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PubMed1993

A model to refine the selection of children with otitis media with effusion for adenoidectomy.

The selection of children with otitis media with effusion for adenoidectomy has been criticized on a number of grounds. In spite of the widespread popularity of the procedure there are few objective guidelines to help the otolaryngologist make a decision which is therefore usually subjective. We present a scientifically derived model using a Generalized Linear Interactive Modelling (GLIM) technique from a population of 122 children who underwent either adenoidectomy or no pharyngeal surgery. The outcome parameter studied was otoscopic clearance in an unoperated ear 1 year after treatment. This was related principally to the operation type (P < 0.001) and to the age at which the operation was performed (P < 0.005). Using a probability of clearance threshold of 50% the model was tested on a prospective population of 166 children and outcome was correctly predicted in over 72%. This model enables the surgeon to determine the probability of clearance of effusion following adenoidectomy in an individual child. At a probability threshold of 50% adenoidectomy would be offered to children with bilateral otitis media with effusion aged between 4.33 and 8.00 years. Over the age of 8 years the probability of clearance occurring spontaneously increases beyond 50%. By careful selection of the probability of clearance thresholds it was possible to produce otoscopic resolution in over 80% of those selected to undergo adenoidectomy. The model also enables the otolaryngologist to determine the proportion of children with established disease that require adenoidectomy at his own chosen probability thresholds which will ensure that a more effective resource allocation is achieved.

Adenoidectomy↗

PubMed2004

Squeezing interval change from ordinal panel data: latent growth curves with ordinal outcomes.

A didactic on latent growth curve modeling for ordinal outcomes is presented. The conceptual aspects of modeling growth with ordinal variables and the notion of threshold invariance are illustrated graphically using a hypothetical example. The ordinal growth model is described in terms of 3 nested models: (a) multivariate normality of the underlying continuous latent variables (yt) and its relationship with the observed ordinal response pattern (Yt), (b) threshold invariance over time, and (c) growth model for the continuous latent variable on a common scale. Algebraic implications of the model restrictions are derived, and practical aspects of fitting ordinal growth models are discussed with the help of an empirical example and Mx script (M. C. Neale, S. M. Boker, G. Xie, & H. H. Maes, 1999). The necessary conditions for the identification of growth models with ordinal data and the methodological implications of the model of threshold invariance are discussed.

Adolescent↗

PubMed2000

Simulation of longitudinal threshold visual field data.

PURPOSE: To describe and evaluate a computer model that simulates longitudinal visual field data. METHODS: A computer model was designed using factors that influence thresholds of normal and glaucomatous visual fields. The simulation model was used to quantify the effects of fluctuation on the outcomes of pointwise linear regression by comparison with simulated gold standard data with no variability. RESULTS: Serial sets of 10 stable and 10 progressive visual fields with different fluctuation levels were generated by simulation and were analyzed using pointwise linear regression. Regression outcome measures used were slopes of -1 dB/year or worse and slopes of -1 dB/year or worse that were also statistically significant. In stable visual fields, the number of locations with regression slopes worse than -1 dB/year increased with fluctuation and defect size and was inversely related to the number of fields. The number of locations with statistically significant slopes remained low and appeared unaffected by these variables. In progressive visual fields, analysis of a small number of visual field test results (<8) overestimated the number of locations with regression slopes worse than -1 dB/year and underestimated the number of locations with statistically significant slopes. CONCLUSIONS: Computer simulation may be used to provide a gold standard outcome that permits evaluation of statistical tools for monitoring progressive glaucomatous visual field loss.

Computer Simulation↗

PubMed1984

Temporal covariance model of human motion perception.

We propose a model of direction-sensitive units in human vision. It is a modified and elaborated version of a model by Reichardt [Z. Naturforsch . Teil B 12, 447 (1957)]. The model is applied to threshold experiments in which subjects view adjacent vertical bars with independently (typically sinusoidally), temporally modulated luminances. The subject must report whether the patterns moved to the left or to the right. According to the model, a basic motion-detecting unit consists of two subunits tuned to opposite directions. Each performs a spatial and temporal linear filtering of its input; outputs of the filters are multiplied, and the multiplied output is integrated (for a time that is long relative to the modulation period). The model's output consists of the difference between the subunit outputs. Direction of movement is indicated by the sign of the model output. Mathematical analysis of the model yielded several predictions that were confirmed experimentally. Specifically, we found that (1) performance with complex patterns can be predicted by spatiotemporal Fourier analysis that results in the segregation and linear addition in the output for different temporal frequencies; (2) under special conditions, performance depends on the product of adjacent bar amplitudes, offering strong support for the multiplication principle; (3) performance is unaffected by addition of stationary patterns; and (4) addition of homogeneous flicker normally produces no effect but under special conditions reverses perceived direction. These and other results confirm our model and reject several other models, including Reichardt 's original model.

Fourier Analysis↗

PubMed1996

Angle judgement: is the whole the sum of its parts?

This study concerns whether the discrimination of a geometric angle depends on the orientations of its bounding lines or on angle size. In Experiment 1, thresholds for angle discrimination were measured in three observers for angles ranging from 15 to 180 deg, oriented either vertically or obliquely. Angle discrimination thresholds were found to depend primarily on angle size for most of the range of angles (angle-dependent, or Weber's law regime). However, in a small region near 90 deg (orientation-dependent regime) angle discrimination depends on the orientations of the bounding lines. When our data in the angle-dependent regime were fitted with a power function, the exponents were close to or < 0.5, suggesting that a step-increment approach was used to calculate angle. In Experiment 2, orientation discrimination thresholds for lines corresponding to the bounding lines of the vertically and obliquely oriented 15, 90 and 165 deg angles were measured. Confirming previous studies, a strong meridional anisotropy in line orientation discrimination was found for all three observers. The orientation discrimination thresholds were then used to predict the discrimination thresholds for the corresponding angles based on a simple statistical model. The predicted angle discrimination thresholds were worse than those measured empirically except for the titled 90 deg angles. This result indicates that angle discrimination thresholds are not limited by the same noise as orientation discrimination for most angles except for the tilted 90 deg angle, where the limiting factor may be the precision in determining the orientations of the bounding lines. In Experiment 3, we show that angle discrimination is quite robust to small amounts of orientation jitter, suggesting that angle judgments are made at a level beyond the early filter representation.

Differential Threshold↗

PubMed2004

Bottom-up excitable models of phytoplankton blooms.

A simple nutrient-phytoplankton model is used to explore the dynamics of phytoplankton blooms. The model exhibits excitable behaviour in the sense that a large scale outbreak can only be triggered when a critical nutrient threshold is exceeded. The model takes into account several features often neglected but whose combined effect proves very important: (i) rapid nutrient recycling associated with the microbial loop and patch formation; (ii) self-shading; and (iii) a bottom-up approach, whereby nutrient levels are responsible for both the triggering and the demise of the bloom. Although the literature is replete with studies of 'top-down' models in which zooplankton grazing control the triggering and demise of the bloom, bottom-up models are nevertheless appropriate in many circumstances. We provide a full mathematical investigation of the effects of these three different features in an excitable system framework.

Models, Biological↗

PubMed2001

An examination of several characteristics that affect the prediction of OSPL90 in hearing aids.

OBJECTIVE: Investigators at the National Acoustic Laboratories have provided a theoretical derivation and experimental validation of a formula for setting the maximum output of hearing aids (Dillon & Storey, 1998; Storey, Dillon, Yeend, & Wigney, 1998). Given that measurement of discomfort levels for setting maximum output can be both time-consuming and of questionable reliability, the use of a prescriptive formula warrants consideration. In this article, an extensive data base was considered and issues of normal hearing, clinical protocol, age and gender were investigated in an effort to further determine optimal maximum output settings. DESIGN: Data were gathered from five previous investigations, for a total of 433 subjects (total ears = 710). Threshold of discomfort (TD) measures were obtained using one of two adaptations of the Ascending Method of Limits, one with category anchors and one without. RESULTS: Subjects with normal hearing had significantly lower TDs than subjects with hearing loss. A different regression line for measured TDs as a function of hearing level was noted for subjects whose hearing threshold levels fell between 20 and 60 dB HL and those with thresholds above 60 dB HL. When all effects (hearing level, method, age and gender) were considered in a single predictive model for the two threshold groups, only method and threshold were significant predictors of TD. However, for the subjects with thresholds between 20 and 60 dB HL, less than 4% of the variance in TD measures could be accounted for by those factors. For subjects with threshold above 60 dB HL, 22% of the variance in TD measures could be accounted for by those variables. CONCLUSIONS: For both groups of subjects (20 to 60 dB HL and above 60 dB HL) methodology and hearing thresholds are significant predictors of discomfort levels. Age and gender are not. Given the small variance accounted for by any of these factors, measures of discomfort using standardized methodologies seem warranted.

Acoustic Stimulation↗

PubMed2005

Testosterone, dehydroepiandrosterone, and physical performance in older men: results from the Massachusetts Male Aging Study.

OBJECTIVE: This manuscript examines the relationships of total testosterone (T), bioavailable T, dehydroepiandrosterone (DHEA), and DHEA sulfate (DHEAS) to measures of physical performance in a large, population-based, random sample of men. METHODS: In the most recent wave of the Massachusetts Male Aging Study, measures of strength and physical performance [seven-item physical performance test (PPT), timed chair stand test, and grip strength] were made in 684 men, aged 55-85 yr. Complete hormone data were also obtained. Initial graphical exploration of performance outcomes as a function of hormone levels showed linear increases in physical performance up to certain threshold hormone concentrations, beyond which the associations were diminished. Regression models were used to estimate threshold locations and standardized regression coefficients quantifying the association between hormones and strength. RESULTS: All hormones exhibited significant age-adjusted positive association with PPT score below, but not necessarily above, the thresholds. DHEA was positively associated with chair stand score below, but was not above the threshold. None of the hormones studied was significantly associated with grip strength. CONCLUSION: Up to certain critical concentrations, elevated levels of TT, total T, bioavailable T, DHEA, and DHEA sulfate are associated with increased physical performance, as indicated by the PPT. However, levels beyond those critical concentrations, as might be achieved through exogenous supplementation, do not appear to confer any additional benefit. In general, hormone concentrations do not appear to be meaningfully associated with grip strength or chair stand scores.

Adult↗

PubMed2006

Resolution of complex motion detectors in the central and peripheral visual field.

We examine how local direction signals are combined to compute the focus of radial motion (FRM) in random dot patterns and examine how this process changes across the visual field. Equivalent noise analysis showed that a loss in FRM accuracy was largely attributable to an increase in local motion detector noise with little or no change in efficiency across the visual field. The minimum separation for discriminating the foci of two overlapping optic flow patterns increased in the periphery faster than predicted from the resolution for a single FRM. This behavior requires that observers average numerous local velocities to estimate the FRM, which enables resistance to internal and external noise and endows the system with the property of position invariance. However, such pooling limits the precision with which multiple looming objects can be discriminated, especially in the peripheral visual field.

Differential Threshold↗

PubMedDate not supplied

[Latent inhibition threshold in Roman high-avoidance rats: a psychogenetic model of abnormalities in attentional filter?].

INTRODUCTION: Basic research devoted to the study of the psychobiological anomalies of schizophrenia, as well as of its treatments, has used animal models in which some psychotic-like symptoms are induced by administration of psychostimulant drugs. There is, however, a growing necessity of having animal models presenting better construct validity, i.e., animal lines spontaneously showing phenotypes associated to the psychotic spectrum (for instance, enhanced sensitivity to psychostimulants, or cognitive and attentional anomalies characteristic of schizophrenic disorders). Several lines of evidence suggest that the RHA (Roman high-avoidance) rat strain presents a neurobehavioral profile which is consistent with such goals. METHODS: RHA rats were compared to Sprague-Dawley (SD) rats (as a standard control strain) for the expression of latent inhibition (in a 100-trial session of two-way active avoidance) under threshold conditions (i.e., only 15 preexposures to the conditioned stimulus were administered). RESULTS: Under such experimental conditions SD rats showed significant latent inhibition of the two-way active avoidance response (both during the first 50 trials and in the whole 100-trial session), while that attentional phenomenon did not appear in the RHA strain. CONCLUSIONS: The experimental results obtained here indicate that RHA rats display a deficit of latent inhibition at threshold conditions, an information processing (or attentional) anomaly which typically appears in schizophrenic patients. It is proposed that RHA rats might be an useful animal model for the study of vulnerability to some schizophrenic symptoms. This conclusion is supported by data that indicate that latent inhibition deficits are a characteristic attentional abnormality of these diseases.

Animals↗

PubMed2006

Age-dependent responses to nerve injury-induced mechanical allodynia.

BACKGROUND: Developmental differences in responses to acute and chronic nerve injury have received minimal attention. This study examines developmental differences in behavioral responses to a proximal (closer to the spinal cord) (L5 and L6 spinal nerve root ligation) or to a more distal (closer to peripheral innervation) (partial sciatic nerve ligation) nerve injury in rats paralleling the infant to young adult human. METHODS: Withdrawal thresholds to von Frey filament testing in the hind paw were determined before and various times after either spinal nerve root ligation or partial sciatic nerve ligation in rats aged 2, 4, and 16 weeks. Control rats of these ages were observed serially without surgery. Times for withdrawal thresholds to mechanical stimuli to return to 80% of that of the hind paw in the control animals were compared among the different ages in the two models. RESULTS: Baseline withdrawal thresholds in younger rats were lower (P < 0.05). In the 2-week-old animals, distal injury partial sciatic nerve ligation did not cause a reduction in withdrawal threshold from baseline. This was different from the spinal nerve root ligation group and the older animals in the partial sciatic nerve ligation group. However, when compared with age-matched control animals, both nerve injuries resulted in reduced withdrawal thresholds (P < 0.05). The resolution of hypersensitivity to mechanical stimulation, as measured by return of threshold to 80% of controls, occurred more quickly in 2-week-old than in 4- and 16-week-old animals in both injury models (P < 0.05). CONCLUSION: These data suggest that resolution of sensitization to A-fiber input occurs more rapidly in young animals. In addition, distal injury has less of a sensitizing effect on A-fiber input than proximal injury in the younger animals. The authors speculate that neuroimmune responses, especially at the site of injury, are developmentally regulated and less likely to produce chronic pain when injury occurs at a young age.

Aging↗

PubMed1999

The spatial properties of a model neuron increase its coding range.

The coding properties of one-compartment and two-compartment model neurons are compared. The membrane depolarization in both models is described as a deterministic leaky integrator. Interspike intervals are identified with the periods between reset of the depolarization after firing and consecutive crossing of a fixed firing threshold. The two-point model has an input in the dendritic compartment and an output in the trigger-zone compartment. it is shown that the sensitivity threshold for the two-point model is shifted to the larger values of the input intensity with respect to the sensitivity threshold of its single-point counterpart. Further, its coding range is substantially larger than the coding range of the single-point model.

Models, Neurological↗

PubMed1996

Determination of the viremia threshold for dental cross-infection in a mouse model.

An animal model of dental virus transmission was developed using the lactate dehydrogenase-elevating virus (LDV) of mice to study cross infection. Mouse-to-mouse cross-infection was carried out by scaling the teeth of LDV-infected donor mice with dental instruments, immediately prior to using the contaminated instruments on the teeth of recipient indicator mice. The level of donor viremia was found to correlate with the rate of virus cross-infection, with a viremia threshold level of 10(7.5) ID50/ml observed for dental cross-infection. The blood volume transferred during dental cross-infection was approximately 10(-4) to 10(-5) ml, demonstrating the inefficiency of virus cross-infection, since deposition of about 1000 virions on dental instruments was associated with the threshold limit. Virus transferred during dental cross-infection rapidly entered the blood circulation, showing that dental cross-infection was not dependent on an oral infection. The results from these model studies predict the general inefficiency of dental instrument virus cross-infection, and a further reduced likelihood of dental cross-infection with appropriately cleaned instruments.

Animals↗

PubMed2002

Effects of high-rate electrical stimulation upon firing in modelled and real neurons.

Many medical devices use high-rate, low-amplitude currents to affect neural function. This study examined the effect of stimulation rate upon action potential threshold and sustained firing rate for two model neurons, the rabbit myelinated fibre and the unmyelinated leech touch sensory cell. These model neurons were constructed with the NEURON simulator from electrophysiological data. Alternating-phase current pulses (0-1250 Hz), of fixed phase duration (0.2 ms), were used to stimulate the neurons, and propagation success or failure was measured. One effect of the high pulse rates was to cause a net depolarisation, and this was verified by the relief of action potential conduction block by 500 Hz extracellular stimulation in leech neurons. The models also predicted that the neurons would maintain maximum sustained firing at a number of different stimulation rates. For example, at twice threshold, the myelinated model followed the stimulus up to 500 Hz stimulation, half the stimulus rate up to 850 Hz stimulation, and it did not fire at 1250 Hz stimulation. By contrast, the unmyelinated neuron model had a lower maximum firing rate of 190 Hz, and this rate was obtained at a number of stimulation rates, up to 1250 Hz. The myelinated model also predicted sustained firing with 1240 Hz stimulation at threshold current, but no firing when the current level was doubled. Most of these effects are explained by the interaction of stimulus pulses with the cell's refractory period.

Animals↗

PubMed2005

The 2001 epidemic of foot-and-mouth disease in the United Kingdom: epidemiological and meteorological case studies.

The possibility of the airborne spread of foot-and-mouth disease during the 2001 epidemic in the uk has been investigated in three epidemiological case studies. On the basis of evidence from field investigations, and a simple meteorological analysis, it is concluded that the spread of disease was consistent with the airborne transport of virus. The distances ranged from less than 1 km to 16 km; six of the farms were over 6 km from the source and involved the passage of virus over the sea combined with meteorological conditions which strongly favoured airborne disease transmission. The results of detailed atmospheric modelling demonstrated that airborne virus could have challenged livestock on all the farms studied. However, with one exception the 24-hour average daily concentrations of the virus were significantly below the experimentally estimated threshold for infection. A detailed model intercomparison established that, under stable atmospheric conditions, peak concentrations of virus up to two orders of magnitude higher might have been experienced for short periods, owing to fluctuations within the plume of virus, and model limitations. This finding would significantly reduce the apparent discrepancy between the experimentally estimated threshold for infection and the modelling results.

Air Microbiology↗

PubMed1998

Estrogen and bone-muscle strength and mass relationships.

The largest voluntary loads on bones come from muscles. To adapt bone strength and mass to them, special strain threshold ranges determine where modeling adds and strengthens bone, and where remodeling conserves or removes it, just as different thermostat settings control the heating and cooling systems in a house. If estrogen lowers the remodeling threshold, two things should occur. First, at puberty in girls, bone mass should begin to increase more than in boys with similar muscle strengths, owing to reduced remodeling-dependent bone losses, while gains from longitudinal bone growth and bone modeling continue normally. That increase in bone mass in girls should plateau when their muscle strength stops increasing, since their stronger bones could then reduce bone strains enough to turn modeling off, but could let remodeling keep conserving existing bone. Second, decreased estrogen secretion [or a related factor(s)], as during menopause, should raise the remodeling threshold and make remodeling begin removing that extra bone. That removal should also tend to plateau after the remaining and weaker bone lets bone strains rise to the higher threshold. Postmenopausal bone loss shows the second effects. Previously unremarked relationships in the data of a 1995 Argentine study showed the first effects. This supports the idea that estrogen can affect human bone strength and mass by lowering the remodeling threshold, and loss of estrogen would raise the threshold and help cause postmenopausal bone loss even if other factors help to do it. The Argentine study also suggested ways to study those things and the roles of muscle strength and other factors in controlling bone strength and mass in children and adult humans. Those factors included, in part, hormones, vitamins, calcium, diet, sex, race, age, medications, cytokines, genetic errors, gene expression patterns, and disease.

Absorptiometry, Photon↗

PubMed2000

Prediction of neural excitation during magnetic stimulation using passive cable models.

A method for predicting neural excitation during magnetic stimulation using passive cable models has been developed. This method uses the information of the threshold capacitor voltage for magnetic stimulation coils to determine the equivalent excitation thresholds for the passive transient (PT) and passive steady-state (PSS) cable models as well as for the activating function. The threshold values for the PT, PSS models, and the activating function vary only with the pulsewidth of the stimulus for a variety of coils at different locations and orientations. Furthermore, the excitation threshold for the PSS model is also independent of axon diameter and best fitted to a simple mathematical function. By comparing the transmembrane potential of the PSS model with the corresponding threshold, the prediction of excitation during magnetic stimulation can be made. Similarly, it is also possible to predict excitation using the PT model and the activating function with the corresponding thresholds provided. By taking advantage of the weighted pulsewidth, this method can even predict the excitation for stimuli with various waveforms, greatly simplifying the determination of neural excitation for magnetic stimulation.

Axons↗

PubMed2004

Evidence of the antiepileptic potential of amiloride with neuropharmacological benefits in rodent models of epilepsy and behavior.

Sodium-hydrogen exchangers (NHEs) in the brain play a key role in regulating neuronal pH and, hence, modulate bioelectric and seizure activity. In this study, we investigated the anticonvulsant effect of amiloride (a NHE inhibitor) on increasing current electroshock (ICES) and pentylenetetrazole (PTZ)-induced seizures in mice. Further, the effect of amiloride on mood, memory, grip strength, and rotarod performance was also evaluated. The forced swimming test (FST) and spontaneous alternation behavior (SAB) models were employed to assess the effects on mood and memory, respectively. Amiloride produced a dose-dependent increase in seizure threshold in both rodent models of epilepsy. It was observed that amiloride reduced behavioral depression in the FST in mice. In addition, it resulted in memory improvement in the SAB model. Amiloride did not affect grip strength and rotarod performance, suggesting it is devoid of behavioral impairment. The results indicate the potential antiseizure activity of amiloride along with additional neurological advantages.

Amiloride↗