Global software development (GSD) which is a growing trend in the software industry is characterized by a highly distributed environment. Performing software project management (SPM) in such conditions implies the need to overcome new limitations resulting from cultural, temporal and geographic separation. The aim of this research is to discover and classify the various tools mentioned in literature that provide GSD project managers with support and to identify in what way they support group interaction. A systematic mapping study has been performed by means of automatic searches in five sources. We have then synthesized the data extracted and presented the results of this study. A total of 102 tools were identified as being used in SPM activities in GSD. We have classified these tools, according to the software life cycle process on which they focus and how they support the 3C collaboration model . The majority of the tools found are standalone tools (77%). A small number of platforms (8%) also offer a set of interacting tools that cover the software development lifecycle. Results also indicate that SPM areas in GSD are not adequately supported by corresponding tools and deserve more attention from tool builders. Globalization as an economic and social trend has relentlessly pushed businesses to turn from national markets toward a global market in which new forms of concurrence and cooperation have been spawned. In the last decade, software development in particular has undergone a drastic change in its business operations and processes. This concerns not only marketing and distribution but also the way in which software is conceived, designed, constructed, tested, and delivered to customers  on the tools used for SPM in the context of GSD. Its purpose is to identify these tools, to classify them using an international standard [ISO/IEC/IEEE 12207 “Systems & Software Engineering—Software Lifecycle Process” ] and to The remainder of this paper is organized as follows: “Early research on the subject was conducted by da Silva et al.  in an SLIn an SMS published in 2012, Portillo-Rodríguez et al. . AccordiTell and Ali Babar  tackle tPortillo-Rodríguez et al.  and TellTo the best of our knowledge, no systematic mapping or review of the tools available in GSD has been performed to date with a focus on SPM activities and processes. As stated above, da Silva et al.  proposedSMSs are designed to provide a classified scheme of a broad research area, and to categorize existing research evidence on a topic and its subsequent results. These results can identify gaps in research, and an SLR can then be used to study these gaps in greater depth. In this section, the research questions (RQs) of this study are first laid down and the search string used to query the digital libraries is constructed based on the PICO method. Afterwards, digital libraries used are specified, Inclusion and Exclusion criteria are defined and a quality assessment method is established. Finally the data extraction method is indicated for each RQ. The protocol of this SMS is based on the recommendations of Kitchenham and Charters , and theThe RQs aim to classify the tools available in terms of features and type and to assess their use in the software industry. The RQs and their motivations are displayed in Table For the purpose of this study, SPM is defined as the project processes of the ISO/IEC/IEEE 12207 “Systems and Software Engineering—Software Lifecycle Process” . ProjectIn order to answer the RQs, a search was conducted using a search string composed of keywords relevant to this study and applied to a number of academic electronic libraries and search engines.Global software development.Population: Software project management.Intervention: Tool.Outcome: Comparison is not relevant in this study, since it is an exploratory study. The Intervention keyword has proven to be limiting the group of studies targeted by this SMS. The reason being that most papers do not talk specifically about SPM but rather SPM processes , consequently, the intervention keyword has been dropped.Following the guidelines of Kitchenham and Charters  using thThe search terms that have similar meanings are organized into groups. Combined terms are obtained using the OR logical operator between search terms in the same group. The final search string is obtained using the AND logical operator between combined terms of different groups. The search string used is: [ AND (software) AND (development OR engineering OR improvement OR project) OR GSD OR DSD OR GSE OR CSE] AND (tool OR technology).The search terms used have been inspired from similar research  .Digital librariesGoogle Scholar .Digital bibliography and library project (DBLP) .Digital search enginesThe digital libraries used in this SMS have been selected by using previous SLRs and SMSs in the same field and with a similar scope as a basis. In studies  used in GSD.The paper was a workshop summary.Paper was not in English.The exclusion criteria were applied using “OR” logical operator between them.Although some tools can be thought not to be related to this research, further investigation of their use in industry and their features proved otherwise. A tool like “Eclipse help system” is primarily a help system for an IDE and would commonly be considered to be outside of this research’s scope. Nonetheless, it allows users to create and modify project documentation, which can then be used by other stakeholders. This feature allows knowledge to be created and shared. This tool can then be included in the group of tools that support the IM process. “Social software” can also be a source of confusion as they are frequently used inside and outside work context, but recent studies indicate that tools such as Skype, Twitter, etc., have been widely used in the GSD industry to provide additional communication channels .Some papers may present a discussion about our research subject but only in a secondary manner, while the main topic of the paper is different. We consider that these papers are not as interesting for our SMS as those which are entirely dedicated to discussing SPM tools in a GSD context.QA2 The study explicitly presents tools with which to support GSD project management activity (yes +1/no +0).Some papers may only present guidelines or recommendations as to how to use or develop tools. These papers might be interesting for our discussion but our main research goal is to list all available tools. The paper achieves the full score if a new tool is presented or an existing tool is assessed.QA3 The study uses empirical results for argumentation (yes +1/no +0). The results and conclusions of the study are strengthened by empirical evidence and it provides important and reliable information about future research and practice  (Conf: (CORE A* +2/CORE A +1.5/CORE B +1/CORE C +0.5).Journal: (Q1 +2/Q2 +1.5/Q3 +1/Q4 +0.5).No ranking: +0.Each of the primary studies selected was assessed to ascertain its quality using four criteria inspired by previous studies , CORE A (excellent conference), CORE B (good conference), CORE C (other ranked conference).Although there is a general opinion that journal papers are better than conference papers, Bowyer  is the oRQ1 The publication source and channel for each paper is listed and the aggregated results will be presented.RQ2 The publication year for each paper is listed, and the aggregated result will provide an overview of the number of related articles per year.RQ3 The research type for each paper is classified into one of the following categories .RQ4 The research approach for each paper is classified into one of the following categories .Experiment An empirical method applied under controlled conditions.Non empirical Non empirical research approaches or a theoretical evaluation of a tool.Other Any approach not listed above.RQ5 Each of the tools proposed or reviewed in any of the selected papers is listed in order to create an exhaustive set of the tools proposed for SPM activity in literature. We have differentiated between the technologies by adopting the classification proposed in Tell and Ali Babar  an independent software application fulfilling a specific design intent.Framework a “semi-complete” application that provides an integrated set of domain-specific structures and functionality.Environment a development environment that comprises a set of processes and programming tools used to create software products. Moreover, an integrated development environment is a subset of this group, which identifies a development environment that has a unified interface.Platform a set of generic components that form a common structure, from which a set of derivative products can be developed.Plug-in a software component that interacts with an existing software application through the use of well defined application programming interfaces (APIs), often designed to enhance it by adding new functionalities. RQ5 also aims to classify tools on the basis of their type of license, and we have adopted the classification used in Portillo-Rodríguez et al. , etc.Research Tools or prototypes developed by research groups which are not freely or commercially available.li Babar :Standalz et al. :CommercRQ6 We have adopted the ISO/IEC/IEEE 12207 “Systems and Software Engineering—Software Lifecycle Process” standard for the classification of the activities related to SPM. The classification of the listed tools will be based on the project processes of (Project planning (PP) The purpose of the PP process is to produce and communicate effective and workable project plans. This process determines the scope of the project management and technical activities, identifies process outputs, project tasks and deliverables, establishes schedules during which the project task will be conducted, including achievement criteria, and states which resources are required to accomplish project tasks.Project assessment and control (PA) The purpose of the PA process is to determine the status of the project and ensure that the project performs according to plans and schedules, and within projected budgets, and that it satisfies technical objectives. This process includes the redirection of the project activities, as appropriate, in order to correct identified deviations and variations from other project management or technical processes. Redirection may include replanning as appropriate.Decision management (DM) The purpose of the DM process is to select the most beneficial course of project action when alternatives exist. This process responds to a request for a decision encountered during the system life cycle, whatever its nature or source, in order to reach specified, desirable or optimized outcomes. Alternative actions are analyzed and a course of action is selected and directed. Decisions and their rationale are recorded to support future decision-making.Risk management (RM) The purpose of the RM process is to identify, analyze, treat and monitor the risks continuously. The RM process is a continuous process by which to systematically address risk throughout the life cycle of a system or software product or service. It can be applied to risks related to the acquisition, development, maintenance or operation of a system.Configuration management (CM) The purpose of the CM process is to establish and maintain the integrity of all identified outputs of a project or process and make them available to the parties concerned.Information management (IM) The purpose of the IM process is to provide relevant, timely, complete, valid and, if required, confidential information to designated parties during and, as appropriate, after the system life cycle. This process generates, collects, transforms, retains, retrieves, disseminates and disposes of information. It manages designated information, including technical, project, organizational, agreement and user information.Measurement (Me) The purpose of the Measurement process is to collect, analyze, and report data relating to the products developed and processes implemented within the organizational unit, to support the effective management of the processes, and to objectively demonstrate the quality of the products.esses of . A tool esses of :ProjectRQ7 This question aims to identify which of the 3C are supported by the listed tools. The classification used is inspired from a systematic review of awareness support in GSD carried out by Steinmacher et al. . Based on their title, keywords, abstract and perhaps the full text, the researchers tasked with the study selection process included those articles that might possibly answer the RQs presented in Table The rules shown in “Kitchenham and Charters  specifieThe main motivation of this quality assessment is to provide an overview of the usefulness of the selected papers in this SMS. Results show that 75% have an average score of 2.5 points or higher. Table Table As shown in Table The main publication target for articles related to the use of SPM tools in a GSD context would generally appear to be conferences, as 70% of the selected articles have been published via this channel. Table 9 in Appendix Figure Of the selected articles, 46% are solution proposals that have been derived solely from academic research. This prevalence hints that there may be a lack of existing tools with which to mitigate the effect of globalization on the software industry. In addition, 25% of the selected papers are evaluation research. Reviews represent 15% of the selected papers while another 14% represent the “Other” category, comprising opinion reports, experience papers and so on.According to the data shown in Fig. These results can be explained using the Redwine & Riddle maturation model Shaw . This moThe existing literature offers a relatively high number of solution proposal studies to support the management activity of GSD projects, and half of them have no empirical data to support their usefulness.As shown in Fig. SPM experiments in GSD are difficult to conduct. GSD settings imply that the subjects of the experiment must be from different geographic locations, different time zones and different cultures. This can complicate the empirical evaluation of tools in industrial environments  are SATs that are intended to satisfy a specific design. However, the use of SATs increases context switches, which can be a source of frustration . Their prominent feature is automatic messaging, which is triggered by new version updates to alert the parties concerned.PA and PP processes are covered by 26 and 17 tools respectively that enhance collaboration in distributed teams. Eight of these tools propose features that cover both PA and PP processes, namely: ActiveCollab  (CMMI Product Team Communication The tools studied integrate a number of features that are intended to reduce the effect of geographical distance and time zone differences on group awareness. These features include instant messaging, forums, e-mail notifications, audio/videoconferencing, and wikis. A total of 49 tools support communication using these features. Lack of communication is considered to be one of the key challenges when managing traditional or agile software development projects in a globally distributed environment (Dullemond et al. Coordination Coordination between project actors is ensured through an improved awareness of team members’ activities. A total of 40 tools support coordination by using e-mail notifications or a dynamic visualization interface of team project members and their respective activities. Tools like ActiveCollab (Portillo-Rodríguez et al. Cooperation According to Gorton et al. (n et al. , the vasThe tools that have been identified have specific features that manage the challenges that high distribution poses as regards the success of GSD projects. Three key areas have been tackled by GSD tools to support group interaction, namely the 3Cs:Figure Practitioners such as tool builders and project managers, along with researchers involved in GSD project management, are eager to know about latest research on the subject. They should view articles published in the proceedings of the ICGSE conference and its affiliated workshops. The latter is the main publication source for studies about SPM in GSD. In the same line, researchers are encouraged to send their articles to this conference.The results of this SMS indicate a lack of empirical validation of the various solutions proposed. Only 24.7% of the tools identified in our study have been empirically validated. A similar percentage was obtained in a previous mapping study of GSD tools (Portillo-Rodríguez et al. A number of tools that are, according to literature, intended to support SPM activities in the GSD context have been listed in this study. Practitioners, and particularly project managers, can use this list to select tools that can support their activities. License type and technology type are specified for each tool in order to ease this choice (see Table 12 in Appendix Decision management, risk management and measurement processes are not adequately supported by tools when compared to the other SPM processes. Researchers are encouraged to engage in lines of research that may enable the reasons for this disparity to be discovered. Practitioners are encouraged to fill the tooling gaps in these processes.The results of this research contribute to GSD in many ways. Firstly, they provide the academic community with a better understanding of the SPM activities and tools used in GSD settings and show gaps in the areas in which tooling is insufficient or inadequate. This information can open up opportunities for future advances by both researchers, who will better understand the landscape of GSD and tackle its challenges, and tool builders, who may wish to provide new tools that are capable of mitigating the effect of globalization on the management of software projects. Secondly, project managers involved in highly distributed projects will have an overview of the existing tools intended to support their activities and identify those which are most likely to meet their needs. The recommendations to researchers and practitioners resulting from the conclusions drawn from this SMS are as follows:The validity of the study is concerned with the trustworthiness of its results. The conclusions of this research may have been threatened by bias resulting from the researchers’ subjective point of view. The categorization used in Runeson et al. , Wohlin Construction validity refers to the extent to which the operational measures that are studied really represent what the RQs aim to answer. In an SMS, two factors can be easily identified as a threat to construct validity. One is the research string used, while the other is the digital libraries researched. In this study, we have performed a systematic search using an extensive range of terms to widen our scope of research. The search keywords were proposed by two authors in several iterations to ensure that all relevant literature would be included in the study. Another threat to the construct validity is the choice of digital libraries used. This risk was mitigated by the identification of three digital libraries as the main source of related articles according to existing literature. Two digital search engines were also used to provide additional sources for related articles. A manual search of the reference lists in the selected studies in order to complement the SMS is strongly suggested by Kitchenham and Charters . This prWith regard to the internal validity of the study, the classification and the decision to assign a specific tool to a specific area of project management can be judged to be subjective. Similar studies (Portillo-Rodríguez et al. Conclusion validity is the degree to which the conclusions we reach about relationships in our data are reasonable and is concerned with the ability to replicate these findings. In an SMS, the threat to conclusion validity is a factor that may lead to an incorrect conclusion being reached about a relationship in the observed data. Bias as regards both selecting and classifying primary studies and tools along with analyzing data may therefore affect the interpretation of the results. In order to mitigate this threat, each step of the selection, extraction and analysis of the data was validated by means of the systematic process and the periodic reviews carried out by the researchers involved in this work. Finally, this SMS has inherited the threats to validity of its primary studies.External validity concerns how far the results of a study can be generalized. In this case, it concerns the external validity of the tools presented. The ratio of academic tools to commercial tools, along with the fact that the selected studies included in this SMS that discuss industrial tools are written by authors who are involved in both research and industrial fields, lead us to believe that our tool list is not exhaustive and that some of the tools used in industry may not have been included. Furthermore, some details about the tools were difficult to obtain, since our tool list was indirectly built, starting from the primary studies and not from the tool’s experience. Nevertheless, the results of this study may serve as a starting point for SPM for GSD researchers and practitioners.This paper reports an SMS that explores how tools support SPM activities in GSD. Seventy-six studies were selected and a total of 102 tools were identified in literature and were classified according to the following criteria: license, type, SPM area they support and dimension of 3C collaboration model they focus on. During the process of this SMS, we noticed that although a large number of standalone tools are proposed, fewer tools cover the whole SPM process. A commercial platform and an environment created by international companies have been encountered during research prior to this SMS: IBM Rational and Microsoft Visual Studio. These tools have integrated a number of commercial standalone tools that collaborate and exchange information in a seamless manner, thus ensuring that the project manager’s activities are consistently supported throughout the development lifecycle.Two studies (Sinha et al. The limitations discussed in “


Meta‐analyses combine the results of multiple studies of a common question. Approaches based on effect size estimates from each study are generally regarded as the most informative. However, these methods can only be used if comparable effect sizes can be computed from each study, and this may not be the case due to variation in how the studies were done or limitations in how their results were reported. Other methods, such as vote counting, are then used to summarize the results of these studies, but most of these methods are limited in that they do not provide any indication of the magnitude of effect.P value and a total sample size from each study . The plot allows an approximate examination of underlying effect sizes and the potential to identify sources of heterogeneity across studies. This is achieved by drawing contours showing the range of effect sizes that might lead to each P value for given sample sizes, under simple study designs. We provide examples of albatross plots using data from previous meta‐analyses, allowing for comparison of results, and an example from when a meta‐analysis was not possible.We propose a novel plot, the albatross plot, which requires only a 1‐sided  They typically focus on estimation of an underlying (average) effect size across studies, and often illustrate the individual study results and the pooled result using a forest plot. Meta‐analyses of this type are, however, not always possible. This is especially the case when data are collected, analysed, or reported in different ways in different studies. Sometimes statistical analysis results are presented in ways that do not facilitate estimation of a comparable effect size for every study. One possibility in this situation is to perform a narrative synthesis of the findings across studies. This can be cumbersome to digest and there is a risk that conscious or unconscious bias may affect the way in which the results are presented.P value threshold are counted. Harvest plots have been proposed as an extension of vote counting, providing a graphical tool for displaying the results from each study.Where statistical test results are available from each study, vote counting might be used, in which the numbers of studies reporting a positive, negative or null association using a predefined P values across studies, for example, using methods of FisherP value for testing the null hypothesis of no effect in every study. However, again, they take no account of the relative sizes of the studies and do not provide an estimate of effect magnitude.More attractive alternatives to vote counting are available. One possibility is a sign test, in which directions of effect are counted rather than conclusions around statistical significance. A second possibility is the statistical combination of exact precise P value and a total sample size. However, unlike the simple methods described above, albatross plots allow an approximate examination of underlying effect sizes and the potential to identify sources of heterogeneity across studies.In this paper, we propose a novel plot to illustrate findings from quantitative studies when insufficient information is available to present results in a forest plot. Our albatross plot is based on minimal statistical information that is usually available from each study, namely, a precise P values; the third is from a systematic review of the association between milk intake and insulin‐like growth factor‐I (IGF‐I), where a meta‐analysis was not possible because of the diverse reporting of studies; and the fourth is from a review of the association between body mass index (BMI) and prostate specific antigen (PSA), where a meta‐analysis was possible but not for all studies.We introduce the albatross plot in 2P values requires information about the sizes of the study from which they come. For example, a P value of 0.2 may arise from a large study with a small underlying effect or from a small study with a large underlying effect. Our basic albatross plot is a scatter plot of study sample sizes against 1‐sided P values. Equivalently, this is a scatter plot of study sample sizes against 2‐sided P values, with results separated according to the observed direction of effect. The albatross plot allows the P values to be interpreted in the context of the study sample size. Small studies appear towards the bottom of the plot and large studies towards the top. Throughout this paper, we plot 2‐sided versions of the P values, separated by direction of effect, because these are much more commonly reported. One‐sided P values can readily be transformed to 2‐sided P values.Appropriate interpretation of P values from strong negative results  appear at the left of the plot and small 2‐sided P values from strong positive results  appear at the right of the plot, with studies with null results towards the middle. We plot both the sample size axis and the P value axis on the log scale for improved visual interpretation.Small 2‐sided P values. These contours will be specific to the type of data  used to calculate the P values and are to be interpreted very approximately. The contours typically resemble large flying birds, giving rise to our proposed name of an albatross plot. We describe some simple derivations for contours in the following section and derive a variety of other possibilities in an online supplement. Second, different subgroups of studies can be drawn using different colours or symbols to facilitate identification of subgroup effects.Two types of enhancement to the basic albatross allow approximate examination of effect sizes and their heterogeneity. First, we superimpose contours on the plot to reflect different hypothetical effect sizes that would have given rise to particular Figures 3P values were derived from Wald tests. A Wald test involves division of the effect size estimate  P value using the same distribution, so we write This statistic is compared to a standard normal distribution to obtain the In general, the SE is proportional to the inverse of the square root of the total number of participants  or drawing a line instead of a point on the albatross plot to show the range of P values compatible with the reported finding . For P > threshold, we suggest that either omitting the study or again using a line to show the range of P values compatible with the reported finding .Some studies do not report explicit 3.1A simple and commonly used effect size is the SMDs, which compares mean responses between two groups, such as an intervention group and a control group in a randomized trial. We will use this effect measure as an example of how we create effect contours for the albatross plot.The SMD is defined asd) of the SMD is obtained by substituting estimates of the means and the pooled SD into Equation where SE=2N . Hence where A better approximationAgain assuming equally sized groups, we obtainP value, does not require any additional information to create. For example, for an SMD of 0.1, a P value of 0.05 (Z = 1.96) would arise from a study of 1539 participants.Thus, the effect contour for a given SMD, which shows the number of participants required for a particular The assumption of equal sample size in both groups may be a reasonable approximation for experimental studies such as randomized trials but may not be appropriate for observational studies. To account for unequal group sizes when the ratio varies across studies, it is possible to adjust the plotting position for individual points to reflect an effective sample size rather than the observed sample size. Such adjustments to the sample size may require assumptions to be made about the magnitude of the effect size. In our experience, the effect of this adjustment is typically minimal.While it is possible to use the contour generation equations to estimate the effect size and SE of each study, this is not the purpose of the equations, nor is it advisable for all studies. Rather, the equations define hypothetical effect contours that can aid in interpretation of the magnitude of effect across many studies and are useful for when there is insufficient information to calculate effect sizes and SEs for all included studies.3.2Under particular assumptions, ORs and correlation coefficients can be transformed to and from an SMD.Similarly, using the approximation for correlation coefficients  from univariable linear regression,44.1To demonstrate the connections between an albatross plot and established techniques, we first illustrate the albatross plot using a data set that was originally analysed using effect sizes and which could, therefore, be illustrated in a forest plot. Zhang et al performed a meta‐analysis of randomized trials evaluating the effect of exercise training after an acute MI.P values and total sample sizes from the contributing studies. The SMD effect size contours are generated using Equation P values are spread horizontally along the graph.The albatross plot for these studies is presented in Figure For studies in which exercise training started during the healing period, the points are mostly clustered around an SMD of 0.25. The exception4.2P values.P values, producing combined P values between 1.99 × 10−4  and 1.25 × 10−16 (using Stouffer's Z). These tests all demonstrated that student ratings of instructors and achievement levels were correlated. Indeed, a meta‐analysis of the correlation coefficients gave an average correlation of 0.36 (obtained via the weighted average of Fisher Z‐transformed correlation coefficients), with no evidence of heterogeneity.Our second example uses data from Cohen,P values and corresponding study sample sizes in Figure We provide the albatross plot for these 4.3Our third example uses data from a review of studies investigating associations between intake of milk products and IGF‐I, a protein found in blood The albatross plot for these studies is presented in Figure As a meta‐analysis was not possible for this review, the albatross plot was useful in determining the likely magnitude of association between milk intake and IGF‐I, as well as in graphically representing all data.4.4Our fourth example uses data from a review of studies investigating the association between BMI and PSA, a protein found in blood often used as a screening test for prostate cancer The albatross plot for these studies is presented in Figure In this example, the albatross plot was useful in determining that the excluded studies were broadly consistent with the included studies so that, had they been included in the meta‐analysis, the effect estimate would have been unlikely to change. However, the plot also identified one inconsistent study. This identification allowed us to consider possible explanations of the difference and to qualify the results, which may not be generalizable to all populations, appropriately.5P value and a sample size from each study . A single plot can show a large amount of information, including multiple results per study. Since most studies will report the number of participants and a P value, albatross plots can be more inclusive than a meta‐analysis. Should some studies be excluded from a meta‐analysis due to insufficient information, an albatross plot can be used as a sensitivity analysis to determine whether the excluded studies were consistent with the studies with sufficient information.Albatross plots provide a versatile and simple way of graphically displaying data from multiple studies when meta‐analysis is not feasible. The basic requirements of the plot are minimal, since it needs only a 1‐sided Heterogeneity can be seen in the spread of points across the plot, and outliers easily identified. Our proposed effect size contours give a broad indication of the magnitude of an association, while different plotting colours and symbols allow for subgroups to be compared informally. As albatross plots are not designed to estimate the magnitude of an association precisely, the effect size contours and any association seen must be interpreted as approximate; precise combined effect estimates can only be obtained through meta‐analysis. A narrative synthesis may be needed to supplement the plot to fully explore the data, especially for outlying studies.The authors declare no conflict of interest.


Reactions involving reactive species produced in electric discharges are frequently characterized by the emission of visible light of many different colors. Some typical afterglows and atomic flames have been photographed, and the observed colors  are discussed with regard to the reactions from which they arise. Laboratory studies of this sort are helpful for the understanding of the energy transfer processes which occur in flames, in electrical discharges, and in the upper atmosphere. Brightly colored afterglows from electric discharges have provided a source of laboratory demonstrations and wonder for nearly 80 years. In 1884 Warburg  and oxygWhen dissociation products from an electric discharge through gases at low pressure 0.01 to 100 mm Hg) are mixed with suitably reacting gases or vapors, bright regions known as atomic flames are observed . In addi0 mm Hg aVery few studies have been made of glows which occur in short times after the discharge. However some observations have been made as part of wind tunnel visualization efforts .Previous attempts to obtain color photographs of these phenomena have met with only partial success. However, recent developments in photographic techniques and color film, as well as improved control of the afterglow and flame systems have made possible the satisfactory photographic recording of these glows. A group of photographs of characteristic glows and flames has been assembled in Afterglows and atomic flames were photographed in the apparatus shown in In order to observe very short time phenomena the apparatus shown in The critical factor for the reproducible photography of these phenomena is the correct control of exposure time. This was achieved through the use of a Honeywell Pen tax 3°/21° exposure meter which has very high sensitivity for an acceptance angle of 3°. The narrow acceptance angle permits exposure readings from restricted regions of the flames and thus adjustments may readily be made for small changes in brightness. These photographs were made on Kodachrome II film (daylight type) rated at an exposure index of 32. Exposure times were varied from 1/25 sec to 30 sec. Changes of exposure time by as little as a factor of 2 made the difference between good and poor color reproduction. For the longer exposures attempts were made to take account of the reciprocity-law failure by the use of color correction filters as recommended by the film manufacturer. However, it was found that the best color reproduction was obtained without the use of filters.Two general methods have been used in the study of afterglows. In a static gas system after pulsing an electrical discharge, changes of color can be observed as function of time. Alternatively, the same phenomena can be observed in a flowing gas system as a function of distance downstream from the discharge. The observed phenomena are caused by various reactions of energetic species. Different color regions indicate different concentrations of atoms, molecules, and ions and the collision processes occurring between these species.4 molecule has been postulated as the energy carrier in this case [There are many known species in discharges in pure atomic and diatomic gases: atoms and diatomic molecules each in several electronic states, ions of atoms and molecules, electrons, and photons. Conditions in the discharge affect the relative concentrations of these species. Concentrations change as a function of time after the discharge with most of the more energetic species decreasing monotonically with time. However in some cases, the collision processes cause increases of certain species. In nitrogen at times a few milliseconds after the discharge, the number of ions  and of ehis case . At timehis case , +2(B 3Πg):M is any third body in the three body collision process. The electronically excited molecule of nitrogen, hv:This is followed by radiation of a photon, 3Πg molecular nitrogen is limited to this amount. As a consequence, the twelfth vibrational level of the B 3Πg state is the highest one that can be populated. The emission from reaction 2 with a non-Boltzmann distribution of vibrational populations, characterized in the visible region by a maximum near v′= 10.Because the energy available in reaction 2 which gives the “straw-yellow” color of 2(B 3Πg) state is changed [v′=8. This corresponds to a shift toward the red end of the spectrum and is observed visually as a change in color of the afterglow. The change in color can be seen by comparing It is this particular emission of the First Positive system of N changed  moving tor of 2B Πg state β bands of NO, B 2Π:hv represents radiation which is brightest in the blue region of the spectrum [2 and oxygen atoms:Impurities in the discharge cause large changes in concentrations and in the kinds of species. Additions of small amounts of oxygen cause large changes in the colors of the afterglow because of reactions with oxygen atoms formed in the discharge. Oxygen atoms can react with nitrogen producing NO molecules in the upper state of the N( 4S)+O P+M→NOβ bands o2 in an electronically excited state [When sufficient oxygen is added all the nitrogen atoms are consumed in the reactions. In the absence of nitrogen atoms, NO reacts with oxygen atoms to give NOed state  and thisReaction 2 [Bright “atomic flames” often can be seen when gases or vapors are added to the atoms produced in the discharge. For example the glow produced by reaction 2 , 9.To observe steady-state afterglows at very short times after a discharge, it has been necessary to use supersonic flows from a discharge in a de Laval nozzle , 19. At Some dissociation of molecules also occurs since emission of atomic oxygen has been observed . In figu


In rural regions with a low population density, distances to health care providers as well as insufficient public transport may be barriers for the accessibility of health care. In this analysis it was examined whether the accessibility of gynecologists and GPs, measured as travel time both by car and public transport has an influence on the utilization of health care in the rural region of Western Pomerania in Northern Germany.Utilization data was obtained from the population based Study of Health in Pomerania (SHIP). Utilization was operationalized by the parameter “at least one physician visit during the last 12 months”. To determine travel times by car and by public transport, network analyses were conducted in a Geographic Information System (GIS). Multivariate logistic regression models were calculated to identify determinants for the utilization of gynecologists and GPs.p < 0.0001), social class  and having persons ≥18 years in the household .There is no significant association between the accessibility by car or public transport and the utilization of gynecologists and GPs. Significant predictors for the utilization of gynecologists in the regression model including public transport are age (OR 0.960, 95% CI 0.950–0.971, In the examined region less utilization of gynecologists is not explainable with long travel times by car or public transport. In sparsely populated rural regions the spatial distribution of healthcare providers cannot be as dense as in urban regions. A possible consequence could be that the spatial accessibility of medical facilities  affects The objective of this analysis was to examine whether accessibility by car and by public transport is associated with the utilization of outpatient general practitioners (GP) and gynecologists as an example for specialized physicians.In contrast to many other countries, the people in Germany have the free choice of doctor, which is incorporated into law. People with a disease or a health problem normally have to consult an outpatient general practitioner (GP) at first. For particular medical problems, the patients will be referred to an outpatient medical specialist. In some cases, e.g. for routine examinations at gynecologists, patients can also go directly to medical specialists without referral .2  and is  of the population-based epidemiologic cohort study “Study of Health in Pomerania” (SHIP) which was performed in 2002–2006 . SHIP-1 Persons in the household ≥18 years were included, because they are old enough to have a driver’s license and might be able to give a ride. A comprehensive description of the SHIP study is published by John et al. .The spatial accessibility of the physician practices was operationalized by calculating travel times between the residential addresses of each participant and the nearest practices by car and by public transport. For both analyzes was an underlying assumption that the participants use the respective transportation mode and that they visit the closest practice.Geographical coordinates of the residential addresses of the female participants of SHIP-1 and the locations of the practices of 248 GPs and 38 gynecologists in the study region were calculated in a geographic information system (GIS) ). Additionally, 93 GPs and 9 gynecologists in a 12 km buffer zone around the study region were included. The addresses of the practices were retrieved from the physician directory of the Association of Statutory Health Insurance Physicians of the federal states of Mecklenburg-Western Pomerania and Brandenburg.The distances between the residential addresses of the participants and the physician practices and the travel times by car were calculated using routable digital street data . The calculation of distances and travel times was conducted with ArcGIS, using the software extension Network Analyst which allows network-based spatial analysis. Not considered were delays due to traffic jams, construction sites etc.The accessibility of the practices by public transport was calculated on the basis of timetables of busses and trains. Geographic coordinates of bus and train stops were partly delivered by public transportation services and partly assessed in the field by project staff with GPS devices.The appointment with the physician was set at Tuesday, 11 am during school times, because public transport is closely aligned with the transport of school children.The pedestrian speed was set at 4 km/h for SHIP-1 participants under 51 years, 3 km/h for 51–69 year old participants and 2 km/h for participants of 70 years and older.Foot walks between the patients homes and the practices and between the homes of the patients and the bus and train stops were limited to a maximum length of 1000 m.Foot walks between the bus and train stops and the practices were limited to a maximum length of 500 m.Foot walks needed to change between busses and/or trains were limited to a maximum length of 250 m.The journey back home after the physician appointment was set to start at 12 pm and had to be finished the same day before midnight.The calculation of the accessibility by public transport included some assumptions:The duration of the appointment with the physician was not included in the travel time. The calculated travel time was the total travel time of the roundtrip.The calculation of the travel times by public transport was conducted on the basis of the Dijkstra algorithm , 36 usinMultivariate logistic regression analyses were conducted to identify determinants for the utilization of gynecologists and GPs. Outcome of the regression analyses was the utilization of the gynecologist and/or GP in the 12 months prior to the assessment . Age was included as a metric variable. The travel times were included as a metric variable in minutes by car and as a categorical variable (categories: ≤ 60 min (min), > 60 – ≤ 120 min, > 120 - ≤ 180 min, > 180 min, no connection at all) by public transport. The social class index is calculated additively as a point sum score  and was SHIP-1 has 3300 participants in total. The flowchart in Fig. Figure Figure n = 968) of the study participants have access to a GP within one hour by public transport (Table n = 29) have no access with public transport. This means that they can’t travel to the nearest GP and travel back within one day. The distance to the practices does not play a major role. Accessibility rather depends on the connection to the public transport network.The accessibility with regard to public transport differs from the accessibility by car Fig. . 83% of rt Table  whereas Compared to GPs the number of the gynecologists as an example for specialized physicians is much lower and their spatial distribution in the region is less dense. This is reflected in longer travel times by car Fig. . AlthougThe accessibility by public transport is also poorer for gynecologists than for GPs Fig. . Just 62Regarding GPs, only the social class index was found to be a significant determinant for utilization in both models. If the social class index increases by 1, the probability of utilization decreases by a factor of 0.947 . In contrast, the odds for utilization increase with increasing social class index. The odds for utilizing a gynecologist are two times higher if the participants live together with persons ≥18 years in their household  would be influenced by the travel times to the practices. A trend can be seen, but the results show, that neither travel time by car nor by public transport is significantly associated with whether or not a gynecologist or GP was consulted in the previous 12 months. Some international studies found associations between spatial accessibility and utilization or issues like regular check-up visits. Jones et al.  and CelaTwo other studies came to the result that distance to the closest mammography facility is indeed a significant risk factor for predicting advanced stage diagnosis in breast cancer , 39. A NThe actual use of public transport in our study region is rather low. A survey of the National Association of Statutory Health Insurance Physicians in 2010 found that only 8% of the respondents used public transport . A recenAs a consequence of the low population density, the public transport network is often poor in rural regions, which in turn tends to reduce the number of users . Where pLiving with other adults in the household was not a significant predictor for the utilization of GPs. It may generally be easier to reach the GP because of the larger geographic density of GP-practices.The results show that the utilization of gynecologists significantly increased with higher social class index. Higher education might foster higher health competence and awareness for health checks . Data frThe determination of accessibility is based on a range of assumptions. For example it was assumed that patients get treatment from the geographically nearest physician’s practice. However, accessibility is not always the determining criterion for the choice of one’s doctor. Also qualifications, reputation and recommendation  of an inThe accessibility by car can be calculated under relatively realistic conditions whereas regarding the public transport accessibility requires several assumptions. These include the length of the foot paths and walking speed. These definitions, in part, determine which patients are assigned to the category “no connection”. The doctor’s appointment  was chosen for all calculations because this was the time with best public transport connections. Both at other times and during the holidays connectivity was found to be lower.Unfortunately, we did not know whether the participants use the car or public transport. A cross-sectional survey assessed the mobility behavior of people aged 60 years and older in the same region as our study region. 33.7% of the participants reported, that they usually walk, use a bike, the bus, or other modes of transportation for the study region .Major sickness as a covariate is not considered in the model. Chronic diseases would be a good parameter for the model. The SHIP data does not provide an appropriate variable. Besides all women are recommended to regularly visit a gynecologist for preventative check-ups. Therefore, we accepted this limitation.p-values of 0.8273 (car) and 0.9188 (public transport). The p-value of the gynecologists-model are slightly below and above alpha 0.05 with 0.0333 (car) and 0.0547 (public transport). The goodness of fit is marginally to low here.The model fit was evaluated with goodness of fit statistic (Hosmer-Lemeshow test). It was computed for all four models. Both models regarding GPs are acceptable models for the data with Strengths of the study are (a) the data from the population based prospective SHIP-cohort and (b) the consideration of real road-networks (including specific speed categories for motorized vehicles as well as for pedestrians), real bus and train schedules and changes of means of transport. Thus, the model uses valid data and depicts the reality in the study region as close as possible.This study showed that accessibility is not significantly associated with the utilization of GPs and gynecologists in a rural region of Western Pomerania. Significant predictors for the utilization of gynecologists are social class index and persons ≥18 years living in the household of the participants.


A frequency ratio of about 1.06 often appears in cochlear mechanics, and the question naturally arises, why? The ratio is close to that of the semitone (1.059) in music, giving reason to think that this aspect of musical perception might have a cochlear basis. Here, data on synchronised spontaneous otoacoustic emissions is presented, and a clustering of ratios between 1.05 and 1.07 is found with a peak at 1.063 ± 0.005. These findings reinforce what has been found from previous sources, which are reviewed and placed alongside the present work. The review establishes that a peak in the vicinity of 1.06 has often been found in human cochlear data. Several possible cochlear models for explaining the findings are described. Irrespective of which model is selected, the fact remains that the cochlea itself appears to be the origin of a ratio remarkably close to an equal-tempered musical semitone, and this close coincidence leads to the suggestion that the inner ear may play a role in constructing a natural theory of music. The outlook for such an enterprise is surveyed. Our understanding of the cochlea has increased enormously since 1978 when Kemp reported that the inner ear produced faint echoes in response to a click . These oQ values of about 10–30 over the range 1–10 kHz  or 1.059 on the equal tempered scale or 16/15 (1.067) in Just intonation. The presence in the cochlea of an approximation to a semitone has been pointed out by several researchers  in terms of cents, where 100 cents = 1 equal-tempered semitone . When thFinally, an analysis by A method for extracting “resonant modes” from OAEs evoked by tone-bursts or clicks was developed by Blinowska and colleagues, a technique which uses a matching pursuit algorithm to identify waveforms of specific frequency in the cochlear responses . A remarThe The present study focuses on analysing the ratios between synchronized SOAEs (SSOAEs) in a group of 81 normally hearing young adults. In addition, some SOAE data from an earlier study is presented in which two bistable emissions alternately switched frequencies, and this unusual and rather rare phenomenon shows that the ratio between the two switching frequencies is 1.0600 ± 0.0005. Some relevant cochlear modelling efforts are then described which attempt to place the findings within the framework of a single physical model, and an assessment is then made of whether the findings could support the proposal that the human musical sense originates within the cochlea itself.Measurements were performed on a group of normally hearing adults: 81 subjects  of age 19–31 years. All subjects underwent pure tone audiometry, tympanometry, and OAE measurement. All had pure tone thresholds better than 25 dB HL over 0.5–8 kHz, type A tympanograms, and no known history of otologic disease. The subjects gave written informed consent prior to participation. Research procedures were approved by the Ethics Committee of the Institute of Physiology and Pathology of Hearing, Poland (IFPS/KB/06/2012).−1) to synchronize SOAEs. SSOAEs were studied over the range 0.5–6 kHz, with frequencies accurate to ±6 Hz. The detection method is described below, and an example of SSOAEs acquired by the technique is shown in OAEs were measured under low-noise ambient conditions using an ILO 292 system . Standard equipment protocols were used for measurement of synchronized SOAEs (SSOAEs). Click stimuli of amplitude 80 ± 3 dB peak SPL were used to evoke a total of 260 responses in each ear. SSOAEs were acquired under the linear protocol using a train of clicks  package.The results of analysing the ratios between all neighbouring SSOAEs from 140 ears of 81 adults are shown in Some SOAEs are bistable, meaning that energy swaps back and forth between two of them. Each state has its own distinct frequency, and the energy alternates every second or so. Bistable emissions are sometimes encountered in OAE studies, although they are rare, and data from a previous study  showing Evidence has been assembled which indicates that frequency ratios of 1.05–1.07, with a peak very close to a semitone (1.060 ± 0.005), are widespread in the cochlear mechanics literature, a finding that has implications for the theoretical underpinnings of music. One possibility, discussed more fully later, is that the observed semitone could be identified as part of a natural theory of music in which the cochlea is able to instantly detect musical ratios. Although such an idea is speculative, there are findings in the literature which give it support. The significance here is a reduction in the amount of neural processing required to recognise a musical ratio, relieving the nervous system, and the brain in general, of perceptual effort .The proposal for an innate cochlear semitone aligns with evidence that music has both harmonic and melodic dimensions . More spFirst, however, a brief survey of the approaches so far used in trying to understand the 1.06 ratio is made. All these perspectives involve different models of cochlear mechanics, and in general the musical implications of the ratio have not been emphasised. The models can be classified under four main headings.The most common explanation for the 1.06 ratio comes from the multiple internal reflection (MIR) model of Shera and Zweig which explains the ratio in terms of reflection of the traveling wave between the stapes and its peak . This meThe MIR model has been developed into what Shera calls “the spiral staircase”  where thMost models of the cochlea emphasise the role of slow traveling waves in the cochlea rather than the direct effect of fast acoustic pressure waves . HoweverOf particular interest is the previous work with “dual resonance” models which combine two processing stages separated by a frequency ratio of 1.06. Based on an original idea of The cochlea has most commonly been modelled as an electrical transmission line which supports a traveling wave of displacement that bends the stereocilia of hair cells . An alteThese frequency plateaus were modelled in detail by A model of the cochlea developed by The preceding survey has indicated that there may be at least two or three physical mechanisms for the origin of the 1.06 ratio in the cochlea, and more work is needed in order to decide which model gives the best account. Some recent work has highlighted the role of elastic coupling along the basilar membrane , and theFollowing through with this coupled oscillator model, the 1.06 interval between OAEs appears because a particular site on the basilar membrane has excess cochlear amplifier gain, generating a spontaneous emission at a corresponding frequency. This local oscillation will in turn entrain neighbouring oscillators, meaning that all the oscillators within a step will oscillate together. The dynamics of a coupled chain mean that if the driving oscillator is near the edge of a step, it may sometimes jump to, and entrain, oscillators on a neighbouring step, and a switching of frequencies will take place . EvidentThe coupled oscillator picture suggests that the cochlea possesses innately tuned resonant elements, and that when these structures are stimulated acoustically, the resulting echoes contain a clear signature of a musically tuned array. The tuned elements might be part of a semitone-detection process related to the automatic pitch-detection mechanism described by Demany and colleagues. Such a mechanism would permit melodies to be identified with minimal neural processing.Whatever the full scenario may be, the evidence suggests there is at least one aspect of the cochlea itself—and not the brain —which is innately musical. This is an intriguing possibility, and is an aspect which calls for closer investigation. The following sections outline evidence from the literature which supports the proposal that the cochlear semitone might be a building block within a natural theory of music.Music is both beautiful and enigmatic. We know many details of how it works, but the bigger picture of why humans experience music in its unique shape and form—most distinctively, why there should be 12 semitones to the octave as found in many cultures—has been elusive . At thisFundamentally, there are two separate but linked aspects of music—harmony and melody First, consider the curious case of the enlarged octave. The octave is the mainstay of music, and in Pythagorean terms it is a frequency ratio of precisely 2:1. Remarkably, however, this is not always experimentally true. Whereas in harmonic terms it is always possible to tune an instrument exactly to an octave by reducing the rate of beating of a fundamental and its first partial to zero, in melodic terms things are slightly different. Psychophysical experiments involving the fine-scale setting of two tones reveal that the octave is almost invariably ‘stretched’ . In geneThe stretched octave, and the implications for how frequency ratios are discriminated, was investigated in detail by Turning to a second strand of evidence, there is one notable study which appears to directly reveal the two-fold nature of musical perception and its fundamental landmarks. The detailed psychophysical investigations of There is the semitone (15:16) at one end of the curve, and the octave (1:2) at the other, but inbetween are two other extrema: an upward peak due to the fifth (2:3) and a downward peak due to the major third (4:5) and perfect fourth (3:4). The authors are puzzled by these extrema and suggest the peaks might be due to a special affinity—a musical sense—surrounding these intervals. In other words, there seems to be an innate ability to pick out certain musical intervals from among a continuum of intervals. One of those is the octave, but the presence of the other peaks suggests the auditory system is able to detect other musical ratios in a similar way, and one of those anchor points includes the semitone.It would be interesting to repeat this important work, confirm its findings, and fill in some of the missing gaps. The intervals plotted in As a matter of detail, a closed loop corresponds in Levelt’s paper to fitting an ellipse rather than a parabola to the data, which is actually noted in their paper as giving a better fit. On theoretical grounds, Levelt and colleagues could not see how an extremely narrow interval  could end up matching a wide one , but it seems that recognising an equivalence between the unison and the octave would provide a way of completing the cycle. Again, more data would clarify the picture and provide more accurate measures of the locations of the peaks—particularly the semitone, which is predicted to be enlarged .The endpoint of this discussion is to bring together an identified objective property of the cochlea—a peak in OAE frequency spacing of 1.060 ± 0.005—and a subjective preference for one of four musical landmarks in the auditory system (notably the semitone). One possibility is that the semitone is built in to the cochlea as an automatic mechanism to identify melodies. More specifically, the semitone could arise through a step-like organisation of the cochlea’s tuned elements . The steThe advantage of the latter model is that it allows melodic intervals to be automatically detected. The coupled oscillator model implies there might be an array of tuned elements in the cochlea, quantised not unlike the way the strings of a harp or piano are. If so, this might give credence to the original Helmholtz model of the cochlea as an array of tuned strings which resonate sympathetically to incoming sound. In fact, there seems to be direct evidence of a regular array of tuned elements—a “keyboard”—in the cochlea. As mentioned earlier, the otoacoustic emissions of subject BD, described by Against this picture, it is worth noting that Helmholtz himself regarded music—scales, keys, diatonic tunings, and the like—as “the work of artistic invention” and were “by no means furnished by the natural formation or natural function of our ear”  cultures in some form or another—appears simply by chance, and it may be that the ratio reflects an innate musical property of the cochlea, a possibility that carries major implications for music theory.In terms of cochlear mechanics, the most common explanation for the 1.06 ratio is in terms of multiple internal reflection, a ‘global’ model with an extensive array of support, both experimental and mathematical. One of its predictions is the generation of a cochlear staircase in which the frequency ratio between the steps is about a semitone. However, the coupled oscillator model also predicts a similar staircase, and more work is needed to decide between them. The advantage of the coupled oscillator model is that it is self-adjusting, so that the steps are formed automatically because of the way oscillators entrain to incoming sound and to each other. This means that each note of a melody could entrain a group of oscillators—a portion of the basilar membrane—to dynamically create a plateau. That is, unlike the multiple internal reflection model, there is no pre-existing staircase (or keyboard); the quantisation only happens in response to an impressed tone. Relative pitch could therefore be extracted by counting steps between neighbouring intervals. Such a mechanism would accord with the “automatic frequency shift detector” described by The coupled oscillator model provides an account of how the semitone could emerge naturally within the cochlea. However, placing this model within the context of music theory will require more development than what has been sketched here ; middle column is signal (dB SPL); right column is noise floor (dB SPL).Click here for additional data file.10.7717/peerj.4192/supp-2Supplemental Information 1Click here for additional data file.10.7717/peerj.4192/supp-3Data S2Left column is time (seconds); middle column is amplitude (linear units) of SOAE at 2,165.5 Hz; right column is amplitude of SOAE at 2,295.5 Hz.Click here for additional data file.


These types of operators enable better error estimation on the interval The purpose of this paper is to introduce a modification of  In 1912, Bernstein ).Let Letbe the operators defined by  a a4.2) anFor the operators fined by  we may wwhere by  we haveNow we claim that the error estimation in Theorem Now we give the rate of convergence of the operators fined in  in termsLet Letbe the operators defined in . From , clearlyTherefore,By taking infimum over all by using , we gett}Q>0 in , we use q-Dunkl analogue of Szász operators. Here we have defined a Dunkl generalization of these modified operators. This type of modification enables better error estimation on the interval The purpose of this paper is to provide a better error estimation of convergence by modification of the perators . We obta


Intuitively, it seems as if we should be able to point accurately to the location of a target object within a room even if we were teleported to a different location and the object removed from view. We measured the precision of pointing to a previously-seen object in a real room, a virtual room with same dimensions  and a sparse virtual scene consisting only of long thin poles at the same locations as the target object and room corners. Participants viewed the target object from one location, walked to another so that the object passed out of view, then turned in complete darkness to point at the location of the previously-viewed target. In a separate experiment, participants viewed a sparse scene consisting of long thin poles (including a target) and had to point to the location of the absent target after teleportation to a new location within the scene. Pointing precision in this case was dramatically reduced (σ ≈ 34°) compared to the conditions in which participants walked in the real room, virtual room or sparse scene. In the latter three conditions, pointing precision was very similar (σ ≈ 15°) despite the removal of prominent distance cues in the sparse condition. Our results show that spatial updating after teleportation is substantially poorer than when walking between two locations. [Supported by Microsoft Research and Wellcome Trust]


We focus on the measurement of both the gate and drain currents and, from the comparative analysis of the current transients, we conclude that there are at least two independent trapping mechanisms: trapping of charges in the silicon oxide substrate, occurring with time constants of the order of tens of seconds and involving charge motion orthogonal to the MoS2 sheet, and trapping at the channel surface, which occurs with much longer time constants, in particular when the device is in a vacuum. We observe that the presence of such slow phenomena makes it very difficult to perform reliable low-frequency noise measurements, requiring a stable and repeatable steady-state bias point condition, and may explain the sometimes contradictory results that can be found in the literature about the dependence of the flicker noise power spectral density on gate bias.We present an experimental investigation of slow transients in the gate and drain currents of MoS This characteristic is of pivotal importance for the fabrication of devices for digital applications, since a large Ino/Iffo ratio can be achieved. On the other hand, MoS2 has a mobility much smaller than graphene, up to only a few hundreds of cm2 V−1 s−1 1. The actual mobility is strongly dependent also on the surface condition of the material, i.e. on the presence of adsorbates, in particular water or oxygen. Late et al.2 have investigated the effect of adsorbates on the hysteretic behavior and on current transients in MoS2 transistors, focusing, in particular, on the former. Further studies of the hysteretic behavior of MoS2 transistors have been performed by Li et al.3, Illarionov et al.4, and Guo et al.5.In the last few years 2-D materials based on metal dichalcogenides have attracted increasing interest, because they share many of the properties of graphene, while having a nonzero bandgap, which makes them more suitable candidates for the implementation of switching devices for digital electronics. Among such 2-D materials, Molybdenum disulfide . In particular, in the presence of a strong hysteresis, it is extremely difficult to obtain repeatable gate bias conditions, since, for the same gate voltage different values of the drain current will be present, depending on the previous bias history.While trying to perform repeatable 2 devices is attributed to their photosensitivity combined with the fluctuations of ambient illumination. Although this may be a concurring cause of the variance, it is not the only one, because we have observed fluctuations also with the devices kept in a completely dark environment, as in most of our measurements.In ref. f noise on the bias condition in MoS2 transistors has been investigated by several authors with results that appear to be often in contrast with one another.Such difficulties and slow transients may contribute to explain why the dependence of 1/et al.6 argue that the normalized flicker noise  scales with the inverse of the gate overdrive, thereby supporting an interpretation in terms of a bulk Hooge formula approach and a mobility fluctuation model, if one assumes that the carrier density in the channel is proportional to the gate overdrive.In particular, Sangwan et al. a somewhat opposite conclusion is reached, attributing flicker noise observed in single-layer MoS2 devices to the McWhorter model, i.e. mainly to carrier number fluctuation with some contribution from mobility modulation still due to trap occupancy fluctuation.In ref. et al.8 argue for a mobility fluctuation, Hooge-like model, but reintroduce the carrier fluctuation model to explain the behavior in the presence of Al203 passivation.For multilayer devices, Na et al. in ref. Renteria et al.10 present an analysis similar to that in ref. Kwon et al.11 argue that number fluctuation is the main source of flicker noise in bilayer MoS2 devices, with a gate voltage dependence of the normalized noise power spectral density that is however quite different from what reported by other authors.Xie et al. attribute the 1/f noise seen in few-layer MoS2 devices to a Hooge-like mechanism on the basis of the fact that they observe a proportionality of the noise power spectral density to the square of the drain current.Finally, in ref. 2 transistors and on their interpretation in terms of charge trapping phenomena. The presence of extremely long time constants and of variable amounts of trapped charge makes a reliable estimation of the bias point dependence of low-frequency noise power spectral density quite challenging.In the following we focus on the measurement of current transients in MoS1. Atomically thin MoS2 flakes were obtained with scotch-tape exfoliation of bulk MoS2 crystals (SPI supplies) on degenerately doped Si substrates covered with 270 nm thick thermally grown SiO2 for optimized contrast detection13. Contacts were fabricated with electron beam lithography using PMMA as resist, followed by evaporation of 90 nm of Au and liftoff in acetone. Furthermore, selected devices were annealed in Ar atmosphere at 200 °C to improve the contact resistance  of the output of a 6 V lead battery included in the same shielding as the amplifier. The capacitor C has the purpose of filtering the noise that could be introduced by the partition. The voltage measured at Vout is proportional to the current flowing out of the DUT source electrode through the value of the feedback resistor. In series with the gate we have a picoammeter (HP 4140B), to measure the gate current, and the voltage bias source.We first report on the results of a measurement performed by applying a gate voltage of 20 V, with the circuit of Fig. iin of the transresistive amplifier over a wide range, and then, from a linear fit of the measured vout vs. iin, we get a good estimate of the actual value of the feedback resistor (from the slope) and of the voltage offset at the output of the amplifier (from the intercept at zero input current). An example of a calibration measurement, together with the associated linear fitting, is reported in Fig. For a precise evaluation of the current in the device channel, we perform a calibration step, with the circuit arrangement shown in Fig. −4 Torr) or in the air.In Fig. 2 sheet into the dielectric substrate and then trapped in the proximity of the MoS2-silicon oxide interface.It is immediately apparent that in a vacuum the gate current and the channel current have a rather different behavior, while in air they are much more similar. Thus in a vacuum there seems to be a further trapping mechanism active, which affects only the channel current. Let us first discuss the gate current behavior: as already stated, it is independent of the presence of air, with a quick exponential decay, which can be associated with trapping of charge carriers in traps with relatively short characteristic times. The gate current is made up of two components: the conduction current and the displacement current. The conduction current is the result of leakage from the gate to the channel through the oxide and is substantially dependent only on the applied gate bias, while the displacement current is the consequence of charge moving orthogonally to the oxide layer, without actually reaching the gate. We interpret the asymptotic value of the gate current as the leakage (conduction) component, while the much larger time-dependent component can be seen as the displacement current resulting from the motion of electrons being injected from the MoS2 sheet to a position slightly closer to the gate. In particular, from the Ramo-Shockley theorem15 or from simple electrostatic considerations, we know that, for each charge qc moving from the channel to a trap at a depth Δz in the SiO2, a charge variation ΔQ = (Δz/tox)(−qc) is induced in the gate.This phenomenon had been excluded in ref. The interpretation of the current transient measured on the gate as associated with charge trapping in the oxide is further strengthened by the results obtained for the gate current transient occurring when the gate bias voltage is switched off, which exhibits an exponential behavior analogous to that of the switch-on transient. This excludes the hypothesis that the switch-on exponential could be due, for example, to a leakage current to the channel following, for some undetermined reason, the same behavior of the channel current.−10 C , thus a fraction of the total oxide thickness. This means that, considering a charge penetration of about 3 nm vs. an oxide thickness of 270 nm, the trapped charge will be almost two orders of magnitude larger than the charge measured in the gate transients, i.e. about 15 × 10−9 C. This corresponds to about 1010 electrons and, considering that the metal leads contact a number of MoS2 flakes, each with an average area of the order of a hundred square microns, the resulting density of trapped charge is around 1015 cm−2, which is consistent with the upper values reported in the literature20. It can actually be lower if we consider that charge trapping will occur also underneath the metal lines  = a exp(−t/τ1) + b exp(−t/τ2) + c for the gate current and id(t) = d exp(−t/τ1) + e exp(−t/τ2) + f exp(−t/τ3) + g for the drain current. The choice of two exponentials for the gate current and of three for the drain current has been made on the basis both of the physical argument that the trapping phenomenon leading to the gate displacement current must impose on the drain current an analogous transient (since the trapped charge will screen the electric field from the gate), with the same time constants and of the observation that this choice leads to the best fit. Accordingly, the fit has been performed for the gate current first, and then the one for the drain current has been completed assuming the two lowest time constant equal to those of the gate current. Overall, the choice of the number of time constants for the fit unavoidably involves some arbitrariness, also because of the critical nature of this type of fittings21.Let us now discuss the features of the drain (channel) current, reported in the right panel of Fig. τ1 = 1.8 s and τ2 = 27 s . For the drain current in air we get τ3 = 994 s , while in a vacuum we have τ3 = 1365 s . We propose an interpretation of these results in terms of the previously mentioned capture of charge carriers in oxide traps with the shorter time constants (those appearing only in the gate current), which lead to the transient in the gate current and to a modulation of the channel current with the same time constants.For the gate transients we obtain, independent of the presence of air: In air the drain current exhibits only the small additional effect of another time constant (994 s), with a relatively small amplitude for the associated exponential. In vacuum, instead, the asymptotic value of the drain current is much larger and the long time constant (1365 s) has an amplitude greater by an order of magnitude than in air, which makes quite unpractical any measurement requiring a steady-state condition to be maintained over a long period of time, as in the case of low-frequency noise measurements.2 sheet and therefore do not involve a vertical motion of charge  which would register in the gate current. The presence of air is modifying these surface traps . We point out that both trapping phenomena, the one associated with the short time constants and that associated with the long time constants, lead to a decrease of the channel current. For the shorter time constants trapping of negative charge in the oxide below the MoS2 channel is consistent with a decrease of the channel current ; for the longer time constants trapping at the surface of the MoS2 of charge that is already present in the channel (and therefore without contribution to the gate current) corresponds to a decrease of the mobile charge, and therefore of the current. The two proposed trapping processes are illustrated in Fig. A possible explanation of such a behavior can be constructed assuming that the longer time constant is associated with traps that are at the exposed surface of the MoSVG = 30 V and drain bias VD = 250 mV. These measurements were performed in a vacuum. For the gate current we have τ1 = 6.2 s and τ2 = 66 s , while for the drain current we get τ3 = 4354 s  We notice that the overall behavior is analogous to that for the previous bias point, although the time constants are somewhat different. Such a difference can be, at least in part, due to the sensitivity of the exponential fitting procedure to small variations. Although it is thus difficult to provide exact numbers, we can conclude that the trapping phenomena associated with the gate current have time constants of the order of seconds or tens of seconds, while in vacuum the drain current exhibits also the effect of a time constant of the order of thousands of seconds.In Fig. In Fig. In Fig. 2 field effect transistors has allowed us to assess the presence of two distinct phenomena, dominated by different time constant: a faster process in which charges move from the channel to traps in the oxide, along a path that determines the appearance of displacement current in the gate, as well as an associated modulation in the drain current; a slower process in which carriers are captured by traps at the surface of the channel, without a significant shift in the direction of the gate (and therefore without an influence on the gate current). This latter process has a much stronger effect in a vacuum, arguably because the associated time constant is reduced by the presence of air or because adsorbates from air act as much more efficient and faster traps. The resulting long transients make it very difficult to perform reliable noise measurements at low frequencies, and may explain the strong variability of results observed in experimental studies of 1/f noise in MoS2 devices.The comparative analysis of the time-dependent behavior of the gate and channel currents in MoS


Pointing is one of the communicative actions that infants acquire during their first year of life. Based on a hypothesis that early pointing is triggered by emergent reaching behavior toward objects placed at out‐of‐reach distances, we proposed a neural network model that acquires reaching without explicit representation of ‘targets’. The proposed model controls a two‐joint arm in a horizontal plane, and it learns a loop of internal forward and inverse transformations; the former predicts the visual feedback of hand position and the latter generates motor commands from the visual input through random generation of the motor commands. In the proposed model, the motor output and visual input were represented by broadly tuned neural units. Even though explicit ‘targets’ were not presented during learning, the simulation successfully generated reaching toward visually presented objects at within‐reach and out‐of‐reach distances. We hypothesized that infants’ primitive pointing behavior emerges as a nonsocial‐orienting action based on the acquisition of visuo‐motor transformations through motor babbling.We proposed a neural network model for the development of reaching and pointing based on visuo‐motor transformations.Through random arm movements (motor babbling), the network learned both forward (motor‐to‐vision) and inverse (vision‐to‐motor) transformations without the presentation of an explicit target for reaching.Although the network only learned these transformations for random movements, the loop of the forward and inverse transformations can generate successive motor commands to reach toward visually presented objects, thereby implementing a feedback control with an internal estimation.The model can also simulate the infants’ primitive pointing behavior toward objects that are placed out of reach.11.1Human beings have the ability to communicate with each other using both language and symbolic gestures. As for symbolic gestures, even 1‐year‐old infants use reaching and pointing in different social contexts. Reaching is used in the imperative‐instrumental context, and pointing is used in the declarative‐referential context : forward and inverse transformations . Infants at 2–3 months of age persistently look at their hands (hand regard) . Specifically, the forward transformation learns the resultant visual hand position of a random motor command with a simple recurrent network where an internal representation of the visual hand position before the movement is held in the hidden layer, and the inverse transformation learns the required motor command that brings the hand from the visual position before the movement to the position after the movement.The visual hand position before the movement is internally represented in the hidden layer of the forward network, and the visual hand position after the movement is visually input. A feedback control using an internal estimation autonomously develops by connecting the hidden layer of the forward transformation to the hidden layer of the inverse transformation.The motor command and the visual feedback are ambiguous. Specifically, these signals are represented in a ‘place code’  without communicative intentions. The behavior is a mere ‘reaction’ that is not driven by internal intention. The network does not learn how to reach toward a ‘target’; it only acquires the forward and inverse transformations through motor babbling. In the simulation of reaching and pointing, the input of the representation of a visual position instead of a hand position drives the hand toward that location, without the need for any mechanism of intention. To avoid the model's confusion by simultaneous visual representation of the hand and the target, the hand vision was removed during reaching and pointing.2A neural network model is proposed that learns the forward and inverse transformations for two‐joint arm movements. The network learns the relationship of the motor command and the hand position through the visual observation of the hand moving randomly Figure . Both thThe arm is modeled by two‐joint limbs moving in the horizontal plane driven by motor commands in the form of relative displacements of the joint angles Figure . The motThe network consists of two streams of three‐layered artificial neural units. Each stream learns the forward and inverse transformations Figure . The forThe inverse network learns the transformation from a visual hand position to a motor command, which is required to achieve the hand position. The information about the hand position before the movement is provided by the connection from the hidden layer of the forward network. Although this scheme provides a ‘retrospective’ mapping of visual and motor information, it only reframed the sensorimotor mapping that was implemented in the DIRECT model  or hand positions (vision) and broad selectivity. In Figure The motor command generated by motor babbling is input to the forward transformation network, and the signal propagates through hidden units to form an output visual representation. Units in the input, hidden, and output layers exhibit activation that ranges from 0 to 1. The activation of the units in the input layer represents the population‐coded motor command described above. The units in the hidden layer have connections with all motor input units, the hidden units (recurrent connection), and the visual input units (for the hand position before the movement). The hidden units receive the signal from all the connected units. The units in the output layer have connections with all the hidden units. The activation of the pre‐synaptic units is multiplied by the connection weights and is input to the post‐synaptic units. The post‐synaptic units conduct a summation of the weighted activation of the pre‐synaptic units and scale the summed signal to the activation ranging from 0 to 1 in a monotonic but nonlinear manner (with a logistic function). The visual representation of the output layer is compared to the visual representation of the actual hand position, which is a consequence of the motor command, and the error signal is used to update the network connection weights following the backpropagation algorithm  =   =  and  in Cartesian space), and the motor commands for the test were 25 combinations of Δα = {−20, −10, 0, 10, 20} and Δβ = {−20, −10, 0, 10, 20}. Figure y*(t + 1) in the output layer. The outputs of the forward transformation approximately predicted the visual input of the hand position after movement, although the prediction was comparatively inaccurate for the motor commands at the limits of the range.The forward transformation test examined the predicted visual feedback when several different motor commands were input into a network whose state was set to represent the initial hand position. The initial hand position was set to  = , and the target positions were the hand positions that would be achieved when the motor commands of 25 combinations of Δα = {−20, −10, 0, 10, 20} and Δβ = {−20, −10, 0, 10, 20} were output. Figure u*(t) of the inverse transformations. The outputs of the inverse transformation moved the hand approximately toward the target positions, although the motor commands were comparatively small for the target at the limits of the motor command range.The inverse transformation test examined the motor commands generated when the inputs into the settled network representations of visual positions were different from the actual hand position. During learning, the visual input was always derived from the hand position of the self after the movement. The movement had no target. Here, if the input was a new visual representation that was not the position of the hand, the inverse transformation network would interpret this position as the hand position after the movement. It would then generate a motor command to achieve the movement toward the new visual input, that is, a ‘target’ position. The initial hand position in this test was 3.3For the inverse transformation test, we placed the targets at distances that were reachable in a single step of the transformation. However, as mentioned in the introduction, infants can reach for a target using a loop of forward and inverse transformations, even in the dark. To simulate this ability, we input the output of the inverse transformation to the forward transformation Figure . Note th = . The transformations were iterated four times. Because the distance traveled by a single motor command issued during learning is much smaller than the movement distance required in the reaching simulation, the network needs to generate successive motor commands to achieve reaching. The loop of the forward and inverse transformations successfully generated the sequence of motor commands toward the target, especially for a target in the ipsilateral space.Figure Even when the targets were placed at an out‐of‐reach distance, the model reached toward the targets Figure . The net44.1The present model simulated emergent reaching toward a visual position at a within‐reach distance and an out‐of‐reach distance. During the simulation of reaching, the vision of the hand was removed. This means that the vision of the hand is not necessary for executing reaching, although it is necessary during learning, which is consistent with the observation noted by Clifton et al. .The simulation results showed that reaching toward a target placed in the contralateral space  is less accurate than in the ipsilateral space. This is mainly because the shoulder joint has to be flexed to reach the targets in the contralateral space, despite a relatively extended initial posture, while the elbow joint has to be fully extended for all of the targets. The targets in the contralateral space are further from the initial configuration in the joint space. This difficulty of reaching toward contralateral targets is also seen in infant behavior. Infants first contact objects in front of the ipsilateral shoulder, and between 9 and 17 weeks, there is an increase in reaching that crosses the midline  from the environment, and infants would discover the relationship between the action and the consequent feedback. If this resultant feedback of the action is a motivational force of infant behavior, the presented object that would lead infants to expect the (tactile) feedback when they reach toward it can be a ‘target’ of intentional reaching behavior. In the case of pointing, observers  of the action will provide social feedback to the infant in some manner. This reaction of the other person will then become the aim of the action, and communicative intention will ultimately emerge. Although the manner in which the reaction is sublimated to the intention remains unclear, the proposed model explains the emergence of actions that trigger reactions.The current model did not include extension movement of the index finger, which plays a key role in typical pointing. While merely reaching toward an out‐of‐reach target without finger extension can be criticized as not being a referent behavior that indicates communication, Blake, O'Rourke, and Borzellino  have rep4.2The proposed model acquired sensorimotor transformations between a visual position in body‐centered polar coordinates and a motor command in joint space. Pointing behavior performed in the dark in adults incurs a pointing error that varies systematically with the azimuth of the target  space with one arm, seven degrees of freedom (DoFs) of the shoulder, elbow, and wrist joints were used. The control problem always involves redundant DoFs. While computationally solving the problem of DoFs is an important issue, how humans  solve this is still being debated. Therefore, for simplicity, our proposed model only moves in two‐dimensional (2D) space with two control parameters. In the future, we would like to solve this problem by introducing mechanical (Hirashima & Oya, During the model's learning, the only visual input was the infant's hand. However, in real life, objects or others’ hands may also appear in infants’ visual fields. Infants may develop the agency of the visual movement through sensorimotor contingency (Pitti, Mori, Kouzuma, & Kuniyoshi, Although the proposed model explained the emergence of arm extending behavior toward targets at out‐of‐reach distances, how the social meanings of the behavior are acquired was not modeled here. We speculate that infants discover the social meanings of the behavior through two steps: First, they will acquire this meaning by touching the object at the end of the reaching movement, wherein they will determine the relationship between the movement and the resultant tactile sensation. The contingency of the movement and the sensation may reinforce the behavior, and the richness of the sensory stimulus when infants manipulate the object would motivate them to reach toward the visual input generated by the object. This would make visually guided reaching within the reaching distance more frequent. Second, if people who observe an infant trying to reach toward out‐of‐reach objects bring the objects to the infant, the resultant tactile stimulus may also motivate the behavior and help infants to use it as a communicative tool to satisfy their motivation. In order to verify these possibilities, experiments involving the longitudinal observation of infants and their caregivers would be helpful in uncovering the order of emergence of behaviors such as reaching, touching, bringing, and pointing (Carpendale & Carpendale, 5We proposed a neural network model that learns forward and inverse sensorimotor transformations and acquires reaching control using these transformations. Sensory inputs and motor commands are coded in place coding with neural units that have broad sensitivities, and the network is learned through motor babbling. Even in the absence of targets during learning, the learned network successfully simulated arm extending action toward targets located at a within‐reach and out‐of‐reach distance. The ability of the proposed model to perform arm extending toward out‐of‐reach targets might be a basis for reaching and pointing behaviors that contain communicative intentions, which leads to social behaviors.