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Biomedical subjects

W R Bobier

Publications and source records attributed to W R Bobier.

At least 19 recordsLinked to original sources
PubMed2004

Gain and movement time of convergence-accommodation in preschool children.

BACKGROUND: Convergence-accommodation is the synkinetic change in accommodation driven by vergence. A few studies have investigated the static and dynamic properties of this cross-link in adults but little is known about convergence-accommodation in children. The purpose of this study was to develop a technique for measuring convergence-accommodation and to study its dynamics (gain and movement time) in a sample of pre-school children. METHOD: Convergence-accommodation measures were examined on thiry-seven normal pre-school children (mean age = 4.0 +/- 1.31 yrs). Stimulus CA/C (sCA/C) ratios and movement time measures of convergence-accommodation were assessed using a photorefractor while subjects viewed a DOG target. Repeated measures were obtained on eight normal adults (mean age = 23 +/- 0.2 yrs). RESULTS: The mean sCA/C ratios and movement times were not significantly different between adults and children (0.10 D/Delta [0.61 D/M.A.], 743 +/- 70 ms and 0.11 D/Delta [0.50 D/M.A.], 787 +/- 216 ms). Repeated measures on adults showed a non-significant mean difference of 0.001 D/Delta. CONCLUSION: The results suggest that the possible differences in crystalline lens (plant) characteristics between children and adults do not appear to influence convergence-accommodation gain or duration.

Accommodation, Ocular↗
PubMed2003

The manifestation of noncycloplegic refractive state in pre-school children is dependent on autorefractor design.

PURPOSE: To investigate the factors that govern the manifestation of hyperopic refractive errors of pre-school children when tested with and without the application of cycloplegics. METHODS: Forty-three pre-school children (mean age, 3.68 +/- 0.59 years) were tested before and during cycloplegia in the following order: retinoscopy with optical fogging; Retinomax K plus; Welch Allyn SureSight (DAV SureSight), and PowerRefractor. In the case of the PowerRefractor, the children viewed a difference of Gaussian target (0.20 cpd) at 3.5 m in addition to viewing the instrument LED sources. RESULTS: Instruments with close working distances (Retinomax) showed the greatest underestimation of hyperopia and the largest variation, followed by the instruments having a greater working distance (PowerRefractor LED view and retinoscopy). The addition of a far target (PowerRefractor difference of Gaussian view) showed the least underestimation, whereas DAV SureSight showed a mean overestimation of the refractive state. CONCLUSION: When autorefractors are applied to vision screening of pre-school children without the use of cycloplegics, autorefractor designs must be developed that both stabilize and relax the child's accommodation. Our results suggest that designs should include large working distances and distant fixation targets.

Accommodation, Ocular↗
PubMed2000

Prism induced accommodation in infants 3 to 6 months of age.

Convergence-accommodation, one of several cross-linkages in the oculomotor system is manifested by opening the accommodative feedback loop and increasing the vergence input. We elicited this response in human infants aged 3-6 months by placing a 15 delta prism (base-out) before one eye while they viewed a diffuse patch of light. Accommodation was measured and ocular alignment was confirmed with a video photorefractor. The convergence-accommodation response is therefore present during a time when blur driven accommodation and disparity vergence are maturing. The gain of convergence-accommodation (expressed as the stimulus CA/C ratio) appeared to be greater for infants than adults.

Accommodation, Ocular↗
PubMed1999

Measurement of the validity of a preschool vision screening program.

OBJECTIVES: The validity (sensitivity and specificity) of a preschool vision screening program was measured over a 3-year period to determine how well strabismus and significant refractive errors could be detected. METHODS: Public health nurses were trained to administer tests of visual acuity, stereoacuity, and ocular alignment. Failure on any test, visual acuity of 6/9 or less, stereoacuity of less than 100 seconds of arc, or an apparent misalignment of the eyes resulted in referral to an eye care practitioner. An age-matched control was also referred. Analysis of practitioner reports used predefined study-based criteria for ocular abnormalities. RESULTS: More than 1100 children were screened each year. The annually calculated prevalence of vision problems ranged between 10.5% and 13.8%. The estimated sensitivity varied from 60.4% to 70.9% (specificity, 69.6% to 79.9%). The yield indicated that a very high percentage of children with vision problems were identified for the first time. CONCLUSIONS: The validity of this screening is comparable to that of other school screenings. The limitations are predictable. Consideration should be given to replacing visual acuity tests with a rapid, objective measure of refractive error and ocular alignment.

Age Factors↗
PubMed1998

An infrared eccentric photo-optometer.

An objective infrared optometer has been designed, based on the optical principles of eccentric photorefraction. A CCD camera with an eccentric infrared light source images the subject's pupil through a Badal optometer. The slope of the light distribution across the pupil is continuously recorded. Accommodative state is measured by moving the camera behind the Badal lens until the slope is zero. This position corresponds to the case where the camera is conjugate with the retina of the observer. In this Badal optometer, the irradiance of light at the pupil plane, the sensitivity of the photorefractor, and the focal setting of the camera lens remain constant for all positions of the camera from the eye. The repeatability of a single measure of refractive state in a cyclopleged eye was less than 0.05 D. Static accommodative responses taken from 3 subjects in both closed and open loop conditions provided expected stimulus/response measures. The instrument can also be adapted to measure dynamic accommodation.

Accommodation, Ocular↗
PubMed1997

Slope-based eccentric photorefraction: theoretical analysis of different light source configurations and effects of ocular aberrations.

A geometrical-optical technique is used to predict the changes in the slope of the eccentric-photorefraction intensity profiles as a function of refractive state. We investigate how the intensity profiles vary with refractive state for different light source configurations and monochromatic aberrations in the eye. The best possible light source configuration extends from zero eccentricity (to increase sensitivity and reduce the dead zone) to a high eccentricity (to increase the working range). An advantage of using the extended light source is that the intensity profile of the eccentric-photorefraction reflex is more linear for extended sources than for point light sources. It is also shown that the change in slope with refractive state is dependent on pupil size. Furthermore, when asymmetric aberrations are present, the change in intensity profile slope with refractive state is dependent on the circumferential position of the light source, but this dependence can be resolved by averaging slope values obtained by using two sources placed on opposite sides of the pupil. The importance of this study to existing eccentric-photorefractor designs is discussed, and recommendations for improved eccentric photorefractors are suggested.

Color Vision Defects↗
PubMed1996

Accuracy and precision of the Tomey ViVA infrared photorefractor.

PURPOSE: The Fortune Optical (Tomey ViVA) VRB-100 video refractor was tested to determine its accuracy and precision in measuring manifest refractions of human eyes with and without cycloplegia. The specific issues addressed included its accuracy in measuring spherical and cylindrical refractive errors and its precision in refracting near emmetropia. METHODS: To determine its ability to measure moderate to high (> 4.00 D) myopia, we compared the VIVA's refractions to those taken by a Canon Autorefractor R1 and a retinoscopist. A spherical lens series from -7.00 to + 7.00 D at 1.00 D intervals, or -5.00 to + 5.00 at 0.50 D intervals, was placed over a subject's eye, which was then covered by an infrared (IR) filter, refracted, and analyzed to determine the VIVA's ability to measure spheres. Subjects with refractive errors of -2.00 to + 2.00 DS (diopters sphere) and 0 to 1.00 DC (diopters cylinder) were refracted 7 to 15 times during 1 sitting to determine the VIVA's precision. The instrument's accuracy in measuring cylinders was tested by placing + 3.00 to -3.00 D cylinders (at 0.50 D intervals) over the eyes of subjects at 0 degree, 45 degrees, 60 degrees, 75 degrees, 90 degrees, 105 degrees, 120 degrees, and 135 degrees. RESULTS: The VIVA measured spheres of +/- 3.00 D with a root mean squared (rms) error of 0.5 +/- 0.1 D. Beyond this power, its accuracy progressively worsened. In some subjects, irregular intensity profiles compromised the VIVA's accuracy even with low spherical refractive errors. The VIVA was very precise in measuring spheres from + 2.00 to -2.00 D and cylinders from 0 to 1.00 D. Although the ViVA adequately measured all cylinder powers at 0 degree and 90 degrees, the accuracy of cylindrical power measurement decreased with obliquity; only cylinders < or = 1.00 D magnitude were accurately measured at 45 degrees and 135 degrees. CONCLUSIONS: We conclude that, although the ViVA offers many attractive features for vision screening, it is seriously limited by its inability to property detect and measure oblique astigmatic errors.

Humans↗
PubMed1996

Geometrical technique to determine the influence of monochromatic aberrations on retinoscopy.

A geometrical-optical analysis is developed to predict the reflex observed in retinoscopy. The analysis can be expanded to explain the reflex for an eye with aberrations. The succession of reflexes across the pupil for each position of the retinoscope is represented in a contour plot. The plots demonstrate that retinoscopy can be considered a measure of the transverse ray aberration of the eye. For an eye with simple defocus this causes the typical with and against motions observed with hyperopic and myopic refractive errors. For an eye with aberrations we predict more-complex retinoscopic reflexes. This theory is confirmed by actual measurements on a human eye with known aberrations.

Color Perception↗
PubMed1996

Gain changes in the accommodative convergence cross-link.

The output of the accommodative vergence cross-link, the AC/A ratio, was measured before and after subjects viewed through a telestereoscope which increased their interocular separation (pd) approximately fourfold. The initial paradigm was designed to force an increase in the AC/A ratio in that subjects alternately fixated targets set at differing distances. In this paradigm, the varying vergence demands could not be met by a constant increase in phasic or tonic vergence responses. In a second paradigm, only one target was viewed at a fixed position from the eye. Now the constant vergence demand could be met by a set increase in phasic and/or tonic vergence. A significant increase in the AC/A ratio was found in the two-target study but not in the single-target study. Increases in the accommodative vergence gain did show considerable individual differences. Tonic vergence changes were interpolated from the data. This parameter increased significantly in both cases but more so when two targets were alternately fixated. This finding is consistent with current near triad models which predict that accommodative convergence input acts along with disparity vergence input to increase tonic levels of convergence. When AC/A ratios were calculated using clinical measures of the phoria at 6 and 0.4 m, increases were found in the two-target study and to a lesser degree in the single-target study. This clinical method of AC/A measure appears to have been confounded by changes in tonic vergence.

Accommodation, Ocular↗
PubMed1996

Retinoscopic reflexes: theoretical basis and effects of monochromatic aberrations.

BACKGROUND: Current literature describing retinoscopy does not provide a full description of the pupil reflexes observed in retinoscopy or of the effects of monochromatic aberrations. The intent of the study was to develop a geometrical model of retinoscopy that provides a more complete analysis. The model can take specific aberrations and predict the succession of reflexes observed in retinoscopy for an eye with these aberrations. METHODS: Theoretical predictions were compared with observations using a hand-held streak retinoscope. CCD-based retinoscopy was performed on subjects with known aberrations. RESULTS: Reflexes observed in retinoscopy can be computed by modeling a light source whose eccentricity from the retinoscope sight-hole is varying in position. Results from study calculations show that the succession of reflexes observed in retinoscopy can be thought of as a measure of the transverse aberration of the eye. Both qualitative and quantitative measures showed agreement between theoretical and experimental results. Transverse aberration of the eye can be observed and measured using retinoscopy. CONCLUSIONS: Monochromatic aberrations have significant and predictable effects on retinoscopy. The best way for a retinoscopist to avoid such aberrations and provide a proper refraction is to neutralize the reflex across the largest visual zone possible and ignore the reflex motion at the edges of the pupil.

Humans↗
PubMed1995

Effect of monochromatic aberrations on photorefractive patterns.

Photorefractive methods have become popular in the measurement of refractive and accommodative states of infants and children owing to their photographic nature and rapid speed of measurement. As in the case of any method that measures the refractive state of the human eye, monochromatic aberrations will reduce the accuracy of the measurement. Monochromatic aberrations cannot be as easily predicted or controlled as chromatic aberrations during the measurement, and accordingly they will introduce measurement errors. This study defines the error or uncertainty by extending the existing paraxial optical analyses of coaxial and eccentric photorefraction. This new optical analysis predicts that, for the amounts of spherical aberration (SA) reported for the human eye, there will be a significant degree of measurement uncertainty introduced for all photorefractive methods. The dioptric amount of this uncertainty may exceed the maximum amount of SA present in the eye. The calculated effects on photorefractive measurement of a real eye with a mixture of spherical aberration and coma are shown to be significant. The ability, developed here, to predict photorefractive patterns corresponding to different amounts and types of monochromatic aberration may in the future lead to an extension of photorefractive methods to the dual measurement of refractive states and aberrations of individual eyes.

Accommodation, Ocular↗
PubMed1995

Geometrical theory to predict eccentric photorefraction intensity profiles in the human eye.

In eccentric photorefraction, light returning from the retina of the eye is photographed by a camera focused on the eye's pupil. We use a geometrical model of eccentric photorefraction to generate intensity profiles across the pupil image. The intensity profiles for three different monochromatic aberration functions induced in a single eye are predicted and show good agreement with the measured eccentric photorefraction intensity profiles. A directional reflection from the retina is incorporated into the calculation. Intensity profiles for symmetric and asymmetric aberrations are generated and measured. The latter profile shows a dependency on the source position and the meridian. The magnitude of the effect of thresholding on measured pattern extents is predicted. Monochromatic aberrations in human eyes will cause deviations in the eccentric photorefraction measurements from traditional crescents caused by defocus and may cause misdiagnoses of ametropia or anisometropia. Our results suggest that measuring refraction along the vertical meridian is preferred for screening studies with the eccentric photorefractor.

Computer Simulation↗
PubMed1992

The influence of chromatic aberration on the static accommodative response.

Previous measurements of static accommodation have consistently shown steady state errors over most of the range; the response lags below the stimulus and, at low levels, the response leads the stimulus. A series of experiments is presented in which the longitudinal and, for the first time, transverse chromatic aberrations of the eye were varied and the resultant stimulus-response functions of accommodation were measured. The results show that the steady state error of accommodation is not influenced by manipulations of the magnitude or the direction of either longitudinal or transverse chromatic aberration. This indicates that a particular wavelength is not preferentially focussed on the retina as a function of stimulus level and supports the negative feedback theory of accommodation.

Accommodation, Ocular↗
PubMed1992

Geometrical optical analysis of photorefractive methods.

Photorefractive methods allow rapid measures of the refractive and accommodative state of infants and young children whose brief attention and co-operation limit the use of more traditional methods such as retinoscopy and autorefraction. Three methods have been defined: orthogonal, isotropic and eccentric. We provide a common geometrical optical analysis for these three methods where the photorefractive pattern is defined at the plane of focus of the camera. Since this plane is conjugate with the detector plane of the camera then the critical optical parameters can be defined without reference to the design of the camera by simply determining the relative magnification of the projected image of an object photographed at the camera's plane of focus. Specifically the pattern width CF (mm) over a photorefractor's working range can be defined for the isotropic and orthogonal methods as: [formula: see text] and for the eccentric method as: [formula: see text] where: K is a myopic refractive error of the eye (dioptres) (K Less than 0); P the distance of the flash source to the eye (dioptres); L the distance in front of the eye of the camera's plane of focus (dioptres) (L less than 0); GH the pupil diameter (mm); e the eccentricity of the flash source from the camera aperture (mm); M magnification of the image measured relative to the camera plane of focus.

Accommodation, Ocular↗
PubMed1988

Quantitative photorefraction using an off-center flash source.

When an eye is refracted by "eccentric photorefraction" with a flash source off-centered from a camera lens, a crescent of light is formed in the margin of the pupil. The size of the crescent varies directly with the eye's refractive error. This photographic method has been used in vision screening studies of young children where the appearance of a crescent indicated that the refractive error was above a certain threshold. Usually quantification of the refraction could not be achieved by the photorefractor but relied upon subsequent testing using retinoscopy. My research aimed to expand eccentric photorefraction so as to enable it to provide quantification of the eye's refractive error. This was achieved by varying the eccentricity of the flash source from the camera lens and then calibrating the instrument over a large range of refractive errors. The calibration modified a previously derived optical relation which defined the eye's refractive error in terms of the eccentricity of the source for a given pupil size. Eccentric photorefraction of 26 infants and children aged 7 to 48 months showed a good correlation with retinoscopy (r = 0.82). It is concluded that this method would be complementary to other photorefractive methods (e.g., isotropic) particularly as it is able to measure a large range of refractive errors once the astigmatic meridians of the eye are known.

Calibration↗
PubMed1985

Eccentric photorefraction: optical analysis and empirical measures.

An optical analysis of a photographic technique, "eccentric photorefraction," designed to measure refraction and accommodative states along a single meridian of the eye, is presented. Empirical measures taken from a model eye support the theoretical derivation. The application of the technique for use with human infants is discussed with reference to measurements taken from human eyes.

Humans↗
PubMed1983

Orthoptic treatment of subjects showing slow accommodative responses.

Five subjects showing slow accommodative responses were given orthoptic treatment. Speed of accommodative response improved after 3 to 6 weeks. No regressions were evident 18 weeks after the cessation of training. Slow accommodative responses were found in subjects with normal phorias and fusion limits. The technique of dynamic photorefraction is introduced for the clinical measurement of accommodative time characteristics.

Accommodation, Ocular↗