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PubMed1987

Oxygen boundary crossing probabilities.

The probability that an oxygen particle will reach a time dependent boundary is required in oxygen transport studies involving solution methods based on probability considerations. A Volterra integral equation is presented, the solution of which gives directly the boundary crossing probability density function. The boundary crossing probability is the probability that the oxygen particle will reach a boundary within a specified time interval. When the motion of the oxygen particle may be described as strongly Markovian, then the Volterra integral equation can be rewritten as a generalized Abel equation, the solution of which has been widely studied.

Biological Transport, Active↗

PubMed1996

Probability distribution of molecular evolutionary trees: a new method of phylogenetic inference.

A new method is presented for inferring evolutionary trees using nucleotide sequence data. The birth-death process is used as a model of speciation and extinction to specify the prior distribution of phylogenies and branching times. Nucleotide substitution is modeled by a continuous-time Markov process. Parameters of the branching model and the substitution model are estimated by maximum likelihood. The posterior probabilities of different phylogenies are calculated and the phylogeny with the highest posterior probability is chosen as the best estimate of the evolutionary relationship among species. We refer to this as the maximum posterior probability (MAP) tree. The posterior probability provides a natural measure of the reliability of the estimated phylogeny. Two example data sets are analyzed to infer the phylogenetic relationship of human, chimpanzee, gorilla, and orangutan. The best trees estimated by the new method are the same as those from the maximum likelihood analysis of separate topologies, but the posterior probabilities are quite different from the bootstrap proportions. The results of the method are found to be insensitive to changes in the rate parameter of the branching process.

Animals↗

PubMed2005

Assessment of ancestry probabilities in the presence of genotyping errors.

This paper extends an approach for estimating the ancestry probability, the probability that an inbred line is an ancestor of a given hybrid, to account for genotyping errors. The effect of such errors on ancestry probability estimates is evaluated through simulation. The simulation study shows that if misclassification is ignored, then ancestry probabilities may be slightly overestimated. The sensitivity of ancestry probability calculations to the assumed genotyping error rate is also assessed.

Alleles↗

PubMed2002

Behavioral and electrocortical evidence of an interaction between probability and task metrics in movement preparation.

Recent neurophysiological evidence suggests that cognitive factors shape neural activity in cortical areas such as parietal (area 5), premotor, and primary motor cortex. The implication of these findings is that behavioral signatures of cognitive factors and movement-specific factors should likewise be interdependent. The present study provides evidence of this interdependence in both behavioral (reaction time) and electrophysiological (P300) measures. Subjects performed a two-choice pointing task, in which the angular distance between the two required movement directions (task metrics) and the probability of the two responses was varied. In a control condition, a single reaction was required in response to both stimuli to test for the influence of stimulus metrics. Results from the pointing task showed a clear interaction between the metrics and effects of probability. When the potential targets were widely separated (120 degrees), stimulus probability influences reaction time and P300 amplitude in the classic fashion (longer reaction times and larger P300 amplitudes to less probable responses). When pointing to targets that were narrowly separated (20 degrees), probability had no effect: both rare and frequent targets were "functionally frequent." The same interaction was not observed in the control condition, indicating that metrics were primarily influencing movement preparation rather than stimulus processing. The results are consistent with the theoretical framework of dynamic field theory and demonstrate that metrics are an important factor that must be taken into account when assessing the processes associated with movement preparation.

Adolescent↗

PubMed1989

International application of a new probability algorithm for the diagnosis of coronary artery disease.

A new discriminant function model for estimating probabilities of angiographic coronary disease was tested for reliability and clinical utility in 3 patient test groups. This model, derived from the clinical and noninvasive test results of 303 patients undergoing angiography at the Cleveland Clinic in Cleveland, Ohio, was applied to a group of 425 patients undergoing angiography at the Hungarian Institute of Cardiology in Budapest, Hungary (disease prevalence 38%); 200 patients undergoing angiography at the Veterans Administration Medical Center in Long Beach, California (disease prevalence 75%); and 143 such patients from the University Hospitals in Zurich and Basel, Switzerland (disease prevalence 84%). The probabilities that resulted from the application of the Cleveland algorithm were compared with those derived by applying a Bayesian algorithm derived from published medical studies called CADENZA to the same 3 patient test groups. Both algorithms overpredicted the probability of disease at the Hungarian and American centers. Overprediction was more pronounced with the use of CADENZA (average overestimation 16 vs 10% and 11 vs 5%, p less than 0.001). In the Swiss group, the discriminant function underestimated (by 7%) and CADENZA slightly overestimated (by 2%) disease probability. Clinical utility, assessed as the percentage of patients correctly classified, was modestly superior for the new discriminant function as compared with CADENZA in the Hungarian group and similar in the American and Swiss groups. It was concluded that coronary disease probabilities derived from discriminant functions are reliable and clinically useful when applied to patients with chest pain syndromes and intermediate disease prevalence.

Algorithms↗

PubMed1992

Worldwide variations in the lifetime probability of reproductive cancer in women: implications of best-case, worst-case, and likely-case assumptions about the effect of oral contraceptive use.

Cancer incidence in countries representative of three patterns of reproductive cancer and age-specific mortality was used to estimate the effect of oral contraceptive use on the lifetime probability of reproductive cancer under three sets of assumptions about the effects of oral contraceptives. Under the set of assumptions considered likely, oral contraceptives were estimated to reduce or increase only slightly the lifetime probability of any reproductive cancer in each setting. Under worst-case assumptions, oral contraceptives were estimated to increase the lifetime probability of reproductive cancer only modestly in settings with low cancer rates and in settings with high rates of breast, ovarian, and endometrial cancer, but it might have a large impact on lifetime probability of reproductive cancer in settings with high cervical cancer rates. Under best-case assumptions, oral contraceptives were estimated to decrease the lifetime probability of reproductive cancer in each setting; this reduction was estimated to be greatest in settings where endometrial and ovarian cancer incidence are high.

Adolescent↗

PubMed1996

Variability of sex ratio, mating probability and egg production in an intestinal nematode in its fluctuating host population.

In dioecious parasites, the chances of co-occurring with the opposite sex depend on the sex ratio, abundance and distribution pattern of parasites within the host population. Theory suggests that if the abundance and degree of aggregation are very low, mating probability may decrease so much that the parasite population is vulnerable to extinction. Our aim is to determine the factors affecting the mating probability and egg production in Heligmosomum mixtum (Heligmosomidae), an intestinal nematode of the bank vole, Clethrionomys glareolus, at Pallasjärvi, Finnish Lapland. We also search for factors responsible for the persistence of H. mixtum in its fluctuating host population. The results showed that during high parasite abundance practically all nematode females co-occurred with males, but during a phase of very low abundance only 15% of females had a chance to mate. Comparison of observed mating probabilities and those predicted by a theoretical model (May & Woolhouse, 1993) showed that deviation from the assumption of complete aggregation between males and females results in underestimation of the mating probability. Sex ratio and the degree of aggregation showed a minor effect on mating probability. The sex ratio (proportion of females) of H. mixtum, which was female-biased (0.58), showed a negative correlation with the mean intensity of infection in the monthly samples (decreasing female-bias at high mean intensity), but no significant relation to the intensity of infection (number of worms in a host individual). The long-term persistence of H. mixtum in its strongly varying host population seems to be due to the high transmission efficiency and long life-span of the parasite.

Animals↗

PubMed1989

The role of location probability in the programming of saccades: implications for "center-of-gravity" tendencies.

Short-latency saccades to targets among nontarget backgrounds are often directed to the center of the entire (target + nontarget) stimulus configuration. This "averaging" or "center-of-gravity" tendency has been attributed to an automatic, reflexive saccadic response to a poorly-resolved visual signal. We investigated the role of high-level processes by varying the probability of the target appearing in one of two locations. Subjects were asked to make a saccade to a target "+" located above-right or above-left of a central fixation point. A nontarget ("x") was in the other location (directional separation = 30 deg). The mean latencies were short (180-230 msec) in accordance with instructions. Mean saccadic direction was shifted to the right by 24-52% of the directional separation of the stimulus pair as the probability of the target appearing on the right increased from 0.2 to 0.8. The difference in saccadic directions as a function of the actual target location was small and independent of probability, showing that probability introduced a bias without affecting the discriminability of the target from the nontarget. The effect of probability was reduced when the discrimination of the target from the nontarget was easier (square vs triangle), and abolished (saccadic accuracy near perfect with the same average latencies) when the target was presented alone. The results show that the direction of short-latency saccades, initiated before the target has been distinguished from a nearby nontarget, is based on the prior history of target locations and expectations about the future location of the target. High-level plans can account for effects of nontargets on saccades. To infer that a reflexive sensorimotor averaging mechanism exists solely on the basis of observed saccadic "centering" tendencies is unwarranted.

Eye Movements↗

PubMed1989

Motivational effects of smoked marijuana: behavioral contingencies and high-probability recreational activities.

Fifteen adult male research volunteers, in five groups of three subjects each, lived in a residential laboratory for up to 20 days. All contact with the experimenters was through a networked computer system, and subjects' behaviors were monitored continuously and recorded. During the first part of each day, subjects remained in their private rooms doing planned work activities, and during the remainder of each day, they were allowed to socialize. One or two cigarettes containing active marijuana (1.3-2.7% delta 9-THC) or placebo were smoked during both the private work period and the period of access to social activities. Two-or three-day contingency conditions requiring subjects to engage in a low-probability recreational activity in order to earn time that could be spent engaging in a high-probability recreational activity were programmed during periods of placebo and active marijuana smoking. During placebo administration, the contingency requirement reliably increased the amount of time that subjects spent engaged in the low-probability instrumental activity and decreased the time spent engaged in the high-probability contingent activity. During active marijuana administration, however, the increases in instrumental activity were consistently smaller than observed under placebo conditions. The decreases in high-probability contingent activity were similar across drug conditions. Smoking active marijuana was thus observed to produce instrumental decrements under motivational conditions involving recreational contingencies.

Adult↗

PubMed1995

The value of exercise electrocardiography testing in the identification of coronary restenosis: a probability analysis.

We studied by means of probability analysis the role of exercise ECG in identifying coronary restenosis. A total of 213 patients were independently evaluated by clinical history, conventional assessment of the exercise ECG ('yes or no' statement), D score (a discriminant function derived from exercise ECG), and coronariography, 5.4 +/- 2.8 months after successful coronary angioplasty. The initial probability of restenosis (30%), that is, the prevalence of the condition, was radically changed by the result of clinical history (77% for patients with angina vs. 17% for those without angina). By contrast, ECG binary assessment, due to its low accuracy (70% vs 82% of clinical history, P < 0.005), was unable to significantly change the established probabilities after symptomatic evaluation. Finally, D score, which greatly enhanced specificity (92% vs. 76% of bivariate assessment, P < 0.0001), proved to be useful in changing the probability (from 32% to 76% or to 25%) of patients (n = 34) with a discordant result (no angina/positive exercise ECG). When this stepwise approach was tested in 46 new patients, predicted and observed probabilities were actually very similar. We conclude that exercise ECG has a very limited role in identifying coronary restenosis if positive responses are not adjusted with a weighted score which takes into account other exercise derived factors.

Angina Pectoris↗

PubMed1978

Anticipatory HR deceleration as a function of perceived control and probability of aversive loud noise: a deployment of attention account.

Five groups of 10 undergraduate subjects each participated in an experiment to examine the effects of perceived control over, and the probability of occurrence of, noxious (white-noise) stimulation on anticipatory heart rate (HR) deceleration. All groups performed an arithmetic task, the numbers for which were changed on each of the 20 trials, but three groups were defined as active groups since they were instructed that the occurrence of noxious stimulation was contingent upon their performance on the task. The actual probability of occurrence of noxious stimulation (i.e., either 0.9, 0.5 or 0.1) was manipulated between these three groups. A fourth (passive) group (0.5 probability of occurrence) was instructed as to the non-contingent relationship between performance and the aversive white noise. The last group was a control group which only performed the task, and thus both provided an estimate of the rate of return to baseline following the expected HR acceleration to the task and eliminated the need for a within-subject control period to assess HR deceleration. The results indicated that all active and passive groups produced significant HR decelerations, relative to the control group, prior to the noxious stimulus. However, all active groups (regardless of probability levels) displayed significantly greater, and required more trials to maximally develop, anticipatory deceleration than the passive group. In addition, the results for the active groups revealed a direct relationship between the probability of occurrence of noxious stimulation and the magnitude of anticipatory deceleration. It is argued that the results cannot be adequately accommodated by the preparatory-adaptive-response theory. A deployment-of-attention account is suggested since it appears both to account for the obtained pattern of results completely and to generate additional testable predictions for the future.

Adaptation, Psychological↗

PubMed1989

Channel probability and Nd: an event-related potential sign of attention strategies.

Event-related potentials were recorded in a two-channel selective attention paradigm designed to assess the effects of channel probability on processing negativity. For channel probabilities of 70% or greater, the subtraction Nd for standard tones was greatly reduced or absent. This change was due to apparent processing negativity in the waveform of the irrelevant standards at high channel probabilities. Deviant long tones showed similar early Nds to standard tones, and also developed late attention-related negativity commencing at about 500 ms from stimulus onset. It was concluded that there was no evidence that processing negativity to the relevant tones, defined as a negative component associated with selective attention, is affected by channel probability. Changes in Nd amplitude as the channel probabilities became more unequal were attributed to changes in strategies used by subjects, which primarily altered ERPs associated with the irrelevant tones.

Adult↗

PubMed1990

"Upstream" regulation of the release probability in sympathetic nerve varicosities.

The results appear to support the following tentative working hypothesis. (1) Nerve impulse-induced transmitter release from sympathetic nerve varicosities is monoquantal and highly intermittent (probability range: 0-0.03). (2) Nerve impulses invade varicosities as all-or-none, Na+ channel-dependent action potentials; invasion failure may be rare. (3) The release probability is not controlled by properties (amplitude or duration) of the invading action potential or the resulting Ca2+ current, but by the availability of an as yet unidentified permissive factor. (4) The permissive factor is actively transported intra-axonally, probably in association with organelles (LDVs?). (5) The activation and/or transport of the permissive factor are controlled "upstream" of the varicosity; they depend on Ca2+ influx through channels insensitive to nifedipine (hence, not of L-type) but blocked by Cd2+ and apparently opened by slight depolarization of the resting membrane, in this respect behaving more as T- than N-type channels. (6) A high resting K+ efflux "upstream" of the varicosity restricts the availability of the permissive factor; it is the main mechanism maintaining the (economically necessary) low release probability. (7) Prejunctional agonists do not inhibit transmitter secretion by causing a conduction block or by reducing the action potential-induced Ca2+ influx into the varicosity itself, but by depressing the Ca2(+)-dependent activation and/or transport of the permissive factor; they act at least in part via receptors "upstream" of the varicosity. (8) This hypothesis for regulation of the release probability in sympathetic nerves may apply, at least in part, to other neurons as well.

Animals↗

PubMed1996

Probability of having hypertension: effects of sex, history of hypertension in parents, and other risk factors.

The objective of the present study was to determine whether paternal and maternal history of hypertension contributes to the probability of an individual having hypertension before and after other measured traits, including sex, are considered. A cross-sectional sample of 217 men and 196 women was selected from the general Caucasion population of Rochester, Minnesota without respect to the hypertension status of subjects and their parents. Logistic regression analyses indicated that when no other information was considered, paternal history of hypertension contributed to the probability of having hypertension in men (chi 2 = 4.14, df = 1, p = 0.042) and in women (chi 2 = 4.12, df = 1, p = 0.042). The odds ratio associated with paternal history of hypertension was 2.80 in men (95% confidence interval [CI] = 1.0-8.0); the odds ratio was 4.11 in women (95% CI = 0.9-19.3). Maternal history of hypertension provided a marginally significant contribution to the prediction of probability of having hypertension in men (chi 2 = 3.86, df = 1, p = 0.049 in men), and less so in women (chi 2 = 3.31, df = 1, p = 0.068). The odds ratios associated with maternal history of hypertension were 2.85 in men (95% CI = 0.9-8.8) and 3.60 in women (95% CI = 0.8-16.9). A stepwise selection algorithm was used to select other predictors of hypertension in men and women. Other predictors of hypertension identified in men were age and sodium-lithium countertransport level. After these other predictors were considered, paternal but not maternal history of hypertension contributed to the probability of having hypertension in men. In a model that contained these other predictors, the partial odds ratio associated with paternal history of hypertension was 3.38 (95% CI = 1.1 = 10.1). Other predictors identified in women were age, apolipoprotein (apo) B, and apo B squared. After these other predictors were considered, neither paternal nor maternal history of hypertension made a statistically significant contribution to the probability of having hypertension in women. These results suggest that evaluation of the hypertension risk of an individual depends on the sex of the individual, the sexes of the individual's hypertensive parents, and the values of other measured risk factor traits.

Algorithms↗

PubMed2005

Clinical experience did not reduce the variance in physicians' estimates of pretest probability in a cross-sectional survey.

BACKGROUND AND OBJECTIVES: In light of the increasing popularity of the threshold approach in clinical decision-making, this study assesses the role of expertise in physicians' agreement in estimating the probability of disease in patients. METHODS: A cross-sectional survey of physicians of different specialties, attending weekly staff meetings in four teaching hospitals in Jerusalem, Israel. An anonymous questionnaire describing three case scenarios of patients with chest pain was administered and participants were asked to estimate pretest probabilities of disease. RESULTS: Eighty-six physicians (practicing cardiology, internal medicine, and family medicine, as well as general practitioners and internists) out of 125 approached (response rate 69%). The mean estimated probabilities were very similar for residents and specialists; however, the standard deviation was higher for specialists in all three cases: 20.7, 21.0, and 19.1 among specialists and 16.4, 20.5, and 14.9 among residents, respectively. CONCLUSION: This study, based on case scenarios, did not find that medical expertise improved agreement among doctors when estimating the probability of disease in patients-despite the common belief that senior physicians should have smaller interobserver differences in probability estimates. The wide variation observed calls into question the applicability of the threshold approach.

Cardiology↗

PubMed2005

Linking retinotopic fMRI mapping and anatomical probability maps of human occipital areas V1 and V2.

Using functional MRI, we characterized field sign maps of the occipital cortex and created three-dimensional maps of these areas. By averaging the individual maps into group maps, probability maps of functionally defined V1 or V2 were determined and compared to anatomical probability maps of Brodmann areas BA17 and BA18 derived from cytoarchitectonic analysis (Amunts, K., Malikovic, A., Mohlberg, H., Schormann, T., Zilles, K., 2000. Brodmann's areas 17 and 18 brought into stereotaxic space-where and how variable? NeuroImage 11, 66-84). Comparison of areas BA17/V1 and BA18/V2 revealed good agreement of the anatomical and functional probability maps. Taking into account that our functional stimulation (due to constraints of the visual angle of stimulation achievable in the MR scanner) only identified parts of V1 and V2, for statistical evaluation of the spatial correlation of V1 and BA17, or V2 and BA18, respectively, the a priori measure kappa was calculated testing the hypothesis that a region can only be part of functionally defined V1 or V2 if it is also in anatomically defined BA17 or BA18, respectively. kappa = 1 means the hypothesis is fully true, kappa = 0 means functionally and anatomically defined visual areas are independent. When applying this measure to the probability maps, kappa was equal to 0.84 for both V1/BA17 and V2/BA18. The data thus show a good correspondence of functionally and anatomically derived segregations of early visual processing areas and serve as a basis for employing anatomical probability maps of V1 and V2 in group analyses to characterize functional activations of early visual processing areas.

Adult↗

PubMed2005

Robust treatment planning for intensity modulated radiotherapy of prostate cancer based on coverage probabilities.

BACKGROUND AND PURPOSE: To evaluate an optimization approach where coverage probabilities are incorporated into the optimization of intensity modulated radiotherapy (IMRT) to overcome the problem of margin definition in the case of overlapping planning target volume and organs at risk. PATIENTS AND METHODS: IMRT plans were generated for three optimization approaches: based on a planning CT plus margin (A), on prostate and rectum contours from five pre-treatment CT plus margin (B), and on coverage probabilities (C). For approach (C), the probability of organ occupation was computed for each voxel from five pre-treatment CTs and the population distribution of systematic setup error and it was used as local weight in the costfunctions. Monte Carlo simulations of treatment courses were used to compute the probability distribution of prostate and rectal wall equivalent uniform dose (EUD). RESULTS: Treatment simulations showed best and most robust results for prostate and rectal wall EUD within the population for (C). For (A) the rectal wall EUD was on average about 1.5 Gy greater than in (C), while the prostate EUD was lower than those from (C) for most of the patients for (B) (especially for those with great organ motion). CONCLUSIONS: The incorporation of coverage probabilities as local weights allows for dose escalation as well as improved rectal sparing and results in a safer and more robust IMRT treatment.

Computer Simulation↗

PubMed2002

Factors influencing the probability of an incident at a junction: results from an interactive driving simulator.

Using data generated from a fixed-base interactive driving simulator, which was used to evaluate a driver decision aid, a model is built to predict the probability of an incident (i.e. an accident or a 'near miss') occurring as a result of a right-turn across left-hand traffic at an unsignalised junction. This can be considered to be the product of two separate probabilities, the first being the probability that the gap between a pair of vehicles in the traffic stream is accepted, and the second the probability that the time needed to cross the on-coming stream of traffic causes the time-to-collision with the nearest vehicle in this traffic stream to be less than a second. The model is developed from the results of experimental trials involving a sample of drivers, the majority of whom were aged 60 years or older, in order to demonstrate the effect of various parameters on these probabilities. The parameters considered include the size of the gap between successive vehicles, vehicle characteristics such as size, colour and velocity, driver characteristics such as age and sex, and both daytime and night-time conditions.

Accidents, Traffic↗