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Biomedical subjects

C R McKenzie

Publications and source records attributed to C R McKenzie.

13 recordsLinked to original sources
PubMed2001

Relation between confidence in yes-no and forced-choice tasks.

Yes-no and forced-choice tasks are common in psychology, but the empirical relation between reported confidence in the 2 tasks has been unclear. The authors examined this relation with 2 experiments. The general experimental method had participants first report confidence in the truth of each of many general knowledge statements (a yes-no task) then report confidence in them again when the statements were put into pairs where it was known that one statement was true and one was false (a forced-choice task). At issue was how confidence in the statements changed between the yes-no task and the forced-choice task. Two models, including the normative one, were ruled out as descriptive models. A linear model and a multiplicative model remain viable contenders.

Choice Behavior↗
PubMed2000

The psychological side of Hempel's paradox of confirmation.

People often test hypotheses about two variables (X and Y), each with two levels (e.g., X1 and X2). When testing "If X1, then Y1," observing the conjunction of X1 and Y1 is overwhelmingly perceived as more supportive than observing the conjunction of X2 and Y2, although both observations support the hypothesis. Normatively, the X2&Y2 observation provides stronger support than the X1&Y1 observation if the former is rarer. Because participants in laboratory settings typically test hypotheses they are unfamiliar with, previous research has not examined whether participants are sensitive to the rarity of observations. The experiment reported here showed that participants were sensitive to rarity, even judging a rare X2&Y2 observation more supportive than a common X1&Y1 observation under certain conditions. Furthermore, participants' default strategy of judging X1&Y1 observations more informative might be generally adaptive because hypotheses usually regard rare events.

Adult↗
PubMed1999

Phosphatidyl choline-mediated inhibition of Streptococcus pneumoniae adherence to type II pneumocytes in vitro.

Nearly 80% of the adherence of several strains of Streptococcus pneumoniae to A549 lung cells was inhibited by dimyristoylphosphatidylcholine (DMPC), as well as by the following mixtures of lipids: DMPC/globoside, DMPC/asialo GM-1 and DMPC/asialo GM-1/globoside liposomes. Control phosphatidylserine liposomes were ineffective at inhibiting bacterial adherence demonstrating the specificity of the interaction between bacteria and liposomes. FITC-labelled bacteria were shown to adhere directly to silica beads coated with DMPC. The proportion of S. pneumoniae bacteria binding to DMPC-coated beads did not exceed 20% of the bacterial population as shown by the binding isotherm. This clearly demonstrates that only a fraction of the bacterial population (a subpopulation) was capable of binding to the beads. The specificity of bacterial binding to DMPC was further demonstrated by surface plasmon resonance. By this latter technique, the affinity between DMPC and bacteria was shown to be high and substantially non-reversible. Finally, we established that in order to be efficient at inhibiting bacterial binding to A549 cells the average liposome diameter must be greater than approximately 200 nm suggesting that a multivalent attachment of the bacterium to a liposome is required for high affinity binding.

Bacterial Adhesion↗
PubMed1999

Value of saphenous vein graft markers during subsequent diagnostic cardiac catheterization.

BACKGROUND: Although saphenous vein graft (SVG) markers have been available for many years, they have not been widely used in coronary artery bypass graft (CABG) surgery. This is likely due to the paucity of data regarding the utility of these markers in postsurgery cardiac catheterization. METHODS: We performed a prospective study of all post-CABG patients undergoing cardiac catheterization at Barnes-Jewish Hospital over a 6-month period to test our hypothesis that SVG markers would have a beneficial effect on these procedures. Differences in total procedure (arterial) time, time to image only the SVGs, fluoroscopy time, amount of contrast used, number of aortotomies, and number of views required were compared in patients with and without markers. RESULTS: Post-CABG patients undergoing catheterization who had markers (n = 76) required significantly less total procedure time (p = 0.007), fluoroscopy time (p = 0.02), and contrast use (p = 0.008). Even after adjusting for the numbers of SVG ostia and numbers of cine views, patients with markers still required less catheterization and fluoroscopy time (p < 0.01, p < 0.02) and time to image only the SVGs (p < 0.05) than those without markers (n = 106). CONCLUSIONS: SVG markers improve the efficiency of post-CABG catheterizations; they decrease the exposure of patients and cardiologists to ionizing radiation, and they decrease the exposure of patients to potentially toxic contrast agents. SVG markers are beneficial to the vast majority of post-CABG patients.

Cardiac Catheterization↗
PubMed1999

(Non)complementary updating of belief in two hypotheses.

Past research has led to the conclusion that two competing hypotheses are represented dependently, and confidence in them is updated in a complementary manner. It is argued here that confidence in two hypotheses can be represented either dependently or independently. Changes in confidence in the former case are always complementary, but changes in the latter case are complementary only under certain conditions. In three simulated medical diagnosis experiments, subjects learned about two illnesses in a manner expected to lead to either dependent or independent confidence. They were then presented with two symptoms sequentially (for each of several patients), updating confidence after each. Experiment 1 demonstrated that changes in confidence in the two illnesses were largely complementary for subjects with dependent, but not independent, confidence. Experiment 2 showed that encouraging consideration of the alternative led to more complementary changes for subjects with independent confidence. Experiment 3 succeeded in producing complementary changes from these subjects. Thus, complementarity does not imply dependent confidence, nor does independent confidence imply non-complementarity.

Adult↗
PubMed1997

The relationship of soluble fibrin and cross-linked fibrin degradation products to the clinical course of myocardial infarction.

Recently, increases in the plasma concentration of soluble fibrin (SF) have been suggested to be sensitive and specific for myocardial infarction (MI). However, the relationship between elevations in the SF concentration and the onset of symptoms and clinical course of MI is unknown. In addition, there are no data regarding the relationship between SF concentrations and concentrations of other markers of procoagulant (fibrinopeptide A [FPA]) and fibrinolytic (cross-linked fibrin degradation products [XL-FDPs]) activity in patients with MI. In this study, concentrations of SF were measured with a novel antigen-based assay for 93 MI patients and 29 control subjects, and the relationship between SF concentrations and those of XL-FDPs and FPA was determined. Increases in SF, FPA, and XL-FDP concentrations were documented in 55.9%, 45.2%, and 73.9%, respectively, of patients with MI, but there was no relationship between the concentrations of these markers. Increases in the concentration of SF or XL-FDPs did not show a relationship to increases in the concentration of FPA. Concentrations of XL-FDPs but not of SF were elevated to a greater extent in patients with MI complications (defined as death, ventricular arrhythmia, severe congestive heart failure, or mural thrombus). Increases in SF and XL-FDPs were not sensitive enough for the diagnosis of MI, but increased concentrations of XL-FDPs appear to predict those patients who are at higher risk for MI-related complications.

Adult↗
PubMed1996

Sustained inhibition of whole-blood clot procoagulant activity by inhibition of thrombus-associated factor Xa.

Progression of arterial thrombosis partly depends on thrombus-associated thrombin and activated factor X (Xa) activity. However, whether Xa or thrombin is the most appropriate target for inhibition of recurrent thrombosis is unknown. This study was designed to determine whether inhibition of Xa results in more sustained attenuation of thrombus-associated precoagulant activity than does inhibition of thrombin. Clots prepared ex vivo from human whole blood and pathological arterial thrombi from patients were preincubated in citrated plasma containing no inhibitor, 0.5 to 1 U/mL heparin, 0.5 to 1 mumol/L hirudin, 5 to 10 mumol/L tick anticoagulant peptide (TAP), 0.15 to 3 mumol/L tissue factor pathway inhibitor (TFPI), or a combination of 1 mumol/L hirudin and 10 mumol/L TAP for 2 hours. After preincubation the clots were removed from first-stage plasma, extensively washed in phosphate-buffered saline, and added to nonanticoagulated whole blood. Clots preincubated in plasma without inhibitors induced marked activation of the coagulation system in whole blood, as characterized by greater increases in the concentration of fibrinopeptide A (FPA) over 7 minutes than in blood without added clots (1522 +/- 568 compared with 117 +/- 170 ng/mL, P < .01). Preincubation of clots with heparin or hirudin did not attenuate the increases in FPA in wholeblood. In contrast, compared with incubation without an inhibitor, preincubation of clots with TAP or TFPI markedly attenuated the increases in FPA when clots were added to whole blood (551 +/- 316 and 508 +/- 208 ng/mL, respectively, P < .01). Similar results were obtained with arterial thrombi from patients. Inhibition of Xa but not of thrombin results in sustained attenuation of thrombus-associated procoagulant activity. Uninhibited thrombus-associated Xa activity may account for the increases in thrombin activity that are commonly observed in clinical trials after discontinuation of thrombin inhibitors.

Anticoagulants↗
PubMed1995

Role of thrombin compared with factor Xa in the procoagulant activity of whole blood clots.

BACKGROUND: Thrombi are known to induce activation of the coagulation system, which may be a mechanism for progression of thrombosis and its recurrence after thrombolysis. This study was designed to characterize the relative role of thrombin and activated factor X (factor Xa) as mediators of procoagulant activity of whole blood clots in blood and plasma. METHODS AND RESULTS: Clots formed from human blood were incubated in recalcified (25 mmol/L CaCl2) citrated plasma or nonanticoagulated blood with increasing concentrations of recombinant desulfatohirudin (hirudin) to inhibit thrombin activity, recombinant tick anticoagulant peptide (TAP) or recombinant tissue factor pathway inhibitor (TFPI) to inhibit factor Xa, or heparin. Fibrinopeptide A (FPA) was assayed serially as an index of procoagulant (thrombin) activity. FPA generation was greatly accelerated by addition of clots to recalcified plasma (from 1251 +/- 211 ng/mL after 15 minutes without clot to 5916 +/- 1412 ng/mL with clot, n = 7, P < .01) or whole blood (4803 +/- 761 ng/mL with clot compared with 546 +/- 233 without clot, n = 5, P < .05) and was attenuated by inhibitors of thrombin (> 90% inhibition of FPA with 0.05 mumol/L hirudin and 1.0 U/mL heparin) and factor Xa (> 90% inhibition of FPA with 1.0 mumol/L TAP and 0.15 mumol/L TFPI) in a concentration-dependent manner. Preincubation of clots with tissue-type plasminogen activator sufficient to induce partial clot lysis increased the rate of thrombin-induced FPA generation by increasing the surface area of clot exposed to plasma. However, procoagulant activity induced by partially lysed clots was attenuated by lower concentrations of both thrombin and Xa inhibitors, presumably because access of the inhibitors to bound procoagulant molecules was facilitated. Comparable results were obtained with incubations in nonanticoagulated blood. CONCLUSIONS: These results indicate that factor Xa is primarily responsible for the procoagulant activity of clots in vitro and suggest a potential molecular mechanism for the observed efficacy of inhibitors of factor Xa in preventing recurrent thrombosis after coronary thrombolysis.

Arthropod Proteins↗
PubMed1992

Allocation of visual attention in younger and older adults.

Younger and older adults were compared in three experiments, using procedures that had been shown to affect the spread of visual attention. The attentional effects found in previous experiments were replicated. A broader focus of attention speeded responses to peripheral targets. In addition, two established findings concerning aging were replicated: Responses were slower in older than in younger adults, and, in certain conditions, they slowed more rapidly as target eccentricity increased. No interactions of age effects with attentional manipulations were found. The results of all three experiments were consistent with the interpretation that younger and older adults do not differ in the allocation of attention.

Adult↗
PubMed1991

Learning from feedback: exactingness and incentives.

In a series of five experiments, exactingness, or the extent to which deviations from optimal decisions are punished, is studied within the context of learning a repetitive decision-making task together with the effects of incentives. Results include the findings that (a) performance is an inverted-U shaped function of exactingness, (b) performance is better under incentives when environments are lenient but not when they are exacting, (c) the interaction between exactingness and incentives does not obtain when an incentives function fails to discriminate sharply between good and bad performance, and (d) when the negative effects of exactingness on performance are eliminated, performance increases with exactingness.

Adult↗
PubMed1991

Attentional and perceptual contributions to the identification of extrafoveal stimuli: adult age comparisons.

Gerontological researchers have been cautioned that conclusions about age differences in attention may have been inferred from data that, in fact, reflected age differences in perceptual processing of stimuli falling outside the fovea (Cerella, 1985). Presumably, the experimental manipulations on which Cerella based his caution induced a broad focus of attention so that changes in perceptual processing would not be confounded with changes in attention. Experiment 1 tested this by comparing a condition similar to Cerella's with another in which attention was narrowly focused at fixation. The results replicated Cerella's findings. In addition, there were greater age differences when attention had been narrowly focused, showing that attentional effects can be separated from the effects reported by Cerella. Experiment 2 showed that age differences in extrafoveal perception could be removed by increasing the duration of the target from 200 to 2000 ms, suggesting that the perceptual deficits in older adults are due to differentially lengthened processing of stimuli outside the fovea.

Adolescent↗