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D R Williams

Publications and source records attributed to D R Williams.

At least 73 records · Page 4Linked to original sources
PubMed2000

Physiology of L- and M-cone inputs to H1 horizontal cells in the primate retina.

In the primate retina, H1 horizontal cells form an electrically coupled network and receive convergent input from long- (L-) and middle- (M-) wavelength-sensitive cones. Using an in vitro preparation of the intact retina to record the light-evoked voltage responses of H1 cells, we systematically varied the L- and M-cone stimulus contrast and measured the relative L- and M-cone input strength for 137 cells across 33 retinas from three Old World species (Macaca nemestrina, M. fascicularis, and Papio anubis). We found that the L- and the M-cone inputs were summed by the H1 cell in proportion to the stimulus cone contrast, which yielded a measure of what we term L- and M-cone contrast gain. The proportion of L-cone contrast gain was highly variable, ranging from 25% to 90% [mean +/- standard deviation, (60 +/- 14)%]. This variability was accounted for by retinal location within an individual, with the temporal retina showing a consistently higher percentage of L-cone gain, and by large overall variation across individuals, with the mean percentage of L-cone gain ranging from 32% to 80%. We hypothesize that the relative L- and M-cone contrast gain is determined simply by the relative number of L and M cones in the H1 cell's receptive field and that the variability in L- and M-cone contrast gain reflects a corresponding variability in the mosaic of L and M cones.

Animals↗
PubMed2000

Functional consequences of the relative numbers of L and M cones.

Direct imaging of the retina by adaptive optics allows assessment of the relative number of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones in living human eyes. We examine the functional consequences of variation in the relative numbers of L and M cones (L/M cone ratio) for two observers whose ratios were measured by direct imaging. The L/M cone ratio for the two observers varied considerably, taking on values of 1.15 and 3.79. Two sets of functional data were collected: spectral sensitivity measured with the flicker electroretinogram (ERG) and the wavelength of unique yellow. A genetic analysis was used to determine L and M cone spectra appropriate for each observer. Rayleigh matches confirmed the use of these spectra. We determined the relative strength of L and M cone contributions to ERG spectral sensitivity by fitting the data with a weighted sum of L and M cone spectra. The relative strengths so determined (1.06 and 3.38) were close to the cone ratios established by direct imaging. Thus variation in L/M cone ratio is preserved at the sites tapped by the flicker ERG. The wavelength of unique yellow varied only slightly between the two observers (576.8 and 574.7 nm). This small variation indicates that neural factors play an important role in stabilizing unique yellow against variation in the L/M cone ratio.

Adult↗
PubMed2000

Race/ethnicity and the 2000 census: recommendations for African American and other black populations in the United States.

This commentary considers the implications of the assessment of racial/ethnic status for monitoring the health of African Americans and other Black populations in the United States. It argues that because racial disparities in health and other social indicators persist undiminished, the continued assessment of race is essential. However, efforts must be made to ensure that racial data are of the highest quality. This will require uniform assessment of racial status that includes identifiers for subgroups of the Black population. Research also indicates that the health of multiracial persons varies by maternal race. Thus, efforts to monitor multiracial status should assess the race of both parents. More attention should also be given to analysis and interpretation of racial data and to the collection of additional data that capture characteristics linked to race (such as socioeconomic factors and racism) that may adversely affect health.

Black or African American↗
PubMed2000

Excess mortality among urban residents: how much, for whom, and why?

OBJECTIVES: The goals of this study were to estimate prospective mortality risks of city residence, specify how these risks vary by population subgroup, and explore possible explanations. METHODS: Data were derived from a probability sample of 3617 adults in the coterminous United States and analyzed via cross-tabular and Cox proportional hazards methods. RESULTS: After adjustment for baseline sociodemographic and health variables, city residents had a mortality hazard rate ratio of 1.62 (95% confidence interval [CI] = 1.21, 2.18) relative to rural/small-town residents; suburbanites had an intermediate but not significantly elevated hazard rate ratio. This urban mortality risk was significant among men (hazard rate ratio: 2.25), especially non-Black men, but not among women. Among Black men, and to some degree Black women, suburban residence carried the greatest risk. All risks were most evident for those younger than 65 years. CONCLUSIONS: The mortality risk of city residence, at least among men, rivals that of major psychosocial risk factors such as race, low income, smoking, and social isolation and merits comparable attention in research and policy.

Activities of Daily Living↗
PubMed2000

Physical and chemical evidence remaining after the explosion of large improvised bombs. Part 1: firings of ammonium nitrate/sugar and urea nitrate.

Recent criminal acts in the United Kingdom, United States and other countries have demonstrated the dangers to public safety from the criminal use of improvised explosives on a large scale. Four sets of trials were carried out over four years, partly in collaboration with the United States Federal Bureau of Investigation, involving the firing of large bombs, mostly fertilizer based. The principal objectives of the firings were to measure the physical effects of the explosions upon objects representative of those that would be found at a real bomb scene and to recover any chemical traces deposited on these objects. The results are intended for use as an aid in determining the approximate size and type of an explosive employed in a terrorist attack. This paper describes the background behind the trials, the procedures for preparation of witness materials and charges, and the collection and analysis of physical and chemical evidence.

Criminology↗
PubMed2000

Understanding and addressing racial disparities in health care.

Racial disparities in medical care should be understood within the context of racial inequities in societal institutions. Systematic discrimination is not the aberrant behavior of a few but is often supported by institutional policies and unconscious bias based on negative stereotypes. Effectively addressing disparities in the quality of care requires improved data systems, increased regulatory vigilance, and new initiatives to appropriately train medical professionals and recruit more providers from disadvantaged minority backgrounds. Identifying and implementing effective strategies to eliminate racial inequities in health status and medical care should be made a national priority.

Delivery of Health Care↗
PubMed1999

The arrangement of the three cone classes in the living human eye.

Human colour vision depends on three classes of receptor, the short- (S), medium- (M), and long- (L) wavelength-sensitive cones. These cone classes are interleaved in a single mosaic so that, at each point in the retina, only a single class of cone samples the retinal image. As a consequence, observers with normal trichromatic colour vision are necessarily colour blind on a local spatial scale. The limits this places on vision depend on the relative numbers and arrangement of cones. Although the topography of human S cones is known, the human L- and M-cone submosaics have resisted analysis. Adaptive optics, a technique used to overcome blur in ground-based telescopes, can also overcome blur in the eye, allowing the sharpest images ever taken of the living retina. Here we combine adaptive optics and retinal densitometry to obtain what are, to our knowledge, the first images of the arrangement of S, M and L cones in the living human eye. The proportion of L to M cones is strikingly different in two male subjects, each of whom has normal colour vision. The mosaics of both subjects have large patches in which either M or L cones are missing. This arrangement reduces the eye's ability to recover colour variations of high spatial frequency in the environment but may improve the recovery of luminance variations of high spatial frequency.

Cell Count↗
PubMed1999

Racism as a stressor for African Americans. A biopsychosocial model.

Various authors have noted that interethnic group and intraethnic group racism are significant stressors for many African Americans. As such, intergroup and intragroup racism may play a role in the high rates of morbidity and mortality in this population. Yet, although scientific examinations of the effects of stress have proliferated, few researchers have explored the psychological, social, and physiological effects of perceived racism among African Americans. The purpose of this article was to outline a biopsychosocial model for perceived racism as a guide for future research. The first section of this article provides a brief overview of how racism has been conceptualized in the scientific literature. The second section reviews research exploring the existence of intergroup and intragroup racism. A contextual model for systematic studies of the biopsychosocial effects of perceived racism is then presented, along with recommendations for future research.

Black or African American↗
PubMed1999

Structure and provision of services in Black churches in New Haven, Connecticut.

Historically, Black (or African American) churches have played a central role as a center of religious and political life and also as a provider of human services and a healing community. This article examined the extent to which African American churches in 1 Northeastern urban environment are involved in the delivery of health and human service programs to their communities. It also explored how comfortable Black clergy are in referring their parishioners to the formal mental health system and identified the actual level of referrals. In addition, the analyses considered the individual and organizational characteristics that predict variations in the levels of support services and the likelihood of referral. Analyses revealed that African American churches deliver a broad range of services to the community. More than two thirds of the clergy feel comfortable in making a referral to a mental health agency or professional, and more than half have actually made a referral. Both service delivery and referral levels varied by several clergy and congregational characteristics. The implications of these findings for research and health policy are considered.

Adult↗
PubMed1999

The monitoring of racial/ethnic status in the USA: data quality issues.

This paper reviews the assessment of racial and ethnic identification in the major data collection systems of the US Department of Health and Human Services. It evaluates the quality of the available data and outlines recommendations for improving the collection of racial data and enhancing our understanding of the role of race in health. Special attention is also given to the role of socioeceonomic status in understanding racial differences in health and the assessment of racial status in data systems in the UK.

Data Collection↗
PubMed1999

Thin diblock copolymer films on patterned surfaces: computer simulations and the Frenkel-Kontorowa model.

We study by direct numerical integration of the dynamical evolution equation the equilibrium configuration of a diblock copolymer thin film melt on a patterned surface. The surface has a large number of stripes and the mismatch between the bulk diblock spacing and the stripe width is small. We investigate primarily the formation of small discommensurations in the incommensurate phase and compare the results with the predictions of an analogous model of solid-state physics, the Frenkel-Kontorowa model.

Journal Article↗
PubMed1999

Compression of a polymer chain by a small obstacle: the effect of fluctuations on the escape transition.

We describe the escape transition of an ideal chain compressed between finite-sized obstacles. Three different theoretical methods are used and each provides a similar description of the escape transition, as predicted by earlier and less detailed mean-field theories. The first two methods show that thermal fluctuations near the transition can blur what was previously described as a sharp transition. The last method is an exact calculation of the partition function that shows unambiguously the character of the escape transition. This exact calculation overcomes the inherent uncertainties associated with previous theory and computer simulation.

Journal Article↗
PubMed1999

Race, socioeconomic status, and health. The added effects of racism and discrimination.

Higher disease rates for blacks (or African Americans) compared to whites are pervasive and persistent over time, with the racial gap in mortality widening in recent years for multiple causes of death. Other racial/ethnic minority populations also have elevated disease risk for some health conditions. This paper considers the complex ways in which race and socioeconomic status (SES) combine to affect health. SES accounts for much of the observed racial disparities in health. Nonetheless, racial differences often persist even at "equivalent" levels of SES. Racism is an added burden for nondominant populations. Individual and institutional discrimination, along with the stigma of inferiority, can adversely affect health by restricting socioeconomic opportunities and mobility. Racism can also directly affect health in multiple ways. Residence in poor neighborhoods, racial bias in medical care, the stress of experiences of discrimination and the acceptance of the societal stigma of inferiority can have deleterious consequences for health.

Black or African American↗
PubMed1999

IncC of broad-host-range plasmid RK2 modulates KorB transcriptional repressor activity In vivo and operator binding in vitro.

The korAB operon of broad-host-range plasmid RK2 encodes five genes, two of which, incC and korB, belong to the parA and parB families, respectively, of genome partitioning functions. Both korB and a third gene, korA, are responsible for coordinate regulation of operons encoding replication, transfer, and stable inheritance functions. Overexpression of incC alone caused rapid displacement of RK2. Using two different reporter systems, we show that incC modulates the action of KorB. Using promoter fusions to the reporter gene xylE, we show that incC potentiates the repression of transcription by korB. This modulation of korB activity was only observed with incC1, which encodes the full-length IncC (364 amino acids [aa]), whereas no effect was observed with incC2, which encodes a polypeptide of 259 aa that lacks the N-terminal 105 aa. Using bacterial extracts with IncC1 and IncC2 or IncC1 purified through the use of a His6 tail and Ni-agarose chromatography, we showed that IncC1 potentiates the binding of KorB to DNA at representative KorB operators. The ability of IncC to stabilize KorB-DNA complexes suggests that these two proteins work together in the global regulation of many operons on the IncP-1 genomes, as well in plasmid partitioning.

Bacterial Proteins↗