Jan Alfred Freund

Carl von Ossietzky Universität Oldenburg · Department of Chemistry and Biology of the Marine Environment (ICBM)

The temporal dynamics of dissolved organic matter (DOM) are inherently linked with the functioning of aquatic ecosystems. Because DOM represents a complex mixture of millions of different compounds, the statistical analysis of DOM dynamics poses a huge challenge. Here, we present a statistical approach based on hierarchical clustering of time serie...

We report the observation of a nontrivial emergent state in a chain of nonidentical, heterogeneously coupled oscillators where a set of weakly coupled oscillators becomes phase synchronized while the strongly coupled ones remain drifting. This intriguing “weak-winner” synchronization phenomenon can be explained by the interplay between nonisochroni...

To understand ecosystem responses to anthropogenic global change, a prevailing framework is the definition of threshold levels of pressure, above which response magnitudes and their variances increase disproportionately. However, we lack systematic quantitative evidence as to whether empirical data allow definition of such thresholds. Here, we summ...

Untargeted molecular analyses of complex mixtures are relevant for many fields of research, including geochemistry, pharmacology, and medicine. Ultrahigh-resolution mass spectrometry is one of the most powerful tools in this context. The availability of open scripts and online tools for specific data processing steps such as noise removal or molecu...

Biodiversity dynamics comprise evolutionary and ecological changes on multiple temporal scales from millions of years to decades, but they are often interpreted within a single time frame. Planktonic foraminifera communities offer a unique opportunity for analysing the dynamics of marine biodiversity over different temporal scales. Our study aims t...

Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is one of the state-of-the-art methods to analyze complex natural organic mixtures. The precision of detected masses is crucial for molecular formula attribution. Random errors can be reduced by averaging multiple measurements of the same mass, but because of limited availabili...

Connectivity between physiological networks is an issue of particular importance for understanding the complex interaction brain-heart. In the present study, this interaction was analyzed in polysomnography recordings of 28 patients diagnosed with obstructive sleep apnea (OSA) and compared with a group of 10 control subjects. Electroencephalography...

In a coupled system, predictive information flows from the causing to the caused variable. The amount of transferred predictive information can be quantified through the use of transfer entropy or, for Gaussian variables, equivalently via Granger causality. It is natural to expect and has been repeatedly observed that a tight coupling does not perm...

Being globally and locally prevalent, beach litter has been monitored at 29 sites along the German North Sea coastline. This study offers an exploratory analysis of data from 1991 to 2016. Schleswig-Holstein exhibited lower mean litter pollution levels than Lower Saxony, possibly because the locations in Lower Saxony are situated along the main coa...

The availability of growth data in N. norvegicus is important for management purposes due to a lack of aging criteria and the commercial importance of fisheries in this species. Growth varies as a function of stock density, hence comparisons of growth rates between stocks at known density is particularly valuable. Growth is also related to starting...

Stock enhancement activities provide an opportunity to examine density-dependent suppression of population biomass which is a fundamental issue for resource management and design of no-take-zones. We document ‘catch-and-wait’ fisheries enhancement where all but the largest lobsters are thrown back, recapturing them later after they have grown to a...

We report the observation of a novel and non-trivial synchronization state in a system consisting of three oscillators coupled in a linear chain. For certain ranges of coupling strength the weakly coupled oscillator pair exhibits phase synchronization while the strongly coupled oscillator pair does not. This intriguing "weak-winner" synchronization...

The rocky intertidal communities of Ireland contain a mix of cold- and warm-adapted species, however the spatial distribution of these communities has not been investigated in a systematic way. Based on a benthic community dataset collected in 2003 at 63 sites, several statistical analyses were combined with the aims of (i) detecting groups of simi...

The statistical concept of Granger causality is defined by prediction improvement, i.e. the causing time series contains unique information about the future of the caused one. Recently we proposed extending this concept to bivariate diffusion processes by defining Granger causality for each point of the state space as the Granger causality of a pro...

Transfer entropy (TE) is a nonlinear metric employed recently in polysomnography (PSG) recordings to quantify the topological characteristics of the brain-heart physiological network. The present study applies the TE to evaluate its usefulness to identify quantitative differences in PSG registers of patients diagnosed with oclusive sleep apnea (OSA)...

We present an encephalography (EEG) connectivity study where 30 healthy male nonsmokers were randomly allocated either to a nicotine group (14 subjects, 7 mg of transdermal nicotine) or to a placebo group. EEG activity was recorded in an eyes-open (EO) and eyes-closed (EC) condition before and after drug administration. This is a reanalysis of a pr...

Granger causality is a statistical concept devised to reconstruct and quantify predictive information flow between stochastic processes. Although the general concept can be formulated model-free it is often considered in the framework of linear stochastic processes. Here we show how local linear model descriptions can be employed to extend Granger...

The quality of water level time series data strongly varies with periods of high- and low-quality sensor data. In this paper we are presenting the processing steps which were used to generate high-quality water level data from water pressure measured at the Time Series Station (TSS) Spiekeroog. The TSS is positioned in a tidal inlet between the isl...

The quality of water level time series data strongly varies with periods of high- and low-quality sensor data. In this paper we are presenting the processing steps which were used to generate high-quality water level data from water pressure measured at the Time Series Station (TSS) Spiekeroog. The TSS is positioned in a tidal inlet between the isl...

The quality of water level time series data strongly varies with periods of high and low quality sensor data. In this paper we are presenting the processing steps which were used to generate high quality water level data from water pressure measured at the Time Series Station (TSS) Spiekeroog. The TSS is positioned in a tidal inlet between the isla...

The quality of water level time series data strongly varies with periods of high and low quality sensor data. In this paper we are presenting the processing steps which were used to generate high quality water level data from water pressure measured at the Time Series Station (TSS) Spiekeroog. The TSS is positioned in a tidal inlet between the isla...

Several linear electroencephalographic (EEG) measures at baseline have been demonstrated to be associated with treatment outcome after antidepressant treatment. In this study we investigated the added value of non-linear EEG metrics in the alpha band in predicting treatment outcome to repetitive transcranial magnetic stimulation (rTMS). Subjects we...

The spiking activity of mammalian cold receptors is described by the Huber-Braun neuron model. Sweeping temperature as a control parameter across a biologically relevant range this model exhibits a complex bifurcation structure seen in the sequence of interspike intervals. The model’s distinctive feature is the interaction between a fast spike gene...

In recent regime shift analyses, the phytoplankton compartment of the marine food web was essentially represented by phytoplankton color or chlorophyll concentration. A detection of changes directly at the species level is highly desirable. The Helgoland Roads data series, a collection of high frequency long-term time series comprising biological a...

This article presents the results of the application of different measures of complexity based on nonlinear dynamics techniques, to evaluate the effect of a neurofeedback therapy in patients with Attention Deficit Hyperactivity Disorder (ADHD) utilizing electroencephalographic (EEG) registers as unique source of information. Every EEG register anal...

This work presents the application of nonlinear dynamics measures to electroencephalograms (EEG) acquired from patients with Attention Deficit/Hyperactivity Disorder (ADHD) before and after a neurofeedback therapy, with the aim to assess the effects of the neurofeedback in a quantitative way. The database contains EEG registers of seven patients ac...

The beneficial role fluctuations can play in the process of phase synchronization is analyzed in terms of a model with binary input and output signals. Special attention is paid to the relation between noise-induced phase synchronization and the well-known phenomenon of stochastic resonance. Analytic predictions are compared with experimental data...

Signal detection in fluctuating background noise is a common problem in diverse fields of research and technology. It has been shown in hearing research that the detection of signals in noise that is correlated in amplitude across the frequency spectrum (comodulated) can be improved compared to uncorrelated background noise. We show that the mechan...

The response of a four-dimensional mammalian cold receptor model to different implementations of noise is studied across a wide temperature range. It is observed that for noisy activation kinetics, the parameter range decomposes into two regions in which the system reacts qualitatively completely different to small perturbations through noise, and...

We investigate simple one-dimensional maps which allow for exact solutions of their related statistical properties. In addition to the originally refined dynamical description a coarsegrained level of description based on certain partitions of the phase space is selected. The deterministic micropscopic dynamics is shifted to a stochastic symbolic d...

The classification of the ecological niche and the grouping of species into generalists and specialists are established ecological concepts. In this study, we compare two methods for the calculation of the realized ecological niche from long-term species data. As the first method, we present a new approach to reconstruct the realized niche not in t...

We explore how the reconstruction efficiency of fast spike population codes varies with population size, population composition and code complexity. Our study is based on experiments with moving light patterns which are projected onto the isolated retina of a turtle Pseudemys scripta elegans. The stimulus features to reconstruct are sequences of ve...

We study the role of the strength of subthreshold currents in a four-dimensional Hodgkin-Huxley-type model of mammalian cold receptors. Since a total diminution of subthreshold activity corresponds to a decomposition of the model into a slow, subthreshold, and a fast, spiking subsystem, we first elucidate their respective dynamics separately and dr...

We present a novel method for regional climate classification that is based on coarse-grained categorical representations of multivariate climate anomalies and a subsequent Markov chain analysis. From the estimated transition matrix several descriptors, such as persistence, recurrence time and entropy , are derived. These descriptors characterise d...

We study a physiologically realistic implementation of internal stochasticity in a four-dimensional Hodgkin-Huxley type model of mammalian cold receptors. We show that in a deterministically tonic firing regime, this stochasticity can drive the neuron into a state of complex bursting behaviour. An explanation of the mechanism behind this effect is...

The footprint of climate change can be seen in many regions on Earth. In Europe, the Iberian Peninsula is most susceptible to climate change. Long term trends indicate that glaciers are melting, species becoming extinct, droughts and storms are increasing and beaches get lost. Less dramatic change is seen in climate variables on shorter time scales...

The Helgoland Roads time series is one of the richest temporal marine data sets available. Running since 1962, it documents changes for phytoplankton, salinity, Secchi disc depths and macronutrients. Uniquely, the data have been carefully quality controlled and linked to relevant meta-data, and the pelagic time series is further augmented by zoopla...

Phytoplankton blooms, in their pivotal position in pelagic seasonal succession require precise classification criteria in order to evaluate such parameters as bloom start, bloom timing, bloom maximum and growth rates. Such bloom parameters are linked directly to species and bloom specific features. Currently the phytoplankton bloom concept, though...

Zusammenfassung: Molekulardynamische Untersuchungen eignen sich zur Simulation des Verhaltens makroskopischer Mengen granularen Materials unter bestimmten, technologisch relevanten Beanspruchungen. Wir untersuchen das Zerkleinerungsverhalten von Mahlgut in einer Kugelmühle, insbesondere im Hinblick auf die Effizienz als Funktion der Drehzahl und au...

Existing climate classifications comprise the genetic classification, which is based on climate genesis factors such as winds and oceanic and continental climate, and the empiric classification based on temperature, precipitation, vegetation and more. We present a novel method for climate classification that is based on dynamic climate descriptors,...

The property of a neuron to phase-lock to an oscillatory stimulus before adapting its spike rate to the stimulus frequency plays an important role for the auditory system. We investigate under which conditions neurons exhibit this phase locking below rate threshold. To this end, we simulate neurons employing the widely used leaky integrate-and-fire...

Measurements of methane (CH4) so far have always shown supersaturation in the entire North Sea relative to the atmospheric partial pressure and the distribution of surface CH4 reveals a distinct increase towards the shore. Since North Sea sediments presumably are an insignificant source for CH4 the coastal contribution via rivers and tidal flats ga...

Many examples of natural noise show common amplitude modulations at different frequency regions. This kind of noise has been termed comodulated noise and is widely examined in hearing research, where an enhanced detectability of pure tones and narrow noise bands in comodulated noise compared to unmodulated noise is well known as the CMR or CDD effe...

We present an analysis of the spike response of a retinal ganglion cell ensemble. The retina of a turtle was stimulated in vitro by moving light patterns. Its non-steady motion was specified by two features: changes of direction and changes of speed. The spike response of a ganglion cell population was recorded extracellularly with a multielectrode...

Natural sounds often exhibit correlated amplitude modulations at different frequency regions, so-called comodulation. Therefore, the ear might be especially adapted to these kinds of sounds. Two effects have been related to the sensitivity of the auditory system to common modulations across frequency: comodulation detection difference (CDD) and com...

We analyze electron microscope X-ray spectroscopy data of recent supratidal marine sediments. Statistical measures are used to characterize the distribution of silicon and calcium in different layers of the sediments. We also use cluster analysis and symbolic dynamics to filter measurement noise and to classify different density regions. This allow...

Many concepts of modern chaos theory are based on the notion of a symbolic dynamics. Usually this method is discussed with restriction to deterministic systems. In this contribution we address the consequences of applying a symbolic dynamics to stochastic processes or, alternatively, of introducing noise at the symbol level. Special attention is pa...

We introduce a symbol sequence generator producing sequences with long-range correlations. We analytically derive the scaling behaviour of block entropies. A similar scaling behaviour was suggested for natural information carrying symbol sequences, e.g. texts.

We study the phase synchronization of a noisy bistable system with an external dichotomic signal of either periodic or Markovian character. Using a generic two-state model, a basic paradigm of stochastic resonance, we analytically prove the occurrence of noise-induced frequency and effective phase locking characterized by a plateau of the mean outp...

Signal detection in fluctuating background noise is a common problem in diverse fields of research and technology. It has been shown in hearing research that the detection of signals in noise that is correlated in amplitude across the frequency spectrum (comodulated) can be improved compared to uncorrelated background noise. We show that the mechan...

With the aim of describing annually recurring phytoplankton blooms we discuss an extension of the phytoplankton–zooplankton model introduced by [Truscott, J.E., Brindley, J., 1994. Ocean plankton populations as excitable media. Bull. Math. Biol. 56, 981–998]. The extension is a seasonal forcing of the phytoplankton growth rate driven by an oscillat...

Processing of external stimuli by sensory neurons often involves bursting, when epochs of fast firing alternate with intervals of quiescence. In particular, sensory neurons of electroreceptors in paddlefish (Polyodon spathula) undergo bursting when stimulated externally with broad-band noise, but otherwise fire spontaneously in a quasiperiodic toni...

We propose a model for a walker moving on an asymmetric periodic ratchet potential. The walker has two ``feet'' represented as two finite-size particles coupled nonlinearly through a double-well potential. In contrast to linear coupling, the bistable potential admits a richer dynamics where the ordering of the particles can alternate. The transitio...

The detection of external stimuli by peripheral sensory neurons and the downstream processing by cortical neurons quite often involves the operational mode of bursting activity, i.e., periods of rapid firing alternating with intervals of quiescence. In some systems, e.g. the electrosense of the paddlefish (Polyodon spathula), a transition from toni...

The decision whether a measured distribution complies with an equidistribution is a central element of many biostatistical methods. High throughput differential expression measurements, for instance, necessitate to judge possible over-representation of genes. The reliability of this judgement, however, is strongly affected when rarely expressed gen...

The phenomenon of frequency and phase synchronization in stochastic systems requires a revision of concepts originally phrased in the context of purely deterministic systems. Various definitions of an instantaneous phase are presented and compared with each other with special attention paid to their robustness with respect to noise. We review the r...

Detection and location of moving prey utilizing electrosense or mechanosense is a strategy commonly followed by animals which cannot rely on visual sense or hearing. In this paper we consider the possibility to detect the source of a localized stimulus that travels along a chain of detectors at constant speed. The detectors are autonomous oscillato...

Given an equidistribution for probabilities p(i)=1/N, i=1, ..., N. What is the expected corresponding rank ordered frequency distribution f(i), i=1, ..., N, if an ensemble of M events is drawn?

The phenomenon of effective phase synchronization in stochastic oscillatory systems can be quantified by an average frequency and a phase diffusion coefficient. A different approach to compute the noise-averaged frequency is put forward. The method is based on a threshold crossing rate pioneered by Rice. After the introduction of the Rice frequency...

A two-dimensional cellular automaton is introduced to model the flow and jamming of vehicular traffic in cities. Each site of the automaton represents a crossing where a finite number of cars can wait approaching the crossing from each of the four directions. The flow of cars obeys realistic traffic rules. We investigate the dependence of the avera...

Zooplankton emit weak electric fields into the surrounding water that originate from their own muscular activities associated with swimming and feeding. Juvenile paddlefish prey upon single zooplankton by detecting and tracking these weak electric signatures. The passive electric sense in this fish is provided by an elaborate array of electrorecept...

We study the nonlinear response of a stochastic bistable system driven by both a weak periodic signal and a dichotomic noise in terms of stochastic phase synchronization. We show that the effect of noise-induced phase synchronization can be significantly enhanced by the addition of a dichotomic noise.

Juvenile paddlefish prey upon single zooplankton by detecting a weak electric signature resulting from its feeding and swimming motions. Moreover, it has recently been shown that paddlefish make use of stochastic resonance near the threshold for prey detection: a process termed behavioral stochastic resonance. But this process depends upon an exter...

Zooplankton emit weak electric fields into the surrounding water that originate from their own muscular activities associated with swimming and feeding. Juvenile paddlefish prey upon single zooplankton by detecting and tracking these weak electric signatures. The passive electric sense in the fish is provided by an elaborate array of electrorecepto...

The theory of stochastic processes provides a huge arsenal of methods suitable for analyzing the influence of noise on a wide range of systems. Noise-induced, noise-supported or noise-enhanced effects sometimes offer an explanation for as yet open problems (information transmission in the nervous system and information processing in the brain, proc...

We investigate autogenous fragmentation of dry granular material in rotating cylinders using two-dimensional molecular dynamics. By evaluation of spatial force distributions achieved numerically for various rotation velocities we argue that comminution occurs mainly due to the existence of force chains. A statistical analysis of theses force chains...

The maximal coherence between an external signal and the output of a noisy bistable system for nonvanishing optimal noise intensity, termed stochastic resonance, can also be observed in the stochastic dynamics of a phase difference between input and output. This formulation connects stochastic resonance with the field of synchronization phenomena....

The phenomenon of noise-induced transport in ratchet devices offers an explanation for directed motion on the molecular scale observed in many biological systems. Net transport through a series of discrete states, occurring in cyclic processes or reactions, can be related to widely investigated continuous ratchet models in the context of thermally...