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PubMed2006

A mathematical model for the kinetics of Methanobacterium bryantii M.o.H. considering hydrogen thresholds.

We develop a kinetic model that builds on the foundation of classic Monod kinetics, but incorporates new phenomena such as substrate thresholds and survival mode observed in experiments with the H2-oxidizing methanogen Methanobacterium bryantii M.o.H. We apply our model to the experimental data presented in our companion paper on H2 thresholds. The model accurately describes H2 consumption, CH4 generation, biomass growth, substrate thresholds, and survival state during batch experiments. Methane formation stops when its Gibbs free energy is equal zero, although this does not interrupt H2 oxidation. The thermodynamic threshold for H2 oxidation occurs when the free energy for oxidizing H2 and transferring electrons to biomass is no longer negative, at approximately 0.4 nM. This threshold is not controlled by the Gibbs free energy equation of methanogenesis from H2 + HCO3- as we show in our companion paper. Beyond this threshold, the microorganisms shift to a low-maintenance metabolism called "the survival state" in response to extended H2 starvation; adding the starvation response as another new feature of the kinetic model. A kinetic threshold (or S (min)), a natural feature of the Monod kinetics, is also captured by the model at H2 concentration of around approximately 2,400 nM. S (min) is the minimum substrate concentration to maintain steady-state biomass concentration. Our model will be useful for interpreting threshold results and designing new studies to understand thresholds and their ecological implications.

Computer Simulation↗

PubMed1997

An approximate model of polygenic inheritance.

The finite polygenic model approximates polygenic inheritance by postulating that a quantitative trait is determined by n independent, additive loci. The 3n possible genotypes for each person in this model limit its applicability. CANNINGS, THOMPSON, and SKOLNICK suggested a simplified, nongenetic version of the model involving only 2n + 1 genotypes per person. This article shows that this hypergeometric polygenic model also approximates polygenic inheritance well. In particular, for noninbred pedigrees, trait means, variances, covariances, and marginal distributions match those of the ordinary finite polygenic model. Furthermore as n --> infinity, the trait values within a pedigree collectively tend toward multivariate normality. The implications of these results for likelihood evaluation under the polygenic threshold and mixed models of inheritance are discussed. Finally, a simple numerical example illustrates the application of the hypergeometric polygenic model to risk prediction under the polygenic threshold model.

Female↗

PubMed1993

Thalamic low threshold calcium current in a genetic model of absence epilepsy.

The low threshold calcium current (IT) in thalamo-cortical neurones contributes to the generation of spike and wave discharges (SWDs) characteristic of generalized, non-convulsive absence epilepsy. The biophysical properties of this current were analysed in dorsal lateral geniculate neurones from the Genetic Absence Epilepsy Rats from Strasbourg (GAERS+). No difference was found in the voltage dependence and kinetics of IT between GAERS+ and rats of the non epileptic control strain (GAERS-). Thus, a dysfunction of IT does not appear to underlie the occurrence of SWDs in absence epilepsy.

Animals↗

PubMed1987

Asbestos exposure dose-bronchoalveolar milieu response in asbestos workers and the sheep model: evidences of a threshold for chrysotile-induced fibrosis.

Epidemiological studies of asbestos workers have proposed a linear dose-response relationship between asbestos cumulative exposure indices and the incidence of asbestos-related lung diseases. However, for chrysotile, several studies have not observed such a linear relationship in low exposure workers. To further study the relationship of chrysotile exposure dose - lung tissue response, we designed in the sheep model one experiment of one exposure at variable intensity and one experiment with variable rates of individual exposures. In the first study, 6 groups of 6 sheep were exposed to either 0, 1, 10, 25, 50 or 100 mg chrysotile in 100 ml saline and lung lavage analyses carried out at day 0, 12, 24, 40 and 60 after. Histopathology was done at day 60. In the second experiment, 4 groups of 12 sheep were exposed to 100 ml saline every week or every two weeks, 100 mg chrysotile in 100 ml saline every week or every two weeks. In the second experiment, lung damage was assessed by changes in vital capacity (VC), lung compliance (Cst) and profusion of chest radiographic opacities (PO). On BAL in the control sheep, there were small and transient early increases in total cells/ml, macrophages/ml, neutrophils/ml, lactate dehydrogenase (LDH)/ml, alkaline phosphatase (AK)/ml, beta-glucuronidase (beta G)/ml and procollagen lll/ml. In the 1, 10, 25, and 50 mg asbestos sheep groups, there were no significant differences. However, in the 100 mg asbestos sheep, there were significant sustained increases in total cells/ml (3-6 times controls), macrophages/ml (2-6 times controls), neutrophils/ml (60-600 times controls), LDH/ml (5-10 times controls), AK/ml (1.5-2 times controls), beta G/ml (3-5 times controls), procollagen lll/ml (2 times controls) and IgG (1.5-3 times controls). These data were correlated with histopathological findings. In the sheep exposed weekly to chrysotile, significant changes in VC, Cst and PO occurred at mean cumulative exposure dose of 1.2 g whereas in the sheep exposed every 2 weeks, similar changes in VC, Cst and PO occurred at the cumulative exposure dose of 3.6 g (P less than 0.05). In conclusion, our data suggest that there is a threshold for chrysotile-induced fibrosis. This level cannot be considered adequately in term of cumulative exposure dose but must take into account intensity and rate of individual exposures.

Animals↗

PubMed1991

Models of the effects of threshold on loudness growth and summation.

The present paper extends previous efforts to compare models of additivity of masking in normal and impaired listeners to measures of loudness. A model that provides a good account of additivity of masking for normal and impaired listeners, the modified power-law model with compressed internal noise, also provides a good description of basic loudness data from normal and impaired listeners. This model has been discussed frequently in the literature on loudness as one means of correcting loudness functions near threshold, but has not been widely accepted. The applicability of this model to loudness growth and the summation of loudness, both monaurally and binaurally, is evaluated and compared to the more generally accepted alternative. The ability of the model reported here to account for growth of loudness near threshold, as well as the additivity of masking, suggests a need to reconsider its application to loudness data, as well as a need to reconsider the relation between loudness and masking.

Attention↗

PubMed2006

Hydrodynamic Lyapunov modes and strong stochasticity threshold in Fermi-Pasta-Ulam models.

The existence of a strong stochasticity threshold (SST) has been detected in many Hamiltonian lattice systems, including the Fermi-Pasta-Ulam (FPU) model, which is characterized by a crossover of the system dynamics from weak to strong chaos with increasing energy density epsilon. Correspondingly, the relaxation time to energy equipartition and the largest Lyapunov exponent exhibit different scaling behavior in the regimes below and beyond the threshold value. In this paper, we attempt to go one step further in this direction to explore further changes in the energy density dependence of other Lyapunov exponents and of hydrodynamic Lyapunov modes (HLMs). In particular, we find that for the FPU-beta and FPU-alpha(beta) models the scalings of the energy density dependence of all Lyapunov exponents experience a similar change at the SST as that of the largest Lyapunov exponent. In addition, the threshold values of the crossover of all Lyapunov exponents are nearly identical. These facts lend support to the point of view that the crossover in the system dynamics at the SST manifests a global change in the geometric structure of phase space. They also partially answer the question of why the simple assumption that the ambient manifold representing the system dynamics is quasi-isotropic works quite well in the analytical calculation of the largest Lyapunov exponent. Furthermore, the FPU-beta model is used as an example to show that HLMs exist in Hamiltonian lattice models with continuous symmetries. Some measures are defined to indicate the significance of HLMs. Numerical simulations demonstrate that there is a smooth transition in the energy density dependence of these variables corresponding to the crossover in Lyapunov exponents at the SST. In particular, our numerical results indicate that strong chaos is essential for the appearance of HLMs and those modes become more significant with increasing degree of chaoticity.

Journal Article↗

PubMed1999

Model of biological evolution with threshold dynamics and infinitely many absorbing states.

We study a model of biological evolution where the survival of a given species depends on its interactions with neighboring species. In the steady state the model has an active phase and an absorbing phase, which are separated by the critical point of the directed percolation universality class. The absorbing phase is infinitely degenerate and the dynamical behavior of our model is found to be nonuniversal.

Journal Article↗

PubMed1997

Exposure-response analysis of risk of respiratory disease associated with occupational exposure to chrysotile asbestos.

OBJECTIVES: To evaluate alternative models and estimate risk of mortality from lung cancer and asbestosis after occupational exposure to chrysotile asbestos. METHODS: Data were used from a recent update of a cohort mortality study of workers in a South Carolina textile factory. Alternative exposure-response models were evaluated with Poisson regression. A model designed to evaluate evidence of a threshold response was also fitted. Lifetime risks of lung cancer and asbestosis were estimated with an actuarial approach that accounts for competing causes of death. RESULTS: A highly significant exposure-response relation was found for both lung cancer and asbestosis. The exposure-response relation for lung cancer seemed to be linear on a multiplicative scale, which is consistent with previous analyses of lung cancer and exposure to asbestos. In contrast, the exposure-response relation for asbestosis seemed to be nonlinear on a multiplicative scale in this analysis. There was no significant evidence for a threshold in models of either the lung cancer or asbestosis. The excess lifetime risk for white men exposed for 45 years at the recently revised OSHA standard of 0.1 fibre/ml was predicted to be about 5/1000 for lung cancer, and 2/1000 for asbestosis. CONCLUSIONS: This study confirms the findings from previous investigations of a strong exposure-response relation between exposure to chrysotile asbestos and mortality from lung cancer, and asbestosis. The risk estimates for lung cancer derived from this analysis are higher than those derived from other populations exposed to chrysotile asbestos. Possible reasons for this discrepancy are discussed.

Adult↗

PubMed1997

Evaluation of alternative methods for establishing safe levels of occupational exposure to vinyl halides.

The facts that reduction of occupational vinyl chloride exposures to levels within or below the 0.5-5 ppm range has so far been successful in eliminating vinyl chloride-induced liver angiosarcoma and that humans appear to be less sensitive to the carcinogenic effect of vinyl chloride than rats offered an opportunity to verify or dispute risk assessment extrapolation models used, and proposed, by the U.S. EPA. Safe occupational vinyl chloride exposures were defined as levels associated with an incidence of one angiosarcoma in 100,000 exposed workers, determined from rat bioassay data using default no-threshold (linearized multistage model and benchmark dose approach with linear extrapolation) and threshold (NOEL/LOEL and benchmark dose uncertainty factor approaches) models, and then compared against the likely protective range of 0.5-5 ppm. Safe levels derived using either no-threshold model are equivalent and are two to three orders of magnitude below the 0.5-5 ppm range. Safe levels derived using either threshold model, when applying uncertainty factors which reflect equal or less sensitivity in humans compared to rats, fall within the 0.5-5 ppm range. Similar results were obtained for vinyl bromide and vinyl fluoride. These results undermine the U.S. EPA default assumption of no-threshold for vinyl halides as well as for other DNA-reactive carcinogens while simultaneously supporting the notion that a practical threshold exists. They further suggest that when threshold models are appropriate, the default assumption of greater sensitivity in humans compared to rats should be carefully evaluated.

Animals↗

PubMed2001

Mechanism-based pharmacokinetic model for paclitaxel.

PURPOSE: To create a model based on known mechanisms of paclitaxel distribution that could describe the pharmacokinetics (PK) of total and unbound plasma concentrations, as well as blood concentrations. In addition, to investigate the relationship between exposure, based on unbound and total concentrations, and neutropenia. PATIENTS AND METHODS: Paclitaxel and Cremophor EL (CrEL) concentrations were obtained from 23 female and three male patients (50 courses in total) with different cancer types that received paclitaxel (Taxol; Bristol-Myers Squibb Co, Princeton, NJ) (135 to 225 mg/m(2)) as 3- or 24-hour intravenous infusions. Seven of the patients received combination therapy with doxorubicin or cisplatin. The population PK model was built to fit three types of data simultaneously: unbound, total plasma, and blood concentrations. The area under the curve, threshold, and general models were used to relate neutrophil survival fraction from 19 patients (29 courses in total) to exposure based on unbound and total plasma concentration, respectively. RESULTS: The PK model included a linear three-compartment model for unbound concentration, binding directly proportional to CrEL, linear and nonlinear binding to plasma proteins, and linear and nonlinear binding to blood cells. The threshold model best described the PK/pharmacodynamic (PD) relationship for total concentration. No distinction could be made between the models for unbound drug. CONCLUSION: Earlier PK models for paclitaxel have been empirical. This study shows that a mechanistic model can be used to describe the nonlinear PK of paclitaxel. There is an indication that the PK/PD relationship is not the same for unbound and total plasma concentrations.

Adult↗

PubMed1994

Dexmedetomidine decreases seizure threshold in a rat model of experimental generalized epilepsy.

BACKGROUND: Dexmedetomidine (DEX) is a highly selective alpha 2 agonist with marked sedative and analgesic properties thought to be mediated via reduction of central noradrenergic transmission. Because an anticonvulsant effect is associated with increased noradrenergic activity, we investigated the possible proconvulsant effects of DEX in an experimental model of generalized seizures. METHODS: Male rats (n = 82) were administered 0.9% saline as placebo (n = 18) or pretreatment drug(s) before initiation of an infusion of pentylenetetrazol (PTZ) at 5.5 mg.kg-1.min-1. The total dose of PTZ required to elicit electroencephalographic (EEG) and behavioral seizure activity was assessed. Blood samples were obtained 15 min after initiation of infusion (82.5 mg/kg) for determination of serum PTZ concentrations by gas chromatography. Pretreatment drug groups included DEX (20 micrograms/kg [n = 11], 100 micrograms/kg [n = 14], and 500 micrograms/kg [n = 10]); L-medetomidine (500 micrograms/kg [n = 7]); the alpha 2 antagonist atipamezole (500 micrograms/kg [n = 9]); and atipamezole (500 micrograms/kg) before DEX (100 micrograms/kg [n = 7] and 500 micrograms/kg [n = 6]). RESULTS: In control animals, PTZ 25-35 mg/kg induced EEG evidence of epileptiform activity. The mean dose to EEG epileptiform activity and clonic convulsions was 30 +/- 5.8 (SE) and 59 +/- 3.2 mg/kg, respectively. Infusion of DEX at 100 and 500 micrograms/kg resulted in a marked sedative response and reduced the EEG seizure threshold of PTZ to 18 +/- 1.5 and 7 +/- 1.8 mg/kg, respectively (P < 0.05 at both doses). The clonic convulsant threshold also was significantly decreased in both groups, to 37 +/- 3.2 and 28 +/- 2.3 mg/kg (P < 0.01 at each dose). Before clonic convulsion, a significantly greater number of motor seizure manifestations were scored in the DEX-treated animals at all three dose levels compared with the number scored in control animals. The proconvulsant action of DEX was not a result of alteration of PTZ kinetics, because serum concentrations did not differ between control and DEX-treated animals. Animals treated with L-medetomidine demonstrated more paroxysmal motor phenomena before clonic seizures than controls (P < 0.01) although the clonic seizure threshold was not altered. Atipamezole alone did not alter background EEG, nor did it affect the clonic convulsant threshold. Atipamezole did, however, block the proconvulsant behavioral action at both doses of DEX, raising clonic seizure threshold from 37 +/- 3.2 to 59 +/- 5.8 mg/kg (100 micrograms/kg DEX, P < 0.05) and from 28 +/- 2.3 to 59 +/- 6.9 mg/kg (500 micrograms/kg DEX, P < 0.01). CONCLUSIONS: DEX exerted a significant proconvulsant action in the PTZ experimental seizure model. The pharmacodynamic effect was dose-dependent and stereospecific and was blocked by the selective alpha 2-receptor antagonist atipamezole. These data are consistent with previous data demonstrating that inhibition of central noradrenergic transmission facilitates seizure expression. Further evaluation of DEX for possible clinical proconvulsant effects may be warranted.

Adrenergic alpha-Agonists↗

PubMed1995

Bistability in cerebellar Purkinje cell dendrites modelled with high-threshold calcium and delayed-rectifier potassium channels.

Phase-plane analysis of the ionic currents underlying dendritic plateau potentials was carried out to study the nonlinear dynamics and steady-state transfer properties of the dendritic tree in cerebellar Purkinje cells. The results of an analysis of the P-type calcium and delayed rectifier potassium channel system are presented in this study. These channels constitute a simple system that can support bistability and plateau potentials. By requiring both the steady-state current-voltage curve and nullclines to mimic basic plateau potential properties, we obtained well-defined ranges of specific conductance that can support bistability. Hysteresis was found to be surprisingly prevalent in this simple ion-channel system. Using the steady-state current voltage relationship, we derive concise, algebraic expressions for the voltage and current thresholds of state transitions as functions of specific conductance. The significance of bistability in this ion-channel system is discussed with respect to the generation of plateau potentials in Purkinje cells dendrites and the role of the cerebellum in motor control.

Action Potentials↗

PubMed2006

Modeling of aqueous transport in rigid porous matrices near the percolation threshold.

PURPOSE: To demonstrate control of passive diffusion of small molecules through rigid ceramic matrices via manipulation of matrix porosity near the percolation threshold, and to model such control using percolation scaling relationships on both infinite and finite lattices. MATERIALS AND METHODS: Rigid alumina disks of controlled porosity were prepared using standard ceramic casting and sintering techniques. Structural void space distributions in sintered disks were measured by dimensional and volume displacement (pycnometry) methods. The impact of void space on transport was determined by tracking the diffusion of ionized benzoic acid across sintered disks mounted in Stokes diffusion cells. Critical percolation thresholds were estimated by fitting structural and transport-dependent results to percolation scaling relationships. Finite-size scaling studies were performed by adding polymer microspheres of known diameter to the disks to generate artificially large pores. RESULTS: Nonlinear least squares techniques were used to fit both structural and transport-dependent properties of rigid alumina disks to total disk porosity using percolation scaling relationships. The critical percolation threshold determined from structural properties (0.129) was lower than that determined from benzoic acid transport (0.169). The transport-derived percolation threshold exactly matched that expected for a tetrakaidecahedral (14 sided) lattice. Finite-size scaling was demonstrated through a nonzero effective volume fraction for transport at the percolation threshold. CONCLUSIONS: Manipulation of total disk porosity near the percolation threshold was shown to be a suitable means of controlling the transport rate of a model small molecule, while deliberate enlargement of individual pores was demonstrated to decrease this threshold without increasing total porosity. The lower-than-expected percolation threshold obtained from the structural model was ascribed to limitations of the measurement technique. The threshold determined from the aqueous transport model was concluded to represent the true threshold for this system.

Algorithms↗

PubMed2005

The impact of environmental fluctuations on structured discrete time population models: resonance, synchrony and threshold behaviour.

External forcing of a discrete time ecological system does not just add variation to existing dynamics but can change the dynamics. We study the mechanisms that can bring this about, focusing on the key concepts of excitation and suppression which emerge when analysing the power spectra of the system in linear approximation. Excitation, through resonance between the system dynamics and the external forcing, is the greater the closer the system is to the boundary of the stability region. This amplification means that the extinction of populations becomes possible sooner than expected and, conversely, invasion can be significantly delayed. Suppression and the consequent redistribution of power within the spectrum proves to be a function both of the connectivity of the network graph of the system and the way that external forcing is applied to the system. It is also established that colour in stochastic forcing can have a major impact, by enhancing resonance and by greater redistribution of power. This can mean a higher risk of extinction through larger fluctuations in population numbers and a higher degree of synchrony between populations. The implications of external forcing for stage-structured species, for populations in competition and for trophic web systems are studied using the tools and concepts developed in the paper.

Ecosystem↗

PubMed1987

Improved cardiac cell excitation with symmetrical biphasic defibrillator waveforms.

According to the most commonly accepted hypothesis, ventricular defibrillation is produced by exciting cells in a critical mass of the ventricle. For monophasic defibrillator waveforms, this hypothesis correctly predicts a direct correlation between defibrillation threshold in the transthoracic calf model and excitation threshold for extracellular field stimulation in the cultured cell model. To further test the hypothesis, we determined whether symmetrical biphasic waveforms, which reduce defibrillation threshold in the calf to approximately 65% of that of the corresponding monophasic waveform (14), decrease excitation threshold in the cultured cell model. Experiments were performed on 100- to 250-microns aggregates from 10- to 12-day-old chick embryos. Excitation threshold strength-duration curves obtained at extracellular potassium (Ko) = 6.5 mM and pacing interval of 1,000 ms showed a significant reduction for symmetrical biphasic rectangular waveforms, when compared with the corresponding monophasic waveforms for durations greater than 3 ms. At the rheobase, the threshold ratio between the biphasic and monophasic waveforms was 0.63 (SE = 0.02). Transmembrane potentials during stimulation showed that excitation takes place during the second portion of the biphasic waveform for intensities that are subthreshold for the monophasic waveform. The relative effectiveness of the biphasic waveform (5-ms duration) increases under "fibrillation conditions" of short pacing interval (300 ms) and high extracellular potassium (10.5 mM). These results show that symmetrical biphasic waveforms decrease excitation threshold in the cultured cell model and that the degree of threshold reduction is dependent on Ko and beat rate.

Animals↗

PubMed1995

Antifibrillary action of class I-IV antiarrhythmic agents in the model of ventricular fibrillation threshold of anesthetized guinea pigs.

We compared the effects of class I-IV antiarrhythmic agents on the ventricular fibrillation threshold (VFT) induced by electrical stimulation directly on the myocardium in anesthetized, open-chest guinea pigs. VFT was assessed by determining the intensity (mA) of electrical current required to induce ventricular fibrillation (VF) and is expressed as a percentage change of the baseline premedication value. The following antiarrhythmic agents or their solvent were administered intravenously (i.v.) to pentobarbital-anesthetized animals (n = 6-12 per group): class I antiarrhythmic agent encainide (1.5 mg/kg); class II antiarrhythmic agents atenolol (2.5 mg/kg), metoprolol (2.5 mg/kg), and nebivolol (2.5 mg/kg); class III antiarrhythmic agents dofetilide (0.08 mg/kg), terikalant (0.04 mg/kg), and DL-sotalolol (10 mg/kg); and class IV antiarrhythmic agent verapamil (0.16 mg/kg). The antiarrhythmic compounds or their solvents resulted in the following changes in the VFT at 15 min after treatment: saline control, 1 +/- 14% (mean +/- SEM) from its baseline value; 10% hydroxypropyl-beta-cyclodextrine (CD), 4 +/- 13%; encainide, 183 +/- 46% (p < 0.05 vs. saline); atenolol, 66 +/- 23% (p > 0.05 vs. saline); metoprolol, 89 +/- 25% (p > 0.05 vs. saline); nebivolol, 224 +/- 58% (p < 0.05 vs. 10% CD); DL-sotalol, 485 +/- 119% (p < 0.05 vs. saline); dofetilide, 357 +/- 69% (p < 0.05 vs. saline); terikalant, 487 +/- 183% (p < 0.05 vs. saline), and verapamil, -17 +/- 21% (p > 0.05 vs. saline). At the doses used, all compounds significantly reduced heart rate (HR).(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic beta-Antagonists↗

PubMed2005

Population-response model for vibrotactile spatial summation.

A computational model based on previous physiological and psychophysical data is presented for the human Pacinian (P) psychophysical channel. The model can predict the probability of detection in simple psychophysical tasks, and hence psychometric functions and thresholds. The model simulates stimulating variable and fixed glabrous skin sites with different-sized contactors and includes spatial variation of monkey P-fiber sensitivities. Therefore, it is especially suitable for studying spatial summation, i.e. the improvement of threshold with increasing contactor area. Selective contributions of neural integration (n.i.) and probability summation (p.s.) are also incorporated into the model. Model predictions are compared to psychophysical results of Gescheider et al. (2005). The performance of the model regarding the effects of contactor size is very good. In addition to predicting approximately 3 dB improvement of thresholds when the contactor area is doubled, the model also reveals nonlinear contributions of p.s. and n.i. Furthermore, the model asserts that thresholds are largely governed by neural integration when small contactors are used. These and other findings discussed in the article show that the presented model is a helpful tool for formulating testable hypotheses. Although the model can also simulate some temporal summation effects, simulation results do not conform well to previous data on temporal response properties. Thus, the model needs to be refined in that respect.

Cell Count↗

PubMed2006

The effects of supplementation with lutein and/or zeaxanthin on human macular pigment density and colour vision.

BACKGROUND: Both yellow-blue (YB) discrimination thresholds and macular pigment optical density (MPOD) measurements in the eye exhibit large variability in the normal population. Although it is well established that selective absorption of blue light by the macular pigment (MP) can significantly affect trichromatic colour matches, the extent to which the MP affects colour discrimination (CD) sensitivity remains controversial. OBJECTIVE: In this study, we assess whether the variability in YB thresholds is attributable to differences in MPOD, both at the fovea and in the paracentral visual field. We also investigated whether higher levels of MP offer any advantage in other visual functions such as red-green (RG) CD sensitivity. DESIGN: CD thresholds and spatial MPOD profiles were measured in 24 normal trichromats supplemented with zeaxanthin (OPTISHARP) and/or lutein. Novel stimulus conditions that isolate YB and RG chromatic mechanisms were employed and MPOD profiles were measured up to an eccentricity of 8 degrees. RESULTS: The data reveal an increase in MPOD in the supplemented subjects that was almost uniform within a centre region around the fovea subtending +/-4 degrees. RG sensitivity was high in all subjects with thresholds well within the normal range. Unexpectedly, YB thresholds were also normal and showed no correlation with MPOD. A model for threshold CD based on appropriate combinations of cone contrast signals was developed to explain the experimental findings. CONCLUSIONS: YB thresholds remain unaffected by supplementation with lutein and/or zeaxanthin rather, at increased MPOD levels, RG vision tends to be improved. The model accounts for the absence of correlation between MPOD and YB thresholds and predicts a marginal improvement in RG discrimination when MPOD is high.

Adult↗