Differences in the sensations of warmth on the anterior torso of normal and spinal-cord transected individuals

The threshold to warming was measured at 10 sites on the anterior torso between the umbilicus and the clavicle of normal and spinal-cord transected individuals. In normal individuals, thresholds were higher on the thorax than on the abdomen. Men had higher and more variable thresholds than women. Magnitude estimations of supra-threshold stimuli showed that men offer verbal estimates of warmth that are about half of the size of the estimates given by women to the same stimuli. The psychometric function shows that in women, the sensation of warmth grows more rapidly than in men after starting from a higher initial value. After spinal-cord injury, thresholds for detection of warming were elevated. This effect was most noticeable within 8 cm of the anesthetic zone, but farther away, thresholds were still elevated but uniform as a function of distance, being about 30% higher than in normal individuals. After spinal-cord injury, the psychometric functions show that small stimuli elicit relatively large sensations and that these sensations grow more slowly with increasing skin temperatures than for normal individuals. Thus, for small warm stimuli spinal-cord-injured patients (both men and women) have a response similar to normal women but the slope of the psychometric function is flat, being similar to the slope observed for normal men.     

Differences in the sensations of warmth on the anterior torso of normal and spinal-cord transected individuals

The threshold to warming was measured at 10 sites on the anterior torso between the umbilicus and the clavicle of normal and spinal-cord transected individuals. In normal individuals, thresholds were higher on the thorax than on the abdomen. Men had higher and more variable thresholds than women. Magnitude estimations of supra-threshold stimuli showed that men offer verbal estimates of warmth that are about half of the size of the estimates given by women to the same stimuli. The psychometric function shows that in women, the sensation of warmth grows more rapidly than in men after starting from a higher initial value. After spinal-cord injury, thresholds for detection of warming were elevated. This effect was most noticeable within 8 cm of the anesthetic zone, but farther away, thresholds were still elevated but uniform as a function of distance, being about 30% higher than in normal individuals. After spinal-cord injury, the psychometric functions show that small stimuli elicit relatively large sensations and that these sensations grow more slowly with increasing skin temperatures than for normal individuals. Thus, for small warm stimuli spinal-cord-injured patients (both men and women) have a response similar to normal women but the slope of the psychometric function is flat, being similar to the slope observed for normal men.     

YAAP: yet another adaptive procedure

YAAP is an implementation of an adaptive psychophysical method, based on Bayesian statistics, for estimating the threshold of a psychometric function. On the basis of Bayesian probability intervals, a dynamic termination criterion allows for threshold estimation within pre-specified confidence limits.     

Psychometric function for nectar volume perception of a flower-visiting bat

For foraging pollinators one aspect of floral quality is the volume of nectar available. Thus, nectar-feeding animals should be able to estimate volumes of received nectar. In this study, we determined the psychometric function for nectar volume discrimination of a Neotropical flower-visiting bat Glossophaga soricina. For this, we examined the ability of bats to discriminate between two nectar volumes in a two-alternative forced-choice paradigm. We used a Bayesian inference approach to determine psychometric functions. From the derived psychometric function we assessed the discrimination threshold at a value of 0.69. G. soricina could clearly distinguish between two volumes, when the difference between the two nectar volumes divided by their average exceeded this value. This indicates that bats possess a sense for the perception and discrimination of volumes of nectar that is better developed than in honeybees.     

A THURSTONIAN VARIANT OF THE BETA-BINOMIAL MODEL FOR REPLICATED DIFFERENCE TESTS

The beta-binomial model is combined with a Thurstonian psychometric function to obtain estimates of the parameters of a distribution applicable to replicated difference tests. A method of estimating the variance of d'obtained from these tests is provided. A formula for determining sample size, which is composed of the number of trials (or panelists) and the number of replications, to determine d'is also given.     

Optimal setups for forced-choice staircases with fixed step sizes

Forced-choice staircases with fixed step sizes are used in a variety of formats whose relative merits have never been studied. This paper presents a comparative study aimed at determining their optimal format. Factors included in the study were the up/down rule, the length (number of reversals), and the size of the steps. The study also addressed the issue of whether a protocol involving three staircases running for N reversals each (with a subsequent average of the estimates provided by each individual staircase) has better statistical properties than an alternative protocol involving a single staircase running for 3N reversals. In all cases the size of a step up was different from that of a step down, in the appropriate ratio determined by García-Pérez (Vision Research, 1998, 38, 1861 - 1881). The results of a simulation study indicate that a) there are no conditions in which the 1-down/1-up rule is advisable; b) different combinations of up/down rule and number of reversals appear equivalent in terms of precision and cost: c) using a single long staircase with 3N reversals is more efficient than running three staircases with N reversals each: d) to avoid bias and attain sufficient accuracy, threshold estimates should be based on at least 30 reversals: and e) to avoid excessive cost and imprecision, the size of the step up should be between 2/3 and 3/3 the (known or presumed) spread of the psychometric function. An empirical study with human subjects confirmed the major characteristics revealed by the simulations.     

Bat sonar: an alternative interpretation of the 10-ns jitter result

In 1990 Simmons et al. reported evidence of a time resolution hitherto unknown in any animal, namely a 10-ns jitter detection threshold in echolocating bats. This result is discussed. The calibration data from the original papers are examined. Observations indicating other cues than delay being presented to the bats are given. We offer an alternative explanation for the psychometric jitter function, based on the assumption of a subtle distortion due to impedance mismatch in the delay-producing apparatus. We also report that effects of impedance mismatch are detectable by a human subject in a model experiment. Accepted: 3 September 1997     

Estimation of the diagnostic threshold accounting for decision costs and sampling uncertainty

Medical diagnostic tests are used to classify subjects as non-diseased or diseased. The classification rule usually consists of classifying subjects using the values of a continuous marker that is dichotomised by means of a threshold. Here, the optimum threshold estimate is found by minimising a cost function that accounts for both decision costs and sampling uncertainty. The cost function is optimised either analytically in a normal distribution setting or empirically in a free-distribution setting when the underlying probability distributions of diseased and non-diseased subjects are unknown. Inference of the threshold estimates is based on approximate analytically standard errors and bootstrap-based approaches. The performance of the proposed methodology is assessed by means of a simulation study, and the sample size required for a given confidence interval precision and sample size ratio is also calculated. Finally, a case example based on previously published data concerning the diagnosis of Alzheimer's patients is provided in order to illustrate the procedure.     

The hockey-stick method to estimate evening dim light melatonin onset (DLMO) in humans

The onset of melatonin secretion in the evening is the most reliable and most widely used index of circadian timing in humans. Saliva (or plasma) is usually sampled every 0.5–1 hours under dim-light conditions in the evening 5–6 hours before usual bedtime to assess the dim-light melatonin onset (DLMO). For many years, attempts have been made to find a reliable objective determination of melatonin onset time either by fixed or dynamic threshold approaches. The here-developed hockey-stick algorithm, used as an interactive computer-based approach, fits the evening melatonin profile by a piecewise linear-parabolic function represented as a straight line switching to the branch of a parabola. The switch point is considered to reliably estimate melatonin rise time. We applied the hockey-stick method to 109 half-hourly melatonin profiles to assess the DLMOs and compared these estimates to visual ratings from three experts in the field. The DLMOs of 103 profiles were considered to be clearly quantifiable. The hockey-stick DLMO estimates were on average 4 minutes earlier than the experts' estimates, with a range of ?27 to +13 minutes; in 47% of the cases the difference fell within ±5 minutes, in 98% within ?20 to +13 minutes. The raters' and hockey-stick estimates showed poor accordance with DLMOs defined by threshold methods. Thus, the hockey-stick algorithm is a reliable objective method to estimate melatonin rise time, which does not depend on a threshold value and is free from errors arising from differences in subjective circadian phase estimates. The method is available as a computerized program that can be easily used in research settings and clinical practice either for salivary or plasma melatonin values.     

Commentary on Holmes et al. (2007): resolving the debate on when extinction risk is predictable

We reconcile the findings of Holmes et al. ( Ecology Letters , 10 , 2007, 1182) that 95% confidence intervals for quasi-extinction risk were narrow for many vertebrates of conservation concern, with previous theory predicting wide confidence intervals. We extend previous theory, concerning the precision of quasi-extinction estimates as a function of population dynamic parameters, prediction intervals and quasi-extinction thresholds, and provide an approximation that specifies the prediction interval and threshold combinations where quasi-extinction estimates are precise (vs. imprecise). This allows PVA practitioners to define the prediction interval and threshold regions of safety (low risk with high confidence), danger (high risk with high confidence), and uncertainty.     

Psychometrics of odor quality discrimination: method for threshold determination

There is no natural physical continuum for odor quality along which an odor quality discrimination (OQD) threshold can be measured. In an attempt to overcome this problem, the substitution-reciprocity (SURE) method defines a framework for the measurement of an OQD threshold. More specifically, it (i) defines a threshold concept for OQD, including the quantification of qualitative change of the stimulus, and (ii) suggests how to avoid perceived intensity as an unwanted cue for discrimination. In doing this, the psychometric properties of odor quality in the case of eugenol and citral are investigated using both discrimination (experiment 1) and scaling (experiment 2). Based on discriminatory responses, a change of approximately one-third in stimulus content was needed to reach the OQD threshold for eugenol and citral.     

Numerical exploration of the influence of neural noise on the psychometric function at low stimulation intensity levels

The relationship between stimulus intensity and the probability of detecting the presence of the stimulus is described by the psychometrical function. The probabilistic nature of this relationship is based on the stochastic behaviour of sensory neural channels and sensory networks involved in perceptual processing (Kiang 1968). This study tries to establish a continuum of variability across different levels of integration in the central nervous system. Once the opening and closing times of ionic channels was simulated, a threshold to the collective behaviour of voltage-gated ionic channels was imposed in order to generate the spike train of a single neuron. Afterwards, the trains of spikes of different neurons were added up, simulating the activity of a sensory nerve. By adding the activity due to the stimulus to the spontaneous neural behaviour, the psychometric function was simulated using a thresholding approach. The results can replicate the stochastic resonance phenomenon, but also open up the possibility that attentional phenomena can be mediated not only by increasing neural activity (bursting or oscillatory), but also by increasing noise at the neural level.     

Item response analysis of a shortened German version of the morningness–eveningness scale

A shortened version of the German adaptation of the morningness–eveningness scale of Horne and Östberg is analysed within a large sample of 994 physicians with respect to dimensionality, reliability, gender differences and validity. The psychometric analysis – which incorporates a highly robust method to check for unidimensionality – shows discrepancies towards unidimensionality and highlights three misfitting items. In addition, hypothesis testing indicates the presence of differential item functioning (DIF) with respect to gender which could be caused by differences in response formats. Although, reliability estimates are satisfactory, an overall lack of adequate psychometric properties of the scale within the population of physicians has to be reported. We derive suggestions for improvement of the original morningness–eveningness questionnaire (MEQ)-scale and provide general comments on how to check for unidimensionality without imposing a restrictive response model.     

On testing the role of genetic asymmetries created by haplodiploidy in the evolution of eusociality in the Hymenoptera

The haplodiploid genetic system found in all Hymenopterans creates an asymmetry in genetic relatedness so that full-sisters are more closely related to each other than a mother is to her daughters. Thus Hymenopteran workers who rear siblings can obtain higher inclusive fitness compared to individuals who rear offspring. However, polyandry and polygyny reduce relatedness between workers and their sisters and thus tend to break down the genetic asymmetry created by haplodiploidy. Since the advent of electrophoretic analysis of variability at enzyme loci, several estimates of intra-colony genetic relatedness in the Hymenoptera have been published. To test the role of the genetic asymmetry created by haplodiploidy in the evolution of eusociality, I assume that workers are capable of investing in their brothers and sisters in their ratio of relatedness to them. I then compute ahaplodiploidy threshold, which is the threshold relatedness to sisters required for workers to obtain a weighted mean relatedness of 0.5 to siblings and thus break even with solitary foundresses. When workers rear mixtures of sisters and brothers in an outbred population, the value of this threshold is 0.604. An examination of the distribution of 185 estimates of mean genetic relatedness between sisters in Hymenopteran colonies shows that the values are well below the expected 0.75 for full sisters, both in higly eusocial as well as in primitively eusocial species although relatedness values in the latter are higher than in the former. Of the 177 estimates with standard error, 49 are significantly lower than the haplodiploidy threshold and 22 are significantly higher. Of the 35 species studied only 6 have one or more estimates that are significantly higher than the haplodiploidy threshold. For more than half the estimates, the probability of the relatedness value being above the haplodiploidy threshold is less than 0.5. Reanalysis of these data using 0.5 as the threshold does not drastically alter these conclusions. I conclude that the genetic asymmetry created by haplodiploidy is, in most cases, insufficient by itself either topromote the origin of eusociality or tomaintain the highly eusocial state.     

Stopping rules in Bayesian adaptive threshold estimation

Threshold estimation with sequential procedures is justifiable on the surmise that the index used in the so-called dynamic stopping rule has diagnostic value for identifying when an accurate estimate has been obtained. The performance of five types of Bayesian sequential procedure was compared here to that of an analogous fixed-length procedure. Indices for use in sequential procedures were: (1) the width of the Bayesian probability interval, (2) the posterior standard deviation, (3) the absolute change, (4) the average change, and (5) the number of sign fluctuations. A simulation study was carried out to evaluate which index renders estimates with less bias and smaller standard error at lower cost (i.e. lower average number of trials to completion), in both yes-no and two-alternative forced-choice (2AFC) tasks. We also considered the effect of the form and parameters of the psychometric function and its similarity with the model function assumed in the procedure. Our results show that sequential procedures do not outperform fixed-length procedures in yes-no tasks. However, in 2AFC tasks, sequential procedures not based on sign fluctuations all yield minimally better estimates than fixed-length procedures, although most of the improvement occurs with short runs that render undependable estimates and the differences vanish when the procedures run for a number of trials (around 70) that ensures dependability. Thus, none of the indices considered here (some of which are widespread) has the diagnostic value that would justify its use. In addition, difficulties of implementation make sequential procedures unfit as alternatives to fixed-length procedures.     

Tactile Discrimination of Gratings: Psychophysical and Neural Correlates in Human and Monkey

Human and monkey performance on discriminating tactile gratings revealed comparable cross-species Weber functions. Neural data obtained while monkeys performed discriminations revealed some matching of neural and psychometric functions. Nearly constant firing rate differences occurred at discrimination threshold for unequal groove widths. Firing rate differences of some cells decreased on trials preceding discrimination errors, and thus predicted performance.     

Tactile discrimination of gratings: psychophysical and neural correlates in human and monkey.

Human and monkey performance on discriminating tactile gratings revealed comparable cross-species Weber functions. Neural data obtained while monkeys performed discriminations revealed some matching of neural and psychometric functions. Nearly constant firing rate differences occurred at discrimination threshold for unequal groove widths. Firing rate differences of some cells decreased on trials preceding discrimination errors, and thus predicted performance.     

Model Specification and the Reliability of fMRI Results: Implications for Longitudinal Neuroimaging Studies in Psychiatry

Functional Magnetic Resonance Imagine (fMRI) is an important assessment tool in longitudinal studies of mental illness and its treatment. Understanding the psychometric properties of fMRI-based metrics, and the factors that influence them, will be critical for properly interpreting the results of these efforts. The current study examined whether the choice among alternative model specifications affects estimates of test-retest reliability in key emotion processing regions across a 6-month interval. Subjects (N = 46) performed an emotional-faces paradigm during fMRI in which neutral faces dynamically morphed into one of four emotional faces. Median voxelwise intraclass correlation coefficients (mvICCs) were calculated to examine stability over time in regions showing task-related activity as well as in bilateral amygdala. Four modeling choices were evaluated: a default model that used the canonical hemodynamic response function (HRF), a flexible HRF model that included additional basis functions, a modified CompCor (mCompCor) model that added corrections for physiological noise in the global signal, and a final model that combined the flexible HRF and mCompCor models. Model residuals were examined to determine the degree to which each pipeline met modeling assumptions. Results indicated that the choice of modeling approaches impacts both the degree to which model assumptions are met and estimates of test-retest reliability. ICC estimates in the visual cortex increased from poor (mvICC = 0.31) in the default pipeline to fair (mvICC = 0.45) in the full alternative pipeline – an increase of 45%. In nearly all tests, the models with the fewest assumption violations generated the highest ICC estimates. Implications for longitudinal treatment studies that utilize fMRI are discussed.     

The transducer model for contrast detection and discrimination: formal relations, implications, and an empirical test

The transducer function mu for contrast perception describes the nonlinear mapping of stimulus contrast onto an internal response. Under a signal detection theory approach, the transducer model of contrast perception states that the internal response elicited by a stimulus of contrast c is a random variable with mean mu(c). Using this approach, we derive the formal relations between the transducer function, the threshold-versus-contrast (TvC) function, and the psychometric functions for contrast detection and discrimination in 2AFC tasks. We show that the mathematical form of the TvC function is determined only by mu, and that the psychometric functions for detection and discrimination have a common mathematical form with common parameters emanating from, and only from, the transducer function mu and the form of the distribution of the internal responses. We discuss the theoretical and practical implications of these relations, which have bearings on the tenability of certain mathematical forms for the psychometric function and on the suitability of empirical approaches to model validation. We also present the results of a comprehensive test of these relations using two alternative forms of the transducer model: a three-parameter version that renders logistic psychometric functions and a five-parameter version using Foley's variant of the Naka-Rushton equation as transducer function. Our results support the validity of the formal relations implied by the general transducer model, and the two versions that were contrasted account for our data equally well.