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PubMed1990

Saturation and purity of near-threshold spectral increments on achromatic backgrounds.

Vector models of color vision predict that the chromatic saturation of near-threshold incremental stimuli should grow with increasing achromatic adaptation. In contrast, colorimetric models predict that saturation should decrease with increasing adaptation in correspondence with excitation purity. An experimental test disagrees with both predictions, but is consistent with a two-process model of chromatic adaptation and contrast. Foveal monochromatic test stimuli 0.6 log unit above threshold were superposed on achromatic backgrounds with retinal illuminances of up to 5.3 log Td. Saturation and purity both decreased as background illuminance increased up to 2 log Td, but the saturation decrease was relatively small. As background illuminance increased above 2 log Td, purity remained nearly constant and saturation often increased, making the overall saturation function U-shaped. Thus, a purity of 0.1 at high background illuminance could sometimes elicit a higher saturation estimate than a purity of 0.9 at low background illuminance.

Adaptation, Ocular↗
PubMed1999

Intrinsic envelope fluctuations and modulation-detection thresholds for narrow-band noise carriers.

A model is presented which calculates the intrinsic envelope power of a bandpass noise carrier within the passband of a hypothetical modulation filter tuned to a specific modulation frequency. Model predictions are compared to experimentally obtained amplitude modulation (AM) detection thresholds. In experiment 1, thresholds for modulation rates of 5, 25, and 100 Hz imposed on a bandpass Gaussian noise carrier with a fixed upper cutoff frequency of 6 kHz and a bandwidth in the range from 1 to 6000 Hz were obtained. In experiment 2, three noises with different spectra of the intrinsic fluctuations served as the carrier: Gaussian noise, multiplied noise, and low-noise noise. In each case, the carrier was spectrally centered at 5 kHz and had a bandwidth of 50 Hz. The AM detection thresholds were obtained for modulation frequencies of 10, 20, 30, 50, 70, and 100 Hz. The intrinsic envelope power of the carrier at the output of the modulation filter tuned to the signal modulation frequency appears to provide a good estimate for AM detection threshold. The results are compared with predictions on the basis of the more complex auditory processing model by Dau et al.

Auditory Threshold↗
PubMed1995

Steroid elution improves the stimulation threshold in an active-fixation atrial permanent pacing lead. A randomized, controlled study. Model 4068 Investigators.

BACKGROUND: Prior work suggests that the addition of a steroid-eluting reservoir to a passive-fixation permanent pacemaker lead improves the stimulation threshold; however, no large randomized study has addressed this tissue. Over the last several years, there has been an increase in enthusiasm for the use of active-fixation permanent pacemaker leads for various reasons in spite of the generally accepted notion that active-fixation leads have higher stimulation thresholds. METHODS AND RESULTS: This multicenter, randomized, controlled study examined the difference in performance between a standard active-fixation atrial lead (Medtronic model 4058) and a steroid-eluting lead (Medtronic model 4068). Stimulation thresholds were obtained in a four-point strength-duration fashion. Evaluations of sensing and impedance were performed as well. These evaluations were performed at implantation, at weeks 1 through 4, and at weeks 6, 12, 24, and 52. Stimulation thresholds were significantly better in the steroid lead than in the nonsteroid lead at each measurement point from 1 week to 12 months. The mean 1.6-V stimulation threshold at 12 months was 0.19 +/- 0.2 ms in the steroid lead and 0.41 +/- 0.30 ms in the control lead. No acute peaking was observed with the steroid lead, whereas significant peaking was observed with the control lead. There was no difference in long-term sensing or impedance. CONCLUSIONS: Inclusion of a steroid-eluting reservoir in an active-fixation permanent pacing lead improved stimulation thresholds in both the subacute and chronic periods and therefore should extend pulse-generator longevity.

Aged↗
PubMedDate not supplied

Patient preference-based treatment thresholds and recommendations: a comparison of decision-analytic modeling with the probability-tradeoff technique.

BACKGROUND: Decision analysis (DA) and the probability-tradeoff technique (PTOT) are patient preference-based methods of determining optimal therapy for individuals. Using aspirin therapy for the primary prevention of stroke and myocardial infarction (MI) in elderly persons as an example, the objective of this study was to determine whether group-level treatment thresholds and individual-level treatment recommendations derived using PTOT are identical to those of DA incorporating the patients' own values. METHODS: Persons in a pilot study of the efficacy of aspirin in the prevention of stroke and MI were asked to participate. Participant values and utilities for pertinent health states (e.g., minor and major stroke, MI, major bleeding episode) were determined. Then, in three hypothetical clinical situations in which the chance of stroke or MI was varied, PTOT was used to directly determine treatment thresholds for aspirin therapy (i.e., the smallest reduction in MI or stroke risk for which participants would be willing to take aspirin). Using DA modeling, with the same probabilities of events as in the PTOT exercise and incorporating participants' own values, treatment thresholds for the three clinical situations were determined. The thresholds determined by the two approaches were compared. Finally, based on these treatment thresholds, using the best estimates of the efficacy of aspirin to prevent first-time stroke and MI, PTOT and DA treatment recommendations for individual participants were compared. RESULTS: The 42 participants reported that a major stroke was the least desirable health state, followed by MI, minor stroke, and major bleeding. The minimum risk reduction required to take aspirin was greater for MI prevention compared with stroke prevention. For the two clinical situations in which the hypothetical efficacy of aspirin to prevent stroke was varied, treatment thresholds for the PTOT versus DA approaches differed (p < 0.04), but this difference was not significant (p = 0.19) for the MI-based clinical situation. Using the best estimate of the efficacy of aspirin to prevent first-time stroke and MI, PTOT and DA treatment recommendations whether or not to take aspirin were discordant for 38% of participants (16 of 42) (p < 0.001). CONCLUSIONS: Patient preference-based group-level treatment thresholds and individual-level treatment recommendations can differ significantly depending on whether PTOT or DA is used, apparently because the two emphasize different aspects of the decision-making process. DA theory assumes that effective therapeutic decision making should maximize both quality and quantity of life; with PTOT, the emphasis for effective clinical decision making allows patients to be fully engaged in the process, thus hopefully leading to fully informed decisions that may result in satisfaction and compliance.

Age Factors↗
PubMed1999

A measure of progress in fold recognition?

We present a retrospective analysis of CASP3 threading predictions, applying evaluation and assessment criteria used at CASP2. Our purpose is twofold. First, we wish to ask whether measures of model accuracy are comparable between CASP3 and CASP2, even though they have been calculated differently. We find that these quantities are effectively the same, and that either may be used to compare model accuracy. Secondly, we wish to assess progress in fold recognition by comparing the numbers of CASP2 and CASP3 models that cross specific accuracy thresholds. We find that the number of accurate models at CASP3 drops sharply as the targets become more difficult, with less extensive similarity to known structures, exactly the pattern seen at CASP2. CASP3 teams do not seem to have predicted accurate models for targets of greater difficulty, and for a given difficulty range the best CASP3 models seem no more accurate than the best models at CASP2. At CASP3, however, we find greater numbers of accurate models for medium-difficulty targets, with extensive similarity to a known structure but no shared sequence motifs. Threading methods would appear to have become more reliable for modeling based on remote evolutionary relationships.

Algorithms↗
PubMed1993

Flow cytometric titration of retroviral expression vectors: comparison of methods for analysis of immunofluorescence histograms derived from cells expressing low antigen levels.

Few quantitative studies addressing immunofluorescence histogram analysis have been published. One study by Overton (Cytometry 9:619-626, 1988) has shown threshold and histogram subtraction methods to be accurate for analysis of well-separated immunofluorescence distributions of positive and negative cells. An evaluation of methods to analyze immunofluorescence histograms when positive and negative immunofluorescence distributions overlap has not, to our knowledge, been reported. In this paper, data obtained from flow cytometry of immunofluorescently stained cells infected with recombinant retroviruses that produce a range of simian virus 40 large T antigen levels were analyzed by threshold, histogram subtraction, and distribution modeling methods. This analysis showed that as the separation between the immunofluorescence distributions of positive and negative cell populations decrease the best methods for histogram analysis are modeling followed, in order, by histogram subtraction, and threshold analysis.

Antigens, Viral, Tumor↗
PubMed1979

Pattern separability in a random neural net with inhibitory connections.

Some interesting properties on pattern separation have been shown through researches by neural models of cerebellar cortex. It seems to us that those results are a part of the properties of pattern separation. A two layer random nerve net with inhibitory connections is given as a model of the cerebellar cortex. The model is composed of threshold elements there. A more general theory of pattern separation than those studied earlier is given, and the pattern separability of the model is considered. It is revealed that the standard deviation of threshold values of threshold elements has a great effect on the pattern separability and the control of the firing rate. The present study is also intended to investigate the pattern separability in such a case that the firing rate of input patterns are not equal, and a pattern includes the other pattern. It is assumed there that the standard deviation is small. Some properties of the degree of pattern separation are cleaned up.

Cerebellum↗
PubMedDate not supplied

Mathematical models of cochlear nucleus onset neurons: I. Point neuron with many weak synaptic inputs.

The cochlear nucleus (CN) presents a unique opportunity for quantitatively studying input-output transformations by neurons because it gives rise to a variety of different response types from a relatively homogeneous input source, the auditory nerve (AN). Particularly interesting among CN neurons are Onset (On) neurons, which have a prominent response to the onset of sustained sounds followed by little or no response in the steady-state. On neurons contrast sharply with their AN inputs, which respond vigorously throughout stimuli. On neurons can entrain to stimuli (firing once per cycle of a periodic stimulus) at up to 1000 Hz, unlike their AN inputs. To understand the mechanisms underlying these response patterns, we tested whether an integrate-to-threshold point-neuron model with a fixed refractory period can account for On discharge patterns for tones, systematically examining the effect of membrane time constant and the number and strength of the exclusively excitatory AN synaptic inputs. To produce both onset responses to high-frequency tone bursts and entrainment to a broad range of low-frequency tones, the model must have a short time constant ( approximately 0.125 ms) and a large number (>100) of weak synaptic inputs, properties that are consistent with the electrical properties and anatomy of On-responding cells. With these parameters, the model acts like a coincidence detector with a threshold-like relationship between the instantaneous discharge rates of the output and the inputs. Onset responses to high-frequency tone bursts result because the threshold effect enhances the initial response of the AN inputs and suppresses their relatively lower sustained response. However, when the model entrains across a broad range of frequencies, it also produces short interspike intervals at the onset of high-frequency tone bursts, a response pattern not found in all types of On neurons. These results show a tradeoff, that may be a general property of many neurons, between following rapid stimulus fluctuations and responding without short interspike intervals at the onset of sustained stimuli.

Acoustic Stimulation↗
PubMed2002

A modelling study of locomotion-induced hyperpolarization of voltage threshold in cat lumbar motoneurones.

During fictive locomotion the excitability of adult cat lumbar motoneurones is increased by a reduction (a mean hyperpolarization of approximately 6.0 mV) of voltage threshold (Vth) for action potential (AP) initiation that is accompanied by only small changes in AP height and width. Further examination of the experimental data in the present study confirms that Vth lowering is present to a similar degree in both the hyperpolarized and depolarized portions of the locomotor step cycle. This indicates that Vth reduction is a modulation of motoneurone membrane currents throughout the locomotor state rather than being related to the phasic synaptic input within the locomotor cycle. Potential ionic mechanisms of this locomotor-state-dependent increase in excitability were examined using three five-compartment models of the motoneurone innervating slow, fast fatigue resistant and fast fatigable muscle fibres. Passive and active membrane conductances were set to produce input resistance, rheobase, afterhyperpolarization (AHP) and membrane time constant values similar to those measured in adult cat motoneurones in non-locomoting conditions. The parameters of 10 membrane conductances were then individually altered in an attempt to replicate the hyperpolarization of Vth that occurs in decerebrate cats during fictive locomotion. The goal was to find conductance changes that could produce a greater than 3 mV hyperpolarization of Vth with only small changes in AP height (< 3 mV) and width (< 1.2 ms). Vth reduction without large changes in AP shape could be produced either by increasing fast sodium current or by reducing delayed rectifier potassium current. The most effective Vth reductions were achieved by either increasing the conductance of fast sodium channels or by hyperpolarizing the voltage dependency of their activation. These changes were particularly effective when localized to the initial segment. Reducing the conductance of delayed rectifier channels or depolarizing their activation produced similar but smaller changes in Vth. Changes in current underlying the AHP, the persistent Na(+) current, three Ca(2+) currents, the "h" mixed cation current, the "A" potassium current and the leak current were either ineffective in reducing Vth or also produced gross changes in the AP. It is suggested that the increased excitability of motoneurones during locomotion could be readily accomplished by hyperpolarizing the voltage dependency of fast sodium channels in the axon hillock by a hitherto unknown neuromodulatory action.

Animals↗
PubMed1983

Progress report on experimental use of CYBEST model 2 for practical gynecologic mass screening. Alterations of the specimen rejection threshold and specimen preparation.

In order to maintain an acceptable performance of CYBEST model 2 in the field of gynecologic mass screening, the algorithm of rejection for the CYBEST assessment of inadequately oligocellular samples was reviewed, and the mean number of detected cells per square millimeter for passing rejection was raised from two cells to four. The results of field tests obtained after the modification showed a false-positive rate of 28.3% for 9,010 samples as compared with 41.8% for 10,549 samples before the modification while the rejection rate showed a moderate yet acceptable increase after the modification from 4.7% to 7.0%. Some modifications of the specimen preparation routine were also introduced to improve the performance of the system.

False Positive Reactions↗
PubMed1998

Elongated physiological summation pools in the human visual cortex.

The visibility of gratings improves with increasing stimulus area. This effect is usually interpreted as being due to probability summation between the outputs of linear, independent spatial filters, although non-linear spatial summation can have similar effects [1]. In order to distinguish between probabilistic and physiological summation models, we measured contrast thresholds using the Visual Evoked Potential (VEP). Our previous work [2] suggests that spatial summation in the VEP is nonlinear and that it occurs preferentially for collinear configurations. Traditional probability summation models predict that areal summation will improve threshold independent of stimulus configuration. Contrast thresholds were derived from VEP contrast response functions for either circular or elongated Gabor patches with aspect ratios up to 6:1. The carrier orientation was either the same as the patch envelope orientation (collinear) or orthogonal to it. Response amplitudes were larger and contrast sensitivity was higher for collinear configurations. The results are consistent with nonlinear, configuration dependent summation that is more extensive along the axis of orientation.

Contrast Sensitivity↗
PubMed1999

Automatic detection of epileptiform activity by single-level wavelet analysis.

We describe a new strategy to automatically identify epileptiform activity in EEG. Our scheme is based upon detecting epileptic spikes, via multiresolution analysis, a relatively new tool in signal processing, which allows for dramatic improvements in the efficiency of basic wavelet analysis. We perform a single-level analysis, which is fast and delivers satisfactory results, provided a wise strategy is adopted. Key points are: the identification of suitable wavelets, in order to gain high computational efficiency; the recognition of a proper resolution level; the computation of an appropriate dynamic threshold, in order to pick out the pathological events. Using a suitable wavelet as the model of a threshold-event proved to be a good choice for devising an algorithm which efficiently performs automatic analysis at high-sensitivity levels. The proposed algorithm was implemented into a C++ multiplatform code having an user-friendly interface, which runs on general-purpose PCs. Results obtained on a set of test tracings, show that the sensitivity of the automatic analysis can be as high as 96%, while less than 5% of the overall recording time is marked. The computational complexity of our algorithm is O (N). Its highly efficient implementation allows for the analysis of up to 310 s of 8 channel EEG, by spending one mere CPU second on a standard PC.

Algorithms↗
PubMed2001

Estimation of the gas exchange threshold in humans: a time series approach.

An autoregressive moving average model is presented for detecting the gas exchange threshold (GET) from gas exchange response data taken during an incremental exercise test. The approach has the advantage of modelling the serial correlation in the noise between observations from the same subject. The model is fitted by maximum likelihood and a breakpoint or threshold in the mean function is the estimated GET. It is shown how an estimate of standard error can be obtained by bootstrapping the residuals from the fitted model. To evaluate the approach, gas exchange data on 14 healthy subjects during a ramp exercise test (20 W x min(-1)) to the limits of tolerance, were analysed. The results are compared to the v-slope method and to the method of maximum likelihood assuming independent noise.

Adult↗
PubMed2000

Analytical estimate of stochasticity thresholds in Fermi-Pasta-Ulam and straight phi(4) models.

We consider an infinitely extended Fermi-Pasta-Ulam model. We show that the slowly modulating amplitude of a narrow wave packet asymptotically satisfies the nonlinear Schrödinger equation (NLS) on the real axis. Using well known results from inverse scattering theory, we then show that there exists a threshold of the energy of the central normal mode of the packet, with the following properties. Below threshold the NLS equation presents a quasilinear regime with no solitons in the solution of the equation, and the wave packet width remains narrow. Above threshold generation of solitons is possible instead and the packet of normal modes can spread out. Analogous results are obtained for the straight phi(4) model. We also give an analytical estimate for such thresholds. Finally, we make a comparison with the numerical results known to us and show that, they are in remarkable agreement with our estimates.

Journal Article↗
PubMedDate not supplied

Threshold fluctuations on temporally modulated backgrounds: a possible physiological explanation based upon a recent computational model.

When a temporally fluctuating background is rapidly modulated (e.g. 30 Hz), the threshold variation of a superimposed flash (the probe) is approximately sinusoidal and in phase with the stimulus. But, with low rates of sinusoidal modulation (e.g. 1 Hz), the threshold variation is distinctly nonsinusoidal in shape. The bases of these aspects of the data, as well as an unmodulated, dc, threshold elevation, are poorly understood. Here 30-Hz and 1-Hz conditions are simulated using a new model of light adaptation (Wilson, 1997). By assuming that the OFF pathway is twice as sensitive as the ON pathway, the model correctly captured the key aspects of both conditions. The results suggest that the 1-Hz data are mediated by a mixture of ON and OFF pathways while the 30-Hz data are largely mediated by the OFF pathway. The probe thresholds on the 30-Hz background appear approximately sinusoidal and approximately in phase with the background stimulus. A number of factors contribute to this deceptively simple observation.

Adaptation, Ocular↗
PubMed2006

Endemic threshold results in an age-duration-structured population model for HIV infection.

In this paper we consider an age-duration-structured population model for HIV infection in a homosexual community. First we investigate the invasion problem to establish the basic reproduction ratio R(0) for the HIV/AIDS epidemic by which we can state the threshold criteria: The disease can invade into the completely susceptible population if R(0)>1, whereas it cannot if R(0)<1. Subsequently, we examine existence and uniqueness of endemic steady states. We will show sufficient conditions for a backward or a forward bifurcation to occur when the basic reproduction ratio crosses unity. That is, in contrast with classical epidemic models, for our HIV model there could exist multiple endemic steady states even if R(0) is less than one. Finally, we show sufficient conditions for the local stability of the endemic steady states.

Age Factors↗