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PubMed1982

Detection of vertical disparties.

Psychometric curves determined for the detection of differences between vertical disparities as a function of the reference disparity for three presentation times (20 msec, 160 msec and 2 sec) and for two eccentricites of the stimulus (0.5 degree and 4 degrees) are presented. The results show that the detectability of disparity differences first increases and then decreases when the reference disparity is increased from zero. It further appears that the only effect of the stimulus presentation time and eccentricity on the psychometric curves and their dependency on the reference disparity is that the effective disparities observed are multiplied by a constant factor. The experimental data can be described by a signal detection model with two special features around zero disparity where binocular single vision occurs, viz. Loss of information about the sign of disparity and presence of intrinsic noise at a level up to 25% higher than the minimum level found at intermediate disparities.

Differential Threshold↗

PubMed2005

Tactile sensitivity in women with sexual arousal disorder.

Evidence suggests that tactile sensitivity may differ between women with sexual arousal difficulties and women with normal sexual functioning. Tactile sensitivity was examined on the distal portion of the dominant hand index finger and on the lower lip in women with female sexual arousal disorder (FSAD) (n = 17) and in normally functioning women (n = 17). The two groups did not differ significantly in age, length of current relationship or on measures of sexual experience and sexual desire. Hierarchical binary logistic regression indicated that finger threshold was significantly associated with FSAD women versus control women, and hierarchical linear regression indicated that finger threshold was associated with severity of arousal dysfunction. Logistic regression showed that 76.5% of participants were correctly classified and 23.5% were incorrectly classified using tactile sensation as a predictor variable. Possible underlying mechanisms and clinical implications are discussed.

Adult↗

PubMed1999

Normal cochlear function in mdx and mdx(Cv3) Duchenne muscular dystrophy mouse models.

OBJECTIVES/HYPOTHESIS: Sensorineural hearing loss has been found in association with inherited muscular dystrophies in humans and in mouse models. An increased brainstem auditory evoked response threshold has been previously reported in the dystrophin-deficient mdx mouse model for Duchenne muscular dystrophy, suggesting that full-length dystrophin (Dp427) is involved in hearing. The objective of the present study was to confirm cochlear dysfunction with this gene defect and determine whether the shorter carboxyl terminus isoforms of dystrophin are also critical in maintaining normal hearing. STUDY DESIGN: Case controlled. Animal model. METHODS: Auditory brainstem response (ABR) audiometry to pure tones was used to evaluate cochlear function. Fourteen mdx, 4 mdx(Cv3), and 13 age-matched control (C57BL/6J and C57BL/10ScSn) male mice were tested at 5 weeks and 11 weeks of age. The ABR thresholds to tone-burst stimuli at 4, 8, 16, and 32 kHz were obtained for each ear and statistically compared (ANOVA) for potential group differences. RESULTS: Both mdx and mdx(Cv3) mice demonstrated normal ABR thresholds when compared with controls. CONCLUSIONS: Both mdx and mdx(Cv3) mouse models have normal hearing by ABR. The authors' data suggest that dystrophin and its carboxyl terminus isoforms do not play a critical role in hearing in the mouse. This was unexpected, as previous studies using the brainstem auditory evoked response method suggested that the mdx mouse has an increased threshold for hearing.

Animals↗

PubMed2006

Transition of absolute instability from global to local modes in a gyrotron traveling-wave amplifier.

The gyrotron traveling-wave amplifier employing the distributed-loss scheme is capable of very high gain and effective in suppressing the global absolute instabilities. This study systematically characterizes the local absolute instabilities and their transitional behavior. The local absolute instabilities are analyzed using a model that incorporates the penetration of the field from the copper section into the lossy section. The axial modes were characterized from the perspective of beam-wave interaction and were found to share many characteristics with the global modes. The transition from global modes to local modes as the distributed loss increases was demonstrated. The electron transit angle in the copper section, which determines the feedback criterion, governs the survivability of an oscillation. In addition, the oscillation thresholds predicted using this model are more accurate than those obtained using a simplified model.

Journal Article↗

PubMed2007

Sex-structured HIV/AIDS model to analyse the effects of condom use with application to Zimbabwe.

We present a sex-structured model for heterosexual transmission of HIV/AIDS in a community. The model is formulated using integro-differential equations, which are shown to be equivalent to delay differential equations with a time delay due to incubation period. The sex-structured HIV/AIDS model divides the population into a two sex-structure consisting of females and males. The threshold and equilibria for the model are determined and stabilities are examined. We extend the model to focus on the effects of condom use as a single-strategy approach in HIV prevention in the absence of any treatment. Initially we model the use of male condoms and further extend the model to incorporate the use of both female and male condoms. The model includes two primary factors in condom use to control HIV that are condom efficacy and compliance. The exposure risk of infection after each intervention is obtained. Basic reproductive numbers for these models are computed and compared to assess the effectiveness of male and female condom use in a community. The models are numerically analysed to assess the effectiveness of condom use on the transmission dynamics of HIV/AIDS using demographic and epidemiological parameters for Zimbabwe. The study demonstrates the use of sex-structured HIV/AIDS models in assessing the effectiveness of female and male condom use as a preventative strategy in a heterosexually active population.

Condoms↗

PubMed2004

Risk characterisation of chemicals in food.

Information on hazard identification, hazard characterisation (including dose-response assessment) and exposure assessment are brought together under risk characterisation in order to provide advice to risk managers. Hazard identification and characterisation define the adverse effects produced by the chemical and are used to determine a health-based guidance value, such as an acceptable daily intake for threshold effects, or an estimate of the intake associated with a predefined level of risk for non-threshold effects. Exposure assessment aims to define average and high intakes, and has to take into account different dietary patterns. Hazard characterisation and exposure data need to be closely inter-related, for example the data should relate to the same life-stages and durations of exposure. Risk characterisation has to be an iterative process in which information on hazard and exposure are matched and any discrepancy taken into account where necessary by the generation of additional data. There is an established method of quantitative risk characterisation for non-threshold effects. A population distribution model is proposed that is applicable to estimate the risk associated with exposure to compounds producing threshold effects.

Animals↗

PubMed1996

Model of a thalamocortical neurone with dendritic voltage-gated ion channels.

To investigate the functional role of dendrites of thalamocortical neurones, we have used our one-compartmental model to construct a multi-compartmental model with dendritic regions chosen according to a representative soma-to-dendritic terminal path of an X cell of the cat dorsal lateral geniculate nucleus. The multi-compartmental model with dendritic low-threshold Ca2+ and delayed rectifier K+ channels yields more accurate results than the one-compartmental model when simulating tonic firing and oscillatory activities, and provides a useful means for the study of propagation of excitation on the dendrites.

Animals↗

PubMed2003

Epidemiological models for the spread of anti-malarial resistance.

BACKGROUND: The spread of drug resistance is making malaria control increasingly difficult. Mathematical models for the transmission dynamics of drug sensitive and resistant strains can be a useful tool to help to understand the factors that influence the spread of drug resistance, and they can therefore help in the design of rational strategies for the control of drug resistance. METHODS: We present an epidemiological framework to investigate the spread of anti-malarial resistance. Several mathematical models, based on the familiar Macdonald-Ross model of malaria transmission, enable us to examine the processes and parameters that are critical in determining the spread of resistance. RESULTS: In our simplest model, resistance does not spread if the fraction of infected individuals treated is less than a threshold value; if drug treatment exceeds this threshold, resistance will eventually become fixed in the population. The threshold value is determined only by the rates of infection and the infectious periods of resistant and sensitive parasites in untreated and treated hosts, whereas the intensity of transmission has no influence on the threshold value. In more complex models, where hosts can be infected by multiple parasite strains or where treatment varies spatially, resistance is generally not fixed, but rather some level of sensitivity is often maintained in the population. CONCLUSIONS: The models developed in this paper are a first step in understanding the epidemiology of anti-malarial resistance and evaluating strategies to reduce the spread of resistance. However, specific recommendations for the management of resistance need to wait until we have more data on the critical parameters underlying the spread of resistance: drug use, spatial variability of treatment and parasite migration among areas, and perhaps most importantly, cost of resistance.

Alleles↗

PubMed2000

Evaluation of threshold stress for bone resorption around screws based on in vivo strain measurement of miniplate.

The purpose of this study is to investigate the critical threshold stress causing bone resorption evaluated from strain measurement in vivo, comparing the various finite element models. In this study strains of miniplates used for mandibular fractures were measured once a week until the strains reduced. The maximum bite force for each patient was applied in the incisal, right molar and left molar region. The strains increased and reached a peak level at 2-4 weeks, whereas the bite forces increased during the period of measurements. A 3-D osteosynthesis model using finite element method showed that the compressive stresses of the bone surrounding screws ranged within approximately -40 MPa under the condition generating the same amounts of strains measured in the miniplates. Furthermore, various finite element models simulating mandibular reconstruction using the fibular graft were constructed. The models for reconstruction using single strut fibula showed distinct stress concentration in the cortical bone surrounding screws, and the peak stress levels were 2 to 3 times as strong as that of the fracture model. We conclude that critical threshold for bone resorption should be approximately -50 MPa (3600 micro strain).

Journal Article↗

PubMed1997

Model of a thalamocortical neurone with dendritic voltage-gated ion channels.

To investigate the functional role of dendrites of thalamocortical neurones, we have used our one-compartmental model to construct a multi-compartmental model with dendritic regions chosen according to a representative soma-to-dendritic terminal path of an X cell of the cat dorsal lateral geniculate nucleus. The multi-compartmental model with dendritic low-threshold CA2+ and delayed rectifier K+ channels yields more accurate results than the one-compartmental model when simulating tonic firing and oscillatory activities, and provides a useful means for the study of propagation of excitation on the dendrites.

Action Potentials↗

PubMed1995

Quantification of the interactions among fatty acid, lysophosphatidylcholine, calcium, dimyristoylphosphatidylcholine vesicles, and phospholipase A2.

The rate of hydrolysis of phosphatidylcholine bilayers by soluble phospholipase A2 (PLA2) is greatly enhanced by the presence in the bilayer of a threshold mole fraction of the reaction products: fatty acid and lysophosphatidylcholine (lyso-PC). The threshold requirement of these products appears to vary as a function of vesicle and calcium concentration. To further identify the roles of myristic acid, lyso-PC, and calcium in promoting optimal PLA2 activity, we have quantified the various interactions among these components and dimyristoylphosphatidylcholine large unilamellar vesicles. The bilayer/water partition coefficient for myristic acid was obtained by competition of vesicles for the binding of the fatty acid to an acrylodan conjugate of an intestinal fatty acid binding protein as monitored by the acrylodan fluorescence emission spectrum. The partition coefficient for lyso-PC was obtained by a similar procedure using the tryptophan emission spectrum of bovine serum albumin. The effect of calcium concentration on these interactions was also quantified. These results were incorporated into an empirical model to describe the threshold requirements for these products in the bilayer. This information is vital for elucidating the mechanism of activation of PLA2 by the hydrolysis products.

Calcium↗

PubMed2006

Implications and cost of pancreatic leak following distal pancreatic resection.

OBJECTIVES: Pancreatic stump leak (PL) after elective distal pancreatic resection significantly impacts cost and increases subsequent health care resource utilization. We sought to provide an economic framework for potential interventions aimed at reducing its occurrence. DESIGN: Retrospective case series and economic evaluation. SETTING: University-affiliated, tertiary care referral center. PATIENTS: Sixty-six patients undergoing elective distal pancreatectomy. MAIN OUTCOME MEASURES: Postoperative complications; hospital and professional costs. RESULTS: Overall postoperative morbidity occurred in 34 patients (52%) with no deaths. The total number of patients with complications directly related to PL was 22 (33%). The mean +/- SD number of total hospital days for the no-PL group was 5.2 +/- 1.7 days (range, 3-12 days) vs 16.6 +/- 14.6 days (range, 4-49 days) for the PL group (P = .001). The average patient with PL-related problems incurred a total cost that was 2.01 times greater than the average patient in the no-PL group. A decision analytic model developed to evaluate threshold costs showed that a hypothetical intervention designed to reduce the complication rate of distal pancreatectomy by one third would be financially justifiable up to a cost of $1418 per patient. CONCLUSIONS: Complications derived from PL following distal pancreatectomy double the cost and dramatically increase health care resource utilization. There is an urgent need to develop strategies that reduce the incidence of this common complication. Interventions aimed at decreasing the incidence of PL should take into account this cost differential. We provide an economic model to serve as a guide for developing these technologies.

Adult↗

PubMed1982

Towards an alternative evolution model.

Lamarck and Darwin agreed on the inconstancy of species and on the exclusive gradualism of evolution (nature does not jump). Darwinism, revived as neo-Darwinism, was almost generally accepted from about 1930 till 1960. In the sixties the evolutionary importance of selection has been called in question by the neutralists. The traditional conception of the gene is disarranged by recent molecular-biological findings. Owing to the increasing confusion about the concept of genotype, this concept is reconsidered. The idea of the genotype as a cluster of genes is replaced by a cybernetical interpretation of the genotype. As nature does jump, exclusive gradualism is dismissed. Saltatory evolution is a natural phenomenon, provided by a sudden collapse of the thresholds which resist against evolution. The fossil record and the taxonomic system call for a macromutational interpretation. As Lamarck and Darwin overlooked the resistance of evolutionary thresholds, an alternative evolution model is needed, the first to be constructed on a palaeontological and taxonomic basis.

Animals↗

PubMed1995

Sensorineural hearing loss alters recovery from short-term adaptation in the C57BL/6 mouse.

Several strains of laboratory mouse (Mus musculus) have a pattern of hearing loss which resembles that found in humans. The C57BL/6 strain of mouse has a genetic defect that results in degeneration of the organ of Corti, originating in the basal, high-frequency region and then proceeding apically over time. The end result is a severe-to-profound sensorineural hearing loss (SNHL) by 14 months of age. In contrast, auditory function of the CBA strain remains normal through its early life span then slowly declines later in life, much like that typified by human presbycusis. The purpose of the present study was to compare ABR (peak 5) forward masking recovery functions in young, normal-hearing CBA and C57BL/6 mice to hearing-impaired C57BL/6 mice. ABR audiograms were obtained prior to collecting the tone-on-tone forward masking data. Masking was defined as a 50% reduction in the P5 component of the ABR, elicited and masked by 12 kHz tone bursts, using masker/probe time delays from 0 to 100 ms. Time constants were computed from an exponential model fit to the recovery functions (masker level vs. time delay). In hearing-impaired animals there was a significant increase in recovery from short-term adaptation as measured by the time constants, as well as a significant latency shift in the P5 component. The effects of SNHL on the recovery of the P5 component from short-term adaptation was comparable to that reported behaviorally for human hearing-impaired listeners and physiologically from the inferior colliculus (IC) of chinchillas suffering permanent threshold shifts.

Acoustic Stimulation↗

PubMed1991

Marginal zinc deficiency and changes in behavioral salt taste threshold and salt preference in mice.

An animal model of marginal zinc deficiency was tested in mice, and salt taste threshold and salt preference were investigated in the state of marginal zinc deficiency. Two experiments were conducted. In Experiment 1, four-week-old male ICR mice were fed diets of three different levels of zinc content (3.17, 9.27, or 48.13 micrograms Zn/g) for 60 days. Food intake, growth, zinc levels in tissues, hepatic metallothionein (MT) content, and activities of selected enzymes (hepatic and RBC delta-amino-levulinic acid dehydratase (ALAD) and plasma alkaline phosphatase (ALP] did not differ among the groups throughout the experiment, and no overt signs of zinc deficiency were manifested in any group. In Experiment 2, four-week-old male ICR mice were fed a zinc-deficient diet (1.98 micrograms Zn/g) or a zinc-adequate diet (49.14 micrograms Zn/g) for 56 days. Food intake and growth did not differ between the two groups, and no overt signs of zinc deficiency were observed throughout the experiment. Zinc levels in the plasma and femur--but not those in the brain, kidneys, liver, and red blood cell (RBC)--and plasma ALP activities were significantly lower on Day 42 and Day 56 of the experiment in the mice fed the zinc-deficient diet (ZnD group) than in those fed the zinc-adequate diet (ZnA group). Hepatic MT contents were lower in the ZnD group than in the ZnA group on Day 56 only. Salt taste threshold was 0.05% in the ZnA group, while it was 0.1% in the ZnD group, between Day 30 and Day 38, and between Day 44 and Day 52 of the experiment. Preference for 0.9% NaCl solution was no different in the two groups when tested between Day 38 and Day 40 or between Day 52 and Day 54, but that for 1.6% NaCl solution was significantly higher in the ZnD group than in the ZnA group between Day 40 and Day 42, and between Day 54 and Day 56.

Alkaline Phosphatase↗

PubMed1998

Temporal processing of the pitch of complex tones.

The effect of tone duration on fundamental frequency (F0) discrimination is greater for complexes containing unresolved harmonics than for those containing resolved harmonics [Plack and Carlyon, J. Acoust. Soc. Am. 98, 1355-1364 (1995)]. Three experiments explored this effect further. The first experiment measured sensitivity (as d') to fundamental frequency (F0) differences for two complexes, both with an F0 of 250 Hz. The first complex was low-pass filtered at 1875 Hz to create a resolved complex and the second was bandpass filtered between 5500 and 7500 Hz to create an unresolved complex. The harmonics for the resolved complex were selected so that no two harmonics were the same between the two observation intervals. Performance for both complexes was measured for tone durations of 20, 40, 80, and 160 ms. For the unresolved complex, the effect of duration was greater than that for the resolved complex and greater than the predictions of a "multiple-looks" model assuming either peripheral (before sampling) or central (after combining samples) sources of variance. The second experiment replicated these results using an F0 of 62.5 Hz with the cutoff frequencies of the bandpass filters divided by four, confirming that the effect is related to resolvability and not to spectral region. In the final experiment, F0 discrimination for pairs of complexes separated by a temporal gap was measured relative to that for one complex. Performance for the resolved and unresolved complexes was similar: Very little effect of gap duration was observed and the results were consistent with the predictions of the peripheral-variance multiple-looks model. Taken together, the results suggest that the pitch mechanism for resolved harmonics uses a relatively short sampling window of around 20 ms, while the mechanism for unresolved harmonics may use a more complex strategy for optimizing the combination of information over time, perhaps involving a flexible integration time.

Adult↗

PubMed2002

Metal-insulator transition in chains with correlated disorder.

According to Bloch's theorem, electronic wavefunctions in perfectly ordered crystals are extended, which implies that the probability of finding an electron is the same over the entire crystal. Such extended states can lead to metallic behaviour. But when disorder is introduced in the crystal, electron states can become localized, and the system can undergo a metal-insulator transition (also known as an Anderson transition). Here we theoretically investigate the effect on the physical properties of the electron wavefunctions of introducing long-range correlations in the disorder in one-dimensional binary solids, and find a correlation-induced metal-insulator transition. We perform numerical simulations using a one-dimensional tight-binding model, and find a threshold value for the exponent characterizing the long-range correlations of the system. Above this threshold, and in the thermodynamic limit, the system behaves as a conductor within a broad energy band; below threshold, the system behaves as an insulator. We discuss the possible relevance of this result for electronic transport in DNA, which displays long-range correlations and has recently been reported to be a one-dimensional disordered conductor.

Crystallization↗

PubMed1979

Analysis of "integrate-to-threshold" neural coding schemes.

Methods of analysis for some deterministic and stochastic variants of the integrate-to-threshold neural coding scheme are presented. Adaptation phenomena are modeled by means of feedforward and feedback adaptive threshold control. Simulations of sinusoidal and step responses reproduce satisfactorily the qualitative characteristics of adaptation as compared with physiological data. It is postulated that such adaptive threshold control may be accomplished by the release, or conformation change, of molecules involved in the control of excitable-channel dynamics.

Action Potentials↗