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At least 415 records · Page 23Linked to original sources
PubMed1978

Effect of sleep deprivation on response threshold for signal detectability parameter, beta.

In contrast to a previous study (Deaton, Tobias, & Wilkinson, 1971), data are presented which demonstrate significant decreases in response bias (beta) as well as sensitivity (d') following one night of sleep deprivation. It is argued that previous failures to observe significant decreases in beta may be related to the disparity in the number of noise and signal-plus-noise trials in these experiments. The use of parametric analysis to test the effect of sleep deprivation on d' and beta is recommended.

Humans↗
PubMedDate not supplied

Relation of observer agreement to accuracy according to a two-receiver signal detection model of diagnosis.

A long-standing problem of medical diagnosis is why rates of observer disagreement are so high. To study this question a model of test interpretation is defined which accounts for disagreement when observers use identical diagnostic strategies. Test subjects are assigned to one of two disease classes on the basis of two parameters: the test score and lambda, the diagnostic certainty, a parameter analogous to signal-to-noise ratio. The rate of disagreement and the mean error rate of independent ideal observers are derived as a function of diagnostic strategy. It follows from the model that: 1) Maximization of the ratio of disagreement to error defines an optimal diagnostic strategy with advantages over maximum likelihood and Bayes diagnostic strategies. 2) Disagreement exceeds error if the sensitivity and specificity of independent observers exceed one-half for all lambda. 3) The ratio of disagreement to error can be no greater than two if test interpretation is dichotomous but can approach infinity for trichotomous interpretation, i.e., if observers are allowed to interpret cases as indeterminate.

Bayes Theorem↗
PubMed1999

A novel local PCA-based method for detecting activation signals in fMRI.

A novel local principal component analysis (LPCA) technique is presented for activation signal detection in functional magnetic resonance imaging (fMRI) without explicit knowledge about the shape of the model activation signal. Unlike the traditional PCA methods, our LPCA algorithm is based on a measure of separation between two clusters formed by the signal segments in active periods and inactive periods, which is computed in an eigen-subspace. In addition, we only applied PCA to the temporal sequence of each individual voxel instead of applying PCA to the fMRI data set. In our algorithm, we first applied a linear regression procedure to alleviate the baseline drift artifact. Then, the baseline-corrected temporal signals were partitioned into active and inactive segments according to the paradigm used for the fMRI data acquisition. Principal components were computed from all these segments for each voxel by PCA. By projecting the segments of each voxel onto a linear subspace formed by the corresponding most dominant principal components, two separate clusters were formed from active and inactive segments. An activation measure was defined based on the degree of separation between these two clusters in the projection space. We show experimental results on the activation signal detection from various sets of fMRI data with different types of stimulation by using the proposed LPCA algorithm and the standard t-test method for comparison. Our experiments indicate that the LPCA algorithm in general provides substantial signal-to-noise ratio improvement over the t-test method.

Algorithms↗
PubMedDate not supplied

Abundance of microembolic signals detected by transcranial doppler ultrasound in a patient with Eisenmenger's syndrome.

BACKGROUND: Clinically silent circulating microemboli can be detected by transcranial Doppler sonography. CASE DESCRIPTION: We report the case of a 45-year-old man with congenital cyanotic heart disease, elevated haematocrit, low platelet count and decreased protein C and protein S activity. Before and following several haemodilutions, we performed 5 1-hour transcranial Doppler recordings from one or both middle cerebral arteries (MCAs) on different days. The number of microembolic signals per hour varied from 54 to 134. During the bilateral recording, the microemboli occurred on both sides (55 in the right and 45 in the left MCA, respectively). There was no relation to red blood cell count, platelet count, the spontaneously lowered prothrombin time or the spontaneously prolonged activated partial thromboplastin time. CONCLUSIONS: The origin of the large number of microembolic signals found in this patient remains unclear, but may be presumed in the intra-abdominal or crural venous system.

Eisenmenger Complex↗
PubMed1999

Signal detection theory, lateral-line excitation patterns and prey capture behaviour of mottled sculpin.

The frequency with which blinded mottled sculpin, Cottus bairdi, oriented towards a dipole current source (50-Hz vibrating sphere) was measured as a function of source distance (2-18 cm) and azimuth (either 0 degrees in front or 90 degrees to the side of the fish). The orienting frequency declined from over 70% to under 50% as source distance increased from 4 to 12 cm for both frontal and lateral sources. When response biases (frequency of responding in the absence of the signal) were taken into account with the performance metric d', threshold distances (distances at which d' fell to 1) for frontal (12.5 cm) and lateral (11.6 cm) sources were 1.35-1.45 times the mean standard length of fish used in this study. At distances less than 8 cm, d' values were considerably higher (i.e. performance was better) for the lateral source, despite the fact that peak stimulus levels at the fish were twice as high for frontal as for lateral sources at any given distance. Performance differences may be related to differences in spatial excitation patterns, in particular the distribution of opposing pressure gradient directions along the lateral-line system, present for lateral sources, but absent for frontal sources. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1999

Evaluation of the AmpliSensor PCR and the SHARP signal detection system for the early prediction of symptomatic CMV infection in solid transplant recipients.

BACKGROUND: Cytomegalovirus (CMV) is associated with high morbidity and mortality in transplant patients. Specific antiviral treatment at an early stage of CMV infection may effectively ameliorate, but not eliminate CMV disease in these patients. Presently, the pp65 antigenemia test on peripheral leukocytes is the method most widely used for predicting and monitoring transplant patients for active CMV infection. Nucleic acid amplification methods are less well defined since they lack standardisation. OBJECTIVE: A seminested fluorometric PCR assay (AmpliSensor-CMV, BAG, Germany) and a one-step PCR with a signal-amplification step (SHARP, Abbott, Germany) specific for the fragments of the CMV UL 122 and UL 123 genes, respectively, were evaluated for the early diagnosis of CMV infection. DESIGN: A total of 26 recipients of heterogeneous solid organs were monitored prospectively for a median of 99 days after transplantation. By testing 371 clinical samples parallel with the pp65-antigen assay and IgM and IgG EIA assays the sensitivity, specificity, correlation and quantitation potential of both PCRs was evaluated. RESULTS: Eight out of 26 patients developed active CMV infection. A total of 48 samples of these patients exceeded a CMV-DNA load threshold of 15 genome equivalents/10(5) leukocytes (AmpliSensor-CMV) and 41 samples exceeded the critical cut-off for the SHARP system. The AmpliSensor PCR exceeded its threshold consistently before the clinical onset of CMV disease (median 8 days). There was very good agreement between symptomatic CMV infection in patients and AmpliSensor-PCR, SHARP PCR, and pp65-antigen results (kappa-coefficient > 0.900). IgM and IgG EIA showed moderate agreement (kappa-coefficient = 0.591 and 0.552, respectively). CONCLUSION: Both PCRs and pp65 antigen assay correlated significantly better with CMV disease than serodiagnosis. The AmpliSensor PCR allowed more precisely than the SHARP system a quantitative determination of viral load and an early and reliable prediction of active CMV infection. The use of AmpliSensor PCR may improve the diagnosis and management of active CMV infection in organ transplant recipients.

Adult↗
PubMed2005

Ambulatory transcranial Doppler cerebral embolic signal detection in symptomatic and asymptomatic carotid stenosis.

BACKGROUND AND PURPOSE: Transcranial Doppler (TCD) ultrasound can detect asymptomatic emboli in carotid stenosis. Current systems are nonportable and can only record for short durations. A novel ambulatory TCD system allows prolonged recording. We applied this to patients with symptomatic and asymptomatic carotid stenosis to determine patterns of embolization in the 2 conditions and optimal recording protocols. METHODS: Ambulatory TCD recordings were performed in 12 symptomatic and 15 asymptomatic carotid stenosis (> or =50%) patients for 8 hours and then repeated on a second occasion. RESULTS: Nine (75%) of symptomatic subjects had embolic signals during the first recording. In this group, repeating the recording did not increase the proportion of positive patients. In asymptomatic patients, 4 (26.7%) had embolic signals on 1 recording, and this proportion increased to 46.7% after 2 recordings. There was significant clustering of embolic signals demonstrating that the process was nonrandom. CONCLUSIONS: Ambulatory TCD is possible in patients with carotid artery stenosis. By increasing the duration of recording, additional information is provided, particularly in asymptomatic patients. Our results also demonstrate clustering of embolic signals. Our study provides baseline data to allow studies in both asymptomatic and symptomatic carotid stenosis to be planned.

Aged↗
PubMed1997

Comparison of hemodynamic cerebral ischemia and microembolic signals detected during carotid endarterectomy and carotid angioplasty.

BACKGROUND AND PURPOSE: There has been concern about carotid percutaneous transluminal angioplasty (PTA) carrying a greater risk of cerebral ischemia than carotid endarterectomy. We set out to compare cerebral hemodynamics and microembolization during carotid PTA and CEA. METHODS: We used transcranial Doppler to monitor the middle cerebral artery of 28 patients undergoing carotid PTA (n = 14) or carotid endarterectomy (CEA) with a shunt (n = 14). Each period during which the internal carotid artery was occluded by PTA balloon or by clamp when the shunt was not in place was timed. Individual periods were summated to give a total occlusion time. Ischemic time was defined as the period for which mean middle cerebral artery velocity fell to a third or less of baseline. Microembolic signals were counted during each procedure. RESULTS: CEA resulted in significantly longer individual and total occlusion time than PTA (mean individual occlusion time, seconds), CEA, 168 +/- 51; PTA, 20 +/- 7; P < .001; mean total occlusion time; CEA, 337 +/- 70; PTA, 26 +/- 10; P < .001. Ischemic time was also significantly longer during CEA than during PTA (CEA, 165 +/- 40; PTA, 17 +/- 5; P = .001). There were significantly more microembolic signals during PTA than during CEA (mean number of microembolic signals during CEA, 52 +/- 64; during PTA, 202 +/- 119; P = .001). There was no correlation between any of the parameters measured and periprocedural stroke, which occurred in one patient in each group. CONCLUSION: PTA results in less hemodynamic ischemia but more cerebral microembolism than CEA. In this small series, however, it is not possible to comment on the relations between ischemic time, microembolism, and stroke.

Aged↗
PubMed2006

Long-term signal detection, segmentation and summarization using wavelets and fractal dimension: a bioacoustics application in gastrointestinal-motility monitoring.

The current paper describes a wavelet-based method for long-term processing and analysis of gastrointestinal sounds (GIS). Windowing techniques are used to select sequential blocks of the prolonged multi-channel recordings and proceed to various wavelet-domain processing stages. De-noising, significant-activity detection, automated segmentation and extraction of summary curves are applied in an integrated mode, allowing for enhanced content manipulation and analysis. The proposed analysis scheme combines flexible long-term graphical representation tools, while maintaining the ability of quick browsing via visualization and auralization of the detected short-term events. This work is part of a project aiming to implement non-invasive diagnosis over gastrointestinal-motility (GIM) physiology. However, the proposed techniques might be applied to any study of long-term bioacoustics time series.

Algorithms↗
PubMedDate not supplied

A signal-detection experiment measuring the effect of computer-aided detection on radiologists' performance.

PURPOSE: To evaluate how the specificity and sensitivity of computer-aided detection (CADe) algorithm outputs affected radiologists' diagnostic performances, the authors studied the effects of 25 simulated CADe algorithms with various sensitivities and specificities (from 60% to 100%). METHODS: Six novice radiologists read 200 images that were produced by computer and randomly displayed on CRT, and their detection performances were evaluated with receiver operating characteristic analysis. RESULTS: There were significant differences in performance among CADe types (p < 0.001). DISCUSSION AND CONCLUSION: The overall accuracy of CADe outputs is the most significant factor affecting radiologists' performances in detection and interpretation of images. There is an approximately linear relationship between the sensitivity (specificity) of the CADe output and the reader's sensitivity (specificity), and the slope of reader sensitivity (specificity) as a linear function of CADe sensitivity (specificity) can be considered to be a positive number less than unity.

Algorithms↗
PubMed1993

Applications of artificial neural networks for ECG signal detection and classification.

The authors have investigated potential applications of artificial neural networks for electrocardiographic QRS detection and beat classification. For the task of QRS detection, the authors used an adaptive multilayer perceptron structure to model the nonlinear background noise so as to enhance the QRS complex. This provided more reliable detection of QRS complexes even in a noisy environment. For electrocardiographic QRS complex pattern classification, an artificial neural network adaptive multilayer perceptron was used as a pattern classifier to distinguish between normal and abnormal beat patterns, as well as to classify 12 different abnormal beat morphologies. Preliminary results using the MIT/BIH (Massachusetts Institute of Technology/Beth Israel Hospital, Cambridge, MA) arrhythmia database are encouraging.

Arrhythmias, Cardiac↗
PubMed1998

Transcription regulatory region analysis using signal detection and fuzzy clustering.

MOTIVATION: Presently available programs for the recognition of potential transcription factor binding sites in genomic sequences generally yield a huge amount of output. These output lists have to be filtered to obtain biologically significant elements, which is highly laborious work to be done manually. RESULTS: We developed a strategy for systematic verification and improvement of the underlying profiles, and for their contextual analysis by a fuzzy clustering approach using non-redundant libraries of search profiles as a prerequisite. AVAILABILITY: The tools mentioned in the paper are available upon request. CONTACT: ewi@gbf.de

Algorithms↗
PubMed1996

Signal detection by the PhoQ sensor-transmitter. Characterization of the sensor domain and a response-impaired mutant that identifies ligand-binding determinants.

The PhoP-PhoQ two-component system is required for virulence and/or regulatory stress responses in enteric bacteria. The PhoQ protein responds to low concentrations of extracellular divalent cations by activating PhoP-mediated transcription of a set of genes. PhoQ is a member of a family of transmembrane proteins that contain a periplasmic sensor domain coupled to a cytoplasmic transmitter domain. Here, we describe the cloning, purification, and properties of a fragment of Escherichia coli PhoQ corresponding to the sensor domain. This fragment is monomeric in solution and has a circular dichroism spectrum indicative of a mixture of alphahelix and beta-sheet. Divalent cations do not affect the oligomeric state, circular dichroism spectrum, or fluorescence spectrum of the sensor domain but do stabilize this domain to denaturation in a fashion expected for a direct binding model. We have also constructed a mutant in which a cluster of acidic amino acids (EDDDDAE) in the sensor domain is replaced with conservative, uncharged residues (QNNNNAQ). The mutant sensor domain is indistinguishable from wild type in terms of oligomeric form and spectral properties but differs in being substantially more stable to urea denaturation, showing no additional stabilization in the presence of divalent cations, and showing little activation of PhoP-mediated transcription in response to divalent-cation starvation in vivo. These data are consistent with a model in which divalent cations bind to the acidic cluster of the wild-type sensor domain and stabilize a conformation that is inactive in signaling. Substituting uncharged residues for the acidic cluster appears to mimic the effect of divalent-cation binding by stabilizing the inactive conformation.

Amino Acid Sequence↗
PubMed1988

[Signal detection by analog/digital conversion using personal computers. III: Examples from general practice and organizational aspects].

Technical and organisational aspects of the setup of an analog/digital conversion are discussed in the light of two clinical reference installations. The first example deals with a closed loop regulation system used to control a cardio-surgical intervention. This setup works with repeated turns of acquisition and evaluation. The second example describes a comprehensive haemodynamic beat-by-beat analysis, involving data acquisition from FM-coded tape, preprocessing on the PC, file transfer to a mainframe and a detailed statistical and graphical evaluation within the package SAS. Special attention is given to certain technical issues and to criteria of performance. Finally, setup procedures are suggested which may help in the avoidance of certain insufficiencies which are likely to occur in a first attempt to establish A/D conversion within clinical research projects.

Analog-Digital Conversion↗
PubMed1989

Magnetic resonance imaging with adiabatic pulses using a single surface coil for RF transmission and signal detection.

In order to overcome the problems that arise from nonuniform B1 fields, there has been interest in developing pulses that are insensitive to large variations in RF power. Pulses derived from adiabatic passage principles that can execute spin inversion, excitation, and 90 degrees and 180 degrees plane rotations in the presence of B1 inhomogeneities have recently been described. When driven with optimized modulation functions, these pulses can execute uniform excitation, refocusing, and slice-selective inversion over a 10-fold or greater variation in B1 magnitude. This insensitivity to B1 strength enables the execution of T1- and/or T2-weighted spin-echo imaging experiments using coils, such as the surface coil, with extremely inhomogeneous B1 profiles. We have successfully acquired images with these pulses at 200 MHz using a single surface coil as the transmitter and receiver. Images of the slice definition, the region over which the excitation and refocusing pulses operate with a surface coil, and brain images obtained with slice planes perpendicular to the plane of the surface coil are presented. Results demonstrate that these pulses can be transmitted with a surface coil to yield high-quality T1- and/or T2-weighted images without B1 artifacts.

Animals↗
PubMed1996

Multiple-Event Forced-Choice Tasks in the Theory of Signal Detectability

Receiver operating characteristic (ROC) analysis is generalized to unidimensional forced-choice tasks involving three or more events. It is shown that the performance of an observer in a unidimensional identification task with n independent events can be represented in n! ROC spaces of dimension n. Each ROC space is associated with a unique pairing of the events and decisions. A hypersurface can be generated in each ROC space by manipulating the observer's decision criteria. Using information theory, a new measure of discriminability based on the hypervolumes under the hypersurfaces is defined. This measure, denoted D, is nonparametric and independent of the criteria. The value of D is shown to increase monotonically with n and to be equal to the channel capacity of an observer in an n-interval forced-choice task. Procedures to compute ROC hypersurfaces from ratings and from ROC curves are given.

Journal Article↗