Biomedical subjects

David R Williams

Publications and source records attributed to David R Williams.

97 records · Page 6Linked to original sources

PubMedDate not supplied

Upstream or downstream. Determinants of consumer willingness to recommend an HMO.

Understanding the attributes that explain an HMO members' willingness to recommend a health plan is considered by many to be critical in a competitive managed care market. The study reported in this article examines the relationship between provider panel composition on overall willingness to recommend a health plan. Our results indicate that a strong association exists between an HMO member's available choice of primary care physician and recommendation of their primary care physician with overall HMO members' recommendation of the health plan. Interestingly, a member's recommendations of hospitals and specialists did not influence HMO member's willingness to recommend a health plan.

Female↗

PubMedDate not supplied

African American girls' smoking habits and day-to-day experiences with racial discrimination.

BACKGROUND: Although it is recognized that African Americans experience racial discrimination, relatively little research has explored the health implications of this experience. Few studies have examined the relationship between racial discrimination and specific health risks. OBJECTIVES: To examine the relationship between smoking habits and perceptions of racial discrimination among African American adolescent girls and to identify and test potential psychological mechanisms through which racial discrimination may operate to increase smoking among this group. METHODS: A sample of 105 African American adolescent girls (mean age 15.45 years) derived from a larger cross-sectional research project comprised the sample. Univariate analyses were conducted to provide descriptive data on the participants of the study, including information about their use of licit and illicit substances. Bivariate correlational analyses were conducted to evaluate the relationship between perceptions of discrimination and smoking habits. The ability of stress to mediate the relationship between discrimination and smoking was examined by using standard analytical procedures for testing mediation models as outlined by Baron and Kenny (1986). RESULTS: The sample (93%) reported experiencing discrimination and racial discrimination was highly correlated with cigarette smoking (r =.35, p >.001). Removing the effects of stress significantly reduced the relationship between racial discrimination and smoking (r =.17, p <.05), indicating that racial discrimination is related to smoking because of its stressful nature. CONCLUSION: Perceptions of racial discrimination are related to the smoking habits of African American adolescent girls.

Adolescent↗

PubMedDate not supplied

Socioeconomic status and health: the role of sleep.

OBJECTIVE: Examine the role of sleep in the relationship between socioeconomic status (SES) and health. METHOD: Self-reported measures of income and education, sleep quantity and quality, and mental and physical health were obtained in a community sample of 1139 adults. RESULTS: More education was associated with higher income (p <.001), and higher income was associated with better physical health (p <.001) and psychological outcomes (p <.001). The effects of income on both mental and physical health were mediated by sleep quality (p values <.01), and sleep quantity was related to both measures of health (p values <.01) but to neither index of SES. CONCLUSION: Sleep quality may play a mediating role in translating SES into mental and physical well-being, and income seems to mediate the effect of education on sleep and, in turn, health.

Adolescent↗

PubMedDate not supplied

Social sources of racial disparities in health.

Racial disparities in mortality over time reflect divergent pathways to the current large racial disparities in health. The residential concentration of African Americans is high and distinctive, and the related inequities in neighborhood environments, socioeconomic circumstances, and medical care are important factors in initiating and maintaining racial disparities in health. Efforts are needed to identify and maximize health-enhancing resources that may reduce some of the negative effects of psychosocial factors on health. Health and health disparities are embedded in larger historical, geographic, sociocultural, economic, and political contexts. Changes in a broad range of public policies are likely to be central to effectively addressing racial disparities.

Black or African American↗

PubMedDate not supplied

Effect of beam size on the expected benefit of customized laser refractive surgery.

PURPOSE: Customized laser surgery attempts to correct higher order aberrations, as well as defocus and astigmatism. The success of such a procedure depends on using a laser beam that is small enough to produce fine ablation profiles needed to correct higher order aberrations. METHODS: Wave aberrations were obtained from a population of 109 normal eyes and 4 keratoconic eyes using a Shack-Hartmann wavefront sensor. We considered a theoretical customized ablation in each eye, performed with beams of 0.5, 1.0, 1.5, and 2.0 mm in diameter. We then calculated the residual aberrations remaining in the eye for the different beam sizes. Retinal image quality was estimated by means of the modulation transfer function (MTF), computed from the residual aberrations. Fourier analysis was used to study spatial filtering of each beam size. RESULTS: The laser beam acts like a spatial filter, smoothing the finest features in the ablation profile. The quality of the correction declines steadily when the beam size increases. A beam of 2 mm is capable of correcting defocus and astigmatism. Beam diameters of 1 mm or less may effectively correct aberrations up to fifth order. CONCLUSION: Large diameter laser beams decrease the ability to correct higher order aberrations. A top-hat laser beam of 1 mm (Gaussian with FWHM of 0.76 mm) is small enough to produce a customized ablation for typical human eyes.

Adult↗

PubMedDate not supplied

Adaptive optics for vision: the eye's adaptation to point spread function.

PURPOSE: Despite the fact that ocular aberrations blur retinal images, our subjective impression of the visual world is sharp, which suggests that the visual system compensates for subjective influence. If the brain adjusts for specific aberrations of the eye, vision should be clearest when looking through a subject's typical wave aberration rather than through an unfamiliar one. We used adaptive optics techniques to control the eye's aberrations in order to evaluate this hypothesis. METHODS: We used adaptive optics to produce point spread functions (PSFs) that were rotated versions of the eye's typical PSF by angles in 45 degrees intervals. Five normal subjects were asked to view a stimulus with their own PSF or with a rotated version, and to adjust the magnitude of the aberrations in the rotated case to match the subjective blur of the stimulus to that seen when the wave aberration was in typical orientation. RESULTS: The magnitude of the rotated wave aberration required to match the blur with the typical wave aberration was 20% to 40% less, indicating that subjective blur for the stimulus increased significantly when the PSF was rotated. CONCLUSION: These results support the hypothesis that the neural visual system is adapted to an eye's aberrations and has important implications for correcting higher order aberrations with customized refractive surgery or contact lenses. The full visual benefit of optimizing optical correction requires that the nervous system compensate for the new correction.

Diagnostic Techniques, Ophthalmological↗

PubMedDate not supplied

Vision improvement by correcting higher-order aberrations with phase plates in normal eyes.

PURPOSE: To psychophysically demonstrate vision improvement when correcting higher-order aberrations with phase plates in normal eyes. METHODS: The wavefront aberrations of three nonsurgical normal eyes were measured with a Shack-Hartmann wavefront sensor. With these measured aberrations, phase plates were fabricated using a lathing technique. Theoretical improvement in retinal image quality was estimated by calculating the optical modulation transfer functions under the white light condition. Visual acuity measurements were also conducted to demonstrate improvement in visual performance after correcting higher-order aberrations with the phase plate. In this visual acuity measurement, a tumbling "E" with high (100%) and low (10%) contrast was used. RESULTS: The phase plate reduced the higher-order root mean square (RMS) wavefront error from 0.39 +/- 0.09 to 0.15 +/- 0.02 microm (mean +/- standard deviation from three eyes) for a 6-mm pupil. With the phase plate, retinal image quality based on the volume of modulation transfer function under 60 cycles per degree (c/deg) was improved by a factor of 1.8 +/- 0.4 over that of the eyes with spherocylindrical correction only. Average improvement in visual acuity achieved by correcting the higher-order aberration was 0.23 lines with high-contrast letters and 1.12 lines with low-contrast letters. All subjects reported subjective improvement in image quality of the letter with the phase plate. CONCLUSION: The phase plate effectively corrected the higher-order aberrations in normal eyes. As a result, both retinal image quality and visual acuity especially with the low-contrast letters were improved. This study demonstrated the feasibility of correcting higher-order aberrations and improving vision with customized optics.

Adult↗