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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
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↗
PubMed1980

Accommodation, convergence, and distance perception in low illumination.

The role of oculomotor adjustments for distance perception in dim light was investigated while preserving the natural interactions between accommodation and convergence. The results show that perceived distance in the dark is biased toward an intermediate value that is correlated with the vergence adjustment assumed in darkness but unrelated to the dark focus of accommodation. A subsequent experimient showed that short-term adaptation to base-out prisms and negative lenses of corresponding strength affects perceived distance and dark convergence but has no influence on the dark focus of accommodation. These findings were interpreted as indicating that convergence is a more important distance cue than accommodation under low illumination and that the physiological resting states of convergence and accommodation are relatively independent.

Accommodation, Ocular↗
PubMed1980

The effectiveness of a retinoscope beam as an accommodative stimulus.

Two experiments with adult subjects and one with infant subjects showed that the beam of a retinoscope, when viewed monocularly in a dark surround, does not stimulate accommodation. In this situation, the eye assumes an intermediate focus that is correlated with the individual's intermediate dark focus or resting state of accommodation. These results provide further evidence that near retinoscopy is a useful method of refraction which controls accommodation by minimizing effective stimulation. The technique is particularly valuable for refracting infants or young children who would otherwise require cycloplegia, and it may be an effective clinical method for the correction of night myopia and related anomalies in adults.

Accommodation, Ocular↗
PubMed1979

Monkeys show an oblique effect.

Monkeys aligned a cursor bar with high-contrast square-wave gratings presented in a variety of orientations. Alignment time increased with increasing spatial frequency from 6 to 24 cycles deg-1 regardless of the orientation of the grating. At higher spatial frequencies, alignment tasks took longer for obliquely oriented gratings than for horizontal and vertical ones. Reducing grating contrast by blurring the image of the 24 cycle deg-1 grating also produced longer alignment times for the obliques. These data indicate that monkeys have an oblique effect similar to that found in humans, implying that the monkey is a useful animal model for investigating the development of meridional anisotropies.

Animals↗
PubMed1979

Evidence for separable binocular processes differentially affected by artificially induced anisometropia.

Two measurable effects are produced by viewing a large disk rotating about the line of sight: a perceived displacement of the apparent horizon (tilt) and cyclotorsion of the eye (torsion). Previous results in our laboratory (see Refs.) using dichoptic presentation techniques show that both effects demonstrate binocular enhancement in normal subjects. Further, stereoblind subjects maintain enhancement for the oculomotor effect, torsion, but not for the perceptual effect, tilt. In this study we have found that this stereoblind pattern of results can be produced optically in normals by introducing a three diopter myopia in one eye. This renders the subject stereoblind and causes suppression of the artificially myopic eye. In this condition torsion still shows enhancement; tilt does not. This procedure may be useful in isolating causes of stereoblindness and in locating different sites of binocular interaction.

Adult↗
PubMed1979

Correction of empty-field myopia on the basis of the dark-focus of accommodation.

The influence of negative spherical corrections on the detection of a small increment flash presented in a uniform field or Ganzfeld were determined. The range investigated included each observer's normal correction as well as values based on the focus assumed by each observer in total darkness (dark-focus of accomodation). A correction equivalent to the dark-focus resulted in maximum sensitivity. The results suggest a convenient technique for the determination of optimal correction for empty-field viewing conditions.

Accommodation, Ocular↗
PubMed1979

The Mandelbaum effect: evidence for an accommodative bias toward intermediate viewing distances.

Previous research has shown that ocular accommodation tends to correspond to an intermediate distance, the dark focus, in the absence of effective stimulation. The present experiments measured accommodative responses in the presence of two adequate, monocular stimuli superimposed optically at different distances. In Experiment I, observers attempted to maintain a matrix of letters in clear focus as a superimposed mesh screen was varied in distance. In experiment II, observers were instructed to focus the "easier" of two similar grating patterns that were presented over a range of distances with a constant separation of two diopters. The results of both experiments showed an accomodative response bias toward target distances near the observers' dark focus of accommodation. The implications of these findings for the theoretical resting state of accomodation and for practical problems of visual performance are discussed.

Accommodation, Ocular↗