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Biomedical subjects

D A Owens

Publications and source records attributed to D A Owens.

At least 19 recordsLinked to original sources
PubMed2000

Stair negotiation in older people: a review.

Locomotion on stairs is among the most challenging and hazardous activities of daily living for older individuals. This is evidenced by the reports that stair falls account for more than 10% of fatal fall accidents. The demands that stairs place on the musculoskeletal and cardiovascular systems are compounded by the need for input from the somatosensory, visual, and vestibular systems at various stages in the task. Many of these collaborating systems deteriorate with aging, thus increasing the difficulty and risk of failure in a task that inherently involves exposure to significant danger. The task itself varies in its degree of challenge depending on many structural and environmental factors that are outside the control of the stair user. This review explores existing literature in the area of stair negotiation by older persons in an effort to define the key factors associated with difficulty and safety on stairs and to interpret these findings in the light of possible interventions that could increase stair safety. Further research is needed to ensure that current recommendations for stair design are suitable for older stair users. A Glossary of Terms is also provided.

Accidental Falls↗
PubMed1999

Predicting optimal accommodative performance from measures of the dark focus of accommodation.

Leibowitz and his colleagues found that accommodation rests at an intermediate distance that shows wide interindividual variation. They proposed that this intermediate dark focus is useful for correcting anomalous refractive errors, but this proposal was later questioned when different measurement techniques yielded discrepant dark focus values. The present study measured dark focus under two levels of visual attentiveness: (a) when performing an open-loop, active viewing task (aDF); and (b) when looking passively into darkness (pDF). These dark focus measures were then compared with an optimal accommodation distance that was derived from accommodative response functions in bright and dim luminance. The aDF measures were found to be more myopic (nearer) than the pDF measures and highly correlated with the optical accommodation distance. No significant relationship was found between pDF and optical accommodation distance. These findings confirm that measures of dark focus are affected by nonoptical aspects of the measurement technique; they also suggest that techniques that demand visual attention (aDF) yield dark focus values that are more useful for optimizing accommodation and potentially reducing fatigue in difficult situations.

Accommodation, Ocular↗
PubMed1998

The assured clear distance ahead rule: implications for nighttime traffic safety and the law.

The assured clear distance ahead (ACDA) rule holds the operator of a motor vehicle responsible to avoid collision with any obstacle that might appear in the vehicle's path. Although widely considered a fundamental responsibility of safe driving, the ACDA rule is routinely violated by most drivers under nighttime conditions. This fact does not necessarily imply reckless disregard for public safety, however. Rather the troublesome discrepancy between drivers' behavior and the principles of safe driving may be understood in terms of vehicle operators' lack of awareness of selective losses of visual function that occur in low light. This theoretical perspective holds basic implications both for traffic regulations and for assessment of liability in nighttime road accidents.

Accidents, Traffic↗
PubMed1998

Dark focus values measured by retinoscopy: a clinical chart review.

The present study investigated the utility of dark retinoscopy as a technique to measure the dark focus of accommodation during routine clinical examinations. Using data from clinical examinations collected during a period of 6 months, a clinical chart review quantified dark focus and mean spherical equivalent refractive error for 480 patients ranging in age from 6 to 55 years. Results showed significant variations in mean dark focus values over the age groups. All groups showed wide interpatient differences, with mean dark focus values that were significantly nearer than the farpoint. Individual dark focus values ranged from 2.25 to -1.00 D (hyperopic), with an overall mean of 0.53 D. Potential clinical applications of dark focus-based correction are discussed.

Accommodation, Ocular↗
PubMed1997

How do the elderly negotiate stairs?

Stair navigation, particularly stair descent, is an extremely challenging and dangerous locomotor task, yet studies suggest that most elderly are unlikely to move to new residences in order to avoid this challenge. The knee and ankle are the key joints where adequate strength and power are required for safe stair descent, and it is not yet clear if sarcopenia in the elderly is likely to result in residual strength below that which is required for successful stair performance. Sensory cues are also critical, and the lack of literature on the specific roles of the various intrinsic and extrinsic factors that affect stair navigation is a clear indication of the need for such research in order to define safer strategies and optimal conditions for elderly individuals to transit between living areas of differing levels.

Aged↗
PubMed1996

Differentiation of visibility and alcohol as contributors to twilight road fatalities.

Elevated road fatality rates at night involve multiple variables, each of which contributes to an unknown degree. In order to determine the relative contributions of reduced visibility and drivers' consumption of alcohol, we performed two quasi-experiments analyzing data recorded by the U.S. Fatal Accident Reporting System from 1980 through 1990. The results confirmed that both variables play a major role in nighttime road fatalities and revealed new evidence that they contribute differentially to two classes of fatal collisions: Degraded visibility in low illumination is associated primarily with collisions involving pedestrians and pedalcyclists, whereas drivers' consumption of alcohol plays a larger role in other fatal collisions.

Accidents, Traffic↗
PubMed1992

Lateral phoria at distance: contributions of accommodation.

Lateral heterophoria at distance often is cited as an index of tonic vergence, yet recent research has shown that vergence in darkness is more convergent than most measures of phoria. Two experiments were conducted to investigate the possible role of accommodation in this discrepancy. Experiment I compared measures of distance phoria of 19 young adults with measures of vergence and accommodation of the same subjects. Dark vergence was found to be correlated with but significantly more convergent than distance phoria. Hierarchical regression analysis showed that individual measures of phoria were related to subjects' negative accommodation (from the dark focus to the distant target) as well as to dark vergence. Experiment II tested 13 young adults to evaluate three simple models of the influence of accommodation on distance phoria. Distance phoria was predicted best by a model that included the subjects' negative accommodation, accommodative vergence, and dark vergence. These findings support the hypothesis that distance phoria is influenced by accommodation for the fixation target. Dark vergence is a simpler index of tonic vergence.

Accommodation, Ocular↗
PubMed1989

Vertical gaze direction and the resting posture of the eyes.

With horizontal gaze, the resting posture of binocular vergence typically corresponds to a distance of about 1 m. The effect of vertical direction of gaze on this basic resting posture was investigated. The dark vergence of twenty-four subjects was measured while they fixated a dim monocular light-point at vertical directions ranging from -45 degree (lowered) to +30 degrees (elevated). In one condition, gaze was varied by changes in eye position with the head held upright; in a second condition, gaze was varied by changes in head inclination with the eyes held in constant (horizontal) position with respect to the head. In both conditions, dark vergence shifted in the convergent (nearer) direction with lowered gaze and in the divergent (farther) direction with elevated gaze. The effect of varied eye inclination was larger, more variable across subjects, and more stable over time than that of varied head inclination. These findings indicate that multiple mechanisms contribute to gaze-related variations of the resting posture of the eyes. They may help to explain the variations of space perception and visual fatigue that are observed with different gaze inclinations.

Eye Movements↗
PubMed1987

Near work, visual fatigue, and variations of oculomotor tonus.

One hour of near work produced adaptive changes in the resting states of accommodation and binocular vergence, which may be responsible for different aspects of visual fatigue. Two groups of college students read from either hard copy or a video display terminal (VDT). Immediately before and after reading, the subjects' distance acuity, dark vergence, dark focus, and accommodative response accuracy for a monocular stimulus were measured. After reading, subjects also rated their subjective feelings of visual fatigue. Reading produced significant changes in both accommodation and vergence, which did not differ for the hard copy and VDT modes of presentation. Dark focus and accommodative responses shifted in the myopic direction by an average of 0.6 D and at least 0.35 D, respectively; dark vergence distance shifted in the convergent direction by an average of 11.4 cm. These changes were greatest for subjects whose initial resting postures corresponded to a far distance. After reading, one third of the subjects exhibited lower visual acuity at distance. This change was significantly correlated with changes in dark focus (r = 0.35) but not with changes in dark vergence (r = -0.12). In contrast, subjective ratings of visual fatigue were not correlated with changes in the dark focus (rho = 0.13), but they were significantly correlated with changes in dark vergence (rho = 0.58). These findings indicate that ordinary near work can produce significant changes in the resting states of accommodation and vergence, whose magnitude depend on the subject's initial oculomotor resting tonus.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1987

Total protection from hypoxic liver damage by fructose.

The purpose of these experiments was to evaluate the effect of carbohydrates on hypoxic cell death in the perfused rat liver. Lactate dehydrogenase (LDH) was released maximally at rates up to 300 U/g/h after 40 to 60 min of hypoxia in control livers from fasted rats or in livers perfused with glucose (20 mM). Fructose (20 mM) prevented the release of LDH completely. Rates of anaerobic glycolysis indexed by release of lactate were around 150 mumol/g/h in livers perfused with fructose but were undetectable in livers perfused with glucose. The results demonstrate that fructose prevents hypoxic damage in the liver presumably by providing glycolytic ATP in the absence of oxygen.

Anaerobiosis↗
PubMed1985

Does performance of tasks affect the resting focus of accommodation?

Measurements of the resting focus of accommodation were obtained with an infrared optometer while subjects engaged in four tasks: (1) passive viewing in darkness; (2) passive viewing of a laser speckle pattern; (3) active judgment of motion in a laser speckle pattern; and (4) mental arithmetic in darkness. The results showed resting focus measures to be comparable under all four conditions for some subjects. However, other subjects exhibited consistent increases or decreases in the resting focus position when asked to judge the direction of speckles within the laser test pattern or perform mental arithmetic. This finding suggests that "effort to see" may influence the resting focus of some subjects. Implications for measurement procedures and population estimates of the resting focus are discussed.

Accommodation, Ocular↗
PubMed1985

Accommodation for flickering stimuli.

A laser optometer was used to measure accommodative responses of three observers for sinusoidal gratings presented in Maxwellian view at optical distances ranging from 0 to 4 diopters. Contrast of the stimuli was modulated spatially at 1.0, 4.2 and 6.5 cycles deg.-1 (cpd), and temporally at six frequencies ranging from 3.0 to 40 Hz. Accommodation was consistently more accurate for the 4.2 cpd than for either the 1.0 or 6.5 cpd gratings. Furthermore, accommodative responsiveness for the 4.2 cpd was not affected by temporal modulation, while that for the other spatial frequencies improved monotonically as a function of temporal frequency. These results reinforce earlier reports that accommodation is most responsive for contrast of intermediate spatial frequencies and they indicate that stimulus flicker generally degrades accommodation for spatial contrast.

Accommodation, Ocular↗
PubMed1983

The accuracy of binocular vergence for peripheral stimuli.

Two experiments were conducted to investigate the roles of dark vergence and retinal eccentricity as factors influencing binocular vergence responses. A nonius alignment technique was used to measure vergence responses in total darkness (dark vergence) and for dim binocular stimuli presented at retinal eccentricities ranging from 2 degrees to 8 degrees over distances ranging from 28.5 to 342 cm. The results indicate that vergence was progressively less accurate with stimuli at increasing retinal eccentricities, and that errors of fixation were biased toward the individual's dark vergence position. The hypothesis that dark vergence represents the functional resting state of the vergence system which influences oculomotor fusional responses under a variety of conditions is discussed.

Adult↗
PubMed1982

Antiallergics: 3-(1H-tetrazol-5-yl)-4H-pyrimido[2,1-b]benzothiazol-4-ones.

A short series of the title compounds was prepared and evaluated for both antiallergic and bronchodilator activity. Members of the series exhibit good oral activity in the rat PCA test, the most potent being the parent compound, 3-(1H-tetrazol-5-yl)-4H-pyrimido[2,1-b]benzothiazol-4-one, and its 8-chloro derivative. The latter two compounds are considerably more potent than either disodium chromoglycate or theophylline as antiallergic agents and also show significant bronchodilator activity.

Animals↗
PubMed1981

Is accommodation colorblind? Focusing chromatic contours.

Two adjacent regions define an edge if they differ in either color or luminance. If the difference is purely chromatic, the edge is said to be isoluminant. Isoluminant contours are often perceptually unstable. Perhaps some of this instability could be explained if isoluminant contours were difficult to bring into focus. To test this hypothesis, a vernier optometer was used to measure the accuracy of steady-state accommodation for the vertical boundary of a red-green bipartite field. This edge was presented at optical distances of 0, 1.5, 3.0, and 4.5 diopters, with brightness contrasts between the two hemifields of 0% (isoluminant), 15%, 58%, and 100%. Accommodation was essentially unresponsiveness to the isoluminant edge and exhibited increasing focusing accuracy with increased brightness contrast. Control experiments replicated this finding for red-orange, green-blue, and white-white fields. These results imply that luminance contrast is a necessary stimulus for monocular accommodation. Inappropriate accommodation may be a factor contributing to the perceptual instability of isoluminant patterns.

Accommodation, Ocular↗
PubMed1981

Effect of maximal versus submaximal expiratory effort on spirometric values.

Data from flow volume curves suggest that flow measured at the mouth may be lower with maximal effort than with submaximal effort. We consequently studied the effect of maximal vs. submaximal expiratory effort on spirometric values. Significant increases in FVC, FEV1 or FEF25-75% with submaximal effort occurred in 12 of 38 subjects. All subjects who improved spirometric values had peak flows with submaximal efforts that varied from 91 to 65% of peak flow with maximal efforts. We conclude that spirometric values in some subjects are larger with submaximal rather than maximal effort.

Adult↗