed to verify the method.As Fig. Periodic pressure can be added to finger by inflating and deflating the air cuff periodically. NIBP (non-invasive blood pressure) is used as a controllable parameter to set the threshold pressure for different people, while the frequency of inflation and deflation are controlled by a high-precision timer. Figure The SvO2 signal extraction process generally includes three steps. Firstly, the stimulated signal is wiped out and the normal SpO2 signal is calculated. Secondly, the signal which is too far from our stimulation frequency is filtered and the peak of artificially stimulated signal is used as input data to calculate Mixed-SpO2 in the normal way which is same to the calculation of SpO2. Thirdly, SvO2 is calculated. In this paper, venous PPG generated by the stimulation signal is used to calculate the venous oxygen saturation, which is the same as arterial oxygen saturation PPG used to calculate the arterial oxygen saturation. The PPG is recorded at 660, 940 nm for red and infrared transmission intensity changes. In theory, if PPG of the arterial blood signal can be used to calculate the arterial oxygen saturation, PPG of the venous blood signal can also be used to calculate the venous oxygen saturation. Because the features of the two signals are the same, SvO2 can be calculated use the same method to calculate SpO2. In fact, the signal the system acquired is the mixed signal. In the case of ideal signal to noise ratio, venous oxygen saturation can be calculated by formula  and formAt last, the invasive blood gas analysis is utilized as the golden criterion to calibrate the data measured by our system, referring to the calibration process of SpO2. The blood gas analysis platform of the laboratory can control the oxygen concentrations by adjusting the intake of oxygen. The system records the values of SaO2 and SvO2 in noninvasive method, at the same time the blood samples of artery and vein are sampled in 100–70 % hypoxic state. 24 samples of venous blood were sampled near from our finger in consistent with the process showed in Fig. In this manuscript, we introduced a system for the continuous and noninvasive measurement of SvO2. Our system utilizes a control system to modulate the adjustable stimulation signal that is essential to the measurement process, in which the stimulation signal rate shouldn’t be the same as patients’ pulse rate. The power spectrum of PPG in Fig. To identify the stability and reliability of the system, we studied nine healthy adults. Five volunteers took part in the invasive experiment and others took part in the noninvasive experiment. Invasive blood gas analysis  was used to benchmark the accuracy and stability of the system. Referring to the standard calibration process of SpO2, samples of venous blood near from subjects’ fingers were collected. The sampling point is approximately set in 6 levels of oxygen saturation, and recorded twice for each subject Fig. . The difThe experimental results indicate that the noninvasive method can be a feasible approach for SvO2 measurement. Adding the external stimulation signal to vein solved the problems of the lack of fluctuation signal and improved signal-to-noise ratio in the measuring process. This new method of SvO2 measurement can potentially meet the unmet clinical needs, reducing the time and cost involved.


We study how categories form and develop over time in a sensemaking task by groups of students employing a collaborative tagging system. In line with distributed cognition theories, we look at both the tags students use and their strength of representation in memory. We hypothesize that categories get more differentiated over time as students learn, and that semantic stabilization on the group level (i.e. the convergence in the use of tags) mediates this relationship. Results of a field experiment that tested the impact of topic study duration on the specificity of tags confirms these hypotheses, although it was not study duration that produced this effect, but rather the effectiveness of the collaborative taxonomy the groups built. In the groups with higher levels of semantic stabilization, we found use of more specific tags and better representation in memory. We discuss these findings with regard to the important role of the information value of tags that would drive both the convergence on the group level as well as a shift to more specific levels of categorization. We also discuss the implication for cognitive science research by highlighting the importance of collaboratively built artefacts in the process of how knowledge is acquired, and implications for educational applications of collaborative tagging environments. Because of the ubiquity of social web technologies, there has been a recent growing interest in how people make sense of large quantities of information when they browse the web . In thisBecause the resources and tags collected by one user can be seen by others, it is usually assumed that individuals are influenced by tags as social cues. In the social web, there is considerable influence of collective information on individual behavior . In collGaining an understanding of the development of categories and the corresponding tag assignments is important for improving information access in the social web. Research on improving information access with tags has been seeking different routes. For example some researchers have suggested enhancing semantic search technologies with social tags e.g. . AnotherHowever, to fully exploit this potential for semantic technologies or recommenders, it is important to understand the process by which shared terminological patterns can emerge without an explicit coordination . This isThe particular contribution of our paper is twofold. First, and rather than studying the information access, we examine the underlying mechanisms of how people develop and extend their categorization in collaborative tagging system. Our assumption is that phenomena of categorization can only be studied by looking at how internal categories and tags people use are coupled. Therefore, we looked both at internal categories students learned, as well as the tags they used.Secondly, we study how the development of categories is mediated by an important social process in collaborative tagging, namely the development of shared language that results from semantic stabilization , a basic level  and a subordinate level . The seminal works of As discussed above, sensemaking when using the web is a process in which internal categories are refined in an iterative manner as understanding in a topic increases. This assumption is based on robust findings from research on human categorization taking place at several levels of abstraction e.g., . TypicalAs the expertise of a person within a particular topic increases, subordinate categories become more differentiated and more easily available in categorization and naming tasks e.g., . Thus, tIn our study, each group of students was instructed to collaboratively generate a hierarchy of tags by means of a taxonomy editor available in the shared Web environment. For measuring the basic level shift, we draw on these taxonomies. Specifically, we regard tags of the first level as basic-level tags and tags of the levels below as subordinate tags .2.3H and b, and students’ basic level shift. Students of groups performing a higher level and faster rate of semantic stabilization within their shared tag vocabulary should – on average – exhibit a more frequent use and a stronger internal representation of subordinate tags than students of groups with a lower level and slower rate of semantic stabilization.As described above, we assume that learning in a shared environment is mediated by shared artifacts, in particular by the extent to which these artifacts support a collaborative exploration and understanding of a topic. For instance, if students succeed in using similar tags for certain aspects of a topic, then subordinate levels of the tag taxonomy should provide helpful tags to categorize newly found Web resources. As a consequence, these subordinate tags should be frequently used and represented in form of strong, internal memory representations. Therefore, we hypothesize an interdependence between attributes of semiotic dynamics on the group level, i.e., H1Users of a collaborative tagging system will develop a more common understanding of the concepts named by the tags when they collaboratively tag for a longer as compared to a shorter duration of time.This means that semantic stabilization should be more pronounced in groups that tag resources for a topic a longer as compared to a shorter amount of time. We measured this convergence time by comparing the number of different tags over time between groups with a short vs. a long engagement with a topic.As our first hypothesis, we therefore assume the following:H2Students in groups with a higher level and faster rate of semantic stabilization within their shared tag vocabulary should – on average – exhibit a more frequent use (hypothesis H2.1) and a stronger internal representation (hypothesis H2.2) of subordinate tags than students of groups with a lower level and slower rate of semantic stabilization.As the second hypothesis we then assume3To test these hypotheses, it was necessary to observe a group of users that naturally use a collaborative tagging system for a sensemaking task over some period of time. We also needed to collect additional data from those users, such as data about the representation of tags in their memory. Therefore, a university course setting provided an adequate setting for conducting our study, as we were able to randomly assign students to groups, give them rather realistic tasks and still control their engagement to a certain degree.For this reason, we chose to conduct a field experiment in which we asked students to collaboratively collect bookmarks related to their course subject and describe them with tags within the shared bookmarking system SOBOLEO. Additionally, by means of the SOBOLEO taxonomy editor, the students had to specify super- and sub-relations between their tags resulting in a collaborative hierarchy of tags with different abstraction levels. That way, we allowed students’ categorization behavior to emerge naturally. At the same time, we introduced an experimental design controlling for unwanted effects, such as the fact that students had to learn to use SOBOLEO at the same time that they were learning about a topic. How exactly the experimental design controlled for these effects will be described in more detail in Section 3.1N = 25, 12 female) were psychology students participating for course credit. Alternatively, they could choose writing an essay for the same course credit, but each of them preferred participating in the study. The average age was 23.3 years (SD = 1.2) ranging from 21 to 25 years.The study took place in the context of a university course on cognitive models in technology enhanced learning at the University of Graz that was conducted in the autumn term 2009/2010. Participants  to the taxonomy tree in the editor (3), and enter textual labels (4). These changes are reflected in the system for all users and are therefore visible to all in the same way. A chat (5) is available for discussing decisions on moving a tag to one branch of the tree rather than another, or about labeling. In the current study, the chat could not be used for technical reasons. Instead, a discussion forum was provided through the WebCT course environment. Here the groups were invited to discuss the building of the shared taxonomy.The use of the taxonomy editor in SOBOLEO allowed us to study the specificity of tags by drawing the tag samples from different levels down the branches of the taxonomies created by the students (see Section 3.2.2To elicit students’ prior knowledge about particular concepts of the topics to be investigated in SOBOLEO (control variable) and students’ internal representations of shared tags (dependent variable), we applied a free association test utilizing (a sample of) tags as open probes. The students were asked to write down all associations for each of the sampled tags coming to their mind for 60 s. After excluding repetitions, we used the number of associations as an indicator for the strength of representation of a tag in students’ semantic memories e.g., . The freWe applied the free association test at the beginning, at half time and at the end of the study. At the beginning, we used it to elicit students’ prior knowledge about the topics and to examine whether different experimental groups were equivalent with respect to prior knowledge (see below). Since at that time no tags had been generated, we chose five concepts  as open probes that we deemed central for the two topics the students were later asked to work on. At half time and at the end, we randomly drew tags from different levels of the taxonomies the students had generated up to the given point in time and used these tags as open probes.3.2.3For me it’s important to exchange views with friends per computer) using a 5-point-Likert scale from agree to do not agree. This scale shows good internal consistency .As a further control variable we used the six items sub-scale FIDEC III of the standardized inventory INCOBI  to elici3.2.4The task helped me to gain a proper understanding of the topic ‘Wikis in enterprises’/‘Weblogs in university courses’), satisfaction with the SOBOLEO system , the communication within the group (Item 4: I’m satisfied with the communication within my group in the SOBOLEO environment) as well as the shared taxonomy (Item 5: I’m satisfied with the collaboratively created taxonomy in SOBOLEO), using a 5-point-Likert-scale from agree to do not agree as well as an open-ended question (Provide some reasoning for your answer). An item analysis revealed a relatively high item difficulty indicating a general tendency of the students to answer the corresponding statements in the negative direction . A test of normal distribution by means of the z-standardized measures of skewness and kurtosis revealed that – except for item 3 – the critical value of crit z= 2.58 was not exceeded . Taken together, we regarded items 1, 2, 4 and 5 as appropriate to differentiate between students’ opinions on the task and software and to better understand results of our statistical analyses.At the end of the study, the students were given a self-developed, five items questionnaire on their general understanding of the topic , we ranked them according to their average number of unique associations to the five open probes. Then, the first four ranked participants were each randomly assigned to one of the four groups. This procedure was repeated for all remaining quadruples such that the final four groups were equivalent according to the free association scores . Additionally, we checked for equivalence with respect to the FIDEC III scale .ld) and the other two groups (2 and 4) to the short duration (sd) condition. Under the ld condition, each of the two groups of students worked collaboratively on one topic (either on “Wikis in enterprises” or on “Weblogs in universities”) for the whole study duration (10 weeks). Under the sd condition, they had to switch the topic (either from topic 1 to 2 or from topic 2 to 1) at half time and, thus, worked on each topic for only five weeks. To control for the development of skills in using the tagging technology and for the formation of social roles, we restricted data analyses to the second half of the study, i.e., to weeks 6–10. We pooled all data of the ld condition  and the sd condition . That way, we could observe the effect of topic duration independent of topic and topic sequence.We assigned two groups (1 and 3) to the long duration  we performed a Welch’s t-test. In accordance with our expectation, there were no significant differences in the average number of associations between the two conditions, t15.3 = .09, n.s. We are therefore confident that potential differences in semantic stabilization and tag specificity that might turn out in the course of the second study period (weeks 6–10) can be attributed to the topic switch at half time, i.e., to the independent variable of topic duration.As described above, we also conducted the free association test at half time to test the assumption that up to this point in time no group differences should exist due to similar experimental conditions. We utilized a sample of randomly drawn tags as open probes where participants were only given tags from their own SOBOLEO instance. The means and standard deviations for the 3.3.2tag specificity, the second independent variable. It was not possible to use a categorization norm to distinguish between superordinate, basic and subordinate categories because the tags generated by the students  were not available in such norms. Therefore, we decided to differentiate tag specificity by considering tags of the first SOBOLEO taxonomy level as general , tags of the second level as medium  and tags of levels below the second one as specific . We utilized the tag taxonomy of each SOBOLEO instance to vary M = 9.82, SD = 2.99) than medium tags  and specific tags  and hence, exhibited a lower information value  again suggesting that general tags exhibit lower information values than medium and specific tags. We take these results as substantiating our use of the SOBOLEO taxonomy as an operationalization of tag specificity.To validate the assumption that the general, medium and specific tags vary in specificity, we firstly checked the number of bookmarks connected to these tags in SOBOLEO. In fact, general tags were associated with a higher number of resources  under the ld and sd condition. That way we obtained a three-way 3 (levels) × 5 (weeks) × 2 (topic durations) contingency table.To measure tag use, we considered all tag assignments (using new tags or reusing existing ones) in the second 5-week period under the 3.4.3We applied the free association test utilizing tags as open probes  and the number of unique associations as an indicator for the strength of internal tag representations.3.5topic duration  and specificity of tags . The main dependent variables were (i) the parameters of equation 2 as a measure of semantic stabilization , (ii) the frequencies of tag use and (iii) the strength of internal representations of tags.The independent variables formed a mixed 2 × 3-design constituted by the factors ld than under the sd condition), we merged the u(t)N distributions of the groups 1 and 3 (ld condition) and of the groups 2 and 4 (sd condition) to get an average distribution as a function of topic duration, uN(t)ld vs. uN(t)sd. Then, we determined the approximately best fitting cumulative exponential given by Eq. (1) (Section uN(t)ld and uN(t)sd separately using Maximum Likelihood Estimation. This procedure resulted in estimates of the asymptote H (amount of agreement in using tags given unlimited time) and the slope b (rate of approaching the asymptote) for the ld and sd condition. Besides contrasting H and b of the two conditions, we tested the differences between uN(t)ld and uN(t)sd performing a Mann–Whitney-U-Test. We applied this non-parametric test since we expected dramatic deviations from a normal distribution , we could detect an effect size of f = 0.37.To test hypothesis H1 , we performed a χ2 tests on the marginal L × W table. Given statistical error probabilities α = β = .05, df = 8, and the total sample size N = 548, we could detect an effect size of w = 0.20. Second, we examined as to whether the association (joint distribution) of L and W changes as the level of T changes. We therefore tested the fit of a model assuming independence of the joint distribution from T. In case of a significant deviation of the joint independence model from the empirical data , it can be concluded that the association differs between the two groups. The fit of the model was evaluated performing a log-linear model with the joint independence assumption (LW/T).To test hypothesis H2.1 (that groups exhibiting stronger semantic stabilization apply tags from the medium and specific taxonomy levels more frequently than groups exhibiting weaker stabilization), we further processed the three-way 3 (levels) × 5 (weeks) × 2 (topic durations) contingency table. In the following we denote these three factors α = β = .05 and N = 24, we could detect an effect size of f = 0.34.With respect to the third dependent variable , per participant we calculated a mean free association score for each of the three taxonomy levels. For instance, if a participant produced 4, 6 and 4 unique associations to the three tags drawn from the first, i.e., general level of the corresponding SOBOLEO taxonomy see , the par3.6n = 6 student groups were equivalent with regard to their attitudes towards the computer as a communication means and prior knowledge  each student was expected to post two relevant bookmarks per week to the SOBOLEO environment and describe them with meaningful tags. The students were also required to collaboratively organize their tag collection with the help of the SOBOLEO taxonomy editor. To facilitate the emergence of consensus, the students were also encouraged to utilize the SOBOLEO chat and an external discussion forum.short duration (sd) condition. The other two groups continued with their prior topic in the long duration (ld) condition. The second five week period was thus the crucial experimental period in which all measures were taken. As reported in Section sd and ld conditions were still equivalent with regards to their knowledge of the topics at the beginning of this period.After five weeks , the SOBOLEO environments of two of the four groups were cleared. They had to start from scratch and to work on the other topic for another five weeks, making them the groups of the At the end of the study after the second five week period, the free association test was administered. Also participants were presented the 5-items questionnaire on their general understanding of the topic and their satisfaction with the collaboration within the group.44.1u(t)N distributions (number of unique tags as a function of consecutive tag assignments) for the sd condition (unfilled circles) and the ld condition (filled circles) as well as the respective, approximately best fitting exponential function given by Eq. R2 = 0.97 (for the sd condition) and R2 = 0.95 (for the ld condition). A glance at the figure reveals that topic duration (independent variable) had a strong impact on semantic stabilization since descriptively, the asymptote H and rate of approaching the asymptote b differ between the two conditions. However, in contrast to our expectation, semantic stabilization  appears to be more pronounced under the sd than under the ld condition . In particular, from the 60th tag assignment, the number of unique tags is lower under the sd than under the ld condition. In accordance with this descriptive data analysis, the Mann–Whitney-U-test yielded a highly significant difference between the two u(t)N distributions .Regarding hypothesis H1, ld condition had already worked on the same topic in the previous five weeks, they exhibited a slower rate of stabilization and were less in agreement on the use of tags in the second 5-week period than the sd groups.This result suggests that there were different levels of semantic stabilization under the two conditions (short and long duration). However, despite the fact that groups in the ld condition gave significantly lower ratings when asked for the understanding of the topic  than students in the sd condition . Additionally, ld groups  perceived a lower quality of their taxonomy than sd groups . Free text answers indicated that especially students in ld groups found it more difficult to collaboratively work on the shared taxonomy in SOBOLEO and they felt that their collaboration had resulted in a chaotic collection of bookmarks and tags where it was rather difficult to keep an overview. Moreover, the discussion forum that had been provided for discussing the shared taxonomy was only used occasionally.To explain this unexpected result, we looked at results of the post hoc questionnaire that had been administered to the students at the end of the semester. First, all groups indicated they had been dissatisfied with the communication mechanisms (the SOBOLEO Chat and discussion forum). Albeit having worked on their topic for a longer time, students in the sd condition that converged more quickly and more successfully than those in the ld condition. We suspect that the reason for this was that because all students were very inexperienced in collaborative tagging, the first five weeks served as a kind of trial period in which students had to learn about how to tag resources effectively and how to build up an effective taxonomy. And while after those five weeks the sd groups were able to start anew because their environments (and the built taxonomy) were cleared at half-time, the ld groups had to continue using the taxonomy they found unhelpful. This obviously helped the students in the sd groups to build a more effective and shared tag taxonomy in the second half of the study and converge to a common tag vocabulary more quickly. The negative effect for ld groups was exacerbated by missing effective direct communication mechanisms in the SOBOLEO system. Further results that we do not report here in more detail also support this interpretation, e.g. that the number of tags that were shared within a group (i.e. used by more than just one person) were much higher in the sd groups than in the ld groups.In conclusion, while our experimental manipulation (duration of engagement with a topic) was obviously effective in producing differences in semantic stabilization in the two conditions as measured by convergence time, it was actually the students working under the H and b, we continue our statistical analyses testing H2.1 and H2.2 in the following sections. We assume that the faster rate of semantic stabilization (under the sd condition) led to a higher usage frequency and a stronger internal representation of medium and specific tags.Because we were successful in manipulating the process of semantic stabilization, i.e., 4.2sd condition, who had turned out to create a more stable tag vocabulary, to exhibit a stronger basic-level shift and thus, to apply medium and specific tags more frequently than students of the ld condition. The frequency analyses reported next are based on the three way 3 (levels) × 5 (weeks) × 2 (topic durations) contingency table  and time (weeks 6–10) summing over topic duration (sd and ld condition). The relative row frequencies are presented in χ2- test on this marginal table yielded a significant deviation from data that would be expected under the null hypothesis of no association between week and specificity (χ2(8) = 18.15, p < .05). Hence, we conclude that students applied general tags constantly and tended to increasingly apply medium and specific tags across the five weeks.To this end, we gathered the marginal frequency table of In a next step, we examined the whole three way contingency table to test H2.1, i.e., as to whether the basic-level shift revealed by sd condition) exhibited a stronger basic-level shift. While under both conditions, medium and specific tags soon started dominating general tags for the categorization of bookmarks, the continuously increasing importance of medium and specific tags was much more pronounced under the sd condition. The log-linear model for the null hypothesis that the time-specificity association does not differ between the two conditions provided a poor representation of the observed data (Pearson’s χ2 (14) = 40.46, p < .001). Therefore, our results do not allow for rejecting H2.1 and support the assumption that conditions conducive for semantic stabilization (in the sd condition in our case) amplify the basic-level shift for all group members.The pattern shown in 4.3sd and ld condition should on average generate an equal number of free associations to general tags, sd students should generate more associations to medium and specific tags than ld students.H2.2 postulates that the basic-level shift, which we assume to depend on the semantic stabilization, does not only become manifest in a particular pattern of tag use but also in relatively strong memory representations (number of unique associations) for medium and specific tags. In particular, H2.2 assumes an interaction between topic duration and tag specificity with respect to memory strength: While students under the Msd = 4.64, SDsd = 0.36; Mld = 4.50, SDld = 0.39). The non-overlapping error bars for the lower two levels of specificity, on the other hand, suggest that students under the sd condition generated more associations to medium and specific tags  than students under the ld condition . The 2 × 3 ANOVA with repeated measures supported this descriptive analysis yielding a main effect for specificity  and a significant interaction between specificity and topic duration . A main effect for topic duration could not be identified .5The results we have reported have a number of implications for research in web science, in cognitive science and for education. Here we discuss three of them. Firstly, the results uncover the exact mechanisms of semantic stabilization that have so far been studied in web science research by statistical modeling of large-scale web environments without direct access to the users’ cognitive mechanisms. Secondly, the result point to the large influence that social mechanisms on the group level (here semantic stabilization mediated by shared artefacts) has on individual categorization which can be potentially overlooked if cognitive science research is conducted in the lab, rather than in real-life settings. And thirdly, we discuss several educational implications of collaborative tagging that our study suggests. We now discuss each of these in turn.5.1Our study is in the tradition of research attempting to uncover the dynamics in collaborative tagging systems. Here we have focused on semiotic dynamics in the form of semantic stabilization, i.e. an emerging consensus on particular terms over time that can be observed despite the missing central control. Semantic stabilization has been previously studied in large scale tagging systems by modeling the emergent consensus by means of statistical models . In thisBy conducting a field experiment that has allowed us to track categorization over time, not only by looking at the use of tags, but also examining the representation of those categories in memory, our study fills the gap between these two perspectives. This allows us to explain how semantic stabilization and individual learning processes play together to produce emergent characteristics of collaborative tagging systems. Specifically, we assume that a driving force behind the processes of semantic stabilization and the basic-level shift is that our students were trying to optimize the information value of the tags they used both for browsing and for indexing resources. sd groups, negotiations on the group level have led to a more optimal information value than in the ld groups and this was also related to higher levels of individual learning (as self-assessed by the students and confirmed with higher numbers of unique associations in the association test) and to higher degrees of satisfaction with the constructed taxonomy.In a sensemaking task employing social tags, a successful system should tend towards tags with a higher information value. In our study, we show that an important prerequisite for reaching a higher information value of more specific categories and their corresponding tags is that semantic stabilization takes place. Semantic stabilization facilitates a quicker shift to more specific levels of abstraction and therefore to a higher information value. In the We intend to employ some of the insights we gained from this study to develop recommendation mechanisms that support more effective collective learning in the context of collaborative tagging. For example, 5.2From a cognitive science perspective, our study shows that formation and variation of individual categorization in sensemaking is mediated by collective phenomena on a group level. In contrast to traditional categorization research which is predominantly conducted in laboratory settings, the present study therefore adds an innovative aspect as we show that individual expertise is to some extent dependent on the semiotic dynamics on a group level. By varying the amount of opportunity in which groups can negotiate a common meaning around the use of artefacts  than ld groups. Additional results we have not reported here point to the fact that the reuse rate of tags (reapplying existing tags from one’s own or others’ vocabulary) was much higher in sd groups than ld groups. This gives further evidence that the external representation was more useful for sd groups as compared to ld groups and provided a better means for learning.The fact that short duration groups were quicker to find consensus is a counterintuitive result. Obviously, the tag collection and the shared taxonomy that the students built seemed to play a more important role for learning than the amount of time students were engaged in the topic. The collaboratively built taxonomies had a significant and differential influence on the development of categories of the Our study also shows the importance of moving cognitive science research out of the laboratory into the “real world” because it points us to phenomena that may be overlooked in the lab . For exa5.3The effectiveness of the shared tag collection together with the taxonomy obviously had a stronger effect on learning than the duration students engaged with the particular topic. Quite clearly, the quality of the learning interaction was more important than the quantity. Recently, ld groups. Obviously it is sometimes better to get rid of a shared representation altogether if it proves unhelpful for learning (as we did in the sd groups). From the perspective of the co-evolution model, it could be argued that accommodation of knowledge was not possible in the ld groups as the collective knowledge was not able to support this kind of operation.We also showed that a stable interpretation needs to develop in a community of learners as a result of students using shared artefacts before individuals can benefit from the collective knowledge. A surprising result was also that the shared artefact in our study was actually hindering learning in the From an educational point of view, recent research has shown that collaborative tagging can support students in learning categories in an exploratory learning task in the classroom and beyond . Our stu6Semantic stabilization is a group process that we assume is dependent on a number of factors. Here we only studied two of them, namely the time of engagement with a topic and the effectiveness of the shared representation. We found that the latter was a much stronger factor in producing convergence than mere collaborative study time. Future studies need verify this result and also disentangle other factors that influence semantic stabilization, such as the trustworthiness of the other persons, the quality of interaction, as well as the design of the tagging environment (such as the type of tag recommendations or how shared tags are displayed in the environment).No competing financial interests exist.


The performance of the tiltmeter in the field was evaluated using tidal data obtained from a closely spaced array of boreholes in Southern California. The long-term stability of the tiltmeter is generally better than 1 μ rad/yr. The data also indicate that instruments in boreholes at least 24 m deepare independent of surface effects. Several different capsules designed to couple the instrument to the surrounding material have been tested. In addition, an experimental method for estimating the magnitudes of local perturbation in the regional tilt field is described.A borehole tiltmeter with a sensitivity of a few nanoradians is described. It is composed of two orthogonal horizontal pendulums with free periods of 1 s. The pendulums are insensitive to barometric pressure fluctuations, and the measured temperature coefficient is less than 30 nrad/°C. The range of the pendulums is about ±5  One measure of the deformation is tilt which is defined ass×d^0)/l,where z^μrad/yr . The value for V0 obtained by combining μrad/yr so most of the data are collected near the electrical center of the system where the effect ts of the nonlinearity are unmeasurable.The electrostatic force is balanced by the gravitational restoring force on the pendulum, so the energy balance equation for the system is(dx)=mgh,where dx pυ(t)of the capacitive transducer is an ac signal whose amplitude is dependent on the magnitude of the displacement of the pendulum from the electrical center of the bridge and whose phase with respect to the driving signal indicates the direction of the displacement. It is calculated from i1 is the current flowing through C1and i2is the current flowing through C2. Using the definitions for the capacitances of the transducer with q⪡d0,the transducer output isϕ is defined byR is infinite, and the output amplitude would simply be V0q/d0. In our case, R is 10 MΩ, so the amplitude of the signal is about 94% of the ideal amplitude, with a phase angle of roughly −20°. The part of the signal which is in phase with the drive voltage to the outer plates has an amplitude ofpV is about ± 10 mV for a capacitance unbalance of ± 1 fF(q = ±1 μm).The output A source follower is connected directly to the pendulum. It decreases the signal by about a factor of 2. The signal is then amplified in two stages with a total gain of about 1440 before entering the phase sensitive detector. The available output from the amplifier is about ±7.2 V corresponding to a capacitance unbalance of ± 1 fF, so that overloading of the amplifier is avoided.The phase sensitive detector is composed of a JFET switch and an integrator. The switch is controlled by the same frequency that drives the outer plates of the transducer so that the integrator only works on the part of the amplified signal which is in phase with the drive frequency. The integrator has a dc gain of 10. The output of the phase sensitive detector is a dc voltage which is related to the transducer output byThe remaining electronics are located at the top of the borehole. The dc voltage tilt signals are sent through a single stage unity gain low-pass filter with about a 100 s time constant to average out microseismic background noise, whose spectrum peaks around a period of 6 s. The filtered signals are recorded on a datalogger at the site.μrad. The digitized data are recorded every 6 min so that there are 10 data points per hour.Each datalogger has eight channel capability, so tilt signals from up to four tiltmeters can be recorded by a single datalogger. The additional channels alternatively can be used to record other local data such as temperature, etc. The filtered signal is digitized with a 12 bit analog to digital converter for the voltage range ± 10 V, making the least count of the digitizer about 2.4 nrad for the nominal sensitivity of 2 V/The datalogger is controlled with commands through the RS-232 interface which uses TTL protocol. This interface can be directly connected to a portable computer in the field, or to a modem for remote access. Data digitized and stored in RAM are retrieved each morning by an automatic dialing program on the computer in Boulder .The calibration of the tiltmeter is determined in the laboratory on a tilt table. It is conducted on a seismic pier in the basement of one of the buildings at the university. The pier is a large concrete block isolated from the building.μrad steps at a rate of one step per minute. Because the table is driven by a screw, the steps are not instantaneous. Each step is completed within 20 s so that the pendulum has at least 40 s to stabilize before the corresponding voltage is determined. The overall sensitivity of the channels is set to be about 2 V/μrad. Successive calibrations agree within 1% even when spaced several years apart [The calibration sweeps across the range of the tiltmeter sensor in approximately 1 rs apart . The nomSince changes in the amplifier circuits cause calibration changes, whenever any components fail in the field, the entire tiltmeter is replaced and the original tiltmeter is brought back to the lab for repair and calibration.The boreholes are drilled by a conventional water well drill. They are lined with a steel casing as shown in The lowest 2.4 m of the casing is stainless steel pipe with 10 cm inner diameter. A flat plate with a 1.3 cm diameter hemisphere in the center is welded to the bottom. The stainless steel instrument compartment is joined to 15 cm diameter carbon steel pipe. The joint is conical on the inside of the casing. As the casing is installed, additional lengths of 15 cm pipe are welded to form a continuous watertight pipe for the remaining depth of the borehole. Boreholes ranging from 15 to 120 m below the surface have been used.Several different versions of the tiltmeter capsule have been designed. All versions are made from stainless steel pipe sections 2 m long. Their lids are pressure tight. The base of the tiltmeter frame is clamped in the capsule by three screws so that the sensor is located midway up the capsule. The differences between the versions is in the way that the capsule is located in the borehole.The original design  locates The second design uses a different technique for centering the capsule in the borehole. The hemisphere not only supports the weight of the capsule but also centers the bottom. A plate with a cone cut into it is welded to the bottom of the capsule. The cone centers the bottom of the capsule on the hemisphere at the bottom of the hole.,The top of the capsule is centered by three spring-loaded pins which are retracted by the weight of the capsule as it is being installed or removed. The capsule weighs approximately 133 N (13.5 kg mass), and each spring has about 26 N pressing against the casing when installed.The third design is similar to the second, except that the bottom of the capsule is located on the conical joint above the instrument compartment. A plate with a spherical edge is welded to the bottom of the capsule. The top of the capsule is centered asbefore with three spring-loaded pins. Since the pins need a larger range of motion in order to press against the larger diameter casing, extra weight is added so that the pins retract completely during installation and removal. This capsule weighs about 267 N (27.2 kg mass), and the force on each spring is about 53 N.The capsule is lowered into the borehole by a steel cable connected to the lid. Provided that the borehole is not too deep (less than 50 m), and the capsule is not too heavy, a capsule can be lowered by hand. For deeper holes and weighted capsules, a winch is used.Once the capsule is lowered, it is important to know its azimuthal orientation. The drill bit wanders around the vertical direction when the boreholes are drilled, so direct observation is not possible except for very shallow holes (<10 m). Two different techniques were used to determine the orientation of the capsule depending on the depth of the borehole.Holes less than 50 m deep use a series of lightweight aluminum rods [The rods were originally calibrated outside of the borehole so that the accuracy of the measurement was a few degrees. Using alignment rods to determine the orientation of a capsule has several disadvantages. First, the alignment rods must be used every time that a tiltmeter is installed. Second, the rods, being made of aluminum to minimize their weight, are relatively easily bent. This may cause degrading of the azimuth measurement for each successive installation. Finally, the system is feasable only for shallow boreholes—the rod stiffness decreases with length, and the additional weight becomes appreciable.For deeper holes a different technique was developed. The lid of the capsule was modified to include an extension with a muleshoe attached to the top . The mulThe tiltmeters have been installed in various locations during the past 12 years. The sites have included boreholes in Colorado , Wyomingμrad/yr) as well as tidal components (amplitude about 10 nrad) with frequencies around one and two cycles per day. The trend in the data is a measure of the secular tilt present at the particular site combined with the instability of the instrument and the installation. For boreholes in Southern California, the expected secular tilt is less than 1 μrad/yr, so the recorded trend in μrad/yr have been observed.In order for geophysical interpretations of the tilt data to be accurate, it is important to understand how the measurements are affected by surface effects, cavity effects, and the existence of local inhomogeneities. Surface effects result from the measurement’s being made near the surface of the earth. Cavity effects arise from the presence of the instrument itself. Inhomogeneities near the instrument cause strain field distortions which can affect the local tilt field. The influence of each of these effects on our borehole tiltmeters has been investigated.The proximity of the earth’s surface affects tilt measurements in several ways. Meertens and Wahr  showed t−3nrad/Pa at nontidal frequencies. Assuming that the coupling is independent of frequency, and since the measured barometric pressure tides have maximum amplitudes of around 60 Pa, the modifications of tidal admittances due to barometric coupling are less than 5%. Because the sensor package is independent of atmospheric pressure changes, the pressure induced tilts are likely to be caused by indirect effects. These effects are true tilts that arise from the spatial variation in the barometric pressure load.Data from the California array have been analyzed to estimate the size of each of these surface effects. The topography at the site is inclined by about 2.5° toward the southwest, so the topographic correction to the data is small and identical for each of the holes. The effect of the temperature and barometric pressure is estimated from the coherence spectra between the meteorological and tilt data. The data only show significant coherence amplitude for the barometric pressure and the tilt data from the 36 m deep hole. The coupling coefficient in this case is 7 × l0The measured tilt data can also be affected by the presence of the instrument itself. The existence of a cavity causes strain field distortions which may produce anomalous tilting of instruments inside the cavity. This anomalous tilting results from either tilting of the cavity walls or differential motion between the tiltmeter capsule and the cavitywalls. Harrison  calculat2(frequency ≃ 1.93 cycles/day), for different installations in a 24 m deep borehole. The phase is measured with respect to the local gravitational potential where a negative value indicates a phase lead. The values in parentheses are the uncertainties of the amplitudes and phases. Consecutive estimates of the tidal components during one installation are stable within 2%over several years. All of the data in this table were obtained with a single tiltmeter. The first two installations used the original capsule design installed along different azimuths. The first installation had the pendulums aligned with 15° and 105° azimuth, while the second installation was rotated to 314° and 440° azimuth. The differences betweenthe two installations is somewhat larger than expected from the performance of the tiltmeter during a single installation. Since measuring the tilt along any two orthogonal azimuths is sufficient to completely determine the local tilt field, these discrepancies indicate differential motion between the borehole and the capsule.The size of the cavity effect arising from differential motion between the cavity and the capsule for the original capsule design is determined by comparing data obtained from multiple installations of a single borehole. Subsequent study of the original capsule design revealed the source of this motion. Although the capsule references only the sides of the borehole, the reference points are asymmetric (two rigid contact points and one spring). This results in a cavity effect as shown in m will be averaged out while longer wavelength phenomenon will be recorded. The magnitude of this effect is estimated by comparing data from the different boreholes.The presence of local inhomogeneities near the tiltmeter can also influence the tilt data. This arises from the fact that the tiltmeters measure the tilt field using a relatively short baseline (2 m). Local disturbancesinthetiltfieldwithwavelengthsshorter than 2 2 amplitudes and phases along 0° and 90° azimuth for each of the three boreholes. The data show marked differences between the boreholes which exceed calibrationuncertainties (1–2%), the long-term stability (2%) and the expected differences from measuring at different depths (<1%). The material properties at the site change between a depth of 15 and 35 m due to the presence of a weathering layer associated with the water table [er table . Investi2 admittances using the original capsule design and the third capsule design. The phase difference along 90° azimuth between the first two installations results from the asymmetry of the reference points as described earlier. Data from the third installation show a phase shift of about 26° along 0° azimuth from using symmetric reference points and changing the depth of the tiltmeter by only 2.4 m. This phase difference exceeds the maximum 10% strain coupling which could result from changing the reference points. Using reasonable estimates of the change in material properties, these measurements constrain the location and size of the weathering layer. The parameters obtained in this way are consistent with direct measurements using well logs and other data.The third capsule design was installed in the 36 m borehole to experimentally determine if there is a variation of material properties near the bottom of the hole. µrad/yr to more than 1 µrad/yr.A geophysical borehole tiltmeter which has a sensitivity of a few nanoradians has been described. The stability of the calibration is less than 1% over several years. Consecutive estimates of the tidaladmittances are stable to 2% for any installation of the tiltmeter. The long term drift of the tiltmeter in the field varies between 0.2 Several items can be noted from the analysis of the California data which should be considered carefully when planning future borehole tiltmeter installations. First, knowledge of the orientation of the tiltmeter is useful for comparisons between different boreholes. Of the two methods described here, determining the orientation using a gyroscope is more desirable, since the uncertainty in the measurement is about ±1° and does not degrade with time. Second, tiltmeters installed at depths of at least 24 m show no evidence of surface temperature effects. This is also true for barometric pressure loading, provided that the sensor is installed far from any boundary layers such as a water table. A drilling log can provide useful information for determining the optimal depth of installation of a tiltmeter in the borehole as well as for interpreting the tilt data. Once a borehole is drilled, changing the location of the tiltmeter in the borehole can be a very sensitive technique for determining the existence of a local perturbation in the tilt field. Finally, the design of the tiltmeter capsule determines the amount of tilt-strain coupling in the instrument itself. We have found that asymmetrically referencing the sides of the borehole near the bottom can produce almost 10% tilt-strain coupling.