Showing papers on "Accommodation published in 1994"

Critical subjective measurement of amplitude of accommodation.

Abstract: The use of fixed print size to measure amplitude of accommodation by the push-up method will result in a range of angular sizes of the print at the nearpoint for patients with different amplitudes. We investigated the effect of this on measured amplitude of accommodation in 60 subjects aged

Clinical and laboratory investigations of the relationship of accommodation and convergence function with refractive error. A literature review.

Abstract: Studies of the relationship of clinical and laboratory measures of accommodation and convergence function with refractive error are reviewed. There are inconsistencies in results from study to study presumably due, in part, to methodological differences. However, some basic trends can be outlined. In studies in young adults, accommodation in darkness (dark focus), optical reflex accommodation, and proximally induced accommodation are less in myopes than in emmetropes and hyperopes. It also appears that nearpoint esophoria is associated with higher rates of myopia progression in children. Implications for myopia etiology are discussed.

Chick eyes under cycloplegia compensate for spectacle lenses despite six-hydroxy dopamine treatment.

Abstract: Purpose. To test whether eye growth changes produced by spectacle lens wear are mediated by changes in ciliary muscle tonus in chicks. Methods. Because there is evidence that deprivation myopia is based on a local-retinal mechanism in the eye that probably remains functional after cycloplegia as well as after ciliary ganglion or Edinger-Westphal lesions, none of these treatments provides insight into whether accommodation tonus is also important in the control of axial eye growth. Because 6-hydroxy dopamine (6-OHDA) suppresses deprivation myopia, to isolate growth changes mediated by accommodation the authors injected 6-OHDA and paralyzed accommodation in addition (by corneal application of vecuroniumbromide). To quantify the state of cycloplegia, the abnormal pecking responses of cyclopleged chickens were studied. Results. The authors found that cycloplegia could be maintained for 3 hours daily by corneal application of vecuroniumbromide. To ensure that visual exposure was restricted to the time period of cycloplegia, chickens were transferred to a 3-hour light/21-hour dark cycle. Control experiments showed that emmetropization was still functional under the changed light cycle. Strikingly, even with suppressed local-retinal growth control mechanisms (as indicated by the lack of deprivation myopia in a 6-OHDA injected group of chickens with occluders) and paralysis of accommodation, the eyes compensated for the defocus imposed by spectacles by changing their axial growth rates to be similar to those of eyes with functional accommodation. Conclusions. The findings show that the ciliary muscle and the activity of the iris sphincter muscle are not involved in emmetropization in chicks. If accommodation mediates the growth effects with lenses, it must happen via another pathway. Based on previous results, the authors propose that either the choroidal nerves from the ciliary ganglion to the choroid are important or that another yet unknown pathway from the Edinger Westphal nucleus to the eye transmits the necessary information. Invest Ophthalmol Vis Sci. 1994; 35:3516-3524. J. here is no doubt that vision plays an important role in the fine tuning of axial eye length to the focal length of the growing chicken: Shifting the focal plane by placing low-power spectacles in front of the eye results in rapid changes in axial eye growth such that the imposed refractive errors are largely compensated for within days.

Amplitude of accommodation for different head positions and different directions of eye gaze.

Abstract: One of the theories of the mechanism of presbyopia is the Hess-Gullstrand theory. This theory predicts that amplitude of accommodation should increase in downgaze because of movement of the lens under the influence of gravity, and that this should be more marked for older than for younger subjects. We assessed this theory by measuring the effect of direction of eye gaze and head posture on amplitude of accommodation for two age groups. Farpoints, nearpoints, and amplitude of accommodation were determined for two subject groups, a young group aged 18 to 25 years and an older group aged 35 to 45 years. Small but significant shifts of nearpoints toward the eye were observed when head position or eye gaze was shifted from above to below the horizontal, for the younger observers only (the maximum mean difference between conditions was 1.1 D, compared with a mean accommodation level of 9.8 D for this young group). Previous studies by others found changes in the same direction for eye gaze and head position, respectively, but our changes were much smaller. We do not believe that particular care is needed in selection of head position and eye gaze during clinical measurements of the amplitude of accommodation. Because the shift of the nearpoint was noted only for the younger group, this study does not support the Hess-Gullstrand theory of presbyopia.

Parental History and Myopia: Taking the Long View

Abstract: To the Editor. —I wish to take issue with the conclusions of Dr Zadnick and colleagues 1 that the slightly greater incidence of myopia and long eyes among the school-age children of myopes is evidence for a strong genetic determination of myopia. I would like to suggest how environmental influences might produce such results, to correct a technical error, and to point out a misleading interpretation. Consider that there are two mechanisms for clearing blurred images: accommodation, which rapidly adjusts the optical power of the lens, and emmetropization, which slowly adjusts the length of the eye during growth (shown clearly by animals compensating for myopia or hyperopia imposed by spectacle lenses 2 ). Because the two mechanisms act in parallel, the better accommodation is, the more in focus images would be, and the less the eye would need to elongate and become myopic to clear the images. Conversely, poor accommodation might

The effect of gravity on the amplitude of accommodation.

Abstract: We measured the amplitude of accommodation in the supine and prone positions of young subjects. We found that gravity does not influence accommodative amplitude. These findings support Schachar's zonular function and accommodative hypotheses that the lens is stable and under tension during accommodation.

The effect of knowledge of object distance on accommodation during instrument viewing.

Abstract: We present data in which instrument accommodation was measured while knowledge of object distance was varied. The accommodative feedback loop was ‘semiopen’—an intermediate state between the closed-loop and open-loop conditions of previous experiments. The semi-open-loop situation mimicked the degraded-image conditions which are frequently encountered during instrument viewing. The results show that for some subjects knowledge of object distance is a more powerful cue for instrument accommodation than is the optical distance of the object; however, for the majority of subjects this is not the case. We also found that subjects whose accommodation is influenced by knowledge of object distance tend to have a more proximal dark focus than those whose accommodation is independent of knowledge of object distance. We propose that the Mandelbaum effect, in which involuntary accommodation occurs when a transparency is superimposed between the observer and the object of regard, could account for the accommodative b...

Natural strabismus in monkeys: accommodative errors assessed by photorefraction and their relationship to convergence errors.

Abstract: Purpose To examine accommodation and accommodative convergence relationships in naturally strabismic monkeys. Methods Photorefraction was used to measure accommodative responses of each eye under monocular and binocular viewing conditions. These accommodative results were then compared to assessments of vergence state made under monocular viewing conditions using methods previously described. Results Accommodation was always accurate under monocular viewing conditions, with the exception of the inability of one myopic eye to focus distant targets. The accommodative response in the two eyes was always consensual. In animals with an anisometropia, the fixating eye was focused accurately on the target, and the fellow eye was in error by an amount predicted by the magnitude of the anisometropia. Some animals alternated fixation, and, under these conditions, control of accommodation and fixation switched in tandem. Accommodative convergence ratios (AC:A) were abnormally low in animals with strabismus syndromes, similar to human essential infantile esotropia, and were excessively high in animals with syndromes, similar to human early onset accommodative esotropia. Conclusions There was no evidence that the strabismus seen in any of the monkeys was related to errors in accommodation. However, the crosslink gain between accommodation and vergence was abnormal in some of the animals. These abnormalities have the effect of driving vergence toward a misalignment that can only be overcome by other factors, such as fusional vergence. Thus, the authors speculate that these abnormalities in synkinesis between accommodation and convergence were present during early postnatal development when fusional vergence is weak, and that they acted as predisposing factors for the development of strabismus.

Relationship among accommodation, focus, and resolution with optical instruments.

Abstract: We studied instrument myopia as an overall effect consisting of interactions among the observer’s accommodation and resolution during instrument viewing and the manner in which the instrument is focused. Previous investigations of instrument myopia generally have been limited to only one of these variables. Our results are consistent with the dark-focus-bias theory of accommodation, which holds that accommodation tends to seek its resting point, or dark focus, whether or not the observer is viewing through an optical instrument. The amount of accommodation measured during instrument viewing was found to be dependent on instrument design features that modulate the bias of accommodation for the dark focus.

VDT screen reflections and accommodation response

Abstract: Reflections in computer screens are imaged behind the screen and therefore potentially create a conflicting cue to appropriate accommodation response to the plane of screen. We investigated the effect of screen reflections on accommodation response by producing reflections (1.33 D accommodation demand) surrounding and overlying the fixation point (2 D accommodation demand) in the screen. We have found little evidence that reflections in visual display terminal screens can influence the accuracy of the users' accommodation responses under normal binocular viewing conditions. The accuracy of the accommodation response was not significantly influenced by the position of the reflections, the presence of surrounding text (peripheral cues), or the subjects' dark focus level. The variability of accommodation response did not significantly change under the various test conditions. However, accommodation errors were apparent to a small degree (< 0.25 D) under some monocular viewing conditions in the presence of screen reflections.

Disorderly Differences: Recognition, Accommodation, and American Law

Abstract: You do not have to be me in order for us to fight alongside each other. I do not have to be you to recognize that our wars are the same. What we must do is commit ourselves to some future that can include each other and to work toward that future with the particular strengths of our individual identities. And in order to do this, we must allow each other our differences at the same time as we recognize our sameness.

Eye Accommodation: Use of Vernier Optometers in Behavioral Research

Abstract: The history of the invention and development of devices suitable for the measurement of eye accommodation in behavioral research settings is briefly reviewed. A series of small experiments involving a Microvernier Optometer that is "worn" on the head demonstrated the practicality of using the device in a variety of dynamic, real-world situations including flying an airplane simulator, driving an automobile, and-with certain constraints-typing at a video display terminal. Some surprising incidental findings are relevant to virtual imaging displays and to the Mandelbaum effect.

Amplitude of Accommodation for Different Head Positions and Different Directions of Eye Gaze

Abstract: One of the theories of the mechanism of presbyopia is the Hess-Gullstrand theory. This theory predicts that amplitude of accommodation should increase in down-gaze because of movement of the lens under the influence of gravity, and that this should be more marked for older than for younger subjects. We assessed this theory by measuring the effect of direction of eye gaze and head posture on amplitude of accommodation for two age groups. Farpoints, nearpoints, and amplitude of accommodation were determined for two subject groups, a young group aged 18 to 25 years and an older group aged 35 to 45 years. Small but significant shifts of nearpoints toward the eye were observed when head position or eye gaze was shifted from above to below the horizontal, for the younger observers only (the maximum mean difference between conditions was 1.1 D, compared with a mean accommodation level of 9.8 D for this young group). Previous studies by others found changes in the same direction for eye gaze and head position, respectively, but our changes were much smaller. We do not believe that particular care is needed in selection of head position and eye gaze during clinical measurements of the amplitude of accommodation. Because the shift of the nearpoint was noted only for the younger group, this study does not support the Hess-Gullstrand theory of presbyopia.

Accommodation During Instrument Viewing Can be Influenced by Knowledge of Object Distance

Abstract: : The level of accommodation during instrument viewing is influenced by the perceived nearness of the object in subjects with proximal, but not distal, dark focuses. Instrument myopia, Accommodations night vision goggles, Known object distance.

A zoom lens before the eyes during imagery: individual differences and strange, unexpected responses.

Abstract: Summary.-The research examined some aspects of an hypothetical involvement of the eyes in the process of imagery and the individual differences in the modulation of the Lnagery-perception interaction of 59 (7 male and 52 female) undergraduate students in psychology. The subjects were asked to image with open eyes and to project, looking through a zoom lens, the "menral" image onto a white screen. While the subject was imaging, the experimenter moved the lever of the zoom lens in the duection of an hypothetical enlargement. This movement evoked different classes of responses: 46% of the subjects had a loss, even if for a short time, of the image, 37% of the subjects observed a strange and unexpected enlargement of the mental image, 7% &d not observe any change in imagery, and 10% had other responses. A psychophys~ological discussion concerned the strange phenomena observed in this research that have not yet been explored enough. In previous researches (Ruggieri, 1991, 1993) some aspects of the interactions between imagery and perception have been examined. Data have indicated that, when a subject imaged with open eyes, a modification in the external stimulation of the retina strongly modified the imaginative activity. The researches showed also the presence of individual differences in the form of the imaginative modification. Another research (Ruggieri & Alfieri, 1992) demonstrated that, during imagery, as in "actual" perception, a mechanism of accommodation of the crystalline lens is present: imaging a "near" or a "far" figure, the bending of the crystalline lens, respectively, increases and reduces. Now it is necessary to investigate the physiological meaning of this involvement of the eyes during imagery. Are the observed modifications only peripheral correlates of central processes or could they have a particular function in the imaginative activity? To examine this question it is important to explore more deeply how, at the level of the retina, processes of imagery and perception interact when a subject is imaging with open eyes, projecting the so-called mental image onto a white screen. In other words, it is interesting to observe how the process of imagery, which I hypothesize involves the retina, interacts with the activity evoked by external luminous beams coming from outside. In earlier investigations the modification of the imaginative activity was produced by covering the eyes of the subjects, i.e., through an interruption of the external luminous stimulation (Ruggieri, 1991, 1993). In the present experiment the exploration of the interaction between mental imagery and external stimula

[Clinical studies of reduced accommodation in patients with HIV-1 infection].

Abstract: PROBLEM Reduced accommodation in HIV-patients has been described. Yet, in literature, statistics on this item cannot be found. PATIENTS AND METHOD In 58 HIV-infected subjects of different infection stages, accommodation was measured using Clement-Clark's accommodometer. Average age was 34.9 years (sigma = 8.3 years) Stages of HIV-infection were defined according to CDC (Centers for Disease Control)-classification. RESULTS The decrease in accommodation did not show any correlation to the stage of HIV-infection neither to its frequency nor to its amount. An antiretroviral therapy may reduce accommodation loss. However, the effect is not statistically significant. Incommodating accommodation loss occurred only in a few cases. CONCLUSION Reduced accommodation is an early ophthalmological symptom of HIV-1-infection. Possible causes of this phenomenon may be either direct neuronal infection by HIV 1, pathologic changes of the lens or the ciliary body.

Does display phosphor bandwidth affect the ability of the eye to focus

Abstract: The focusing response of the eye (accommodation) is degraded in monochromatic light for many subjects. To find out whether this degradation also occurs in narrowband polychromatic light, we measured the accommodation of nine young adult volunteer subjects across three bandwidths (10, 20, and 80 nm) and a broadband control (white light). The peak wavelength was 550 nm for each of the bandpass filtered stimuli, and the luminance of all targets was 10 cd/m2. Accommodation was measured with a dynamic infrared optometer while the subjects viewed threshold size, high-contrast letters under both dynamic and steady-state conditions. In the former, the optical distance of the target was varied sinusoidally from 0.0 to 2.0 diopters (optical infinity to 50 cm) at a temporal frequency of 0.5 Hz, while in the latter it was held constant at 1.0 diopter (1.0 m). We found that, under dynamic conditions, accommodative accuracy steadily improved in a statistically significant way with increases in spectral bandwidth. Under steady-state conditions, there was no statistically significant trend. These results suggest that accommodation might suffer from the use of narrowband phosphors in helmet-mounted displays under dynamic conditions, i.e., the observer might accommodate inaccurately to the display if frequent changes in focus to and from the display are required.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Apparent image quality of magnifiers depends on amplitude of accommodation

Abstract: We investigated the importance of the role of amplitude of accommodation in assessing the usefulness of magnifiers. Accommodation must be exerted to scan across the image produced by a magnifier that exhibits astigmatism and curvature of field. In general, increasing amounts of negative accommodation are required to scan from the center to the edge of the image field of view (FOV) when a simple magnifier is set for nominal magnification. It is necessary to refocus the magnifier (move it closer to the object plane) to convert the demand to positive accomodation. We used the OSLO program to design and evaluate the oblique astigmatism of four common types of magnifiers: Equiconvex, Best-form, Aspheric, and the Steinheil triplet, in powers of 10, 20, 40, 60, and 80 D. We found that the circle of least confusion (CLC), produced by the Equiconvex and Best-form lenses set at nominal magnification, lie on a surface that demands as much as 2.75 D of negative accommodation to view the CLC at the edge of a 30 degrees FOV. By refocusing the magnifier, the CLC at the edge of the field can be imaged at infinity, i.e., have zero vergence, and the demand for negative accommodation may be eliminated, but then about 2.4 D of positive accommodation is required to scan to the center of the field. The Aspheric and Steinheil magnifiers largely corrected the astigmatic errors and flattened the field. Less than 0.75 D of accommodation was sufficient to scan from the edge to the center of the object. Depending on the degree of uncorrected astigmatism and field curvature, presbyopic patients, lacking accommodative ability, will perceive constricted in-focus zones in the image field. They will continuously have to refocus the magnifier to bring adjacent zones into best-focus. This condition may cause a presbyope to reject a magnifier thought to be of good quality by a young clinician who can accommodate for best-focus across the entire FOV.

Is Increased Accommodation a Necessary Condition for Instrument Myopia

Abstract: : Emmetropic subjects focus optical instruments as though they were myopic when the level of accommodation with instrument viewing is no greater than it is without

Accommodation of Pregnancy-Related Disabilities on the Job

Abstract: The author analyzes current legal protections afforded pregnant workers who want to continue working but need some degree of accommodation in order to stay on the job, concludes that they are inadequate, and calls for various reforms. Her focus is on employers with policies that deny accommodation to all temporarily disabled workers, and on the applicability of disparate impact analysis under Title VII to protect the jobs of pregnant workers in such settings. The comment discusses the concept and the incidence of pregnancy-related job disability, the impact of nonaccommodation policies on women workers and their families, and the current status of relevant federal and state laws. The author calls for requiring reasonable accommodation of temporary pregnancy-related disabilities, expanding eligibility for pregnancy-disability leave, and expanded availability of state short-term disability insurance to protect all workers sidelined by temporary disabilities.