CiteULike: Tag threshold

A comparison of fixed-step-size and Bayesian staircases for sensory threshold estimation

Alcala-Quintana — 2007-04-17T11:14:12-00:00

Spatial Vision, Vol. 20, No. 3. (2007), pp. 197-218.

Tuning curves, neuronal variability, and sensory coding.

DA Butts — 2006-04-12T03:10:49-00:00

PLoS Biol, Vol. 4, No. 4. (April 2006)

Tuning curves are widely used to characterize the responses of sensory neurons to external stimuli, but there is an ongoing debate as to their role in sensory processing. Commonly, it is assumed that a neuron's role is to encode the stimulus at the tuning curve peak, because high firing rates are the neuron's most distinct responses. In contrast, many theoretical and empirical studies have noted that nearby stimuli are most easily discriminated in high-slope regions of the tuning curve. Here, we demonstrate that both intuitions are correct, but that their relative importance depends on the experimental context and the level of variability in the neuronal response. Using three different information-based measures of encoding applied to experimentally measured sensory neurons, we show how the best-encoded stimulus can transition from high-slope to high-firing-rate regions of the tuning curve with increasing noise level. We further show that our results are consistent with recent experimental findings that correlate neuronal sensitivities with perception and behavior. This study illustrates the importance of the noise level in determining the encoding properties of sensory neurons and provides a unified framework for interpreting how the tuning curve and neuronal variability relate to the overall role of the neuron in sensory encoding.

Threshold values in toxicology - useful or not?

R Wennig — 2007-02-18T08:17:46-00:00

Forensic Sci Int, Vol. 113, No. 1-3. (11 September 2000), pp. 323-330.

In many fields of toxicology, numbers are used as threshold values, e.g. as "acceptable daily intake values" resulting in maximum permissible concentrations in food or in animal feed by using "safety factors"; maximal admissible concentrations of toxic substances in the air at the workplace; cut-off values in analytical toxicology; limit values for biological specimens in the case of driving under the influence of drugs, guidance values for environmental specimens, etc. The philosophy behind these values must be well understood and they should only be applied to real cases by persons with enough toxicological background. The bad use of these numbers in toxicology can have dramatic consequences. Especially in regulatory toxicology their use should be made with great care. Moreover, tremendous improvements in analytical methodology, e.g. the decreasing of the limits of detection for many potentially toxic substances in recent years, should not end up in an overestimation of risks to humans. To avoid these abuses careful interpretations of analytical findings by qualified toxicologists are of paramount importance. The use and abuse of some of these threshold values will be outlined in several applications from analytical toxicology, risk assessment issues, forensic toxicology in post-mortem cases, as well as from the drugs and driving cases. Generally, if threshold values are considered as guidance values and not as the "absolute truth" in toxicology, they may be very useful in the interpretation of toxicology data.

Inductive Risk and Values in Science

Heather Douglas — 2006-11-09T18:45:56-00:00

Philosophy of Science, Vol. 67, No. 4. (2000), pp. 559-579.

Although epistemic values have become widely accepted as part of scientific reasoning, non-epistemic values have been largely relegated to the "external" parts of science (the selection of hypotheses, restrictions on methodologies, and the use of scientific technologies). I argue that because of inductive risk, or the risk of error, non-epistemic values are required in science wherever non-epistemic consequences of error should be considered. I use examples from dioxin studies to illustrate how non-epistemic consequences of error can and should be considered in the internal stages of science: choice of methodology, characterization of data, and interpretation of results.

Nondetects, Detection Limits, and the Probability of Detection

Diane Lambert — 2007-07-09T12:21:03-00:00

Journal of the American Statistical Association, Vol. 86, No. 414. (1991), pp. 266-277.

When chemists cannot quantify the concentration in a field sample, they report nondetect instead of a numerical measurement. A data analyst faced with environmental data containing nondetects might assume that all nondetects are zeros, all nondetects are smaller than the smallest detect (numerical measurement), or, if a detection limit is reported, that all nondetects are below the detection limit. This article shows that these assumptions can be incorrect and suggests better alternatives. The measurements that are likely to be reported as nondetect can be described by a new concept, the probability of acceptance, where acceptance means that a measurement passes the requirements for being reported as a detect. The 90th percentile of the probability of acceptance curve is a reasonable upper bound or censoring limit for a measurement reported as nondetect. The probability of acceptance also suggests the complexity of the data analysis task. For example, assuming that all nondetects are smaller than the smallest detect is reasonable if the probability of acceptance rises sharply from 0 to 1, but not if the probability of acceptance is nonmonotone. We show how to estimate the probability of acceptance and interpret the estimated probabilities for five soil pollutants from the 1988 Love Canal study. These examples show that, over the range of measurements seen in a study, the probability of acceptance need not be monotone. Statisticians must understand which measurements are likely to be reported as nondetect, because data analysis involves combining nondetects and numerical measurements. But health effects depend on true concentration, not measured concentration, and the public and toxicologists are more concerned with which true, field concentrations are likely to be detected. Since field concentrations are often measured with bias and imprecision, the probability of acceptance does not describe the probability of detection. We show how to estimate the probability of detection from field measurements and quality control data. We compare its 90th percentile, here called the minimum reliably detected concentration, to the detection limit that chemists usually report to describe which field concentrations can be detected. In the Love Canal study, for example, we find that the estimated probability of detecting a field concentration at the detection limit was sometimes almost 1 but sometimes only .5.

Theory of collective firing induced by noise or diversity in excitable media

CJ Tessone — 2007-02-23T13:26:36-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Large variety of physical, chemical, and biological systems show excitable behavior, characterized by a nonlinear response under external perturbations: only perturbations exceeding a threshold induce a full system response (firing). It has been reported that in coupled excitable identical systems noise may induce the simultaneous firing of a macroscopic fraction of units. However, a comprehensive understanding of the role of noise and that of natural diversity present in realistic systems is still lacking. Here we develop a theory for the emergence of collective firings in nonidentical excitable systems subject to noise. Three different dynamical regimes arise: subthreshold motion, where all elements remain confined near the fixed point; coherent pulsations, where a macroscopic fraction fire simultaneously; and incoherent pulsations, where units fire in a disordered fashion. We also show that the mechanism for collective firing is generic: it arises from degradation of entrainment originated either by noise or by diversity.

Threshold voltage characteristics of depletion-mode MOSFET's

MR Wordeman — 2006-10-19T19:59:40-00:00

Electron Devices, IEEE Transactions on, Vol. 28, No. 9. (1981), pp. 1025-1030.

This paper presents the results of a study of the characteristics of the depletion-mode MOSFET. In particular, it is shown that the threshold voltage of this device is a function of its mode of operation (linear or saturated) due to a change in dominant conduction mechanisms caused by the finite depth of donor impurities in the channel. The effect of these impurities on the short channel behavior of the devices also is examined.

Quantifying persuasion effects on choice with the decision threshold of the stochastic choice model

Claudia Gonzalez-Vallejo — 2006-07-24T12:02:12-00:00

Organizational Behavior and Human Decision Processes, Vol. 100, No. 2. (July 2006), pp. 250-267.

Three studies tested the stochastic difference choice model (proportional difference, PD, version in Gonzalez-Vallejo, 2002) in the domain of decision making under certainty. Consumer services and products, hotels defined by price and quality and MP3 players defined by price and memory size, served as choice pairs. The ordinal prediction relating the proportional difference variable, d (computed from stimuli pairs), and the observed choice proportions was supported. Model fitting showed that PD's estimated decision threshold measured within-person sensitivity to value attribute differences both at baseline and after persuasion manipulations. The threshold was also related to whether individuals were low or high in Need for Cognition (NFC, Cacioppo & Petty, 1982). Cross-validation strategies also showed PD to be descriptive and robust.

Diagnosing Adult Attention Deficit Hyperactivity Disorder: Are Late Onset and Subthreshold Diagnoses Valid?

SV Faraone — 2007-11-10T19:13:46-00:00

American Journal of Psychiatry, Vol. 163, No. 10. (1 October 2006), pp. 1720-1729.

OBJECTIVE: Diagnosing attention deficit hyperactivity disorder (ADHD) in adults is difficult when diagnosticians cannot establish an onset before the DSM-IV criterion of age 7 or if the number of symptoms recalled does not achieve DSM's diagnosis threshold. METHOD: The authors addressed the validity of DSM-IV's age-at-onset and symptom threshold criteria by comparing four groups of adults: 127 subjects with full ADHD who met all DSM-IV criteria for childhood-onset ADHD, 79 subjects with late-onset ADHD who met all criteria except the age-at-onset criterion, 41 subjects with subthreshold ADHD who did not meet full symptom criteria for ADHD, and 123 subjects without ADHD who did not meet any criteria. The authors hypothesized that subjects with late-onset and subthreshold ADHD would show patterns of psychiatric comorbidity, functional impairment, and familial transmission similar to those seen in subjects with full ADHD. RESULTS: Subjects with late-onset and full ADHD had similar patterns of psychiatric comorbidity, functional impairment, and familial transmission. Most children with late onset of ADHD (83%) were younger than 12. Subthreshold ADHD was milder and showed a different pattern of familial transmission than the other forms of ADHD. CONCLUSIONS: The data about the clinical features of probands and the pattern of transmission of ADHD among relatives found little evidence for the validity of subthreshold ADHD among such subjects, who reported a lifetime history of some symptoms that never met DSM-IV's threshold for diagnosis. In contrast, the results suggested that late-onset adult ADHD is valid and that DSM-IV's age-at-onset criterion is too stringent.

Neuropsychological Studies of Late Onset and Subthreshold Diagnoses of Adult Attention-Deficit/Hyperactivity Disorder

SV Faraone — 2007-12-15T22:12:27-00:00

Biological Psychiatry, Vol. 60, No. 10. (15 November 2006), pp. 1081-1087.

Background Diagnosing attention-deficit/hyperactivity disorder (ADHD) in adults is difficult when the diagnostician cannot establish an onset prior to the DSM-IV criterion of age 7 or if the number of symptoms recalled does not achieve the DSM-IV threshold for diagnosis. Because neuropsychological deficits are associated with ADHD, we addressed the validity of the DSM-IV age at onset and symptom threshold criteria by using neuropsychological test scores as external validators.Methods We compared four groups of adults: 1) full ADHD subjects met all DSM-IV criteria for childhood-onset ADHD; 2) late-onset ADHD subjects met all criteria except the age at onset criterion; 3) subthreshold ADHD subjects did not meet full symptom criteria; and 4) non-ADHD subjects did not meet any of the above criteria.Results Late-onset and full ADHD subjects had similar patterns of neuropsychological dysfunction. By comparison, subthreshold ADHD subjects showed few neuropsychological differences with non-ADHD subjects.Conclusions Our results showing similar neuropsychological underpinning in subjects with late-onset ADHD suggest that the DSM-IV age at onset criterion may be too stringent. Our data also suggest that ADHD subjects who failed to ever meet the DSM-IV threshold for diagnosis have a milder form of the disorder.

Thresholds in development

J Lewis — 2007-07-04T13:13:03-00:00

Journal of Theoretical Biology, Vol. 65, No. 3. (7 April 1977), pp. 579-590.

The interpretation of gradients in positional information is considered in terms of thresholds in cell responses, giving rise to cell states which are discrete and persistent. Equilibrium models based on co-operative binding of control molecules do not show true thresholds of discontinuity, though with a very high degree of co-operativity they could mimic them; in any case they do not provide the cells with any memory of a transient signal. A simple kinetic model based upon positive feedback can account both for memory and for discontinuities in the pattern of cell states. The model is an example of a bistable control circuit, and transitions from one state to another may be brought about not only by morphogenetic signals, but also by disturbances in the parameters determining the kinetics of the system. This might explain some aspects of transdetermination in insects. An attempt is made to analyse the precision with which a spatial gradient of a diffusible morphogen could be interpreted by a kinetic threshold mechanism, in terms of the length of the field, the steepness of the concentration gradient, and the intrinsic random variability of cells. It is concluded that it would be possible to specify as many as 30 distinct cell states in a positional field 1 mm long with a concentration span of 103. Mechanisms for reducing the positional error are considered.

Brain responses for the subconscious recognition of faces.

M Hoshiyama — 2005-06-07T16:14:15-00:00

Neurosci Res, Vol. 46, No. 4. (August 2003), pp. 435-442.

We investigated the event-related responses following subthreshold and suprathreshold stimulation with facial and non-facial figures using magnetoencephalography (MEG) and EEG recordings to clarify the physiological nature of subconscious perception. Event-related magnetic fields and potentials were recorded from the right hemisphere in eight healthy subjects. Three types of stimulus, i.e., facial image (Face), letters of the alphabet (Letters) and random patterns of dots (Dots), with different presentation periods, subthreshold (16 ms), intermediate (32 ms) and suprathreshold (48 ms) were visually presented in a random order. A psychological discrimination task using the same stimuli was also employed. Clear MEG and EEG responses were recorded for all the stimuli, but the amplitude of the responses was largest for Face and smallest for Dots even in the subthreshold stimulation. The equivalent current dipoles (ECDs) for Face were located around the fusiform gyrus, although the correlation coefficients for ECDs were low under subthreshold and intermediate conditions. The ECDs for Letters and Dots were not estimated with reliable correlation coefficients. The results from the psychological task correlated with the dominancy of face recognition. Face perception was processed differently in the subthreshold condition as well as suprathreshold condition. The subconscious recognition of face might be processed around the fusiform gyrus.

Automatic measurement of sister chromatid exchange frequency.

GW Zack — 2006-10-07T01:00:27-00:00

J Histochem Cytochem, Vol. 25, No. 7. (July 1977), pp. 741-753.

An automatic system for detecting and counting sister chromatid exchanges in human chromosomes has been developed. Metaphase chromosomes from lymphocytes which had incorporated 5-bromodeoxyuridine for two replication cycles were treated with the dye 33258 Hoechst and photodegraded so that the sister chromatids exhibited differential Giemsa staining. A computer-controlled television-microscope system was used to acquire digitized metaphase spread images by direct scanning of microscope slides. Individual objects in the images were identified by a thresholding procedure. The probability that each object was a single, separate chromosome was estimated from size and shape measurements. An analysis of the spatial relationships of the dark-chromatid regions of each object yielded a set of possible exchange locations and estimated probabilities that such locations corresponded to sister chromatid exchanges. A normalized estimate of the sister chromatid exchange frequency was obtained by summing the joint probabilities that a location contained an exchange within a single, separate chromosome over the set of chromosomes from one or more cells and dividing by the expected value of the total chromosome area analyzed. Comparison with manual scoring of exchanges showed satisfactory agreement up to levels of approximately 30 sister chromatid exchanges/cell, or slightly more than twice control levels. The processing time for this automated sister chromatid exchange detection system was comparable to that of manual scoring.

A recursive thresholding technique for image segmentation

M Cheriet — 2006-10-09T18:23:58-00:00

Image Processing, IEEE Transactions on, Vol. 7, No. 6. (1998), pp. 918-921.

In this correspondence, we present a general recursive approach for image segmentation by extending Otsu's (1978) method. The new approach has been implemented in the scope of document images, specifically real-life bank checks. This approach segments the brightest homogeneous object from a given image at each recursion, leaving only the darkest homogeneous object after the last recursion. The major steps of the new technique and the experimental results that illustrate the importance and the usefulness of the new approach for the specified class of document images of bank checks is presented

Survey over image thresholding techniques and quantitative performance evaluation

Mehmet Sezgin — 2006-10-09T18:10:16-00:00

Journal of Electronic Imaging, Vol. 13, No. 1. (2004), pp. 146-168.

We conduct an exhaustive survey of image thresholding methods, categorize them, express their formulas under a uniform notation, and finally carry their performance comparison. The thresholding methods are categorized according to the information they are exploiting, such as histogram shape, measurement space clustering, entropy, object attributes, spatial correlation, and local gray-level surface. 40 selected thresholding methods from various categories are compared in the context of nondestructive testing applications as well as for document images. The comparison is based on the combined performance measures. We identify the thresholding algorithms that perform uniformly better over nondestructive testing and document image applications. ©2004 SPIE and IS&T.

A polybasic motif allows N-WASP to act as a sensor of PIP(2) density.

V Papayannopoulos — 2005-08-15T16:05:58-00:00

Mol Cell, Vol. 17, No. 2. (21 January 2005), pp. 181-191.

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) activates the actin regulatory protein N-WASP by binding to a short polybasic region involved in N-WASP autoinhibition. Here, we show that unlike canonical lipid binding modules, such as PH domains, this polybasic motif binds PIP(2) in a multivalent, cooperative manner. As a result, PIP(2) activation of N-WASP-mediated actin polymerization in vitro and in extracts is ultrasensitive: above a certain threshold, N-WASP responds in a switch-like manner to a small increase in the density of PIP(2) (Hill coefficient n(H) = approximately 20). We show that the sharpness of the PIP(2) activation threshold can be tuned by varying the length of the polybasic motif. This sharp activation threshold may help suppress N-WASP activation by quiescent PIP(2) levels yet leave it poised for activation upon subtle, signaling-induced perturbations in PIP(2) distribution.

Mathematical modeling reveals threshold mechanism in CD95-induced apoptosis.

M Bentele — 2005-11-03T05:33:46-00:00

J Cell Biol, Vol. 166, No. 6. (13 September 2004), pp. 839-851.

Mathematical modeling is required for understanding the complex behavior of large signal transduction networks. Previous attempts to model signal transduction pathways were often limited to small systems or based on qualitative data only. Here, we developed a mathematical modeling framework for understanding the complex signaling behavior of CD95(APO-1/Fas)-mediated apoptosis. Defects in the regulation of apoptosis result in serious diseases such as cancer, autoimmunity, and neurodegeneration. During the last decade many of the molecular mechanisms of apoptosis signaling have been examined and elucidated. A systemic understanding of apoptosis is, however, still missing. To address the complexity of apoptotic signaling we subdivided this system into subsystems of different information qualities. A new approach for sensitivity analysis within the mathematical model was key for the identification of critical system parameters and two essential system properties: modularity and robustness. Our model describes the regulation of apoptosis on a systems level and resolves the important question of a threshold mechanism for the regulation of apoptosis.

Transcriptional control: rheostat converted to on/off switch.

FM Rossi — 2005-05-05T14:50:16-00:00

Mol Cell, Vol. 6, No. 3. (September 2000), pp. 723-728.

Individual cells translate concentration gradients of extracellular factors into all-or-none threshold responses leading to discrete patterns of gene expression. Signaling cascades account for some but not all such threshold responses, suggesting the existence of additional mechanisms. Here we show that all-or-none responses can be generated at a transcriptional level. A graded rheostat mechanism obtained when either transactivators or transrepressors are present is converted to an on/off switch when these factors compete for the same DNA regulatory element. Hill coefficients of dose-response curves confirm that the synergistic responses generated by each factor alone are additive, obviating the need for feedback loops. We postulate that regulatory networks of competing transcription factors prevalent in cells and organisms are crucial for establishing true molecular on/off switches.

Precipitation-vegetation coupling and its influence on erosion on the Loess Plateau, China

Xu Jiongxin — 2006-03-03T13:59:09-00:00

CATENA, Vol. 64, No. 1. (30 November 2005), pp. 103-116.

The relationships between precipitation, vegetation and erosion are important and are unsolved issues in the field of earth surface processes. Based on data from the Loess Plateau of China, some non-linear relationships between forest cover (Cf), mean annual rainfall erosivity (Re) and annual precipitation (Pm) have been found. A threshold has been identified at Pm = 450 mm, that is, when Pm is Cf is low and basically does not vary with Pm; when Pm exceeds 450 mm, Cf increases rapidly. Furthermore, two thresholds are identified in the relationship between rainfall erosivity and annual precipitation. When Pm is Re is low and basically does not vary with Pm. When Pm exceeds 300 mm, Re increases rapidly; when Pm becomes > 530 mm, the rate at which Re increases with Pm becomes higher. Based on these relationships, the non-linear relationship between erosion intensity and annual precipitation (i.e., the erosion intensity increases with annual precipitation to a peak and then declines) is explained. The implication of these thresholds for erosion control on the Loess Plateau is discussed.

Optimal Aggregation Algorithms for Middleware

Ronald Fagin — 2008-05-15T12:51:38-00:00

(2001)

: Assume that each object in a database has m grades, or scores, one for each of m attributes. For example, an object can have a color grade, that tells how red it is, and a shape grade, that tells how round it is. For each attribute, there is a sorted list, which lists each object and its grade under that attribute, sorted by grade (highest grade first). There is some monotone aggregation function, or combining rule, such as min or average, that combines the individual grades to obtain an...

Quantile estimation to derive optimized test thresholds for random field statistics

H Hinrichs — 2006-05-03T20:41:57-00:00

NeuroImage, Vol. 27, No. 1. (1 August 2005), pp. 116-129.

We present a numerical method to estimate the true threshold values in random fields needed to determine the significance of apparent signals observed in noisy images. To accomplish this, a quantile estimation algorithm is applied to derive the threshold with a predefined confidence interval from a large number of simulated random fields. Also, a computationally efficient method for generating a random field simulation is presented using resampling techniques. Applying these techniques, thresholds have been determined for a large variety of parameter settings (smoothness, voxel size, brain shape, type of statistics). By means of interpolation techniques, thresholds for additional arbitrary settings can be quickly derived without the need to run individual simulations. Compared to the parametric approach of Worsley et al. (1996) (Worsley, K.J., Marrett, S., Neelin P., Vandal, A.C., Friston, K.J., Evans, A.C., 1996. A unified statistical approach for determining significant signals in images of cerebral activation. Hum. Brain Mapp. 4, 58-73) and Friston et al. (1991) (Friston, K.J., Frith, C.D., Liddle, P.F., Frackowiak, R.S. 1991. Comparing functional (PET) images: the assessment of significant change. J. Cereb. Blood Flow Metab. 11(4), 690-699), and to the Bonferroni approach, these optimized thresholds lead to higher levels of significance (i.e., lower p values) with a specific amount of activation especially with fields of moderate smoothness (i.e., with a relative full width half maximum between 2 and 6). Alternatively, the threshold for a specified level of significance can be lowered. This improved statistical sensitivity is illustrated by the analysis of an actual event related functional magnetic resonance data set, and its limitations are tested by determining the false positive rate with experimental MR noise data. The grid of estimated threshold values as well as the interpolation algorithm to derive thresholds for arbitrary parameter settings are made available over the internet (http://neuro2.med.uni-magdeburg.de/quantile_estimation/).

Near threshold TDCS for photo-double ionization of helium

G Dawber — 2007-11-15T11:09:49-00:00

Journal of Physics B: Atomic, Molecular and Optical Physics, Vol. 28, No. 8. (1995), pp. L271-L277.

TDCS for photo-double ionization of helium have been measured in a PhotoElectron-PhotoElectron COincidence (PEPECO) experiment. The TDCS have been obtained for the first time at very low excess energies E, 0.6 eV<E<2 eV, for both equal and unequal energy sharing between the two outgoing electrons. The measured data are compared with the predictions of the Wannier model, and also with recent, non-Wannier, ab initio calculations. In addition, also for the first time, the relative magnitudes of the various TDCS measured have been determined in this excess energy region, suggesting a departure from the predictions of the Wannier model at the largest excess energy studied, E=2 eV.

Double-excitation and double-escape processes studied by photoelectron spectroscopy near threshold

George King — 2007-11-14T11:08:52-00:00

Journal of Physics B: Atomic, Molecular and Optical Physics, Vol. 33, No. 16. (2000), pp. R215-R284.

Recent experimental work using photons to study double excitation, double escape and related processes near their respective thresholds is reviewed. These processes are of significant current interest because they are completely determined by electron-electron correlations. In particular, experimental studies based on the techniques of low-energy electron spectroscopy are described. By using selected examples from coincidence and non-coincidence experiments the various kinds of dynamic and spectroscopic information that can be obtained are illustrated. Recent results, mainly in the rare gases, are described and their interpretation is discussed. Future perspectives are considered.

Fail-safe human/robot contact in the safety space

Y Yamada — 2008-06-09T21:15:56-00:00

Robot and Human Communication, 1996., 5th IEEE International Workshop on (1996), pp. 59-64.

We discuss a way to achieve a fail-safe human/robot contact system. Most of our discussion is based on the human pain tolerance, evaluated for the purpose of establishing the human safety space. First we review our previous work on the human-oriented design of a safe robot and the procedure of covering a robot with a viscoelastic material to achieve both impact force attenuation and contact sensitivity, keeping within the human pain tolerance limit. The safe robot design is verified through a demonstration that the robot exerts a contact force much less than the human pain tolerance and gives no pain to humans. Next, we propose a more efficient human/robot system which attains velocity reduction on the robot side activated by the incipient contact detection at the surface and gives the human side an interval margin for the reflexive withdrawal motion to avoid the more severely interactive situation. The experimental result shows the effectiveness of the velocity reduction of the robot in a fail-safe manner

The threshold conditions for initiation of action potentials by excitable cells.

D Noble — 2007-08-17T10:57:39-00:00

J Physiol, Vol. 187, No. 1. (November 1966), pp. 129-162.

1. The relation between the strength and duration of a just threshold stimulus (strength-duration curve) is analysed for an excitable membrane polarized uniformly and for an excitable cable polarized at one point.2. The effect on the strength-duration curve of non-linearity in the membrane current-voltage curve has been analysed. The strength-duration curve can be derived if the membrane current-voltage relation is independent of time. The effects of changes in the current-voltage curve with time due to the existence of a finite membrane activation time and membrane accommodation are analysed.3. The strength-duration curve for the Hodgkin-Huxley membrane equations (Hodgkin & Huxley, 1952) is compared to that of Hill's (1936) two-time constant model.4. The relation between membrane current-voltage curves and those for a point-polarized cable are derived for the steady-state condition. The cable properties tend to linearize the current-voltage curve and to sharpen the voltage threshold.5. The strength-duration curve for a point-polarized cable whose membrane obeys the Hodgkin-Huxley equations is computed numerically. There is an additional large effect on the cable strength-duration curve arising from the redistribution of charge during passage of a constant current; and the resulting strength-duration curve lies within the range of curves predicted by Hill's model.6. The conditions required for a constant charge threshold (Hodgkin & Rushton, 1946) are shown to be satisfied for short, intense stimuli applied to a cable at one point.7. The results are discussed with reference to the experimental studies available.

Optimal aggregation algorithms for middleware

Ronald Fagin — 2008-05-19T11:30:21-00:00

J. Comput. Syst. Sci., Vol. 66, No. 4. (June 2003), pp. 614-656.

Breakdown Limits on Gigavolt-per-Meter Electron-Beam-Driven Wakefields in Dielectric Structures

MC Thompson — 2008-06-02T14:14:13-00:00

Physical Review Letters, Vol. 100, No. 21. (2008)

First measurements of the breakdown threshold in a dielectric subjected to GV/m wakefields produced by short (30–330 fs), 28.5 GeV electron bunches have been made. Fused silica tubes of 100 µm inner diameter were exposed to a range of bunch lengths, allowing surface dielectric fields up to 27 GV/m to be generated. The onset of breakdown, detected through light emission from the tube ends, is observed to occur when the peak electric field at the dielectric surface reaches 13.8±0.7 GV/m. The correlation of structure damage to beam-induced breakdown is established using an array of postexposure inspection techniques.

Changing the percolation threshold of a carbon black/polymer composite by a coupling treatment of the black

Ho Yoon — 2007-05-10T12:08:19-00:00

Carbon, Vol. 42, No. 8-9. (2004), pp. 1877-1879.

Fine Structure of Thresholds in a Micromaser Pumped with Atom Clusters

GM D'Ariano — 2008-04-12T16:30:03-00:00

Physical Review Letters, Vol. 74, No. 6. (6 February 1995), 900.

Allergy and depression: a neurochemical threshold model of the relation between the illnesses.

PS Marshall — 2007-05-29T14:24:18-00:00

Psychol Bull, Vol. 113, No. 1. (January 1993), pp. 23-43.

Empirical studies suggest a very high prevalence of atopic disorder in people with depression. Research indicates that individuals with allergy have cholinergic hyperresponsiveness and beta-adrenergic hyporesponsiveness in the autonomic nervous system. Evidence is reviewed that similar imbalances in central nervous system cholinergic-adrenergic activity play a casual role in depression behaviors. It is hypothesized that the allergic state or allergic reactions can accentuate cholinergic-adrenergic activity imbalances in the central nervous system of a small subgroup of people at risk for endogenous depression thereby producing depression symptomatology.

Thresholding of Statistical Maps in Functional Neuroimaging Using the False Discovery Rate

Christopher Genovese — 2005-09-15T15:01:07-00:00

NeuroImage, Vol. 15, No. 4. (April 2002), pp. 870-878.

Findingobjective and effective thresholds for voxelwise statistics derived from neuroimaging data has been a long-standing problem. With at least one test performed for every voxel in an image, some correction of the thresholds is needed to control the error rates, but standard procedures for multiple hypothesis testing (e.g., Bonferroni) tend to not be sensitive enough to be useful in this context. This paper introduces to the neuroscience literature statistical procedures for controlling the false discovery rate (FDR). Recent theoretical work in statistics suggests that FDR-controlling procedures will be effective for the analysis of neuroimaging data. These procedures operate simultaneously on all voxelwise test statistics to determine which tests should be considered statistically significant. The innovation of the procedures is that they control the expected proportion of the rejected hypotheses that are falsely rejected. We demonstrate this approach using both simulations and functional magnetic resonance imaging data from two simple experiments.

Laterality index in functional MRI: methodological issues

Mohamed Seghier — 2008-06-05T15:08:16-00:00

Magnetic Resonance Imaging, Vol. 26, No. 5. (June 2008), pp. 594-601.

In functional magnetic resonance imaging (fMRI), hemispheric dominance is generally indicated by a measure called the laterality index (LI). The assessment of a meaningful LI measure depends on several methodological factors that should be taken into account when interpreting LI values or comparing between subjects. Principally, these include the nature of the quantification of left and right hemispheres contributions, localisation of volumes of interest within each hemisphere, dependency on statistical threshold, thresholding LI values, choice of activation and baseline conditions and reproducibility of LI values. This review discusses such methodological factors and the different approaches that have been suggested to deal with them. Although these factors are common to a range of fMRI domains, they are discussed here in the context of fMRI of the language system.

A Simplified Approach to Threshold and Proactive RSA

Tal Rabin — 2007-11-12T14:24:43-00:00

Advances in Cryptology — CRYPTO '98 (1998), pp. 349-369.

We present a solution to both the robust threshold RSA and proactive RSA problems. Our solutions are conceptually simple, and allow for an easy design of the system. The signing key, in our solution, is shared at all times in additive form, which allows for simple signing and for a particularly efficient and straightforward refreshing process for proactivization. The key size is (up to a very small constant) the size of the RSA modulus, and the protocol runs in constant time, even when faults occur, unlike previous protocols where either the size of the key has a linear blow-up (at best) in the number of players or the run time of the protocol is linear in the number of faults. The protocol is optimal in its resilience as it can tolerate a minority of faulty players. Furthermore, unlike previous solutions, the existence and availability of the key throughout the lifetime of the system, is guaranteed without probability of error. These results are derived from a new general technique for transforming distributed computations for which there is a known n-out-n solution into threshold and robust computations. Keywords: RSA, threshold signatures, proactive signatures, threshold and proactive RSA

Hydrologic modeling and forecasting: role of thresholds

B Sivakumar — 2005-10-24T17:14:05-00:00

Environmental Modelling & Software, Vol. 20, No. 5. (May 2005), pp. 515-519.

This communication discusses the potential role of 'thresholds' in hydrologic modeling and forecasting. Based on representative examples of studies employing data learning techniques in hydrology (e.g. artificial neural networks, chaos theory, optimization procedures), some of the problems in the existing data calibration and validation procedures are highlighted. Deriving an analogy between human behavior and catchment behavior, emphasizing their internal characteristics and their responses to external events, the concept of thresholds and its use in catchment hydrology are presented. In view of recent calls to focus on dominant processes in hydrology, a proposal is made for integration of concepts of Dominant Processes and Thresholds. The promises and limitations of this proposal are also explained in brief.

Noise Thresholds for Optical Quantum Computers

Christopher Dawson — 2007-04-18T01:17:04-00:00

Physical Review Letters, Vol. 96, No. 2. (2006)

In this Letter we numerically investigate the fault-tolerant threshold for optical cluster-state quantum computing. We allow both photon loss noise and depolarizing noise (as a general proxy for all local noise), and obtain a threshold region of allowed pairs of values for the two types of noise. Roughly speaking, our results show that scalable optical quantum computing is possible for photon loss probabilities <3×10-3, and for depolarization probabilities <10-4.

Coherence and stochastic resonance in threshold crossing detectors with delayed feedback

Robert Morse — 2007-01-10T13:05:56-00:00

Physics Letters A, Vol. 359, No. 6. (11 December 2006), pp. 640-646.

We consider the dynamical behavior of threshold systems driven by external periodic and stochastic signals and internal delayed feedback. Specifically, the effect of positive delayed feedback on the sensitivity of a threshold crossing detector (TCD) to periodic forcing embedded in noise is investigated. The system has an intrinsic ability to oscillate in the presence of positive feedback. We first show conditions under which such reverberatory behavior is enhanced by noise, which is a form of coherence resonance (CR) for this system. Further, for input signals that are subthreshold in the absence of feedback, the open-loop stochastic resonance (SR) characteristic can be sharply enhanced by positive delayed feedback. This enhancement is shown to depend on the stimulus period, and is maximal when this period is matched to an integer multiple of the delay. Reverberatory oscillations, which are particularly prominent after the offset of periodic forcing, are shown to be eliminated by a summing network of such TCDs with local delayed feedback. Theoretical analysis of the crossing rate dynamics qualitatively accounts for the existence of CR and the resonant behavior of the SR effect as a function of delay and forcing frequency.

Solution of an infection model near threshold

David Kessler — 2007-01-19T16:29:55-00:00

(17 Jan 2007)

We study the Susceptible-Infected-Recovered model of epidemics in the vicinity of the threshold infectivity. We derive the distribution of total outbreak size in the limit of large population size $N$. This is accomplished by mapping the problem to the first passage time of a random walker subject to a drift that increases linearly with time. We recover the scaling results of Ben-Naim and Krapivsky that the effective maximal size of the outbreak scales as $N^2/3$, with the average scaling as $N^1/3$, with an explicit form for the scaling function.

Revised estimate of minimum audible pressure: Where is the ”missing 6 dB”?

Mead Killion — 2006-01-24T22:15:10-00:00

The Journal of the Acoustical Society of America, Vol. 63, No. 5. (1978), pp. 1501-1508.

Eardrum pressures at hearing threshold have been calculated from both earphone data (ISO R389–1964 and ANSI S3.6–1969) and free-field data (ISO R226–1961). When head diffraction, external-ear resonance, and an apparent flaw in ISO R226 are accounted for in the free-field data, and real-ear versus coupler differences and physiological noise are accounted for in the earphone data, the agreement between the two derivations is good. At the audiometric frequencies of 125, 250, 500, 1000, 2000, 4000, and 8000 Hz, the estimated eardrum pressures at absolute threshold are 30, 19, 12, 9, 15, 13, and 14 dB SPL, respectively. Except for the effects of physiological noise at low frequencies, no evidence of the ”missing 6 dB” is seen, an observation consistent with the experimental results of several recent studies. doi:10.1121/1.381844 PACS: 43.66.Cb, 43.66.Sr Additional Information Full Text: [ PDF (771 kB) GZipped PS

STOP: searching for Transcription Factor motifs using gene expression.

Libi Hertzberg — 2007-05-22T23:43:50-00:00

Bioinformatics (8 May 2007)

MOTIVATION: Existing computational methods that identify transcription factor (TF) binding sites on a gene's promoter are plagued by significant inaccuracies. Binding of a TF to a particular sequence is assessed by comparing its similarity score, obtained from the TF's known position weight matrix (PWM), to a threshold. If the similarity score is above the threshold, the sequence is considered a putative binding site. Determining this threshold is a central part of the problem, for which no satisfactory biologically based solution exists. RESULTS: We present here a method that integrates gene expression data with sequence-based scoring of TF binding sites, for determining a global score threshold for each TF. We validate our method, STOP (Searching TFs Of Promoters), in several ways: 1) We calculate the average expression values of groups of human putative target genes of each TF, and compare them to similar averages derived for random gene groups. The groups of putative targets show significantly higher relative average expression. 2) We find high consistency between the induced lists of putative targets in human and in mouse. 3) The expression patterns associated with human and mouse genes (ordered by PWM scores for each TF) exhibit high similarity between human and mouse, indicating that our method has firm biological basis. 4) Comparison of results obtained by STOP and PRIMA (Elkon, et al., 2003) suggests that determining the score threshold using gene expression, as is done in STOP, is more biologically tuned. AVAILABILITY: Software package will be available for academic users upon request.

Dependence of defibrillation threshold upon extracellular/intracellular K+ concentrations.

CF Babbs — 2006-07-11T17:34:40-00:00

J Electrocardiol, Vol. 13, No. 1. (1980), pp. 73-78.

The effect of increasing extracellular potassium concentration (Ko) upon electrical ventricular defibrillation threshold was investigated in pentobarbital anesthetized dogs treated with intravenous potassium chloride. Defibrillation threshold fell duirng potassium intoxication. The percent decrease in defibrillation threshold was linearly related to the logarithm of Ko and to the potassium equilibrium potential (Ek), calculated from measured extracellular and intracellular potassium concentrations of ventricular muscle. In dogs supported by left ventricular bypass in order to maintain the circulation during potassium intoxication, the values of Ko and Ek required for spontaneous, K+ induced defibrillation (electrical defibrillation threshold = zero) were 16.6 mEq/L and -46 mV compared to the normal values of 3.9 mEq/L and -84 mV. Changes in defibrillation threshold related to changes in Ek may be significant events in digitalis intoxication and in myocardial anoxia during prolonged fibrillation.

Temporal modulation transfer functions based upon modulation thresholds

Neal Viemeister — 2006-02-10T22:58:40-00:00

The Journal of the Acoustical Society of America, Vol. 66, No. 5. (1979), pp. 1364-1380.

The detectability of amplitude modulation in the absence of spectral cues provides a quantitative description of temporal resolution for steady-state signals with relatively small amplitude changes. Modulation thresholds for sinusoidally amplitude-modulated wideband noise were measured as a function of modulation frequency. The resulting ”Temporal Modulation Transfer Function” (TMTF) shows a lowpass characteristic for modulation frequencies below about 800 Hz. The lowpass characteristic is extended up to approximately 2 kHz when the increment in average power produced by modulation is eliminated. The important parametric effects are summarized as follows: (1) TMTFs are independent of overall level, except at very low intensities; (2) the time constant indicated by the TMTF decreases as the center frequency of the band-limited, modulated noise is increased; (3) modulation thresholds generally decrease with increasing duration of modulation, particularly at low modulation frequencies; (4) when the carrier is gated for the duration of modulation, the TMTF shows a highpass segment at low modulation frequencies. Although the TMTFs are not directly consistent with the attenuation characteristic of a simple lowpass filter, a model which incorporates such a filter, with a time constant of 2.5 ms, describes the entire TMTF and also describes the modulation functions obtained with square-wave and pulse modulation. The wide bandwidth of initial filtering indicated by the model raises the important question of the role of peripheral filtering in determining the detectability of high-frequency modulation. doi:10.1121/1.383531 PACS: 43.66.Mk, 43.66.Ba, 43.66.Fe Additional Information Full Text: [ PDF (1625 kB) GZipped PS

Auditory intensity discrimination at high frequencies in the presence of noise.

NF Viemeister — 2006-01-24T22:36:40-00:00

Science, Vol. 221, No. 4616. (16 September 1983), pp. 1206-1208.

Over a wide range of intensities, subjects were able to detect small differences in the intensity of a high-frequency band of noise that was presented with a relatively intense, complementary band-reject noise. This indicates that neither of two possible mechanisms for peripheral intensity coding, those based on timing and on spread of excitation, is necessary for the large dynamic range of human hearing. It is shown that the information available in the firing rate of a small number of nerve fibers can account for these data.

The Ear as a Frequency Analyzer

R Plomp — 2006-01-24T22:31:49-00:00

The Journal of the Acoustical Society of America, Vol. 36, No. 9. (1964), pp. 1628-1636.

Ohm's acoustical law, as formulated by von Helmholtz, states that the human ear is able to analyze a periodic sound wave into its sinusoidal components. As far as the writer knows, there does not exist any systematic study about the limits of this law. In this paper, the results of some experiments on the number of distinguishable partials of multitone signals with harmonic as well as inharmonic components are communicated. The frequency of the lowest partial was varied between 44 and 2000 cps. The results are in agreement with the critical-band concept, indicating that the partials of a complex sound can be "heard out" only if their frequency separation exceeds the critical bandwidth. This fact supports the supposition that the critical bands correspond with the widths of the stimulating pattern of simple tones on the basilar membrane. The masking pattern of a complex tone of 500 cps, determined over the frequency range between 300 and 4000 cps, points in the same direction. An investigation of the minimum frequency separation required to distinguish the pitches of two-tone stimuli resulted in somewhat smaller values than the critical bandwidth, especially in the low-frequency range. ©1964 Acoustical Society of America

Is There a Sensory Threshold?

John Swets — 2006-01-21T23:30:18-00:00

On dichotomizing phenotypes in family-based association tests: quantitative phenotypes are not always the optimal choice

David Fardo — 2007-06-13T20:50:24-00:00

Genetic Epidemiology, Vol. 31, No. 5. (2007), pp. 376-382.

In family-based association studies, quantitative traits are thought to provide higher statistical power than dichotomous traits. Consequently, it is standard practice to collect quantitative traits and to analyze them as such. However, in many situations, continuous measurements are more difficult to obtain and/or need to be adjusted for other factors/confounding variables which also have to be measured. In such scenarios, it can be advantageous to record and analyze a ?simplified/dichotomized? version of the original trait. Under fairly general circumstances, we derive here rules for the dichotomization of quantitative traits that maintain power levels that are comparable to the analysis of the original quantitative trait. Using simulation studies, we show that the proposed rules are robust against phenotypic misclassification, making them an ideal tool for inexpensive phenotyping in large-scale studies. The guidelines are illustrated by an application to an asthma study. Genet. Epidemiol. 2007. © 2007 Wiley-Liss, Inc.

Bayes factor for testing the genetic background of quantitative threshold traits

J Casellas — 2006-09-11T20:57:32-00:00

Journal of Animal Breeding and Genetics, Vol. 123, No. 5. (October 2006), pp. 301-306.

Estimation of genetic parameters and prediction of breeding values for multivariate threshold and continuous data in a simulated horse population using Gibbs sampling and residual maximum likelihood

KF Stock — 2007-09-18T08:37:04-00:00

Journal of Animal Breeding and Genetics, Vol. 124, No. 5. (2007), pp. 308-319.

Summary Simulated horse data were used to compare multivariate estimation of genetic parameters and prediction of breeding values (BV) for categorical, continuous and molecular genetic data using linear animal models via residual maximum likelihood (REML) and best linear unbiased prediction (BLUP) and mixed linear-threshold animal models via Gibbs sampling (GS). Simulation included additive genetic values, residuals and fixed effects for one continuous trait, liabilities of four binary traits, and quantitative trait locus (QTL) effects and genetic markers with different recombination rates and polymorphism information content for one of the liabilities. Analysed data sets differed in the number of animals with trait records and availability of genetic marker information. Consideration of genetic marker information in the model resulted in marked overestimation of the heritability of the QTL trait. If information on 10 000 or 5000 animals was used, bias of heritabilities and additive genetic correlations was mostly smaller, correlation between true and predicted BV was always higher and identification of genetically superior and inferior animals was - with regard to the moderately heritable traits, in many cases - more reliable with GS than with REML/BLUP. If information on only 1000 animals was used, neither GS nor REML/BLUP produced genetic parameter estimates with relative bias <=25% and BV correlation >50% for all traits. Selection decisions for binary traits should rather be based on GS than on REML/BLUP breeding values.

Motion velocity thresholds in deaf signers: changes in lateralization but not in overall sensitivity.

CJ Brozinsky — 2005-04-20T02:35:03-00:00

Brain Res Cogn Brain Res, Vol. 21, No. 1. (September 2004), pp. 1-10.

In a series of three experiments, we tested whether deaf native signers process motion velocity information differently from hearing nonsigners. In Experiment 1, participants watched radially moving dots and were asked to detect the quadrant in which the velocity of the dots had changed. Similar 79% thresholds were observed in the two populations. In Experiments 2 and 3, peripheral and central thresholds were assessed separately as previous studies suggest early deafness leads mainly to changes in the processing of visual peripheral information. Neither condition produced an overall population difference. These negative results were not due to a lack of sensitivity in our experiments. Indeed, as has been previously reported, deaf native signers exhibited better thresholds in the right than in the left visual field, whereas the opposite pattern was observed in the hearing. This effect appears triggered by experience with American Sign Language (ASL) rather than deafness per se. Overall, this study confirms that early deafness does not enhance motion processing, and suggests that most of the changes previously described in the literature are instead attributable to changes in attention, and possibly special alterations of attention-to-motion processes.

On the development of stage IV hardening using a model based on the mechanical threshold

S Kok — 2007-09-26T13:52:31-00:00

Acta Materialia, Vol. 50, No. 7. (19 April 2002), pp. 1653-1667.

A two state variable hardening law is developed that is capable of simulating stage IV hardening. It is based on a physically motivated dislocation density evolution equation originally intended for use in a detailed FEM analysis, where individual grains are modeled with numerous finite elements. The evolution equation contains a contribution from geometrically necessary dislocations (net/excess dislocations), which can be associated with the slip plane lattice incompatibility. FEM analyses show that slip plane lattice incompatibility evolves linearly with accumulated slip. This observation forms the basis for the evolution equation of the second state variable. Compression and tension of copper are simulated with a rate and temperature dependent viscoplastic polycrystal model, in which the developed hardening law is embedded. The model is capable of predicting the experimentally observed enhanced hardening of small grain sized materials. The material parameter estimates and the state variable initial conditions are obtained by solving an identification problem.

Acquisition of PN code with adaptive threshold for DS/SS communications

Chang-Joo Kim — 2008-05-15T17:42:33-00:00

Electronics Letters, Vol. 33, No. 16. (1997), pp. 1352-1354.

An adaptive acquisition scheme of PN sequences is presented for DS/SS communications. The basic idea of the scheme is to apply the radar CFAR technique to the acquisition of PN sequences. The detection performance is analysed and an example of adaptive thresholding for multipath Rayleigh fading channels is given