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PubMed2004

Geometrical computations explain projection patterns of long-range horizontal connections in visual cortex.

Neurons in primary visual cortex respond selectively to oriented stimuli such as edges and lines. The long-range horizontal connections between them are thought to facilitate contour integration. While many physiological and psychophysical findings suggest that collinear or association field models of good continuation dictate particular projection patterns of horizontal connections to guide this integration process, significant evidence of interactions inconsistent with these hypotheses is accumulating. We first show that natural random variations around the collinear and association field models cannot account for these inconsistencies, a fact that motivates the search for more principled explanations. We then develop a model of long-range projection fields that formalizes good continuation based on differential geometry. The analysis implicates curvature(s) in a fundamental way, and the resulting model explains both consistent data and apparent outliers. It quantitatively predicts the (typically ignored) spread in projection distribution, its nonmonotonic variance, and the differences found among individual neurons. Surprisingly, and for the first time, this model also indicates that texture (and shading) continuation can serve as alternative and complementary functional explanations to contour integration. Because current anatomical data support both (curve and texture) integration models equally and because both are important computationally, new testable predictions are derived to allow their differentiation and identification.

Animals↗

PubMed1985

Increases in intrathoracic pressure do not explain the rise in left ventricular end-diastolic pressure that occurs during exercise in patients with chronic obstructive pulmonary disease.

During exercise, patients with chronic obstructive pulmonary disease (COPD) increase their pulmonary arterial wedge (Ppaw) and left ventricular (LV) end-diastolic pressures more than normal control subjects. The increase in pressure is commonly attributed to an increase in intrathoracic pressure (Pit). However, mean esophageal pressure (Pes) does not increase with supine exercise in patients with COPD. Because changes in Pes may not represent changes in Pit when recorded in the supine position, we measured Ppaw and Pes during upright exercise in 8 patients with severe air-flow limitation (mean +/- SD) FEV1, 0.88 +/- 0.27 L secondary to COPD and no history or electrocardiographic abnormalities suggesting a previous myocardial infarct, history of angina, evidence of systemic hypertension, or use of cardiac medications. In addition, all patients completed a progressive exercise test to exhaustion without angina or ST segment changes, and all had normal LV function at rest assessed by equilibrium radionuclide ventriculography. The Ppaw increased a mean of 7.2 +/- 4.3 mmHg, whereas Pes increased a mean of only 1.3 +/- 1.6 mmHg. By multiple linear regression analysis, Ppaw was significantly associated with the work level performed (p less than 0.01), but had no significant association with Pes (p greater than 0.1). The change in Ppaw could not be attributed to changes in Pes. If changes in Pes during upright exercise are representative of changes in Pit or juxtacardiac pressure, a rise in Pit does not explain the exercise-induced increase in Ppaw and LV end-diastolic pressure that occurs in patients with COPD.

Blood Gas Analysis↗

PubMed1997

Lack of high affinity fiber receptor activity explains the resistance of ciliated airway epithelia to adenovirus infection.

Although recombinant adenoviruses are attractive vectors for gene transfer to airway epithelia, they have proven to be relatively inefficient. To investigate the mechanisms of adenovirus-mediated gene transfer to airway epithelia, we examined the role of adenovirus fiber and penton base, the two proteins involved in attachment to and entry of virus into the cell. We used human airway epithelia grown under conditions that allow differentiation and development of a ciliated apical surface that closely resembles the in vivo condition. We found that addition of fiber protein inhibited virus binding and vector-mediated gene transfer to immature airway epithelia, as well as to primary cultures of rat hepatocytes and HeLa cells. However, fiber protein had no effect on vector binding and gene transfer to ciliated airway epithelia. We obtained similar results with addition of penton base protein: the protein inhibited gene transfer to immature epithelia, whereas there was no effect with ciliated epithelia. Moreover, infection was not attenuated with an adenovirus containing a mutation in penton base that prevents the interaction with cell surface integrins. These data suggest that the receptors required for efficient infection by adenovirus are either not present or not available on the apical surface of ciliated human airway epithelia. The results explain the reason for inefficient gene transfer and suggest approaches for improvement.

Adenoviridae↗

PubMed2005

Enhanced passive Ca2+ reabsorption and reduced Mg2+ channel abundance explains thiazide-induced hypocalciuria and hypomagnesemia.

Thiazide diuretics enhance renal Na+ excretion by blocking the Na+-Cl- cotransporter (NCC), and mutations in NCC result in Gitelman syndrome. The mechanisms underlying the accompanying hypocalciuria and hypomagnesemia remain debated. Here, we show that enhanced passive Ca2+ transport in the proximal tubule rather than active Ca2+ transport in distal convolution explains thiazide-induced hypocalciuria. First, micropuncture experiments in mice demonstrated increased reabsorption of Na+ and Ca2+ in the proximal tubule during chronic hydrochlorothiazide (HCTZ) treatment, whereas Ca2+ reabsorption in distal convolution appeared unaffected. Second, HCTZ administration still induced hypocalciuria in transient receptor potential channel subfamily V, member 5-knockout (Trpv5-knockout) mice, in which active distal Ca2+ reabsorption is abolished due to inactivation of the epithelial Ca2+ channel Trpv5. Third, HCTZ upregulated the Na+/H+ exchanger, responsible for the majority of Na+ and, consequently, Ca2+ reabsorption in the proximal tubule, while the expression of proteins involved in active Ca2+ transport was unaltered. Fourth, experiments addressing the time-dependent effect of a single dose of HCTZ showed that the development of hypocalciuria parallels a compensatory increase in Na+ reabsorption secondary to an initial natriuresis. Hypomagnesemia developed during chronic HCTZ administration and in NCC-knockout mice, an animal model of Gitelman syndrome, accompanied by downregulation of the epithelial Mg2+ channel transient receptor potential channel subfamily M, member 6 (Trpm6). Thus, Trpm6 downregulation may represent a general mechanism involved in the pathogenesis of hypomagnesemia accompanying NCC inhibition or inactivation.

Animals↗

PubMed1998

Polyvariant mutant cystic fibrosis transmembrane conductance regulator genes. The polymorphic (Tg)m locus explains the partial penetrance of the T5 polymorphism as a disease mutation.

In congenital bilateral absence of the vas deferens patients, the T5 allele at the polymorphic Tn locus in the CFTR (cystic fibrosis transmembrane conductance regulator) gene is a frequent disease mutation with incomplete penetrance. This T5 allele will result in a high proportion of CFTR transcripts that lack exon 9, whose translation products will not contribute to apical chloride channel activity. Besides the polymorphic Tn locus, more than 120 polymorphisms have been described in the CFTR gene. We hypothesized that the combination of particular alleles at several polymorphic loci might result in less functional or even insufficient CFTR protein. Analysis of three polymorphic loci with frequent alleles in the general population showed that, in addition to the known effect of the Tn locus, the quantity and quality of CFTR transcripts and/or proteins was affected by two other polymorphic loci: (TG)m and M470V. On a T7 background, the (TG)11 allele gave a 2.8-fold increase in the proportion of CFTR transcripts that lacked exon 9, and (TG)12 gave a sixfold increase, compared with the (TG)10 allele. T5 CFTR genes derived from patients were found to carry a high number of TG repeats, while T5 CFTR genes derived from healthy CF fathers harbored a low number of TG repeats. Moreover, it was found that M470 CFTR proteins matured more slowly, and that they had a 1.7-fold increased intrinsic chloride channel activity compared with V470 CFTR proteins, suggesting that the M470V locus might also play a role in the partial penetrance of T5 as a disease mutation. Such polyvariant mutant genes could explain why apparently normal CFTR genes cause disease. Moreover, they might be responsible for variation in the phenotypic expression of CFTR mutations, and be of relevance in other genetic diseases.

Alleles↗

PubMed2004

Explaining the discrepancy between intentions and actions: the case of hypothetical bias in contingent valuation.

An experiment was designed to account for intention-behavior discrepancies by applying the theory of planned behavior to contingent valuation. College students (N = 160) voted in hypothetical and real payment referenda to contribute $8 to a scholarship fund. Overestimates of willingness to pay in the hypothetical referendum could not be attributed to moderately favorable latent dispositions. Instead, this hypothetical bias was explained by activation of more favorable beliefs and attitudes in the context of a hypothetical rather than a real referendum. A corrective entreaty was found to eliminate this bias by bringing beliefs, attitudes, and intentions in line with those in the real payment situation. As a result, the theory of planned behavior produced more accurate prediction of real payment when participants were exposed to the corrective entreaty.

Adolescent↗

PubMed2006

Anger and guilt about ingroup advantage explain the willingness for political action.

Three studies examined non-Aboriginal Australians' guilt and anger about their ingroup's advantage over structurally disadvantaged Aborigines. Study 1 showed that participants who perceived their ingroup as relatively advantaged perceived this inequality as unfair and felt guilt and anger about it. Anger, and to a lesser degree guilt, predicted the willingness to engage in political action regarding ingroup advantage. Study 2 showed both guilt and anger to be relatively self-focused because both were associated with appraising the ingroup's (rather than the government's) discrimination as responsible for ingroup advantage. Study 3 examined on participants especially willing to engage in political action to bring about systemic compensation to Aborigines. Anger about ingroup advantage was a potent predictor. Although guilt was associated with the abstract goal of systemic compensation, guilt did not explain willingness for political action. Results underline the importance of examining specific group-based emotions in intergroup relations.

Adolescent↗

PubMedDate not supplied

Explaining distortions in utility elicitation through the rank-dependent model for risky choices.

The standard-gamble (SG) method has been accepted as the "gold standard" for the elicitation of utility when risk or uncertainty is involved in decisions, and thus for the measurement of utility in medical decisions. It is based on the assumptions of expected-utility theory. Unfortunately, there is now abundant evidence that expected utility is not empirically valid, and that the SG method overestimates risk aversion and the utilities of impaired health states. This paper shows how rank-dependent utility theory, a newly developed theory in decision science, can explain the main violations of expected utility. Thus it provides a means for correcting the SG method and for improving the assessments of quality-adjusted life years for medical decisions in which there is uncertainty about outcomes.

Bias↗

PubMedDate not supplied

Does nitric oxide help explain the differential healing capacity of the anterior cruciate, posterior cruciate, and medial collateral ligaments?

This study compared the ability of rabbit medial collateral ligament, posterior cruciate ligament, and anterior cruciate ligament tissue to synthesize nitric oxide, and determined its effects on matrix synthesis, an important component of ligament repair. It is not known whether ligament cells can produce nitric oxide and, if so, whether it influences healing of ligament injuries. The anterior cruciate and posterior cruciate ligament tissue produced large amounts of nitric oxide in response to the inflammatory cytokine interleukin-1. Medial collateral ligament, in contrast, produced only modest amounts of nitric oxide. Furthermore, anterior cruciate ligament and, to some degree, posterior cruciate ligament synthesized nitric oxide spontaneously in culture, whereas medial collateral ligament never did so. When nitric oxide was supplied to these tissues, it strongly inhibited collagen synthesis by the two cruciate ligaments, but had little effect on collagen synthesis by the medial collateral ligament. Endogenously synthesized nitric oxide was also able to inhibit collagen synthesis as well as proteoglycan synthesis by the two cruciate ligaments, but had little effect on matrix synthesis by the medial collateral ligament. We propose a novel hypothesis, based on nitric oxide production and matrix synthesis, that may help explain why the two cruciate ligaments have such limited healing capacity compared with the medial collateral ligament.

Animals↗

PubMed2000

Illness intrusiveness explains race-related quality-of-life differences among women with systemic lupus erythematosus.

Our objective was to investigate whether quality of life in systemic lupus erythematosus (SLE) differs across ethnoracial groups and to identify factors that may explain race-related differences. Self-administered questionnaire data from 335 White, 40 Black, and 30 Asian women with SLE were obtained from a multi-center database. Measures assessed illness intrusiveness, psychological well-being, depressive symptoms, musculoskeletal pain, and learned helplessness. Extent of SLE disease activity was indexed by self-reported functional-system involvement. Educational attainment was indicated by number of years in school. Principal-components analysis reduced the four psychosocial measures to a single factor score. This represented psychosocial well-being In path analysis. Psychosocial well-being differed significantly across the three groups, with Whites reporting the highest, and Blacks the lowest, levels. Path analysis indicated that illness intrusiveness accounted for this race-related difference. Although disease activity was significantly associated with psychosocial well-being, it did not differ across ethnoracial groups. Illness intrusiveness and educational attainment emerged as independent mediators of the race-related difference in psychosocial well-being. We conclude that race-related quality-of-life differences exist among women with SLE and are mediated independently by illness intrusiveness and educational attainment.

Asia↗

PubMed2004

The effect of the health belief model in explaining exercise participation among Jordanian myocardial infarction patients.

This study describes the effect of health belief model (HBM) in explaining exercise participation among Jordanian myocardial infarction (MI) patients. A convenient sample of 98 MI patients was recruited from four governmental hospitals in northern Jordan. A self-reported questionnaire and structured interview were designed to obtain the needed information. Study results indicated that Jordanian MI patients had a high score in perceived severity and a low score in perceived barriers. Results also showed a significant correlation between exercise participation and health belief variables and sociodemographics such as age, annual income, level of education, and physician recommendation. These findings have implications for designing intervention programs aimed at improving physical activity by all MI patients. These programs should consider culture, socioeconomic status, personal system, and demographics. Further research is needed to develop a culturally sensitive instrument that takes into consideration the cultural variation and the specific needs of MI patients.

Culture↗

PubMedDate not supplied

Explaining low ratings of patient satisfaction among Asian-Americans.

Asian-Americans report lower levels of satisfaction with health care than other racial or ethnic groups. To determine potential reasons for this observation, we analyzed data from a national household telephone survey examining various aspects of health and health care across racial or ethnic groups. As in previous studies, Asians reported lower satisfaction with their physicians and health care services than other racial or ethnic groups, even after adjusting for demographic and health-related variables. However, Asians also reported lower satisfaction with their health insurance and with life in general and were less likely than non-Asians to have changed physicians due to dissatisfaction. We conclude that lower satisfaction ratings among Asians may be due to different response tendencies rather than to differences in quality of care. Further research is needed to explain this phenomenon and assess its potential impact on quality-of-care evaluations for clinicians and health care organizations providing care for large numbers of Asian-Americans.

Adult↗

PubMed1999

Acquired von Willebrand disease due to increasing platelet count can readily explain the paradox of thrombosis and bleeding in thrombocythemia.

The arterial microvascular thrombotic events in thrombocythemia including erythromelalgia, atypical and typical cerebral and ocular transient ischemic attacks, and acute coronary syndromes already occur at platelet counts > 400 x 10(9)/L and are sensitive to low-dose aspirin, which does not elicit bleedings at platelet counts < 1,000 x 10(9)/L. An increasing platelet count in thrombocythemia to above 1,000 x 10(9)/L is accompanied by the acquisition of a von Willebrand factor deficiency due to the loss of intermediate and large von Willebrand factor multimers. The arterial thrombotic condition in thrombocythemia changes into an overt spontaneous bleeding tendency at mean platelet counts of about 2,000 +/- 1,000 x 10(9)/L due to an acquired von Willebrand disease type II with normal values for von Willebrand factor antigen concentration but absence of intermediate and large von Willebrand factor multimers in plasma. At platelet counts between 1,000 and 2,000 x 10(9)/L, thrombosis and bleeding frequently occur in sequence or paradoxically, and low-dose aspirin does prevent thrombotic complications but aggravates or may elicit bleeding symptoms. Reduction of the platelet count < 1,000 x 10(9)/L significantly restores the von Willebrand factor deficiency with the reappearance of the intermediate and some of the large von Willebrand factor multimers and the disappearance of the bleeding tendency, but the thrombotic tendency persists as long as platelet counts are above the upper limit of normal. The acquisition of von Willebrand factor deficiency at increasing platelet counts can readily explain the paradox of thrombosis and bleeding in thrombocythemia and has important clinical implications.

Blood Platelets↗

PubMed2005

Explaining variation in physician practice patterns and their propensities to recommend services.

Variations in physician practice patterns have important implications for quality and cost. The purpose of this article is to explain variation in physicians' practice patterns in terms of physician personal characteristics, practice setting, patient population, and managed care involvement. Data on 2,455 primary care physicians were derived from the Community Tracking Study Physician Survey (1996-1997). Factor scores were determined based on responses to three clinical scenarios that represent discretionary medical decisions. These scenarios include a specialist referral for benign prostatic hyperplasia, prescription drugs for elevated cholesterol, and an office visit for vaginal discharge. Physician age, being a foreign medical school graduate, being a solo practitioner, and having a larger proportion of Medicaid patients were all associated with higher factor scores, a greater likelihood of ordering a service. Being board certified was associated with lower factor scores. Managed care involvement was not a significant predictor of factor scores.

Adult↗

PubMed2005

Explaining trends in hospitalizations for pneumonia and influenza in the elderly.

From 1987-99, influenza and pneumococcal vaccination rates among elderly Medicare beneficiaries increased by 300 percent and 500 percent, respectively. Despite these gains, annual rates of hospitalizations for pneumonia and influenza (P&I) have not decreased; rather, they have increased steadily. The authors investigate whether this paradoxical increase in hospitalization rates reflects an increasing burden of P&I or the effects of a changing healthcare environment. They find that from 1987-99, P&I hospitalizations per one thousand beneficiaries increased from 15.1 to 23.4. Of this increase, 23 percent was due to an aging Medicare population, 2.4 percent was due to increased rates of rehospitalization, and at most 5 percent was due to upcoding. There was no evidence that physicians were increasingly admitting patients with less complicated cases of P&I. The changing healthcare environment only partially explained the paradoxical increase in P&I hospitalizations. P&I appears to be an increasing burden to the elderly, despite increased vaccination rates.

Aged↗

PubMed2006

Do psychological characteristics explain socioeconomic stratification of self-rated health?

This study evaluated whether negative emotions explain socioeconomic status (SES) stratification of self-rated health (SRH) and whether this putative relation is independent of established SRH determinants. Mood disorders, trait negative affect and health status indices were assessed in a representative cross-sectional survey of 3032 adults in the National Survey of Midlife Development in the United States (MIDUS). Adjustment for health behaviors and health status appreciably reduced SES influence on SRH, but adjustment for negative emotions did not. However, both psychological resources (e.g. social support, extraversion) and negative emotions independently predicted SRH. Detection of SRH determinants was sensitive to binary versus ordinal SRH definitions.

Adult↗

PubMed2001

Lack of flow regulation may explain the development of arteriovenous malformations.

In the normal vasculature, vessels of widely different sizes maintain shear stress within a narrow range. Recently, investigators have had great success using mathematical models to explore the relationship of structure to function in normal vascular beds. When investigators first explored how vascular beds adapt to set shear stress at appropriate levels, however, some vessels tended to regress, and some tended to grow into arteriovenous shunts. Degeneration of the arterial tree is prevented when flow regulation is added to the model. The present work explores the implication of this theoretical development and illustrates how it may explain the genesis of arteriovenous malformations (AVMs). We use a simple model to illustrate how impairing local control of blood flow causes models to become structurally unstable, yielding a structure and behavior similar to AVMs. This work shows how the lack of local flow control can be the cause, not just the result, of arteriovenous malformations. With insight gained from this modeling approach, specific, focused experiments can be designed.

Adaptation, Physiological↗

PubMed1999

Does redox regulation of cell function explain why antioxidants perform so poorly as therapeutic agents?

In normal health, there is a balance between the formation of oxidising chemical species and their effective removal by protective antioxidants. Antioxidants are a diverse group of molecules with diverse functions. For example, they range from large highly specific proteinaceous molecules with catalytic properties to small lipid- and water-soluble molecules with non-specific scavenging or metal chelating properties. Antioxidants control the prevailing relationship between reducing or oxidising (redox) conditions in biological systems. Such control offers two major advantages: (i) the ability to remove toxic levels of oxidants before they damage critical biological molecules; and (ii) the ability to manipulate changes, at the subtoxic level, of molecules that can function as signal, trigger or messenger carriers. If cellular functions are signalled through redox control mechanisms, it would explain why we see such a poor response to antioxidants as therapeutic agents in human medicine.

Animals↗