PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “threshold model”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,387 records · Page 77Linked to original sources
PubMed2004

Threshold-linear analysis of measures of fertility in artificial insemination data and days to calving in beef cattle.

Mating and calving records for 47,533 first-calf heifers in Australian Angus herds were used to examine the relationship between days to calving (DC) and two measures of fertility in AI data: 1) calving to first insemination (CFI) and 2) calving success (CS). Calving to first insemination and calving success were defined as binary traits. A threshold-linear Bayesian model was employed for both analyses: 1) DC and CFI and 2) DC and CS. Posterior means (SD) of additive covariance and corresponding genetic correlation between the DC and CFI were -0.62 d (0.19 d) and -0.66 (0.12), respectively. The corresponding point estimates between the DC and CS were -0.70 d (0.14 d) and -0.73 (0.06), respectively. These genetic correlations indicate a strong, negative relationship between DC and both measures of fertility in AI data. Selecting for animals with shorter DC intervals genetically will lead to correlated increases in both CS and CFI. Posterior means (SD) for additive and residual variance and heritability for DC for the DC-CFI analysis were 23.5 d2 (4.1 d2), 363.2 d2 (4.8 d2), and 0.06 (0.01), respectively. The corresponding parameter estimates for the DC-CS analysis were very similar. Posterior means (SD) for additive, herd-year and service sire variance and heritability for CFI were 0.04 (0.01), 0.06 (0.06), 0.14 (0.16), and 0.03 (0.01), respectively. Posterior means (SD) for additive, herd-year, and service sire variance and heritability for CS were 0.04 (0.01), 0.07 (0.07), 0.14 (0.16), and 0.03 (0.01), respectively. The similarity of the parameter estimates for CFI and CS suggest that either trait could be used as a measure of fertility in AI data. However, the definition of CFI allows the identification of animals that not only record a calving event, but calve to their first insemination, and the value of this trait would be even greater in a more complete dataset than that used in this study. The magnitude of the correlations between DC and CS-CFI suggest that it may be possible to use a multitrait approach in the evaluation of AI and natural service data, and to report one genetic value that could be used for selection purposes.

Animals↗
PubMed2006

Association analyses between the prion protein locus and reproductive and lamb weight traits in Ripollesa sheep.

The objective of this study was to analyze the association between the haplotypes of the prion protein (PrP) locus and several reproductive and lamb weight traits in Ripollesa sheep. Prion protein genotypes were available for a total of 310 sheep (7 rams, 114 ewes, and 189 lambs), all of them belonging to the purebred Ripollesa flock of the Universitat Autònoma of Barcelona, for which all sheep had a known pedigree. In addition, the genotype of 24 historical descendants of the previously genotyped adult individuals was reconstructed, provided that both parents were homozygous for PrP haplotypes. Only 3 haplotypes (ARR, ARQ, and ARH) were observed in the PrP locus of the sheep sampled. Reproductive traits included conception rate and litter size, whereas birth BW and 90-d BW were the lamb weight traits studied. The additive effect of PrP haplotypes was analyzed through Bayesian animal threshold and linear models, for reproduction and weight traits, respectively. Ewe reproductive data belonged to 89 ewes that gave 492 conception rate records and 440 litter size records. Analyses of BW at birth and at 90 d of age were made on 323 and 164 lamb records, respectively. No associations between PrP haplotypes and conception rate and BW traits were observed. For litter size, the effect of the ARH haplotype was greater than that of the ARQ haplotype. Differences between ARH and ARR haplotypes also suggested an advantage for the ARH. As a whole, our results indicated that the selection favorable to increase litter size in Ripollesa ewes may also increase the ARH haplotype frequency, which contradicts the recommendations of the current European Union legislation aiming to increase the genetic resistance to scrapie. As a consequence, scrapie genotyping needs to be included as a new selection criterion in the breed.

Animals↗
PubMed1992

Models for studying the progression of hearing loss caused by noise.

A computer-controlled fixed-frequency Békésy audiometer has been used together with simplified mathematical models for hearing threshold measurements and analysis in 1,796 subjects from seven different noise environments with exposures varying from less than 80 dB(A) to greater than 95 dB(A). The methods of measurement and analysis have been used to determine hearing loss caused by noise for various age groups for men and women. It has also been possible to estimate the equivalent noise level to which workers are nowadays subjected under hearing protectors and the reduction in exposure level over the past 40 years. The results illustrate the effects of noise-reduction measures and the use of hearing protectors on the development of hearing loss suffered by workers in the actual Swedish industries.

Adult↗
PubMed2005

The remember response: subject to bias, graded, and not a process-pure indicator of recollection.

Recognition memory judgments have long been assumed to depend on the contributions of two underlying processes: recollection and familiarity. We measured recollection with receiver-operating characteristic (ROC) data and remember-know judgments. Under standard remember-know instructions, the two estimates of recollection diverged. When subjects were told they might need to justify their remember responses to the experimenter, the two estimates were more likely to agree. The data support the conclusion that remember responses are generally based on a continuous underlying process but that specific task instructions can produce data that appear consistent with a high-threshold recollective process. Models based on signal detection theory provide a better account of these data than does the dual-process model (Yonelinas, 1994) or process-pure interpretations.

Affect↗
PubMed1996

Individual smoothing of blood pressure profiles to define responders or non-responders to drugs.

OBJECTIVE: To compare different methods of individual therapyh efficacy assessment in order to define responding subjects. METHODS: Hypertensive patients were included in three double-blind clinical trials (placebo versus bisoprolol, lisinopril and amlodipine) and ambulatory blood pressure measurements (four per hour) were performed at the end of each month. We analysed the effect of therapy (placebo minus treatment) according to the following criteria: type of model (hourly mean, moving average, fast Fourier analysis), determination of the time to the peak effect (the lowest value of the modelled blood pressure) and the sampling time around this peak (1, 2,., 24 h). RESULTS: Regardless of the type of model, the level of individual therapy efficacy is significantly higher than that of the overall subjects (group efficiency), when the sampling time around this peak decreases. The proportion of responders decreases as the sampling time used to calculate the drop in blood pressure increases, whatever the kind of model and the threshold used to define responders (5 or 10 mmHg in systolic blood pressure). CONCLUSION: By this method, it is possible to appreciate the percentage of subjects considered individuallyas responders according to the time around the peak. This evaluation complements information given by the trough: peak ratio.

Journal Article↗
PubMed1999

Epilepsy in women: the science of why it is special.

Epilepsy is a common neurological disorder that may be affected by reproductive hormones and may complicate reproductive health. Many women with epilepsy experience changes in seizure frequency and severity with changes in reproductive cycles, including at puberty, over the menstrual cycle, with pregnancy and at menopause. Ovarian steroids alter neuronal excitability at the membrane and in the genome. Altered protein synthesis as a consequence of changes in RNA mediated gene transcription is one mechanism for steroid mediated effects on excitability. These genomic effects are delayed and sustained. In contrast, membrane effects are immediate and short duration. These effects are mediated at both the GABA-A and NMDA receptors. Estrogen also dynamically alters synaptic connectivity. Estrogen enhances excitability and lowers the seizure threshold, whereas progesterone enhances inhibition and increases the seizure threshold. In experimental models of epilepsy, estrogen is proconvulsant and progesterone is anticonvulsant. The net effect of these steroid actions is to alter neuronal excitability over physiological cycles. Some epilepsy syndromes are expressed or worsened at puberty. One third to one half of women with epilepsy have catamenial seizure patterns, with seizures most likely to occur in the perimenstrual period and at ovulation. More research is needed to understand the effects of menopause on epilepsy. Antiepileptic drugs may exacerbate the risk of reproductive endocrine disorders in women with epilepsy. Fertility rates are lower for women with epilepsy. Women with epilepsy are more likely to have anovulatory menstrual cycles, abnormal pituitary LH release and altered ovarian steroid concentrations. Polycystic ovaries are detected more often in women with epilepsy, particularly those on valproate. Treatment of hormone sensitive seizures relies on standard AEDs. Small trials suggest that adjunctive progesterone therapy is sometimes helpful. The newer AEDs, gabapentin and lamotrigine may have some advantages for women with epilepsy. These drugs do not alter levels of steroid hormones and do not interfere with effectiveness of hormonal contraception. Experience in pregnancy is limited. The dynamic effects of hormones on seizure expression and of seizures on reproductive health complicate the management of epilepsy in women. Newer AEDs may offer advantages for women with epilepsy in the reproductive years.

Epilepsy↗
PubMed1993

A comparison of the pharmacokinetic and pharmacodynamic properties of quinine and quinidine in healthy Thai males.

1. Eight healthy Thai males, aged 19-27 years, received quinine or quinidine dihydrochloride 10 mg kg-1 body weight by intravenous infusion over 1 h. At least 1 week later, the alternative alkaloid was administered. 2. The terminal elimination half-time of quinidine was shorter than that of quinine (median [range]; 5.7 [5.0-10.0] vs 9.9 [8.8-15.1] h, P < 0.01), the volume of distribution at steady state (Vss) for quinidine was larger than that for quinine (3.5 [2.5-5.6] vs 3.1 [1.8-4.1] 1 kg-1; P = 0.02) and quinidine was less bound to plasma proteins (% free drug: 22.8 [15.4-47.2] vs 9.4 [7.3-15.0]%, P < 0.01). Total clearance was greater for quinidine (7.7 [3.9-11.4] vs 3.4 [1.8-4.6] ml min-1 kg-1, P < 0.01) but not for clearance of unbound drug (32.2 [14.6-50.4] vs 29.9 [20.2-50.9] ml min-1 kg-1 respectively, P > 0.2). 3. Side-effects, including transient hypotension after quinidine in two cases, were mild. 4. Both drugs produced prolongation of the rate-corrected QT interval (QTc), with similar rates of elimination from the cardiac conduction 'effect' compartment (keo; 4.14 [0.03-15.33] h-1 for quinine, 3.74 [1.63-13.14] h-1 for quinidine, P > 0.19). Using a linear concentration-response model, the intercept ('threshold') for quinidine effect was lower than that for quinine (P = 0.004) but the slopes (change in QTc for a given change in free drug concentration) were similar (P = 0.56).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2004

Predicting the threshold of pulse-train electrical stimuli using a stochastic auditory nerve model: the effects of stimulus noise.

The incorporation of low levels of noise into an electrical stimulus has been shown to improve auditory thresholds in some human subjects (Zeng et al., 2000). In this paper, thresholds for noise-modulated pulse-train stimuli are predicted utilizing a stochastic neural-behavioral model of ensemble fiber responses to bi-phasic stimuli. The neural refractory effect is described using a Markov model for a noise-free pulse-train stimulus and a closed-form solution for the steady-state neural response is provided. For noise-modulated pulse-train stimuli, a recursive method using the conditional probability is utilized to track the neural responses to each successive pulse. A neural spike count rule has been presented for both threshold and intensity discrimination under the assumption that auditory perception occurs via integration over a relatively long time period (Bruce et al., 1999). An alternative approach originates from the hypothesis of the multilook model (Viemeister and Wakefield, 1991), which argues that auditory perception is based on several shorter time integrations and may suggest an NofM model for prediction of pulse-train threshold. This motivates analyzing the neural response to each individual pulse within a pulse train, which is considered to be the brief look. A logarithmic rule is hypothesized for pulse-train threshold. Predictions from the multilook model are shown to match trends in psychophysical data for noise-free stimuli that are not always matched by the long-time integration rule. Theoretical predictions indicate that threshold decreases as noise variance increases. Theoretical models of the neural response to pulse-train stimuli not only reduce calculational overhead but also facilitate utilization of signal detection theory and are easily extended to multichannel psychophysical tasks.

Algorithms↗
PubMed2001

Analysis of monophasic and biphasic electrical stimulation of nerve.

In an earlier study, biphasic and monphasic electrical stimulation of the auditory nerve was performed in cats with a cochlear implant. Single-unit recordings demonstrated that spikes resulting from monophasic and biphasic stimuli have different thresholds and latencies. Monophasic thresholds are lower and latencies are shorter under cathodic stimulation. Results from stochastic simulations of a biophysical model of electrical stimulation are similar. A simple analysis of a linear, "integrate to threshold" membrane model accounts for the threshold and latency differences observed experimentally and computationally. Since biphasic stimuli are used extensively in functional electrical stimulation, this analysis greatly simplifies the biophysical interpretation of responses to clinically relevant stimuli by relating them to the responses obtained with monophasic stimuli.

Action Potentials↗
PubMed2006

Obsessive-compulsive disorder, tics and anxiety in 6-year-old twins.

BACKGROUND: Previous reports of genetic influences on obsessive-compulsive disorder (OCD) symptoms have suggested moderate heritability. Family history studies of co-morbidity have found familial aggregation with tics, especially for early-onset OCD, and familial aggregation with anxiety disorders. METHOD: Heritability of OCD and familial aggregation of OCD, tics and anxiety disorders were investigated in a community sample of 6-year-old twins using a two-phase design in which 4662 twin pairs were sampled and 854 pairs were assessed in the second phase by maternal-informant diagnostic interview using DSM-IV criteria. RESULTS: In the multivariate model combined additive genetic and common environmental effects were estimated as 47% for sub-threshold OCD, and the model was unable to distinguish these sources of familial aggregation. There were strong familial aggregations between sub-threshold OCD and tics and between sub-threshold OCD and other anxiety disorders (80% and 97% respectively), although again specific sources could not be distinguished. CONCLUSIONS: The findings are consistent with the hypothesis of a tic-related early-onset OCD phenotype, but also with the hypothesis of an anxiety-related early-onset OCD phenotype.

Anxiety Disorders↗
PubMedDate not supplied

The relationship of somatization and depression to experimental pain response in women with temporomandibular disorders.

OBJECTIVE: Patients with temporomandibular pain disorders (TMD) have greater experimental pain perception when compared with pain-free controls. Common psychological features of TMD include somatization and depression. The impact of depression on experimental pain perception has received considerable attention. However, the role of somatization on experimental pain in a chronic pain population has not been explored. METHODS: Fifty-six women with TMD and 59 pain-free controls underwent three experimental pain procedures, including palpation at fixed amounts of pressure, pressure pain thresholds, and an ischemic pain task. Levels of depression and somatization were assessed using the Research Diagnostic Criteria for TMD. Multiple regression analyses were performed to determine the extent to which depression and somatization were associated with experimental pain response. RESULTS: After controlling for characteristic pain intensity and depression, somatization explained a significant proportion of variance in numbers of masticatory sites rated as painful (R2 change = 6.7%, p = .046) with the full model explaining 16.4% of the variance (p = .024). This did not meet an adjusted level of statistical significance (p = .008). After controlling for characteristic pain, only depression added significantly to the model predicting ischemic pain threshold and tolerance. The full models including characteristic pain and depression explained 49% and 20% of the variance in ischemic pain threshold and tolerance, respectively. CONCLUSIONS: These findings suggest that depression and somatization are associated with different measures of experimental pain. Somatization may be related to more attentional and perceptual measures of clinically relevant pain while depression may be related to more behavioral measures of pain.

Adolescent↗
PubMed2002

Effects of furosemide applied chronically to the round window: a model of metabolic presbyacusis.

Hearing thresholds in elderly humans without a history of noise exposure commonly show a profile of a flat loss at low frequencies coupled with a loss that increases with frequency above approximately 2 kHz. This profile and the relatively robust distortion product otoacoustic emissions that are found in elderly subjects challenge the common belief that age-related hearing loss (presbyacusis) is based primarily on sensory-cell disorders. Here, we examine a model of presbyacusis wherein the endocochlear potential (EP) is reduced by means of furosemide applied chronically to one cochlea of a young gerbil. The model results in an EP that is reduced from 90 to approximately 60 mV, a value often seen in quiet-aged gerbils, with no concomitant loss of hair cells. Resulting measures of cochlear and neural function are quantitatively similar to those seen in aging gerbils and humans, e.g., a flat threshold loss at low frequencies with a high-frequency roll-off of approximately -8.4 dB/octave. The effect of the EP on neural thresholds can be parsimoniously explained by the known gain characteristics of the cochlear amplifier as a function of cochlear location: in the apex, amplification is limited to approximately 20 dB, whereas in the base, the gain can be as high as 60 dB. At high frequencies, amplification is directly proportional to the EP on an approximately 1 dB/mV basis. This model suggests that the primary factor in true age-related hearing loss is an energy-starved cochlear amplifier that results in a specific audiogram profile.

Action Potentials↗
PubMed1997

Orientation tuning of the spatial-frequency-tuned mechanisms of the red-green channel.

Significant orthogonal masking for color stimuli [Invest. Ophthalmol. Visual Sci. 34, 782 (No. 405-54) (1993)] but not for achromatic stimuli [J. Opt. Soc. Am. A 1, 226 (1984)] under sustained presentation led us to investigate the orientation tuning of the spatial-frequency- (SF-) tuned color mechanisms. The Red-Green channel was isolated from the achromatic channel by the minimum flicker technique and from the Yellow-Blue channel by the hue cancellation technique. Contrast sensitivity functions, threshold elevation versus mask orientation curves (measured by orientation masking), and threshold elevation versus mask contrast curves were measured by the method of constant stimuli and a two-interval forced-choice technique on two normal observers. Test targets were spatially localized (D6), vertical color patterns, and masks were sinusoidal color patterns oriented 15 degrees-90 degrees from the vertical in 15 degrees steps and had the same SF's as those of test patterns. Mask contrasts were varied between 1.2% and 60%. The orientation tuning curves of the SF-tuned color mechanisms were extracted by obtaining the best fit to contrast sensitivity and threshold elevation data simultaneously at a given SF with use of the masking model. Results show that threshold elevations depend on test SF, mask SF, mask orientation, and mask contrast. Half-bandwidths at half-height (with respect to 15 degrees from the vertical) of threshold elevation versus mask orientation curves range from 90 degrees to 29 degrees depending on SF's. The slopes of threshold elevation versus mask contrast curves range from 0.76 to 0.29 on octave-octave coordinates depending on SF's. Orientation half-bandwidths at half-height of orientation tuning curves of the SF-tuned color mechanisms (1) range from 79 degrees to 28 degrees and (2) average from 68 degrees to 30 degrees for SF's 0.063-8 cycles per degree (cpd). Data suggest that the orientation tuning curves of the SF-tuned chromatic mechanisms are broader (except at 2 cpd) than those of the achromatic mechanisms (orientation half-bandwidths: 32 degrees-15 degrees for 0.5-11.3 cpd [J. Opt. Soc. Am. A 1, 226 (1984)]); moreover, the orientation bandwidths are SF dependent.

Algorithms↗
PubMed1997

Threshold-dependent Limit Cycles in a Nonlinear Network Model.

A mathematical model of the dynamics of a neural network, in which synaptic strength varies simultaneously with activation, is analyzed. This feedback network, with infinite-gain transfer function, is described by a system of ordinary differential equations. It is shown that the resulting discontinuous nonlinear dynamical system has multiple stable periodic solutions. The existence of these periodic attractors depends critically on the value of the threshold parameter; and by varying the threshold the network can be made to cycle through a sequence of attracting states. Copyright 1997 Elsevier Science Ltd.

Journal Article↗
PubMed2006

Multi-state epidemic processes on complex networks.

Infectious diseases are practically represented by models with multiple states and complex transition rules corresponding to, for example, birth, death, infection, recovery, disease progression, and quarantine. In addition, networks underlying infection events are often much more complex than described by meanfield equations or regular lattices. In models with simple transition rules such as the SIS and SIR models, heterogeneous contact rates are known to decrease epidemic thresholds. We analyse steady states of various multi-state disease propagation models with heterogeneous contact rates. In many models, heterogeneity simply decreases epidemic thresholds. However, in models with competing pathogens and mutation, coexistence of different pathogens for small infection rates requires network-independent conditions in addition to heterogeneity in contact rates. Furthermore, models without spontaneous neighbor-independent state transitions, such as cyclically competing species, do not show heterogeneity effects.

Communicable Diseases↗
PubMed2005

Encoding of vowel-like sounds in the auditory nerve: model predictions of discrimination performance.

The sensitivity of listeners to changes in the center frequency of vowel-like harmonic complexes as a function of the center frequency of the complex cannot be explained by changes in the level of the stimulus [Lyzenga and Horst, J. Acoust. Soc. Am. 98, 1943-1955 (1995)]. Rather, a complex pattern of sensitivity is seen; for a spectrum with a triangular envelope, the greatest sensitivity occurs when the center frequency falls between harmonics, whereas for a spectrum with a trapezoidal envelope, greatest sensitivity occurs when the center frequency is aligned with a harmonic. In this study, the thresholds of a population model of auditory-nerve (AN) fibers were quantitatively compared to these trends in psychophysical thresholds. Single-fiber and population model responses were evaluated in terms of both average discharge rate and the combination of rate and timing information. Results indicate that phase-locked responses of AN fibers encode phase transitions associated with minima in these amplitude-modulated stimuli. The temporal response properties of a single AN fiber, tuned to a frequency slightly above the center frequency of the harmonic complex, were able to explain the trends in thresholds for both triangular- and trapezoidal-shaped spectra.

Acoustic Stimulation↗
PubMed1988

Pharmacokinetic modeling of the anticonvulsant response of oxazepam in rats using the pentylenetetrazol threshold concentration as pharmacodynamic measure.

This investigation developed strategies along which the anticonvulsant effect of oxazepam in the rat could be pharmacokinetically modeled. After determination of the pharmacokinetics of oxazepam, which could be described with a two-compartment model (half-lives of distribution and elimination 6 and 52 min, respectively), the drug was administered iv to groups of animals to achieve a serum concentration range of 0.1-2.5 mg/L at 10, 45, and 120 min after administration. At these time points pentylenetetrazol (PTZ) was infused slowly until the first myoclonic jerk occurred. The anticonvulsant response, expressed as the elevation of the serum or brain threshold concentration of PTZ, was modeled versus the serum (both total and free) and brain oxazepam concentration, according to the sigmoid Emax model. The total serum and brain oxazepam EC50 values are about 0.5 mg/L and 1.1 mg/kg, respectively, and Emax 120 mg/L PTZ. No marked differences in pharmacodynamic parameters between the three time groups were found, which indicates that serum and brain are pharmacokinetically indistinguishable from the effect compartment, that there is no (inter)activity of oxazepam metabolites and absence of development of acute tolerance during the investigated time frame. An interfering role of metabolites was also excluded by a direct radioreceptor assay of oxazepam, yielding very similar results as the specific chromatographic assay. It is concluded that the conception-anticonvulsant effect relationship of oxazepam can satisfactorily be described by the sigmoid Emax model, when utilizing the employed experimental strategies.

Animals↗