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At least 1,621 records · Page 90Linked to original sources

PubMed2001

Time-varying threshold integral pulse frequency modulation.

Several methods have been proposed so far for the analysis of the integral pulse frequency modulation (IPFM) model and detecting its corresponding physiological information. Most of these methods rely on the low-pass filtering method to extract the modulating signal of the model. In this paper, we present an entirely new approach based on vector space theory. The new method is developed for a more comprehensive form of the IPFM model, namely the time-varying threshold integral pulse frequency modulation (TVTIPFM) model. The new method decomposes the driving signals of the TVTIPFM model into a series of orthogonal basis functions and constructs a matrix identity through which the input signals can be obtained by a parametric solution. As a particular case, we apply this method to R-R intervals of the SA node to discriminate between its autonomic nervous modulation and the stretch induced effect.

Heart Rate↗

PubMed2005

The contribution of spike threshold to acoustic feature selectivity, spike information content, and information throughput.

Hypotheses of sensory coding range from the notion of nonlinear "feature detectors" to linear rate coding strategies. Here, we report that auditory neurons exhibit a novel trade-off in the relationship between sound selectivity and the information that can be communicated to a postsynaptic cell. Recordings from the cat inferior colliculus show that neurons with the lowest spike rates reliably signal the occurrence of stereotyped stimulus features, whereas those with high response rates exhibit lower selectivity. The highest information conveyed by individual action potentials comes from neurons with low spike rate and high selectivity. Surprisingly, spike information is inversely related to spike rates, following a trend similar to that of feature selectivity. Information per time interval, however, was proportional to measured spike rates. A neuronal model based on the spike threshold of the synaptic drive accurately accounts for this trade-off: higher thresholds enhance the spiking fidelity at the expense of limiting the total communicated information. Such a constraint on the specificity and throughput creates a continuum in the neural code with two extreme forms of information transfer that likely serve complementary roles in the representation of the auditory environment.

Acoustic Stimulation↗

PubMedDate not supplied

Quadrature subunits in directionally selective simple cells: spatiotemporal interactions.

I explore here whether linear mechanisms can explain directional selectivity (DS) in simple cells of the cat's striate cortex, a question suggested by a recent upswing in popularity of linear DS models. I chose a simple cell with a space-time inseparable receptive field (RF), i.e. one that shows gradually shifting latency across space, as the RF type most likely to depend on linear mechanisms of DS. However, measured responses of the cell to a moving bar were less modulated, and extended over a larger spatial region than predicted by two different popular "linear" models. They also were more DS in exhibiting a higher ratio of total spikes for the preferred direction. Each of the two models used for comparison has a single "branch" with a single spatiotemporally inseparable linear filter followed by a threshold, hence, a- "1-branch" model. Nonlinear interactions between pairs of bars in a 2-bar linear superposition test of the cell also disagreed in time-course with those of the 1-branch models. The only model whose 1-bar and 2-bar predictions matched the measured cell (including a complete "4-branch" motion energy model that matches complex cells) has two branches that differ in phase by about 90 deg, i.e. in quadrature. Each branch has its own threshold that helps define the preceding spatiotemporal unit as a subunit even after the outputs of the two branches are summed. As subunit phases differ by only 90 deg, flashing bar responses of the 2-subunit model are similar to those of the 1-subunit model. Therefore, the number of subunits is hidden from view when testing with a conventional stationary bar. In summary, movement responses and nonlinear interactions between pairs of bars in the measured cell matched those of the 2-subunit model, while they disagreed with the popular 1-subunit model. Thus, multiple nonlinear subunits appear to be necessary for DS, even in simple cortical cells.

Animals↗

PubMed1992

Lower threshold of motion for one and two dimensional patterns in central and peripheral vision.

Lower motion thresholds for discriminating opposing motion directions were compared for one dimensional (grating) and two dimensional (plaid) stimuli in central and peripheral vision. The results were consistent with a two-stage model of motion sensitivity in which threshold-limiting noise occurs at both stages, and the speed as well as the direction of the resultant motion is determined by intersection-of-constraints (IOC) from the component motions. The results do not support a purely geometric interpretation of the IOC model, in which thresholds for plaid stimuli are related to thresholds of component gratings by a geometric factor. Neither do the data favour explanations in which local luminance features (i.e. blobs) are detected and their velocity determined. Monte-Carlo simulations of the two-stage process predict thresholds across variations in component direction, contrast, and visual field eccentricity. Lower motion thresholds for gratings and plaids both follow a saturating function of contrast; the fit between grating and plaid data is improved when the plaid contrast is expressed in terms of the contrast of its components. Although less contrast saturation was found in the periphery, in relative terms, plaid and grating motion thresholds were similar in central and peripheral vision, implying cortical magnifications are similar for mechanisms which process grating and plaid motion.

Contrast Sensitivity↗

PubMed1996

The organization of seismicity on fault networks.

Although models of homogeneous faults develop seismicity that has a Gutenberg-Richter distribution, this is only a transient state that is followed by events that are strongly influenced by the nature of the boundaries. Models with geometrical inhomogeneities of fracture thresholds can limit the sizes of earthquakes but now favor the characteristic earthquake model for large earthquakes. The character of the seismicity is extremely sensitive to distributions of inhomogeneities, suggesting that statistical rules for large earthquakes in one region may not be applicable to large earthquakes in another region. Model simulations on simple networks of faults with inhomogeneities of threshold develop episodes of lacunarity on all members of the network. There is no validity to the popular assumption that the average rate of slip on individual faults is a constant. Intermediate term precursory activity such as local quiescence and increases in intermediate-magnitude activity at long range are simulated well by the assumption that strong weakening of faults by injection of fluids and weakening of asperities on inhomogeneous models of fault networks is the dominant process; the heat flow paradox, the orientation of the stress field, and the low average stress drop in some earthquakes are understood in terms of the asperity model of inhomogeneous faulting.

Journal Article↗

PubMed2000

Thresholds for step initiation induced by support-surface translation: a dynamic center-of-mass model provides much better prediction than a static model.

The need to initiate a step in order to recover balance could, in theory, be predicted by a static model based solely on displacement of the center of mass (COM) with respect to the base of support (BOS), or by a dynamic model based on the interaction between COM displacement and velocity. The purpose of this study was to determine whether the dynamic model provides better prediction than the static model regarding the need to step in response to moving-platform perturbation. The COM phase plane trajectories were determined for 10 healthy young adults for trials where the supporting platform was translated at three different acceleration levels in anterior and posterior directions. These trajectories were compared with the thresholds for step initiation predicted by the static and dynamic COM models. A single-link-plus-foot biomechanical model was employed to mathematically simulate termination of the COM movement, without stepping, using the measured platform acceleration as the input. An optimization routine was used to determine the stability boundaries in COM state space so as to establish the dynamic thresholds where a compensatory step must be initiated in order to recover balance. In the static model, the threshold for step initiation was reached if the COM was displaced beyond the BOS limits. The dynamic model showed substantially better accuracy than the static model in predicting the need to step in order to recover balance: 71% of all stepping responses predicted correctly by the dynamic model versus only 11% by the static model. These results support the proposition that the central nervous system must react to and control dynamic effects, i.e. COM velocity, as well as COM displacement in order to maintain stability with respect to the existing BOS without stepping.

Adult↗

PubMed1983

A central spectrum model: a synthesis of auditory-nerve timing and place cues in monaural communication of frequency spectrum.

A probabilistic psychophysical model for monaural communication from the auditory nerve to the brain is given in the form of a tonotopic display of stimulus spectrum, termed central spectrum. The model builds upon prior research demonstrating the potential of neural timing cues from the auditory nerve for conveying information on complex spectra, and was designed to meet the quantified demands of the psychophysics of frequency measurement. The central spectrum magnitude at each frequency is determined by the response of the auditory-nerve fibre with characteristic frequency matching that frequency. An interval histogram from each fiber is passed through a filter matched to the characteristic frequency of the fiber. This output versus characteristic frequency defines the central spectrum. Detailed analysis demonstrates that efficient probabilistic processing of the central spectrum described known psychophysical properties of frequency measurement in discrimination and periodicity pitch experiments. Psychophysical models based upon the central spectrum model followed by optimum probabilistic pattern recognition are potentially relevant for predicting human communication limits in response to arbitrary sounds of speech and music.

Cues↗

PubMed2002

Predicting pregnancy and spermatogenesis by survival analysis during gonadotrophin treatment of gonadotrophin-deficient infertile men.

BACKGROUND: Predictors of fertility or spermatogenesis during gonadotrophin therapy of gonadotrophin-deficient men remain poorly defined. METHODS AND RESULTS: In order to evaluate potential predictors, this study evaluated 29 consecutive gonadotrophin-deficient men all desiring paternity who received 43 courses of therapy in one centre between 1982 and 1998. The Kaplan-Meier survival analysis estimates of median (SE) time to a sperm concentration of >0, >5 and >20 x 10(6)/ml were 5.5 (1.1), 12.4 (2.3) and 29.1 (1.9) months respectively. Conception occurred in 22/43 cycles (with eight men achieving two pregnancies) with a median (SE) Kaplan-Meier estimate of 20.5 (4.7) months. The median sperm concentration at conception was 5.0 (SE 2.0; range 0.0-59.5) x 10(6)/ml. Multivariate correlated Cox proportional hazards models predicting these same sperm thresholds and conception were developed by forward stepwise variable selection with verification of the model by backward stepping. Larger testicular volume, prior gonadotrophin therapy, completion of puberty, older age, the absence of adverse fertility factors and the absence of multiple pituitary hormone deficiency predicted a favourable response. Multivariate modelling suggests that the two most important predictors of sperm output are testicular volume and pubertal status. The most important potentially modifiable predictor was prior gonadotrophin therapy. The efficacy of recombinant and urinary FSH were similar. Prior androgen therapy and partner's age did not appear to be significant. CONCLUSIONS: Since prolonged treatment may be required to induce spermatogenesis, attention to these predictors may allow appropriate early use of advanced reproductive technologies.

Adult↗

PubMed2002

Ultrasound, a new tool for surface matching in computer-navigated surgery.

OBJECTIVE: The object of this study was to investigate the feasibility of generating a bone surface from data provided by an ultrasound examination and to match this surface with the previous computed tomography (CT) scan. METHODS: From a CT data set of a training model of the pelvis, a three-dimensional surface was extracted by global thresholding-based segmentation. The same model was placed in a water basin, and ultrasound images were taken with a guided ultrasound transducer. The three-dimensional surface was generated from the ultrasound data set, and the two surfaces were matched in a semiautomatic mode. RESULTS: With special segmentation methods, a surface could be extracted automatically from the CT and the ultrasound data set. From these segmented ultrasound slices, a volume data set of the model was generated. After approximate initial matching, the local matching process was completed automatically. CONCLUSION: One of the limitations in computer-assisted surgery is the complicated matching process. Using special algorithms, a surface was extracted from the data set of an ultrasound examination and matched in a semiautomatic mode with the surface of a CT data set, facilitating the matching process.

Algorithms↗

PubMed1988

Pulsed CO2 laser tissue ablation: measurement of the ablation rate.

Ablation of guinea pig skin using a CO2 laser emitting 2-mu sec-long pulses has been quantified by measuring the mass of tissue removed as a function of incident fluence per pulse. The mass-loss curves show three distinct regimes in which water evaporation, explosive tissue removal, and laser-induced plasma formation dominate. The data are fit to two models that predict that the mass removed depends either linearly or logarithmically on fluence. Although the data are best fit by a linear dependence upon fluence, plasma formation at high fluences prohibited obtaining data over a wide enough fluence range to differentiate unambiguously between the two models. Ablation efficiency, ablation thresholds, and the optical penetration depth at 10.6 micron were obtained from the measurements.

Animals↗

PubMed2005

Analgesic activity of 5-HT3 receptor antagonists.

The effects of 5-HT3 receptor antagonists tropisetron and condensed derivative of benzimidazole (laboratory code 64B) on nociceptive threshold were examined on models with activation of peripheral and central nociceptive mechanisms. The examined substances demonstrated pronounced analgesic effects in peripheral pain.

Analgesics↗

PubMed2003

A mathematical model of lipid-mediated thrombin generation.

Thrombin is an enzyme that is generated in both vascular and non-vascular systems. In blood coagulation, a fundamental process in all species, thrombin induces the formation of a fibrin clot. A dynamical model of thrombin generation in the presence of lipid surfaces is presented. This model also includes the self-regulating thrombin feedback reactions, the thrombomodulin-protein C-protein S inhibitory system, tissue factor pathway inhibitor (TFPI), and the inhibitor, antithrombin (AT). The dynamics of this complex system were found to be highly lipid dependent, as would be expected from experimental studies. Simulations of this model indicate that a threshold lipid level is required to generate physiologically relevant amounts of thrombin. The dependence of the onset, the peak levels, and the duration of thrombin generation on lipid was saturable. The lipid concentration affects the way in which the inhibitors modulate thrombin production. A novel feature of this model is the inclusion of the dynamical protein C pathway, initiated by thrombin feedback. This inhibitory system exerts its effects on the lipid surface, where its substrates are formed. The maximum impact of TFPI occurs at intermediate vesicle concentrations. Inhibition by AT is only indirectly affected by the lipid since AT irreversibly binds only to solution phase proteins. In a system with normal plasma concentrations of the proteins involved in thrombin formation, the combination of these three inhibitors is sufficient both to effectively stop thrombin generation prior to the exhaustion of its precursor, prothrombin, and to inhibit all thrombin formed. This model can be used to predict thrombin generation under extreme lipid conditions that are difficult to implement experimentally and to examine thrombin generation in non-vascular systems.

Animals↗

PubMed2001

Using thresholding at varying intervals to obtain different temporal patterns.

We show how stroboscopic threshold mechanisms can be effectively employed to obtain a wide range of stable cyclic behavior from chaotic systems, by simply varying the frequency of control. We demonstrate the success of the scheme in a prototypical one-dimensional map, as well as in a three-dimensional system modeling lasers where the threshold action is implemented on any one of the variables. It is evident that thresholding is capable of yielding exact limit cycles of varying periods and geometries when implemented at different intervals (even when very infrequent). This suggests a simple and potent mechanism for selecting different regular temporal patterns from chaotic dynamics.

Journal Article↗

PubMed2001

Enhancement and stabilization of traffic flow by moving in groups.

We study the traffic behavior of vehicles moving in groups analytically and numerically. A car-following model of traffic is extended to take into account a binary mixture of vehicles. It is shown that the movement in groups stabilizes the traffic flow. The jamming transition among the free traffic, the inhomogeneous traffic, and the homogeneous congested traffic occurs at a higher density than the threshold of the original model. The traffic current is highly enhanced at a high-density region by keeping a short headway without jam. The jamming transition is analyzed by using the linear stability method. It is found that the theoretical neutral stability curve agrees with the transition line obtained by the simulation.

Journal Article↗

PubMed2000

Background adaptation in children with a history of mild retinopathy of prematurity.

PURPOSE: In children with a history of mild retinopathy of prematurity (ROP), test the hypothesis that elevation of the parafoveal over peripheral dark-adapted threshold is due to photoreceptor rather than postreceptor dysfunction. METHODS: A forced choice procedure was used to measure thresholds, for detection of 2 degrees diameter, 50 msec, blue stimuli presented 10 degrees (parafoveal) or 30 degrees (peripheral) eccentric in the dark and in the presence of steady red backgrounds (-4 to +2 log scot td). Four ROP and four control subjects were tested at both eccentricities. A model of the increment threshold function was fit to the data to calculate the eigengrau and dark-adapted threshold. RESULTS: Both ROP subjects with elevated parafoveal thresholds also have elevated parafoveal eigengraus. On the other hand, parafoveal and peripheral eigengraus are equal in ROP subjects without parafoveal threshold elevation. Nevertheless, the dark-adapted thresholds of all ROP subjects are higher than those of any control subject at both sites. CONCLUSIONS: The parafoveal threshold elevation is due to rod dysfunction. There is also evidence of peripheral rod photoreceptor involvement in the subjects with ROP.

Adolescent↗

PubMed1990

Mathematical modelling of energy and redox metabolism of G6PD-deficient erythrocytes.

A mathematical model aimed at the study of erythrocyte metabolism under normal conditions and under glucose 6-phosphate dehydrogenase (G6PD) deficiency has been developed. The degree of deficiency as predicted by the model on the basis of calculated upper limits of oxidative load as well as of maximal methylene blue stimulation correlates with the individual clinical manifestation of the metabolic disease. Therefore, the model permits to judge the degree of metabolic disorder in the presence of G6PD enzymopathies if the kinetic properties of the defect enzyme are known. Experimentally accessible parameters for an assessment of the oxidative load capacity of cells in vivo are proposed. Furthermore, the model predicts that the threshold of tolerance as to energetic load is drastically reduced in the case of severe G6PD deficiency.

Energy Metabolism↗

PubMed2005

The relationship between task performance and functional magnetic resonance imaging response.

We compared psychophysical and functional magnetic resonance imaging (fMRI) responses within areas V1-V3 and MT+ during both a speed and a contrast discrimination task. We found that fMRI responses did not depend significantly on task in any of these areas. Moreover, responses in V1-V3 were larger than those in MT+ for both the speed and the contrast discrimination tasks across a wide range of contrasts. This pattern of results demonstrates that localizing function based on finding those regions of cortex that show greater activity to a given task-stimulus combination than to other tasks and stimuli may, under certain conditions, be misleading. However, a simple ideal observer model assuming that perceptual thresholds are dependent on neuronal population responses does successfully show that V1 has neuronal properties consistent with our subjects' contrast discrimination performance, and that MT+ has neuronal properties consistent with subjects' performance on a speed discrimination task.

Adult↗

PubMed2004

Plant responses to precipitation in desert ecosystems: integrating functional types, pulses, thresholds, and delays.

The 'two-layer' and 'pulse-reserve' hypotheses were developed 30 years ago and continue to serve as the standard for many experiments and modeling studies that examine relationships between primary productivity and rainfall variability in aridlands. The two-layer hypothesis considers two important plant functional types (FTs) and predicts that woody and herbaceous plants are able to co-exist in savannas because they utilize water from different soil layers (or depths). The pulse-reserve model addresses the response of individual plants to precipitation and predicts that there are 'biologically important' rain events that stimulate plant growth and reproduction. These pulses of precipitation may play a key role in long-term plant function and survival (as compared to seasonal or annual rainfall totals as per the two-layer model). In this paper, we re-evaluate these paradigms in terms of their generality, strengths, and limitations. We suggest that while seasonality and resource partitioning (key to the two-layer model) and biologically important precipitation events (key to the pulse-reserve model) are critical to understanding plant responses to precipitation in aridlands, both paradigms have significant limitations. Neither account for plasticity in rooting habits of woody plants, potential delayed responses of plants to rainfall, explicit precipitation thresholds, or vagaries in plant phenology. To address these limitations, we integrate the ideas of precipitation thresholds and plant delays, resource partitioning, and plant FT strategies into a simple 'threshold-delay' model. The model contains six basic parameters that capture the nonlinear nature of plant responses to pulse precipitation. We review the literature within the context of our threshold-delay model to: (i) develop testable hypotheses about how different plant FTs respond to pulses; (ii) identify weaknesses in the current state-of-knowledge; and (iii) suggest future research directions that will provide insight into how the timing, frequency, and magnitude of rainfall in deserts affect plants, plant communities, and ecosystems.

Age Factors↗