1,720,979 research outputs found
Cardiovascular and respiratory responses to pychophysiological tasks : methodological issues for assessing autonomic regulation
No full textThe current work studies the correlation between birth-weight and autonomic cardiovascular modulation in adult life, in order to investigate the physiological mechanisms underlying the fetal origins of cardiovascular disease. However, factors other than autonomic modulation may strongly influence the estimation of cardiovascular indexes. In this thesis, two such confounding factors were investigated in detail. Firstly, the between-task and inter-individual differences in respiratory patterns, especially in tasks involving speech were found to be strongly reflected in cardiovascular indexes. Clear evidence was found that a very significant part of changes in indexes during the psychophysiological experimental protocol considered can be explained by modifications in respiration, without assuming between-tasks or inter-individual differences in autonomic activation elicited by psychological/cognitive processes. The second factor is the presence of within-task dynamics in the cardiovascular reaction to psychophysiological tasks. The common approach in psychophysiological investigations is to estimate cardiovascular indexes as average values over the whole length of the task. However, the results found show that such an approach may obscure significant within-task changes in the indexes that might carry useful psychophysiological information. Choosing shorter epochs within the tasks for estimating the indexes has also a notable impact in terms of assessing changes elicited by the tasks. Since these two factors are intrinsic in the reaction to psychophysiological tasks, they can have a profound impact on the indirect estimates of autonomic reaction through cardiovascular indexes. Controlling them during psychophysiological experiments may be difficult (if not impossible). However, their effects should be minimized, for example by avoiding tasks involving speech and choosing appropriate data epochs for the analysis.</p
Estimation of confidence limits for descriptive indexes derived from autoregressive analysis of time series: Methods and application to heart rate variability
Full text linkThe growing interest in personalized medicine requires making inferences from descriptive indexes estimated from individual recordings of physiological signals, with statistical analyses focused on individual differences between/within subjects, rather than comparing supposedly homogeneous cohorts. To this end, methods to compute confidence limits of individual estimates of descriptive indexes are needed. This study introduces numerical methods to compute such confidence limits and perform statistical comparisons between indexes derived from autoregressive (AR) modeling of individual time series. Analytical approaches are generally not viable, because the indexes are usually nonlinear functions of the AR parameters. We exploit Monte Carlo (MC) and Bootstrap (BS) methods to reproduce the sampling distribution of the AR parameters and indexes computed from them. Here, these methods are implemented for spectral and information-theoretic indexes of heart-rate variability (HRV) estimated from AR models of heart-period time series. First, the MS and BC methods are tested in a wide range of synthetic HRV time series, showing good agreement with a gold-standard approach (i.e. multiple realizations of the "true" process driving the simulation). Then, real HRV time series measured from volunteers performing cognitive tasks are considered, documenting (i) the strong variability of confidence limits' width across recordings, (ii) the diversity of individual responses to the same task, and (iii) frequent disagreement between the cohort-average response and that of many individuals. We conclude that MC and BS methods are robust in estimating confidence limits of these AR-based indexes and thus recommended for short-term HRV analysis. Moreover, the strong inter-individual differences in the response to tasks shown by AR-based indexes evidence the need of individual-by-individual assessments of HRV features. Given their generality, MC and BS methods are promising for applications in biomedical signal processing and beyond, providing a powerful new tool for assessing the confidence limits of indexes estimated from individual recordings
Low-frequency heart rate variability is related to the breath-to-breath variability in the respiratory pattern
No full textChanges in heart rate variability (HRV) at “respiratory” frequencies (0.15–0.5?Hz) may result from changes in respiration rather than autonomic control. We now investigate if the differences in HRV power in the low-frequency (LF) band (0.05–0.15?Hz, HRVLF) can also be predicted by respiration variability, quantified by the fraction of tidal volume power in the LF (VLF,n). Three experimental protocols were considered: paced breathing, mental effort tasks, and a repeated attentional task. Significant intra- and interindividual correlations were found between changes in HRVLF and VLF,n despite all subjects having a respiratory frequency above the LF band. Respiratory parameters (respiratory period, tidal volume, and VLF,n) could predict up to 79% of HRVLF differences in some cases. This suggests that respiratory variability is another mechanism of HRVLF generation, which should be always monitored, assessed, and considered in the interpretation of HRV changes
Gain and coherence estimates between respiration and heart-rate: Differences between inspiration and expiration
No full textThe interaction of respiration and heart-rate variability (HRV), leading to respiratory sinus arrhythmia (RSA) and, in the inverse direction, cardioventilatory coupling has been subject of much study and controversy. A parametric linear feedback model can be used to study these interactions. In order to investigate differences between inspiratory and expiratory periods, we propose that models are estimated separately for each period, by finding least mean square estimates only over the desired signal segments. This approach was tested in simulated data and heart-rate and respiratory air flow signals recorded from 25 young healthy adults (13 men and 12 women), at rest, breathing spontaneously through a face mask for 5 min. The results show significant differences (p < 0.05) between the estimates of coherence obtained from the whole recording, and the inspiration and expiration periods. Simple and causal coherence from respiration to HRV was higher during inspiration than expiration. The estimates of gain also differed significantly in the high frequency (HF) band (0.15–0.5 Hz) between those obtained from the whole recording, and the inspiratory and expiratory periods. These results indicate that a single linear model fitted to the whole recording neglects potentially important differences between inspiration and expiration, and the current paper shows how such differences can be estimated, without the need to control breathing.<br/
Sex-specific programming of cardiovascular physiology in children
No full textAims: Increasing evidence suggests that adverse prenatal environments, as indicated by low birth weight, cause long-term changes in cardiovascular physiology that predispose to circulatory disease. The mechanisms are poorly understood, most human studies have been carried out in adults and little is known about early pathophysiological changes. Therefore, we have assessed the relationship between birth weight and cardiovascular physiology in children. Methods and results: In 140 healthy boys and girls (aged 7–9 years), born at term and followed prospectively, we continuously recorded blood pressure, electrocardiograms and cardiac impedance before, during, and after 10 min of psychosocial stress (Trier Social Stress Test for Children). In boys, an association of lower birth weight with higher resting systemic arterial pressure (? = –6.8 mmHg/kg, P= 0.03) and a trend towards higher vascular resistance (? = –87 dyne s/cm5/kg, ns) were substantially strengthened following stress (? = –9.5 mmHg/kg, P= 0.003 and ? = –139 dyne s/cm5/kg, P = 0.02, respectively). In girls, lower birth weight was associated with a shorter pre-ejection period (? = 8.0 ms/kg, P = 0.005) and corrected QT interval (? = 11.9 ms/kg, P = 0.003) at rest and little changed by stress. Conclusion: Smaller size at birth is associated with sex-specific alterations in cardiac physiology; boys had higher systemic vascular resistance and girls had increased cardiac sympathetic activation
Heart-rate and blood-pressure variability during psychophysiological tasks involving speech: influence of respiration
No full textChanges in heart-rate and systolic arterial pressure variability (HRV and SAPV) indexes have been used in psychophysiology to assess autonomic activation, including during tasks involving speech. The current article clearly demonstrates in a sample of 25 adult subjects that the erratic and broadband respiratory patterns during such tasks violate the usual assumption that respiration is limited to the high-frequency band (0.15-0.4 Hz). For these tasks, interindividual differences and rest-task changes in HRV and SAPV in the low-frequency band (0.04-0.15 Hz) can be explained, to a large extent, by variations in the respiratory volume signal. This makes the use of HRV and SAPV as markers of autonomic function during these tasks highly questionable. Furthermore, a number of subjects with long respiratory period at rest were identified, whose presence in the sample can bias the estimation of baseline rest values
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
