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PubMed2006

The effect of white-noise mask level on sinewave contrast detection thresholds and the critical-band-masking model.

It is known that visual noise added to sinusoidal gratings changes the typical U-shaped threshold curve which becomes flat in log-log scale for frequencies below 10c/deg when gratings are masked with white noise of high power spectral density level. These results have been explained using the critical-band-masking (CBM) model by supposing a visual filter-bank of constant relative bandwidth. However, some psychophysical and biological data support the idea of variable octave bandwidth. The CBM model has been used here to explain the progressive change of threshold curves with the noise mask level and to estimate the bandwidth of visual filters. Bayesian staircases were used in a 2IFC paradigm to measure contrast thresholds of horizontal sinusoidal gratings (0.25-8 c/deg) within a fixed Gaussian window and masked with one-dimensional, static, broadband white noise with each of five power density levels. Raw data showed that the contrast threshold curve progressively shifts upward and flattens out as the mask noise level increases. Theoretical thresholds from the CBM model were fitted simultaneously to the data at all five noise levels using visual filters with log-Gaussian gain functions. If we assume a fixed-channel detection model, the best fit was obtained when the octave bandwidth of visual filters decreases as a function of peak spatial frequency.

Bayes Theorem↗
PubMedDate not supplied

[A stochastic approach to solving the problem of sensory system thresholds].

On the basis of an analysis of mathematical models realizing concept of discrete and signal detection probability of the threshold it is shown that utilization for describing the discrete model concepts of probability and the apparatus of Dirac delta functions and root-mean-square of noise distribution being reduced to zero in the continuous model the analytical expressions of the models are identical. The evidence obtained shows inner unity of the examined thresholds models and universal nature of the thresholds model of Swets, Tanner and Birdsall built on the basis of the statistical theory of signal detectability. It provides solution of one of the central problems of psychophysics--that of the threshold of the sensory systems.

Models, Theoretical↗
PubMed2006

Arsenic cancer risk confounder in southwest Taiwan data set.

Quantitative analysis for the risk of human cancer from the ingestion of inorganic arsenic has been based on the reported cancer mortality experience in the blackfoot disease (BFD) -endemic area of southwest Taiwan. Linear regression analysis shows that arsenic as the sole etiologic factor accounts for only 21% of the variance in the village standardized mortality ratios for bladder and lung cancer. A previous study had reported the influence of confounders (township, BFD prevalence, and artesian well dependency) qualitatively, but they have not been introduced into a quantitative assessment. In this six-township study, only three townships (2, 4, and 6) showed a significant positive dose-response relationship with arsenic exposure. The other three townships (0, 3, and 5) demonstrated significant bladder and lung cancer risks that were independent of arsenic exposure. The data for bladder and lung cancer mortality for townships 2, 4, and 6 fit an inverse linear regression model (p < 0.001) with an estimated threshold at 151 microg/L (95% confidence interval, 42 to 229 microg/L) . Such a model is consistent with epidemiologic and toxicologic literature for bladder cancer. Exploration of the southwest Taiwan cancer mortality data set has clarified the dose-response relationship with arsenic exposure by separating out township as a confounding factor. Key words: arsenic, blackfoot disease, bladder cancer, cancer risk, confounder, dose-response relationship, southwest Taiwan, threshold model.

Arsenic↗
PubMed1989

Measurement and modeling of peripheral detection and discrimination thresholds.

This report describes experimental measurements of threshold contrasts as a function of the angle to the visual axis (peripheral threshold contrasts). The visual tasks consist in detection (perception of presence) and discrimination (perception of a form feature) of simple visual signs during a fixation period realistic observing conditions being chosen. Proceeding from the experimental findings a model for forecasting off-axis threshold contrast functions on different visual conditions is developed based upon spatial frequency filters. Further with the aid of a known model visibility fields are calculated.

Discrimination, Psychological↗
PubMed2000

Complete numerical solution of electron-hydrogen model collision problem above the ionization threshold

Benchmark results are presented for electrons colliding with hydrogen atoms in the S-wave (Temkin-Poet) model collision problem, which neglects angular momentum. Complete results (elastic, inelastic, and ionization), accurate to 1%, are obtained by numerically integrating Schrodinger's equation subject to correct asymptotic boundary conditions. This marks the first time direct matching to asymptotic boundary conditions has been shown to yield convergent ionization amplitudes for a Coulomb three-body problem. Results are presented for impact energies of 54.4 and 40.8 eV, where comparison with other theories is available.

Journal Article↗
PubMed1986

Invariance of the slope of the psychometric function with spatial summation.

The high-threshold probability summation model for improved detection of near-threshold gratings with increased spatial extent of the patterns assumes that the detection psychometric function is a Weibull function. This model predicts that the slope of the psychometric function should not change as the number of mechanisms stimulated increases, although other models predict that the slope should vary. We confirm for a two-alternative, forced-choice paradigm that the slope parameter does not vary systematically with spatial frequency or with number of periods of the grating, although there are reliable differences in average slope between observers.

Female↗
PubMed1997

Red-green and achromatic temporal filters: a ratio model predicts contrast-dependent speed perception.

We simultaneously measured detection and identification performance by using isoluminant red-green (RG) and achromatic flickering stimuli and fitted these data with a modified line-element model that does not make high-threshold assumptions. The modeling shows that detection and identification data are adequately described by postulating only two underlying temporal filters each for RG and achromatic vision, even when more than two threshold classifications are evident. We use a spatial frequency of 1.5 cycles per degree (c/deg) and compare the derived temporal impulse response functions with those obtained previously with the use of 0.25 c/deg stimuli under otherwise identical conditions [J. Opt. Soc. Am. A 13, 1969 (1996)]. We find that at 1.5 c/deg the luminance impulse response functions peak later and integrate out to longer times compared with those measured at 0.25 c/deg. For RG stimuli, although their relative overall sensitivities change, the impulse response functions are similar across spatial frequency, indicating a constancy of chromatic temporal properties across spatial scales. In a second experiment, we measured RG and achromatic flicker discrimination over a wide range of suprathreshold contrasts. These data suggest a common nonlinear contrast response function operating after initial temporal filtering. Using a ratio model of speed perception in which both RG and achromatic filters are combined at a common motion site, we can predict (1) the perceived slowing of RG stimuli compared with the perceived drift of achromatic drifting stimuli, (2) the contrast dependency of speed perception for RG and achromatic drifting stimuli, and (3) how this dependency changes with base speed. Thus we conclude that there is no need to postulate separate mechanisms for fast and slow motion [Nature (London) 367, 268 (1994)], since a unified ratio model can explain both RG and achromatic contrast-speed dependency.

Color Perception↗
PubMed2002

Self-organized criticality: robustness of scaling exponents.

We investigate a deterministic, conservative, undirected, critical height sandpile model with dissipation of an energy at boundaries that can simulate avalanche dynamics. It was derived from the Bak-Tang-Wiesenfeld model [P. Bak, C. Tang, and K. Wiesenfeld, Phys. Rev. Lett. 59, 381 (1987)] introducing an additional second-higher threshold so the model has two distinct thresholds. Our computer simulations for a two-dimensional lattice show that scaling properties of the model depend on the higher-threshold values and site concentrations. These results are not therefore consistent with the present self-organized criticality hypothesis where the scaling properties are independent of the model parameters.

Journal Article↗
PubMed2001

The antinociceptive effect of moclobemide on the vocalization threshold to paw pressure in a rat model of unilateral mononeuropathy.

The aim of our study was to investigate the antinociceptive activity of moclobemide on the vocalization threshold to paw pressure in a rat model of unilateral mononeuropathy. The neuropathy was produced by ligation of the sciatic nerve and nociceptive thresholds were determined 15-21 days after surgery by a modification of the Randall-Sellito method. Group 1 (n= 10) received 0.2 ml peroral (p.o.) saline, Group 2 (n= 10) 5 mg x kg(-1), Group 3 (n= 10) 10 mg x kg(-1) and Group 4 (n= 10) 20 mg x kg(-1) p.o. moclobemide. Nociceptive pressure thresholds were then measured every 20 minutes after drug administration. Analysis of variance, Tukey's test and a paired Student's t-test were employed for statistical analysis. The perorally administered moclobemide (5, 10 and 20 mg x kg(-1)) produced an antinociceptive effect on both lesioned and non-lesioned hind paws ( P< 0.05). However, the analgesic effect on the lesioned paw was significantly more potent than the non-lesioned paw. The peak value ( p) remained constant while the maximal increment between the control threshold and the peak value ( I(max)) was significantly more pronounced for the lesioned paw ( P< 0.001). The results of this study may suggest that moclobemide can be a therapeutic alternative to treat some clinical symptoms in peripheral neuropathic conditions.

Analgesics↗
PubMed1989

A clinical study of perimetric probability maps.

Perimetric probability maps depict visual field results in terms of the frequency with which the measured findings are seen in a normal population. We tested clinically the importance of the model of the normal visual field used to calculate such maps. Forty-one eyes of 41 normal subjects and 58 eyes of 46 glaucomatous patients were studied. Probability maps were calculated by means of two different models of the normal visual field. The first model assumed gaussian threshold distributions with constant variability across the field. The second used empirically determined nongaussian location-dependent threshold distributions. Probability maps using the empiric model allowed better separation between glaucomatous and normal eyes, and the number of significant points in normal subjects was in better agreement with the theoretically expected number. The gaussian model yielded an unacceptably high frequency of significant points in normal fields, particularly in the midperiphery. The clinical usefulness of perimetric probability maps depends critically on the choice of normal visual field model.

Adult↗
PubMed1995

The origin of synergistic symbiosis.

A dominant theme in the history of life has been the evolutionary innovations of cooperative symbioses: the first genomes near the origin of life, integrated prokaryotic cells, the complex symbiotic communities that evolved into modern eukaryotic cells, lichens, mycorrhizae, and so on. In this paper, a model of cooperative symbiosis that shows a threshold condition for the evolution of cooperation is analyzed. The threshold is not easily passed, but cooperative evolution proceeds rapidly once a symbiosis overcomes the threshold. In the model presented here, each species has genetic variability for a symbiotic trait. The trait imposes a reproductive cost on its bearer but enhances the reproduction of its partner species. For example, in the origin of genetic systems, the trait may cause biochemical synergism for the rate of replication of primitive RNA strands as in Eigen and Schuster's hypercycle model. Models of growth are contrasted with synergism, which are most appropriate for the evolution of genetic systems and for mutualisms such as lichens, with the strategic and psychological applications of the Prisoner's Dilemma model.

Biological Evolution↗
PubMed1999

Binary regression for risks in excess of subject-specific thresholds.

We describe models for binary valued data to be used to explain the incidence of disease given the level of a known risk factor. Every individual has an unobservable tolerance of the risk. Risk levels below the individual tolerance do not increase the disease incidence above the background, unexposed rate. We estimate parameters from both the tolerance distribution and the risk function for a large group of mice exposed to very low levels of a known carcinogen.

2-Acetylaminofluorene↗
PubMed2000

Tissue-level thresholds for axonal damage in an experimental model of central nervous system white matter injury.

In vivo, tissue-level, mechanical thresholds for axonal injury were determined by comparing morphological injury and electrophysiological impairment to estimated tissue strain in an in vivo model of axonal injury. Axonal injury was produced by dynamically stretching the right optic nerve of an adult male guinea pig to one of seven levels of ocular displacement (Nlevel = 10; Ntotal = 70). Morphological injury was detected with neurofilament immunohistochemical staining (NF68, SM132). Simultaneously, functional injury was determined by the magnitude of the latency shift of the N35 peak of the visual evoked potentials (VEPs) recorded before and after stretch. A companion set of in situ experiments (Nlevel = 5) was used to determine the empirical relationship between the applied ocular displacement and the magnitude of optic nerve stretch. Logistic regression analysis, combined with sensitivity and specificity measures and receiver operating characteristic (ROC) curves were used to predict strain thresholds for axonal injury. From this analysis, we determined three Lagrangian strain-based thresholds for morphological damage to white matter. The liberal threshold, intended to minimize the detection of false positives, was a strain of 0.34, and the conservative threshold strain that minimized the false negative rate was 0.14. The optimal threshold strain criterion that balanced the specificity and sensitivity measures was 0.21. Similar comparisons for electrophysiological impairment produced liberal, conservative, and optimal strain thresholds of 0.28, 0.13, and 0.18, respectively. With these threshold data, it is now possible to predict more accurately the conditions that cause axonal injury in human white matter.

Analysis of Variance↗
PubMed2003

A multiple regression model of combined duplex criteria for detecting threshold carotid stenosis and predicting the exact degree of carotid stenosis.

BACKGROUND AND PURPOSE: Combined duplex criteria are commonly used in vascular laboratories for evaluating carotid stenosis. However, most of these combinations are empirical, and systemic validation is lacking. This study was completed using a multiple regression method to evaluate the accuracy of different combined duplex criteria for detecting threshold carotid stenosis and predicting the exact degree of carotid stenosis on angiography. METHODS: Two hundred sixty-six sets of unilateral carotid duplex and angiographic data were randomly divided into 2 sets: a derivation set and a validation set. The derivation set was used to develop a multiple logistic regression model for detecting 70% threshold carotid stenosis. Age, sex, systolic blood pressure, diastolic blood pressure, Doppler peak systolic velocity (PSV), Doppler and diastolic velocity (EDV), the systolic carotid ratio (SCR), and ophthalmic artery flow direction were tested as independent variables. A multiple linear regression model was also developed for predicting the exact degree of carotid stenosis on angiogram. The validation set was then used to evaluate the accuracy of these models. RESULTS: According to the logistic regression strategy, the best multiple logistic regression model was as follows: probability of threshold carotid stenosis = exp(2.6 PSV - 6.2)/[1 + exp(2.6 PSV - 6.2)]. The best linear regression model was as follows: degree of carotid stenosis = 20.2 PSV - 7.4 EDV + 0.4 SCR + 8.5. Both models proved to be valid following an evaluation of the validation set. CONCLUSIONS: This study illustrates that Doppler parameters may be of use in predicting the exact degree of carotid stenosis and the probability of threshold carotid stenosis. This is important if duplex criteria are going to replace angiography as the only tool for selecting endarterectomy candidates.

Aged↗
PubMed2007

Neurokinin-1 receptor antagonism in a human model of visceral hypersensitivity.

BACKGROUND: Substance P acting via the neurokinin-1 receptor is involved in the development of hyperalgesia, although studies using neurokinin-1 receptor antagonists (NK-1RA) in human somatic pain have been disappointing. AIM: To evaluate whether Substance P is involved in the development of human visceral pain/hyperalgesia using a selective NK-1RA. METHODS: Using a validated human model of acid-induced oesophageal allodynia, pain thresholds to electrical stimulation (mA) were measured in the proximal oesophagus and the foot (somatic control), pre- and for 4 h postdistal oesophageal acid in 14 healthy subjects, using a double-blind, randomized, two-period, crossover study. Measurements were taken on the third day of dosing with either an oral NK-1RA or matching placebo, with 2 weeks washout between periods. RESULTS: Baseline pain threshold did not differ between treatments (proximal oesophagus 37 +/- 7.4 mA NK-1RA vs. 38 +/- 10.1 placebo P = 0.81, foot 40 +/- 15 mA NK-1RA vs. 38 +/- 14 placebo P = 0.68). NK-1RA did not attenuate the reduction in pain threshold in the proximal oesophagus postacid infusion (AUC-394 +/- 279 NK-1RA vs. -262 +/- 397 placebo P = 0.54). CONCLUSIONS: The lack of effect of NK-1RA on oesophageal pain threshold in our model does not support a role for Substance P in the development of acid-induced oesophageal allodynia.

Adult↗
PubMed2003

The heritability of bipolar affective disorder and the genetic relationship to unipolar depression.

BACKGROUND: Twin studies of bipolar affective disorder (BPD) have either been small or have not used explicit diagnostic criteria. There has been little use of genetic model fitting and no analyses to explore the etiological overlap with unipolar depression (UPD). METHODS: Sixty-seven twin pairs, 30 monozygotic and 37 dizygotic, in which the proband had BPD were ascertained, and lifetime diagnoses were made using DSM-IV criteria. Univariate models were applied to estimate the contribution of additive genetic and environmental effects. Bipolar data were then combined with those from 68 monozygotic and 109 dizygotic pairs in which the proband had UPD. Two models were explored: a classic 2-threshold approach, in which BPD and UPD occupy the same continuum of liability but differ in severity, and a correlated liability model of mania and depression. RESULTS: Heritability of BPD was estimated at 85% (95% confidence interval [CI], 0.73-0.93) using narrow concordance and 89% (95% CI, 0.61-1.0) using broad concordance, with no shared environmental effects detected. A 2-threshold model was an unsatisfactory fit. Fitting a correlated liability model revealed a genetic correlation of 0.65 (95% CI, 0.58-0.75) between mania and depression and a correlation of 0.59 (95% CI, 0.15-0.84) for nonfamilial environment. Approximately 71% of the genetic variance for mania was not shared with depression. CONCLUSIONS: As defined by the DSM-IV, BPD is highly heritable. There are substantial genetic and nonshared environmental correlations between mania and depression, but most of the genetic variance in liability to mania is specific to the manic syndrome.

Bipolar Disorder↗
PubMed1998

Effects of left atrial dilatation on the endocardial atrial defibrillation threshold: a study in an ovine model of pacing induced dilated cardiomyopathy.

Left atrial (LA) dilation is a common finding in patients with chronic atrial fibrillation (AF). Progressive dilatation may alter the atrial defibrillation threshold (ADFT). In our study, epicardial electrodes were implanted on the LA free wall and right ventricular apex of eight adult sheep. Large surface area, coiled endocardial electrodes were positioned in the coronary sinus and right atrium (RA). LA dilatation was induced by rapid ventricular pacing (190 beats/min) for 6 weeks and echocardiographically assessed weekly along with the ADFT (under propofol anesthesia). LA effective refractory period (ERP) was measured every 2-3 days using a standard extra stimulus technique and 400 ms drive. The AF cycle length (AFCL) was assessed from LA electrograms. During the 6 weeks of pacing the mean LA area increased from 6.1 +/- 1.5 to 21.3 +/- 2.4 cm2. There were no significant changes in the mean ADFT (122 +/- 15 V), circuit impedance (46 +/- 5 omega), or LA AFCL (136 +/- 23 ms). There was a significant increase in the mean LA ERP (106 +/- 10 ms at day 0, and 120 +/- 13 ms at day 42 of pacing). In this study, using chronically implanted defibrillation leads, the minimal energy requirements for successful AF were not significantly altered by ongoing left atrial dilatation. This finding is a further endorsement of the efficiency of the coronary sinus/RA shock vector. Furthermore, the apparent stability of the AF present may be a further indication of a link between the type of AF and the ADFT.

Animals↗