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Results for “Surrogate variables analysis”

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PubMed2006

Oocyte degeneration after intracytoplasmic sperm injection: a multivariate analysis to assess its importance as a laboratory or clinical marker.

OBJECTIVE: Oocyte degeneration has historically been associated with the intracytoplasmic (ICSI) technique. We sought to determine whether oocyte degeneration rates were associated with the technician performing the procedure, the baseline characteristics of the patient, and/or ovarian stimulation variables. We also evaluated whether the degeneration rate could serve as a surrogate marker for implantation potential. DESIGN: Cohort study. SETTING: Academic medical center. PATIENT(S): Couples undergoing ICSI. INTERVENTION(S): Six thousand six hundred fifty-three injected oocytes were analyzed to determine whether the degeneration rate was technician dependent. Two hundred thirty first-entry down-regulated cycles were examined to identify predictors associated with oocyte degeneration. Multivariate analyses were performed using generalized linear model routines. MAIN OUTCOME MEASURE(S): Oocyte degeneration rates and implantation rates. RESULT(S): Neither the ICSI technician nor the stripping technician was associated with the oocyte degeneration rate. However, the day 3 FSH, number of mature oocytes retrieved, and E2 levels on the day of hCG were significant independent predictors of degeneration rate. Physician-adjustable ovarian stimulation variables were not associated with the degeneration rate. The degeneration rate did not appear to be associated with the implantation rate. CONCLUSION(S): These data suggest that oocyte degeneration is not technician or physician dependent. Degeneration is likely a function of the inherent oocyte quality in women who underwent ovarian stimulation. However, the remaining cohort of retrieved oocytes appears to be unaffected by virtue of an uncompromised implantation rate.

Adult↗

PubMed2002

Temporal trends of organochlorine concentrations in umbilical cord blood of newborns from the lower north shore of the St. Lawrence river (Québec, Canada).

This study describes the time trends of organochlorines [OCs; 14 polychlorinated biphenyls (PCBs) and 11 chlorinated pesticides] in umbilical cord plasma of newborns in a remote Canadian coastal population. We analyzed 408 cord blood samples collected between 1993 and 2000 for PCBs, chlordanes, dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyltrichloroethylene (DDE), hexachlorobenzene (HCB), and n-3 fatty acids. We also gathered information on the mothers (age, past and present residence, ethnic group, use of tobacco during pregnancy, and breast-feeding during previous pregnancies). From 1993 to 2000, mean concentrations of PCBs, chlordanes, DDT/DDE, and HCB in cord blood decreased by 63%, 25%, 66%, and 69%, respectively (p < 0.0001). Multiple regression analysis with the year of birth as the main independent variable yielded a strong significant exponential decrease for all contaminants (in all age and ethnic groups). We detected no monthly or seasonal pattern. We used n-3 fatty acids concentration as a surrogate of maternal fish consumption. Fish consumption declined only slightly between 1993 and 2000, but this decrease did not contribute significantly to the reduction of OCs. These results show that prenatal exposure to persistent OCs has declined significantly between 1993 and 2000 in this population.

Adult↗

PubMed2006

Assessing moral distress in respiratory care practitioners.

OBJECTIVE: To test the reliability and validity of a modified moral distress tool, originally developed for the nursing profession, on respiratory care practitioners. To describe the relationship between moral distress, career dissatisfaction, and job turnover in respiratory care. DESIGN: A 28-question survey was developed. Three categories of survey questions were predefined: "individual responsibility," "not in the patient's best interest," and "deception." Additional questions measured career dissatisfaction, job turnover, and demographic information. SETTING: University Hospital at the University of Virginia Health System, a 552-bed tertiary care hospital. SUBJECTS: Fifty-seven of 115 (49.6%) of respiratory care practitioners responded to the survey. INTERVENTIONS: Exploratory factor analysis was used to investigate the underlying factor structure. After we extracted theoretically meaningful factors, reliability of each factor was estimated. Multiple regression analysis was conducted to test if the underlying factors predicted career dissatisfaction and job turnover. MEASUREMENTS AND MAIN RESULTS: The factor analysis yielded a five-factor structure. Several questions in the "not in patient's best interest" category scored the highest moral distress including disagreements with surrogate decision makers and providing futile care. Higher scores were also found with questions regarding the perception of unsafe staffing and passively or actively participating in deception. None of the demographic variables predicted career dissatisfaction or job turnover. However, the perception of unsafe staffing was found to be a significant factor in predicting career dissatisfaction and job turnover. CONCLUSIONS: In this one-center pilot study, respiratory care practitioners reported experiencing moral distress in many areas of their practice. Distress related to the perception of unsafe staffing may be related to career dissatisfaction and job turnover. Further exploration of the factors that contribute to respiratory care practitioners' moral distress is needed, as well as implementing ways to ameliorate it.

Adult↗

PubMed2004

Correlation between ankle-brachial index, symptoms, and health-related quality of life in patients with peripheral vascular disease.

OBJECTIVE: Improving health-related quality of life (HRQL) is the main goal of surgery to treat peripheral vascular disease (PVD); however, HRQL is rarely measured directly. Rather, most surgeons use other measures, such as patient symptoms and ankle-brachial index (ABI) to determine the need for intervention in PVD. The accuracy of these surrogates in representing HRQL has been untested. The purpose of this study was to determine the correlation of these measures with HRQL in patients undergoing evaluation for intervention in symptomatic PVD. METHODS: Patients (n=108) referred to the vascular surgery service with symptoms of PVD were enrolled in a prospective study of HRQL. Patients completed two validated HRQL questionnaires: the short form-36 (SF-36) and the Walking Impairment Questionnaire (WIQ). All patients had symptoms consistent with PVD, including claudication (n=69; 63.9%), ischemic rest pain (n=17; 15.7%), or tissue loss (n=22; 20.4%). ABI was measured at presentation. RESULTS: The mean ABI was 0.53 (range, 0.00-0.98). The maximal correlation between SF-36 score and ABI was reflected in the Physical Component Summary score (r=0.25). WIQ score also exhibited modest correlation with ABI, with maximal correlation noted for stair climbing (r=0.26). Both SF-36 and WIQ scores exhibited a highly significant association with symptoms. Patients with more severe symptoms, such as lifestyle-limiting claudication or limb-threatening ischemia, had lower HRQL scores compared with patients with non-lifestyle-limiting claudication. Multivariate analysis demonstrated that SF-36 and WIQ physical summary scores are better predicted by symptoms than by ABI (P<.01). CONCLUSIONS: HRQL in patients with PVD correlates weakly with ABI, but exhibits a closer association with vascular symptoms. However, neither variable fully expresses patient HRQL. These data suggest that sole reliance on these surrogates may not accurately reflect the effect of PVD on HRQL, or the potential benefit of vascular surgery in improving HRQL.

Adult↗

PubMed1999

The poor accuracy of indirect measurements of cadaveric donor kidney weights.

Reports that examined the issue of whether transplantation of inadequate nephron mass may be a risk factor for long-term allograft failure yielded conflicting results. One of the more accurate methods of estimating glomerular mass is kidney weight. Most of the clinical studies used body surface area (BSA) or kidney length as estimates of kidney weight. To test the hypothesis that indirect measures of kidney weight are accurate estimates of kidney weight, we compared the kidney weight of 41 consecutive cadaveric kidneys to donor BSA, dimensions measured with calipers at the time of transplantation, and dimensions supplied by the Organ Procurement Agency (OPA). Linear regression analysis was used with kidney weight as the dependent variable and BSA, kidney length, or kidney volume as the independent variable. Kidney length measured with calipers was also compared to kidney length supplied by the OPA. Kidney weight had the best correlation with kidney volume and kidney length determined by caliper measurements (r = 0.640 and 0.646, respectively). The regression analysis showed that the correlation of kidney weight with BSA was 0.487. The correlation of OPA-provided kidney length with kidney weights was poor (r = 0.410). The linear regression of caliper-measured kidney length versus OPA length yielded a slope of 0.360, instead of an ideal slope of 1. The assumption has been made that kidney weight or a surrogate of kidney weight has an excellent correlation with nephron mass. Some of the variability in studies that attempted to examine the effect of transplanted nephron mass on allograft outcome may be due to inaccurate estimates used for kidney weight. Our data suggest that surrogate measurements of kidney weight may not be accurate. We recommend that measured kidney weight should be used in studies examining the effect of donor renal mass on allograft outcomes.

Body Surface Area↗

PubMed2000

Mean arterial pressure, pregnancy induced hypertension, and preeclampsia. Evaluation as independent risk factors and as surrogates for high maternal serum alpha-protein in estimating breast cancer risk.

PURPOSE: Data from a nested case-control study were analyzed to examine high mean arterial pressure (MAP), hypertension of pregnancy and preeclampsia as independent predictors, and as surrogate markers for elevated Alpha-fetoprotein (AFP) levels in evaluating breast cancer risk.METHODS: Cases (n = 205) were identified by the California Cancer Registry from a cohort of pregnant women who were part of the Kaiser Health Plan and took part in the Child Health and Development Studies initiated by the University of California, Berkeley, from June 1959 to September 1966. Controls (n = 337) were selected by randomized recruitment from the same cohort probability matched to cases by distribution of birth dates of cases.RESULTS: High MAP was associated with breast cancer risk, and was different across quartiles of age at first full term pregnancy. Odds Ratios across quartiles were 0.24 (95% CI = 0.08-0.71), 0.84 (95% CI = 0.39-1.66), 1.00 (referent), and 2.50 (95% CI = 1.21-5.13). Neither diagnosed preeclampsia nor hypertension of pregnancy showed any association with breast cancer risk. When both high Alpha-fetoprotein and high mean arterial pressure were entered into the same analysis neither changed the odds ratio for the other more than eight percent. Additionally, AFP level was not a linear function of MAP.CONCLUSIONS: Although the pattern of odds ratios across quartiles of age at first full term pregnancy were similar for the two variables it can not be concluded that high MAP is an adequate surrogate for high levels of maternal serum AFP, but rather represents some related process which is in and of itself a risk factor for breast cancer.

Journal Article↗

PubMed2006

Liver iron is a surrogate marker of severe fibrosis in chronic hepatitis C.

BACKGROUND/AIMS: Patients with chronic hepatitis C have frequently mild to moderate liver iron overload which increases with fibrosis stage. Thus, it has been postulated that iron could enhance the progression of fibrosis. However, the real impact of iron is still controversial. The study was undertaken to determine the effect of confounding variables. All factors known to influence both iron overload and fibrosis were taken into account. METHODS: Five hundred and eighty-six patients, who had liver biopsy performed prior to antiviral treatment, were included. Serum ferritin and liver iron were correlated with clinical, biological and histological variables in univariate and multivariate analysis. The impact of iron on fibrosis was evaluated in multivariate analysis in the whole group and in the subgroup of 380 patients with available date of infection. RESULTS: Hyperferritinemia, encountered in 27%, was associated with liver iron deposits in only 46% of cases. Liver iron was elevated in 17%, and correlated with age, male sex, and alcohol intake. The univariate strong link which existed between liver iron and fibrosis disappeared after adjustment for confounding variables. CONCLUSIONS: According to the results of this study, liver iron should be considered more as a surrogate marker for disease severity than as a fibrogenic factor per se.

Adult↗

PubMedDate not supplied

Residential independence of former special education high school students: a second look.

The residential independence of postsecondary students was assessed in 5,462 parents or surrogate parents of students with disabilities from the National Longitudinal Transition Study who had left United States high schools between 1985 and the time of the questionnaire in 1990. An index of residential placement independence served as the dependent variable in a hierarchical regression analysis that featured 43 community, family, student, and school program characteristics entered as block-wise predictors (i.e., entered in a controlled order). This analysis produced a multiple R2 of .376: missing data, youths' daily living skills, youths' social skills, youths' maladaptive ("problem") behaviors, and community characteristics all contributed significantly to the prediction of the postschool residential independence of former special education students during the first 5 years after they had left secondary school.

Activities of Daily Living↗

PubMed2005

Modelling malaria risk in East Africa at high-spatial resolution.

OBJECTIVES: Malaria risk maps have re-emerged as an important tool for appropriately targeting the limited resources available for malaria control. In Sub-Saharan Africa empirically derived maps using standardized criteria are few and this paper considers the development of a model of malaria risk for East Africa. METHODS: Statistical techniques were applied to high spatial resolution remotely sensed, human settlement and land-use data to predict the intensity of malaria transmission as defined according to the childhood parasite ratio (PR) in East Africa. Discriminant analysis was used to train environmental and human settlement predictor variables to distinguish between four classes of PR risk shown to relate to disease outcomes in the region. RESULTS: Independent empirical estimates of the PR were identified from Kenya, Tanzania and Uganda (n = 330). Surrogate markers of climate recorded on-board earth orbiting satellites, population settlement, elevation and water bodies all contributed significantly to the predictive models of malaria transmission intensity in the sub-region. The accuracy of the model was increased by stratifying East Africa into two ecological zones. In addition, the inclusion of urbanization as a predictor of malaria prevalence, whilst reducing formal accuracy statistics, nevertheless improved the consistency of the predictive map with expert opinion malaria maps. The overall accuracy achieved with ecological zone and urban stratification was 62% with surrogates of precipitation and temperature being among the most discriminating predictors of the PR. CONCLUSIONS: It is possible to achieve a high degree of predictive accuracy for Plasmodium falciparum parasite prevalence in East Africa using high-spatial resolution environmental data. However, discrepancies were evident from mapped outputs from the models which were largely due to poor coverage of malaria training data and the comparable spatial resolution of predictor data. These deficiencies will only be addressed by more random, intensive small areas studies of empirical estimates of PR.

Animals↗

PubMed2003

Relation between insulin resistance and hematological parameters in elderly Koreans-Southwest Seoul (SWS) Study.

In this study, we investigated the relation between insulin resistance and hematological parameters in elderly Koreans. This study included 1314 non-diabetic subjects over the age of 60, selected from a cross-sectional study, which was conducted in 1999 in Seoul, Korea. We measured fasting and post-load 2 h plasma glucose, insulin levels, lipid profiles, anthropometric measures, and hematological parameters. The degree of insulin resistance was assessed using the homeostasis model assessment (HOMA). We found a correlation between insulin resistance and hemoglobin concentrations in non-smoking men (r=0.20, P=0.0186). In non-smoking women, insulin resistance correlated with hemoglobin (r=0.10, P=0.0017) and with white blood cell (WBC) count (r=0.15, P=0.001). Hemoglobin concentrations and WBC counts were also associated with other components of the insulin resistance syndrome such as body mass index, blood pressure, lipid profiles and fasting plasma insulin levels (surrogate for insulin resistance). Furthermore, the group in the upper quartile for insulin resistance showed higher hemoglobin concentrations and WBC counts than the lower quartile, independent of smoking status and serum iron concentrations. Using HOMA-IR as a dependent variable in a multiple regression analysis, age, body mass index (BMI), waist-to-hip ratio (WHR), systolic blood pressure, HDL cholesterol, triglyceride, WBC count, hemoglobin, hematocrit and serum TIBC were significant. Our results provide support for a relation between insulin resistance/hyperinsulinemia and hematological parameters such as hemoglobin concentrations and WBC counts in elderly Koreans. This suggests that increased erythropoiesis and subclinical inflammation could be part of the metabolic syndrome in elderly Koreans.

Aged↗

PubMed2002

Genome-wide analysis of hepatic fibrosis in inbred mice identifies the susceptibility locus Hfib1 on chromosome 15.

BACKGROUND & AIMS: Host genetic factors are likely to contribute to the variable course of hepatic fibrosis in response to chronic liver injury. Similarly, the fibrotic response differs among inbred mouse strains after challenge with CCl(4). Our aim was to identify unknown susceptibility loci for hepatic fibrosis in a cross between fibrosis-susceptible and -resistant inbred mice. METHODS: Seven inbred mouse strains were treated with CCl(4), and hepatic fibrosis was phenotypically characterized by histology, hepatic hydroxyproline levels, and serum surrogate markers. F(1) hybrids of susceptible BALB/cJ and resistant A/J inbred strains were intercrossed to obtain 358 F(2) progeny. Quantitative trait loci (QTL) that determine hepatic fibrosis were identified by genome-wide interval mapping and haplotype analysis. RESULTS: In this model, marked strain differences in fibrosis susceptibility exist, with BALB/c inbred mice being most susceptible. The hydroxyproline levels of F(1) mice resemble the resistant parental strains, indicating that fibrosis susceptibility is a recessive trait. QTL analysis identifies a susceptibility locus on chromosome 15 that significantly affects the stage of fibrosis and hydroxyproline levels. According to standard nomenclature, this locus is called Hfib1 (hepatic fibrogenic gene 1). Hfib1 is defined by genetic markers D15Mit26 and D15Mit122. A suggestive QTL on chromosome 2 colocalizes with the complement factor 5 gene, known to be mutated in the resistant strain A. CONCLUSIONS: The set of inbred strains provides a framework for systematic analysis of fibrogenic genes. QTL mapping is useful to identify genetic susceptibility loci for hepatic fibrosis that might harbor new molecular targets for antifibrotic drug design.

Animals↗

PubMedDate not supplied

Using surrogate modeling in the prediction of fibrinogen adsorption onto polymer surfaces.

We present a Surrogate (semiempirical) Model for prediction of protein adsorption onto the surfaces of biodegradable polymers that have been designed for tissue engineering applications. The protein used in these studies, fibrinogen, is known to play a key role in blood clotting. Therefore, fibrinogen adsorption dictates the performance of implants exposed to blood. The Surrogate Model combines molecular modeling, machine learning and an Artificial Neural Network. This novel approach includes an accounting for experimental error using a Monte Carlo analysis. Briefly, measurements of human fibrinogen adsorption were obtained for 45 polymers. A total of 106 molecular descriptors were generated for each polymer. Of these, 102 descriptors were computed using the Molecular Operating Environment (MOE) software based upon the polymer chemical structures, two represented different monomer types, and two were measured experimentally. The Surrogate Model was developed in two stages. In the first stage, the three descriptors with the highest correlation to adsorption were determined by calculating the information gain of each descriptor. Here a Monte Carlo approach enabled a direct assessment of the effect of the experimental uncertainty on the results. The three highest-ranking descriptors, defined as those with the highest information gain for the sample set, were then selected as the input variables for the second stage, an Artificial Neural Network (ANN) to predict fibrinogen adsorption. The ANN was trained using one-half of the experimental data set (the training set) selected at random. The effect of experimental error on predictive capability was again explored using a Monte Carlo analysis. The accuracy of the ANN was assessed by comparison of the predicted values for fibrinogen adsorption with the experimental data for the remaining polymers (the validation set). The mean value of the Pearson correlation coefficient for the validation data sets was 0.54 +/- 0.12. The average root-mean-square (relative) error in prediction for the validation data sets is 38%. This is an order of magnitude less than the range of experimental values (i.e., 366%) and compares favorably with the average percent relative standard deviation of the experimental measurements (i.e., 17.9%). The effects of each of the user-defined parameters in the ANN were explored. None were observed to have a significant effect on the results. Thus, the Surrogate Model can be used to accurately and unambiguously identify polymers whose fibrinogen absorption is at the limits of the range (i.e., low or high) which is an essential requirement for assessing polymers for regenerative tissue applications.

Adsorption↗

PubMed2004

Physical inactivity is an important lifestyle determinant of insulin resistance in hypertensive patients.

The purpose of the study is to assess the relative impact of lifestyle factors including physical inactivity, cigarette smoking, and alcohol intake on insulin resistance in hypertensive patients. In total, 872 hypertensive patients, of Chinese and Japanese origin, from the Stanford Asia and Pacific Program for Hypertension and Insulin Resistance were included for the current analysis. Homeostasis model assessment for insulin resistance (HOMA-IR) and the insulin sensitivity index ISI0,120 were chosen as surrogate measures of insulin resistance. Standardized interview-administered questionnaires were used to obtain information on demographic and lifestyle characteristics. The sedentary hypertensive patients were more insulin resistant than the non-sedentary hypertensive. There were significant differences in Log (HOMA-IR) (0.06-unit increases, p < 0.01) and Log (ISI0,120) (0.05-unit decreases, p < 0.01) between sedentary and non-sedentary hypertensive patients after controlling demographic variables. There were no differences in insulin sensitivity in subjects with different smoking status. Neither smoking nor alcohol intake was persistently associated with insulin resistance in the analysis. Our results suggest that physical inactivity is an important lifestyle determinant of insulin resistance in hypertensive patients. The influences of smoking and alcohol intake on insulin resistance are less significant than physical inactivity in hypertensive subjects.

Adult↗

PubMed1996

Use of ICD-9-CM codes in the estimation of alcohol-involved injury: search for a surrogate II.

This study presents a follow-up analysis of 22,427 injury cases drawn from the California Regional Trauma Registry. Whereas the earlier analysis developed a surrogate for alcohol-involved injuries using E-codes as reflective of injury type, this analysis explores the possibility of using ICD-9-CM diagnosis codes that have a longer history and are available in more jurisdictions. Findings reported herein indicate that the original demographic patterns predicting testing and alcohol involvement patterns were maintained in ICD-9-CM diagnostic code-based models. Moreover, although variables representing demographic background, time of day, and day of week clearly were the most powerful model predictors, ICD-9-CM diagnostic codes reflecting physician assessment that the patient had a chronic alcohol problem did improve the fit of models, and thus provide additional information concerning testing and blood alcohol concentration patterns. In contrast, ICD-9-CM diagnostic codes reflecting the specific nature of the injury, although statistically significant, explained little additional variability in alcohol involvement. Nevertheless, the overall model did accurately classify approximately 75% of those in the sample for whom blood alcohol concentration status information was available, suggesting its appropriateness for surrogate development.

Adolescent↗

PubMed2000

A quantitative study of gamma-band activity in human intracranial recordings triggered by visual stimuli.

This paper studies gamma-band responses from two implanted epileptic patients during a simple visual discrimination task. Our main aim was to ascertain, in a reliable manner, whether evoked (stimulus-locked) and induced (triggered by, but not locked to, stimuli) responses are present in intracranial recordings. For this purpose, we introduce new methods adapted to detect the presence of gamma responses at this level of recording, intermediary between EEG-scalp and unicellular responses. The analysis relies on a trial-by-trial time-frequency analysis and on the use of surrogate data for statistical testing. We report that visual stimulation reliably elicits evoked and induced responses in human intracranial recordings. Induced intracranial gamma activity is significantly present in short oscillatory bursts (a few cycles) following visual stimulation. These responses are highly variable from trial to trial, beginning after 200 ms and lasting up to 500 ms. In contrast, intracranial-evoked gamma responses concentrate around 100 ms latencies corresponding to evoked responses observed on the scalp. We discuss our results in relation to scalp gamma response in a similar protocol [Tallon-Baudry et al. (1996) J. Neurosci., 16, 4240-4249] and draw some conclusions for bridging the gap between gamma oscillations observed on the scalp surface and their possible cortical sources.

Adolescent↗

PubMed1999

Effect of race on insurance coverage and health service use for HIV-infected gay men.

OBJECTIVE: To determine whether race is associated with health insurance coverage and health service use among gay and bisexual men in the Baltimore center of the Multicenter AIDS Cohort Study. METHODS: Data from eight semiannual study visits between 1991 and 1996 were used. Descriptive, stratified, and logistic regression analyses were conducted to determine whether race is associated with insurance coverage, medical, or dental service use, after controlling for socioeconomic variables. RESULTS: No difference was found between blacks' and whites' likelihood of having health insurance, private insurance, using inpatient, emergency department services, or antiretroviral medications. Whites were more likely to use outpatient services, particularly if CD4 cell counts were high, and were more likely to use dental services, although blacks were more likely to have dental insurance. CONCLUSIONS: Further research must be conducted to examine cultural, social, and psychological factors that help explain why white gay men use more outpatient and dental services, when other service use is unrelated to race. Investigators should be precise when using race as a variable in health services and epidemiologic research, emphasizing when racial differences truly exist versus when the variable race is a surrogate for another factor.

Adult↗

PubMedDate not supplied

Resource measurement by health care providers.

The need to use health care resources effectively and efficiently has led to increased interest in developing a "should cost" approach to performance measurement. The development of appropriate standards and the separation of fixed costs into surrogate variable and capacity components can provide a useful tool for managers to measure performance. This article develops a framework for evaluating the utilization of fixed costs in providing output.

Analysis of Variance↗

PubMed2004

Statistical method for detection of phase-locking episodes in neural oscillations.

In many networks of oscillatory neurons, synaptic interactions can promote the entrainment of units into phase-coupled groups. The detection of synchrony in experimental data, especially if the data consist of single-trial runs, can be problematic when, for example, phase entrainment is of short duration, buried in noise, or masked by amplitude fluctuations that are uncorrelated among the oscillating units. In the present study, we tackle the problem of detecting neural interactions from pairs of oscillatory signals in a narrow frequency band. To avoid the interference of amplitude fluctuations in the detection of synchrony, we extract a phase variable from the data and utilize statistical indices to measure phase locking. We use three different phase-locking indices based on coherence, entropy, and mutual information between the phase variables. Phase-locking indices are calculated over time using sliding analysis windows. By varying the duration of the analysis windows, we were able to inspect the data at different levels of temporal resolution and statistical reliability. The statistical significance of high index values was evaluated using four different surrogate data methods. We determined phase-locking indices using alternative methods for generating surrogate data and found that results are sensitive to the particular method selected. Surrogate methods that preserve the temporal structure of the individual phase time series decrease substantially the number of false positives when tested on a pair of independent signals.

Acoustic Stimulation↗